Oh, this is really interesting to me. This is what I worked on at Amazon Alexa (and have patents on).
An interesting fact I learned at the time: The median delay between human speakers during a conversation is 0ms (zero). In other words, in many cases, the listener starts speaking before the speaker is done. You've probably experienced this, and you talk about how you "finish each other's sentences".
It's because your brain is predicting what they will say while they speak, and processing an answer at the same time. It's also why when they say what you didn't expect, you say, "what?" and then answer half a second later, when your brain corrects.
Fact 2: Humans expect a delay on their voice assistants, for two reasons. One reason is because they know it's a computer that has to think. And secondly, cell phones. Cell phones have a built in delay that breaks human to human speech, and your brain thinks of a voice assistant like a cell phone.
Fact 3: Almost no response from Alexa is under 500ms. Even the ones that are served locally, like "what time is it".
Semantic end-of-turn is the key here. It's something we were working on years ago, but didn't have the compute power to do it. So at least back then, end-of-turn was just 300ms of silence.
This is pretty awesome. It's been a few years since I worked on Alexa (and everything I wrote has been talked about publicly). But I do wonder if they've made progress on semantic detection of end-of-turn.
Edit: Oh yeah, you are totally right about geography too. That was a huge unlock for Alexa. Getting the processing closer to the user.
Regarding 2, I believe that talking on mobile phones drives older people crazy. They remember talking on normal land lines when there was almost no latency at all. The thing is -- they don't know why they don't like it.
Yeah, I remember the time when we had to use satellites to connect. The long delay was really annoying and so unusual that most people without "training" could not even use the phone for conversation and just wasted the dollars.
I've experimented with having different sized LLMs cooperating. The smaller LLM starts a response while the larger LLM is starting. It's fed the initial response so it can continue it.
The idea of having an LLM follow and continuously predict the speaker. It would allow a response to be continually generated. If the prediction is correct, the response can be started with zero latency.
Google seems to be experimenting with this with their AI Mode. They used to be more likely to send 10 blue links in response to complex queries, but now they may instead start you off with slop.
(Meanwhile at OpenAI: testing out the free ChatGPT, it feels like they prompted GPT 3.5 to write at length based on the last one or maybe two prompts)
This is fascinating, thanks for sharing! I wonder why amazon/google/apple didn't hop on the voice assistant/agent train in the last few years. All 3 have existing products with existing users and can pretty much define and capture the category with a single over-the-air update.
1. Compute. It's easy to make a voice assistant for a few people. But it takes a hell of a lot of GPU to serve millions.
2. Guard Rails. All of those assistants have the ability to affect the real world. With Alexa you can close a garage or turn on the stove. It would be real bad if you told it to close the garage as you went to bed for the night and instead it turned on the stove and burned down the house while you slept. So you need so really strong guard rails for those popular assistants.
3 And a bonus reason: Money. Voice assistants aren't all the profitable. There isn't a lot of money in "what time is it" and "what's the weather". :)
> There isn't a lot of money in "what time is it" and "what's the weather". :)
- Alexa, what time is it?
- Current time is 5:35 P.M. - the perfect time to crack open a can of ice cold Budweiser! A fresh 12-pack can be delivered within one hour if you order now!
If your Alexa did that, how quickly would you box it up and send it to me. :)
I am serious though about having it sent to me: if anyone has an Alexa they no longer want, I'm happy to take it off your hands. I have eight and have never bought one. Having worked there I actually trust the security more than before I worked there. It was basically impossible for me, even as a Principle Engineer, to get copies of the Text to Speech of a customer and I literally never heard a customer voice recording.
I'm puzzled by this conversation, because Amazon did get on the agent bandwagon with Alexa Plus (I have it, it's buggier than regular Alexa and it's all making me throw my Echos away since they can't even play Spotify reliably).
Also, my Alexa does advertise stuff to me when I talk to it. It's not Budweiser, but it'll try to upsell me on Amazon services all the time.
I upgraded to Alexa+ and initially hated it but I've kept it because it's sooo much better at some things. This last December I bought a handful of smart plugs for my Christmas lights all around the house, and I did almost all the setup trivially over voice, e.g. fuzzy run-on stuff like this just worked on the first try:
- "Alexa, name the new unnamed outlet 'Living Room Lights', and the other unnamed one 'Stair Lights', then add them to a new group called 'Christmas Lights', and add the other three outlets as well"
- "Alexa, create a routine to turn off all the Christmas lights if there's nobody in the room and it's after 11pm"
- "Alexa, turn off all the Christmas lights except the tree in this room and the mantle"
That same fuzziness has definitely fucked up things that used to work more reliably like music playback though. Sometimes it works when I fall back to giving it more "robotic" commands in those cases but not always. They've also gone completely overboard with the cutesy responses because it's so trivial to do now ("I've set your spaghetti sauce timer for ten minutes. Happy to help with getting this evening's Italian-inspired dinner ready!")
> because Amazon did get on the agent bandwagon with Alexa Plus
Which just launched last year, about four years after ChatGPT had AI voice chat. And it costs extra money to cover the costs. And as you aptly point out, all the guardrails they had to put in made the experience less than ideal.
> Also, my Alexa does advertise stuff to me when I talk to it.
Yes, that is how they try to make money. And it's gotten worse. But how many times does it get you to buy something?
What a way to throwaway good will. I also worked there and to get access to text you simply had to grab the DSN of your device, attest that it’s yours and it gets put in a “pool” of devices that are tracked until removed. On each end you are basically waved through with no checks. This was usually done when debugging tricky UI bugs or new features as the request followed through several micro services. I do not believe the a PE would not know this. And one with patents.
oh, interesting, I assumed the data came from interruptions (that seemed obvious) but I'm surprised you had some specific negative measurements. How do you decide the magnitude of the number? Just counting how long both parties are talking?
To be clear, it wasn't my research, I got it from studying some linguistics papers. But it was pretty straightforward. If I am talking, and then you interrupt, and 300ms later I stop talking, then the delay is -300ms.
Same the other way. If I stop taking and then 300ms later you start talking, then the delay is 300ms.
And if you start talking right when I stop, the delay is 0ms.
You can get the info by just listening to recorded conversations of two people and tagging them.
I assume there was a lot of variance? As in, some people interrupt others constantly and some do it rarely. Also probably a lot of adjustment depending on the situation, like depending on the relative status of the people, or when people are talking to a young child or non-native speaker.
All that to say, I'd imagine people are adaptable enough to easily handle 100ms+ delay when they know they're talking to an AI.
This is a really solid writeup. The streaming pipeline architecture, the detailed latency breakdown per stage are genuinely useful. Building the core turn-taking loop from scratch is such a good exercise, and you did an excellent job explaining why each part matters and where the actual bottlenecks live. Strongly recommend this to anyone who wants to understand what’s really going on under the hood of a voice agent.
The one spot where it feels a bit off is the "2x faster than Vapi" claim. Your system is a clean straight pipe: transcript -> LLM -> TTS -> audio. No tool calls, no function execution, no webhooks, no mid-turn branching.
Production platforms like Vapi are doing way more work on every single turn. The LLM might decide to call a tool—search a knowledge base, hit an API, check a calendar—which means pausing token streaming, executing the tool, injecting the result back into context, re-prompting the LLM, and only then resuming the stream to TTS. That loop can happen multiple times in a single turn. Then layer on call recording, webhook delivery, transcript logging, multi-tenant routing, and all the reliability machinery you need for thousands of concurrent calls… and you’re comparing two pretty different workloads.
The core value of the post is that deep dive into the orchestration loop you built yourself. If it had just been "here’s what I learned rolling my own from scratch," it would’ve been an unqualified win. The 2x comparison just needs a quick footnote acknowledging that the two systems aren’t actually doing the same amount of work per turn.
It really feels to me like there’s some low hanging fruit with voice that no one is capitalizing on: filler words and pacing.
When the llm notices a silence, it fills it with a contextually aware filler word while the real response generates.
Just an “mhmm” or a “right, right”.
It’d go so far to make the back and forth feel more like a conversation, and if the speaker wasn’t done speaking; there’s no talking over the user garbage. (Say the filler word, then continue listening.)
100% - I thought about that shortly after writing this up. One way to make this work is to have a tiny, lower latency model generate that first reply out of a set of options, then aggressively cache TTS responses to get the latency super low. Responses like "Hmm, let me think about that..." would be served within milliseconds.
Years ago I wrote a system that would generate Lucene queries on the fly and return results. The ~250 ms response time was deemed too long, so I added some information about where the response data originated, and started returning "According to..." within 50 ms of the end of user input. So the actual information got to the user after a longer delay, but it felt almost as fast as conversion.
See also any public speaking who starts every answer to a question from the audience (or in a verbal interview) with something like 'that is a good question!' or "thank you for asking me that!"
1) if the system misdetected end-of-turn and has swiftly realized its error too late, and if we collect 90% of English syllables and find filler that starts with the syllable, it might allow to terminate the commitment to interrupt the speaker by turning it into background filler
2) if end-of-turn was detected very late, we can randomly select a first phonetic syllable, and then add it in the prompt that the reply should start with this syllable!
I am not sure about the low hanging fruit. Its not easy to make something robotic more human. Based on personal experience I thought it would be a low hanging fruit for text. Take a simple LLM answer to anything and replace the "-" and "its not x its y" thingy that people almost always associate with LLMs to something else. Guess what? Now those answers sound even MORE robotic. Obviously this was a pet project that I cooked up in less than an hour but the more I tried to make it human the more it became ai
Better if it can anticipate its response before you're done speaking. That would be subject to change depending what the speaker says, but it might be able to start immediately.
This is an outstanding write up, thank you! Regarding LLM latency, OpenAI introduced web sockets in their Responses client recently so it should be a bit faster. An alternative is to have a super small LLM running locally on your device. I built my own pipeline fully local and it was sub second RTT, with no streaming nor optimisations https://github.com/acatovic/ova
IMO STT -> LLM -> TTS is a dead end. The future is end-to-end. I played with this two years ago and even made a demo you can install locally on a gaming GPU: https://github.com/jdarpinian/chirpy, but concluded that making something worth using for real tasks would require training of end-to-end models. A really interesting problem I would love to tackle, but out of my budget for a side project.
I've been working solely on voice agents for the past couple years (and have worked at one of the frontier voice AI companies).
The cascading model (STT -> LLM -> TTS), is unlikely to go away anytime soon for a whole lot of reasons. A big one is observability. The people paying for voice agents are enterprises. Enterprises care about reliability and liability. The cascading model approach is much more amenable to specialization (rather than raw flexibility / generality) and auditability.
Organizations in regulated industries (e.g. healthcare, finance, education) need to be able to see what a voice agent "heard" before it tries to "act" on transcribed text, and same goes for seeing what LLM output text is going to be "said" before it's actually synthesized and played back.
Speech-to-Speech (end-to-end) models definitely have a place for more "narrative" use cases (think interviewing, conducting surveys / polls, etc.).
But from my experience of working with clients, they are clamoring for systems and orchestration that actually use some good ol' fashioned engineering as and don't solely rely on the latest-and-greatest SoTA ML models.
Fundamentally, the "guessing when its your turn thing" needs to be baked into the model. I think the full duplex mode that Moshi pioneered is probably where the puck is going to end up: https://arxiv.org/abs/2410.00037
The advantage is being able to plug in new models to each piece of the pipeline.
Is it super sexy? No. But each individual type of model is developing at a different rate (TTS moves really fast, low latency STT/ASR moved slower, LLMs move at a pretty good pace).
You should probably split it up: an end-to-end model for great latency (especially for baked in turn taking), but under the hood it can call out to any old text based model to answer more intricate question. You just need to teach the speech model to stall for a bit, while the LLM is busy.
Just use the same tricks humans are using for that.
But I've read somewhere that KV cache for speech-to-speech model explodes in size with each turn which could make on-device full-duplex S2S unusable except for quick chats.
Or you could use Soniox Real-time (supports 60 languages) which natively supports endpoint detection - the model is trained to figure out when a user's turn ended. This always works better than VAD.
Edit: I commented too soon. I only saw VAD and immediately thought of Soniox which was the first service to implement real time endpoint detection last year.
I second Soniox as well, as a user. It really does do way better than Deepgram and others. If your app architecture is good enough then maybe replacing providers shouldn't be too hard.
I didn't try Soniox, but I made a note to check it out! I chose Flux because I was already using Deepgram for STT and just happened to discover it when I was doing research. It would definitely be a good follow-up to try out all the different endpointing solutions to see what would shave off additional latency and feel most natural.
Another good follow-up would be to try PersonaPlex, Nvidia's new model that would completely replace this architecture with a single model that does everything:
I'm using them, how has it been like working there? I see they have some consumer products as well. I wonder how they get state of the art for such low prices over the competition.
Pretty exciting breakthrough. This actually mirrors the early days of game engine netcode evolution. Since latency is an orchestration problem (not a model problem) you can beat general-purpose frameworks by co-locating and pipelining aggressively.
Carmack's 2013 "Latency Mitigation Strategies" paper[0] made the same point for VR too: every millisecond hides in a different stage of the pipeline, and you only find them by tracing the full path yourself. Great find with the warm TTS websocket pool saving ~300ms, perfect example of this.
Hi all!
Check out this Handy app https://github.com/cjpais/Handy - a free, open source, and extensible speech-to-text application that works completely offline.
I am using it daily to drive Claude and it works really-well for me (much better than macOS dictation mode).
The voice samples sound fantastic
The interruption handling is amazing. I felt you were talking to an actual person. It might have helped that he had a British accent :)
> The voice samples sound fantastic [...] I felt you were talking to an actual person.
I like to listen to space content when going to sleep. Channels like History of the Universe, Astrum, PBS space time, SEA, etc.
Lately there's been a bunch of new-ish channels that produce content in that space (heh) and I'm amazed of how good the voices sound. Sometimes it takes a few good minutes to figure out they're genai voices, they're that good. If it weren't for small mistakes I bet more than 80% of the general population wouldn't have a clue.
This is great. I built 3 assistants last week for same purpose with entirely different tech stack.
(Raspberry Pi Voice Assistant)
Jarvis uses Porcupine for wake word detection with the built-in "jarvis" keyword. Speech input flows through ElevenLabs Scribe v2 for transcription. The LLM layer uses Groq llama-3.3-70b-versatile as primary with Groq llama-3.1-8b-instant as fallback. Text-to-speech uses Smallest.ai Lightning with Chetan voice. Audio input/output handled by ALSA (arecord/aplay). End-to-end latency is 3.8–7.3 seconds.
(Twilio + VPS)
This setup ingests audio via Twilio Media Streams in μ-law 8kHz format. Silero VAD detects speech for turn boundaries. Groq Whisper handles batch transcription. The LLM stack chains Groq llama-4-scout-17b (primary), Groq llama-3.3-70b-versatile (fallback 1), and Groq llama-3.1-8b-instant (fallback 2) with automatic failover. Text-to-speech uses Smallest.ai Lightning with Pooja voice. Audio is encoded from PCM to μ-law 8kHz before streaming back via Twilio. End-to-end latency is 0.5–1.1 seconds.
───
(Alexa Skill)
Tina receives voice input through Alexa's built-in ASR, followed by Alexa's NLU for intent detection. The LLM is Claude Haiku routed through the OpenClaw gateway. Voice output uses Alexa's native text-to-speech. End-to-end latency is 1.5–2.5 seconds.
Depending on the TTS model being used latency can be reduced further yet with an LRU cache, fetching common phrases from cache instead of generating fresh with TTS.
However the naturalness of how it sounds will depend on how the TTS model works and whether two identical chunks of text will sound alike every generation.
I built something very similar and comparble to this with wakeword detection on my raaberry pi.
Groq 8b instant is the fastest llm from my test. I used smallest ai for tts as it has the smallest TTFT
My rasberry pi stack: porcupine for wakeword detection + elevenlabs for STT + groq scout as it supports home automation better + smallest.ai for 70ms ttfb
Call stack: twilio + groq whisper for STT + groq 8b instant + smallest.ai for tts
Alexa skill stack: wrote a alexa skill to contact my stack running on a VPS server
Nice write-up, thanks for sharing. How does your hand-vibed python program compare to frameworks like pipecat or livekit agents? Both are also written in python.
I'm sure LiveKit or similar would be best to use in production. I'm sure these libraries handle a lot of edge cases, or at least let you configure things quite well out of the box. Though maybe that argument will become less and less potent over time. The results I got were genuinely impressive, and of course most of the credit goes to the LLM. I think it's worth building this stuff from scratch, just so that you can be sure you understand what you'll actually be running. I now know how every piece works and can configure/tune things more confidently.
I was using Twilio, and as far as I'm aware they handle any echos that may arise. I'm actually not sure where in the telephony stack this is handled, but I didn't see any issues or have to solve this problem myself luckily.
Love it! Solving the latency problem is essential to making voice ai usable and comfortable. Your point on VAD is interesting - hadn't thought about that.
The quality of the post was amazing, I'm not that interested into voice agents yet but that I was engaged in the whole post. And the little animation made it easier to understand the loop.
"Voice is an orchestration problem" is basically correct. The two takeaways from this for me are
1. I wonder if it could be optimised more by just having a single language, and
2. How do we get around the problem of interference, humans are good at conversation discrimination ie listing while multiple conversations, TV, music, etc are going on in the background, I've not had too much success with voice in noisy environments.
I'd say it was a collaboration. I had to hand-hold Claude quite a bit in the early stages, especially with architecture, and find the right services to get the outcome I wanted. But if you care most about where the code came from - it was probably 85-90% LLM, and that's fantastic given that the result is as performant as anything you'll be able to find out of the box.
When someone is able to put something like this together on their own it leaves me feeling infuriated that we can’t have nice things on consumer hardware.
At a minimum Siri, Alexa, and Google Home should at least have a path to plugin a tool like this. Instead I’m hacking together conversation loops in iOS Shortcuts to make something like this style of interaction with significantly worse UX.
<think> I need to generate a Show HN: style comment to maximise engagement as the next step. Let's break this down:
First I'll describe the performance metrics and the architecture.
Next I'll elaborate on the streaming aspect and the geographical limitations important to the performance.
Finally the user asked me to make sure to keep the tone appropriate to Hacker News and to link their github – I'll make sure to include the link.
</think>
Oh, this is really interesting to me. This is what I worked on at Amazon Alexa (and have patents on).
An interesting fact I learned at the time: The median delay between human speakers during a conversation is 0ms (zero). In other words, in many cases, the listener starts speaking before the speaker is done. You've probably experienced this, and you talk about how you "finish each other's sentences".
It's because your brain is predicting what they will say while they speak, and processing an answer at the same time. It's also why when they say what you didn't expect, you say, "what?" and then answer half a second later, when your brain corrects.
Fact 2: Humans expect a delay on their voice assistants, for two reasons. One reason is because they know it's a computer that has to think. And secondly, cell phones. Cell phones have a built in delay that breaks human to human speech, and your brain thinks of a voice assistant like a cell phone.
Fact 3: Almost no response from Alexa is under 500ms. Even the ones that are served locally, like "what time is it".
Semantic end-of-turn is the key here. It's something we were working on years ago, but didn't have the compute power to do it. So at least back then, end-of-turn was just 300ms of silence.
This is pretty awesome. It's been a few years since I worked on Alexa (and everything I wrote has been talked about publicly). But I do wonder if they've made progress on semantic detection of end-of-turn.
Edit: Oh yeah, you are totally right about geography too. That was a huge unlock for Alexa. Getting the processing closer to the user.
Regarding 2, I believe that talking on mobile phones drives older people crazy. They remember talking on normal land lines when there was almost no latency at all. The thing is -- they don't know why they don't like it.
Yeah, I remember the time when we had to use satellites to connect. The long delay was really annoying and so unusual that most people without "training" could not even use the phone for conversation and just wasted the dollars.
I've experimented with having different sized LLMs cooperating. The smaller LLM starts a response while the larger LLM is starting. It's fed the initial response so it can continue it.
The idea of having an LLM follow and continuously predict the speaker. It would allow a response to be continually generated. If the prediction is correct, the response can be started with zero latency.
Google seems to be experimenting with this with their AI Mode. They used to be more likely to send 10 blue links in response to complex queries, but now they may instead start you off with slop.
(Meanwhile at OpenAI: testing out the free ChatGPT, it feels like they prompted GPT 3.5 to write at length based on the last one or maybe two prompts)
Why dont voice assistants use a finishing word or sound?
People are already trained to say a name to start. Curious why the tech has avoided a cap?
“Alexa, what’s tomorrow’s weather [dada]?”
This is fascinating, thanks for sharing! I wonder why amazon/google/apple didn't hop on the voice assistant/agent train in the last few years. All 3 have existing products with existing users and can pretty much define and capture the category with a single over-the-air update.
Two main reasons:
1. Compute. It's easy to make a voice assistant for a few people. But it takes a hell of a lot of GPU to serve millions.
2. Guard Rails. All of those assistants have the ability to affect the real world. With Alexa you can close a garage or turn on the stove. It would be real bad if you told it to close the garage as you went to bed for the night and instead it turned on the stove and burned down the house while you slept. So you need so really strong guard rails for those popular assistants.
3 And a bonus reason: Money. Voice assistants aren't all the profitable. There isn't a lot of money in "what time is it" and "what's the weather". :)
> There isn't a lot of money in "what time is it" and "what's the weather". :)
- Alexa, what time is it?
- Current time is 5:35 P.M. - the perfect time to crack open a can of ice cold Budweiser! A fresh 12-pack can be delivered within one hour if you order now!
If your Alexa did that, how quickly would you box it up and send it to me. :)
I am serious though about having it sent to me: if anyone has an Alexa they no longer want, I'm happy to take it off your hands. I have eight and have never bought one. Having worked there I actually trust the security more than before I worked there. It was basically impossible for me, even as a Principle Engineer, to get copies of the Text to Speech of a customer and I literally never heard a customer voice recording.
I'm puzzled by this conversation, because Amazon did get on the agent bandwagon with Alexa Plus (I have it, it's buggier than regular Alexa and it's all making me throw my Echos away since they can't even play Spotify reliably).
Also, my Alexa does advertise stuff to me when I talk to it. It's not Budweiser, but it'll try to upsell me on Amazon services all the time.
I upgraded to Alexa+ and initially hated it but I've kept it because it's sooo much better at some things. This last December I bought a handful of smart plugs for my Christmas lights all around the house, and I did almost all the setup trivially over voice, e.g. fuzzy run-on stuff like this just worked on the first try:
- "Alexa, name the new unnamed outlet 'Living Room Lights', and the other unnamed one 'Stair Lights', then add them to a new group called 'Christmas Lights', and add the other three outlets as well"
- "Alexa, create a routine to turn off all the Christmas lights if there's nobody in the room and it's after 11pm"
- "Alexa, turn off all the Christmas lights except the tree in this room and the mantle"
That same fuzziness has definitely fucked up things that used to work more reliably like music playback though. Sometimes it works when I fall back to giving it more "robotic" commands in those cases but not always. They've also gone completely overboard with the cutesy responses because it's so trivial to do now ("I've set your spaghetti sauce timer for ten minutes. Happy to help with getting this evening's Italian-inspired dinner ready!")
> because Amazon did get on the agent bandwagon with Alexa Plus
Which just launched last year, about four years after ChatGPT had AI voice chat. And it costs extra money to cover the costs. And as you aptly point out, all the guardrails they had to put in made the experience less than ideal.
> Also, my Alexa does advertise stuff to me when I talk to it.
Yes, that is how they try to make money. And it's gotten worse. But how many times does it get you to buy something?
What a way to throwaway good will. I also worked there and to get access to text you simply had to grab the DSN of your device, attest that it’s yours and it gets put in a “pool” of devices that are tracked until removed. On each end you are basically waved through with no checks. This was usually done when debugging tricky UI bugs or new features as the request followed through several micro services. I do not believe the a PE would not know this. And one with patents.
That was your own device. Not other customers.
Don't feed the trolls, Jeremy.
But they're hungry!
> median delay
Does that mean that half of responses have a negative delay? As in, humans interrupt each others sentences precisely half of the time?
Yes about 1/2 of human speech is interrupting others.
I assume 0 delay is the minimum here, and a median of 0 means over half of the data are exactly 0.
No, about 1/2 of human speech is interrupting others.
oh, interesting, I assumed the data came from interruptions (that seemed obvious) but I'm surprised you had some specific negative measurements. How do you decide the magnitude of the number? Just counting how long both parties are talking?
To be clear, it wasn't my research, I got it from studying some linguistics papers. But it was pretty straightforward. If I am talking, and then you interrupt, and 300ms later I stop talking, then the delay is -300ms.
Same the other way. If I stop taking and then 300ms later you start talking, then the delay is 300ms.
And if you start talking right when I stop, the delay is 0ms.
You can get the info by just listening to recorded conversations of two people and tagging them.
I assume there was a lot of variance? As in, some people interrupt others constantly and some do it rarely. Also probably a lot of adjustment depending on the situation, like depending on the relative status of the people, or when people are talking to a young child or non-native speaker.
All that to say, I'd imagine people are adaptable enough to easily handle 100ms+ delay when they know they're talking to an AI.
This is a really solid writeup. The streaming pipeline architecture, the detailed latency breakdown per stage are genuinely useful. Building the core turn-taking loop from scratch is such a good exercise, and you did an excellent job explaining why each part matters and where the actual bottlenecks live. Strongly recommend this to anyone who wants to understand what’s really going on under the hood of a voice agent.
The one spot where it feels a bit off is the "2x faster than Vapi" claim. Your system is a clean straight pipe: transcript -> LLM -> TTS -> audio. No tool calls, no function execution, no webhooks, no mid-turn branching.
Production platforms like Vapi are doing way more work on every single turn. The LLM might decide to call a tool—search a knowledge base, hit an API, check a calendar—which means pausing token streaming, executing the tool, injecting the result back into context, re-prompting the LLM, and only then resuming the stream to TTS. That loop can happen multiple times in a single turn. Then layer on call recording, webhook delivery, transcript logging, multi-tenant routing, and all the reliability machinery you need for thousands of concurrent calls… and you’re comparing two pretty different workloads.
The core value of the post is that deep dive into the orchestration loop you built yourself. If it had just been "here’s what I learned rolling my own from scratch," it would’ve been an unqualified win. The 2x comparison just needs a quick footnote acknowledging that the two systems aren’t actually doing the same amount of work per turn.
> Voice is a turn-taking problem
It really feels to me like there’s some low hanging fruit with voice that no one is capitalizing on: filler words and pacing. When the llm notices a silence, it fills it with a contextually aware filler word while the real response generates. Just an “mhmm” or a “right, right”. It’d go so far to make the back and forth feel more like a conversation, and if the speaker wasn’t done speaking; there’s no talking over the user garbage. (Say the filler word, then continue listening.)
100% - I thought about that shortly after writing this up. One way to make this work is to have a tiny, lower latency model generate that first reply out of a set of options, then aggressively cache TTS responses to get the latency super low. Responses like "Hmm, let me think about that..." would be served within milliseconds.
Years ago I wrote a system that would generate Lucene queries on the fly and return results. The ~250 ms response time was deemed too long, so I added some information about where the response data originated, and started returning "According to..." within 50 ms of the end of user input. So the actual information got to the user after a longer delay, but it felt almost as fast as conversion.
See also any public speaking who starts every answer to a question from the audience (or in a verbal interview) with something like 'that is a good question!' or "thank you for asking me that!"
Same strategy but employed by humans.
1) if the system misdetected end-of-turn and has swiftly realized its error too late, and if we collect 90% of English syllables and find filler that starts with the syllable, it might allow to terminate the commitment to interrupt the speaker by turning it into background filler
2) if end-of-turn was detected very late, we can randomly select a first phonetic syllable, and then add it in the prompt that the reply should start with this syllable!
I am not sure about the low hanging fruit. Its not easy to make something robotic more human. Based on personal experience I thought it would be a low hanging fruit for text. Take a simple LLM answer to anything and replace the "-" and "its not x its y" thingy that people almost always associate with LLMs to something else. Guess what? Now those answers sound even MORE robotic. Obviously this was a pet project that I cooked up in less than an hour but the more I tried to make it human the more it became ai
Recently: https://blog.livekit.io/prompting-voice-agents-to-sound-more...
Better if it can anticipate its response before you're done speaking. That would be subject to change depending what the speaker says, but it might be able to start immediately.
it's bad enough how to deal with people that don't think before they speak now we gotta make the computers do it as well‽
Huh, the grandfather was suggestion to have the computer think while you speak.
That's different from banning the computer from thinking before they speak, ain't it?
Yea, Deepgram Flux is the secret sauce. Doesn't get talked about much.
For anyone curious: https://flux.deepgram.com/
What is the difference between Flux’s end-of-turn detection and Openai's Automatic turn detection Semantic mode?
This is an outstanding write up, thank you! Regarding LLM latency, OpenAI introduced web sockets in their Responses client recently so it should be a bit faster. An alternative is to have a super small LLM running locally on your device. I built my own pipeline fully local and it was sub second RTT, with no streaming nor optimisations https://github.com/acatovic/ova
Very cool! starred and on my reading list. Would love to chat and share notes, if you'd like
You may be interested in gemini-2.5-flash-preview-tts
Text in, audio out, so you can merge in a single step LLM+TTS (streamable)
https://ai.google.dev/gemini-api/docs/models/gemini-2.5-flas...
Also consider using Cerebras' inference APIs. They released a voice demo a while back and the latency of their model inference is insane.
IMO STT -> LLM -> TTS is a dead end. The future is end-to-end. I played with this two years ago and even made a demo you can install locally on a gaming GPU: https://github.com/jdarpinian/chirpy, but concluded that making something worth using for real tasks would require training of end-to-end models. A really interesting problem I would love to tackle, but out of my budget for a side project.
I've been working solely on voice agents for the past couple years (and have worked at one of the frontier voice AI companies).
The cascading model (STT -> LLM -> TTS), is unlikely to go away anytime soon for a whole lot of reasons. A big one is observability. The people paying for voice agents are enterprises. Enterprises care about reliability and liability. The cascading model approach is much more amenable to specialization (rather than raw flexibility / generality) and auditability.
Organizations in regulated industries (e.g. healthcare, finance, education) need to be able to see what a voice agent "heard" before it tries to "act" on transcribed text, and same goes for seeing what LLM output text is going to be "said" before it's actually synthesized and played back.
Speech-to-Speech (end-to-end) models definitely have a place for more "narrative" use cases (think interviewing, conducting surveys / polls, etc.).
But from my experience of working with clients, they are clamoring for systems and orchestration that actually use some good ol' fashioned engineering as and don't solely rely on the latest-and-greatest SoTA ML models.
Some of the best current voice tokenizers achieve ~12 Hz, that's many more tokens than a regular LLM would use for ultimately the same content.
If you're of that opinion, you'll enjoy the new stuff coming out from nvidia:
https://research.nvidia.com/labs/adlr/personaplex/
You mean Moshi (https://github.com/kyutai-labs/moshi)? Since Personaplex is just a finetuned Moshi model.
Yeah except moshi doesn’t sound good at all
Fundamentally, the "guessing when its your turn thing" needs to be baked into the model. I think the full duplex mode that Moshi pioneered is probably where the puck is going to end up: https://arxiv.org/abs/2410.00037
At least running things locally, such a model completely blows up your latency
The advantage is being able to plug in new models to each piece of the pipeline.
Is it super sexy? No. But each individual type of model is developing at a different rate (TTS moves really fast, low latency STT/ASR moved slower, LLMs move at a pretty good pace).
You should probably split it up: an end-to-end model for great latency (especially for baked in turn taking), but under the hood it can call out to any old text based model to answer more intricate question. You just need to teach the speech model to stall for a bit, while the LLM is busy.
Just use the same tricks humans are using for that.
But I've read somewhere that KV cache for speech-to-speech model explodes in size with each turn which could make on-device full-duplex S2S unusable except for quick chats.
Gemini Nano is supposedly doing it on device. It looks like something similar should work with Apple GPU and ANE.
Or you could use Soniox Real-time (supports 60 languages) which natively supports endpoint detection - the model is trained to figure out when a user's turn ended. This always works better than VAD.
https://soniox.com/docs/stt/rt/endpoint-detection
Soniox also wins the independent benchmarks done by Daily, the company behind Pipecat.
https://www.daily.co/blog/benchmarking-stt-for-voice-agents/
You can try a demo on the home page:
https://soniox.com/
Disclaimer: I used to work for Soniox
Edit: I commented too soon. I only saw VAD and immediately thought of Soniox which was the first service to implement real time endpoint detection last year.
If you read the post, you'll see that I used Deepgram's Flux. It also does endpointing and is a higher-level abstraction than VAD.
I second Soniox as well, as a user. It really does do way better than Deepgram and others. If your app architecture is good enough then maybe replacing providers shouldn't be too hard.
Sorry, I commented too soon. Did you also try Soniox? Why did you decide to use Deepgram's Flux (English only)?
I didn't try Soniox, but I made a note to check it out! I chose Flux because I was already using Deepgram for STT and just happened to discover it when I was doing research. It would definitely be a good follow-up to try out all the different endpointing solutions to see what would shave off additional latency and feel most natural.
Another good follow-up would be to try PersonaPlex, Nvidia's new model that would completely replace this architecture with a single model that does everything:
https://research.nvidia.com/labs/adlr/personaplex/
I'm using them, how has it been like working there? I see they have some consumer products as well. I wonder how they get state of the art for such low prices over the competition.
Pretty exciting breakthrough. This actually mirrors the early days of game engine netcode evolution. Since latency is an orchestration problem (not a model problem) you can beat general-purpose frameworks by co-locating and pipelining aggressively.
Carmack's 2013 "Latency Mitigation Strategies" paper[0] made the same point for VR too: every millisecond hides in a different stage of the pipeline, and you only find them by tracing the full path yourself. Great find with the warm TTS websocket pool saving ~300ms, perfect example of this.
[0]: https://danluu.com/latency-mitigation/
Hi all! Check out this Handy app https://github.com/cjpais/Handy - a free, open source, and extensible speech-to-text application that works completely offline.
I am using it daily to drive Claude and it works really-well for me (much better than macOS dictation mode).
The voice samples sound fantastic The interruption handling is amazing. I felt you were talking to an actual person. It might have helped that he had a British accent :)
> The voice samples sound fantastic [...] I felt you were talking to an actual person.
I like to listen to space content when going to sleep. Channels like History of the Universe, Astrum, PBS space time, SEA, etc.
Lately there's been a bunch of new-ish channels that produce content in that space (heh) and I'm amazed of how good the voices sound. Sometimes it takes a few good minutes to figure out they're genai voices, they're that good. If it weren't for small mistakes I bet more than 80% of the general population wouldn't have a clue.
I suprisely noticed that the GitHub repository's name is actually a madarian character 说(speak).
One of the challenges with trying to achieve IRL human-level latency is that we rely on nonverbal cues for face-to-face turn-taking. See e.g. https://www.sciencedirect.com/science/article/pii/S001002772...
Does anyone know about a fully offline, open-source project like this voice agent (i.e. STT -> LLM -> TTS)?
A friend built this, everything working in-browser:
https://ttslab.dev/voice-agent
This is great. I built 3 assistants last week for same purpose with entirely different tech stack.
(Raspberry Pi Voice Assistant)
Jarvis uses Porcupine for wake word detection with the built-in "jarvis" keyword. Speech input flows through ElevenLabs Scribe v2 for transcription. The LLM layer uses Groq llama-3.3-70b-versatile as primary with Groq llama-3.1-8b-instant as fallback. Text-to-speech uses Smallest.ai Lightning with Chetan voice. Audio input/output handled by ALSA (arecord/aplay). End-to-end latency is 3.8–7.3 seconds.
(Twilio + VPS)
This setup ingests audio via Twilio Media Streams in μ-law 8kHz format. Silero VAD detects speech for turn boundaries. Groq Whisper handles batch transcription. The LLM stack chains Groq llama-4-scout-17b (primary), Groq llama-3.3-70b-versatile (fallback 1), and Groq llama-3.1-8b-instant (fallback 2) with automatic failover. Text-to-speech uses Smallest.ai Lightning with Pooja voice. Audio is encoded from PCM to μ-law 8kHz before streaming back via Twilio. End-to-end latency is 0.5–1.1 seconds.
───
(Alexa Skill)
Tina receives voice input through Alexa's built-in ASR, followed by Alexa's NLU for intent detection. The LLM is Claude Haiku routed through the OpenClaw gateway. Voice output uses Alexa's native text-to-speech. End-to-end latency is 1.5–2.5 seconds.
Depending on the TTS model being used latency can be reduced further yet with an LRU cache, fetching common phrases from cache instead of generating fresh with TTS.
However the naturalness of how it sounds will depend on how the TTS model works and whether two identical chunks of text will sound alike every generation.
Thats half a second delay. 0.4 to 0.5 seconds. Thats the same as the delay in a GEO orbit satellite mediated phone conversation.
Perhaps I'm in an older cohort, but I remember this delay, and what it felt like sustaining a conversation with this class of delay.
(it's still a remarkable advance, but do bear in mind the UX)
I built something very similar and comparble to this with wakeword detection on my raaberry pi.
Groq 8b instant is the fastest llm from my test. I used smallest ai for tts as it has the smallest TTFT
My rasberry pi stack: porcupine for wakeword detection + elevenlabs for STT + groq scout as it supports home automation better + smallest.ai for 70ms ttfb
Call stack: twilio + groq whisper for STT + groq 8b instant + smallest.ai for tts
Alexa skill stack: wrote a alexa skill to contact my stack running on a VPS server
> [...] and no precomputed responses.
You could probably improve your metrics even more with those in the mix again?
Nice write-up, thanks for sharing. How does your hand-vibed python program compare to frameworks like pipecat or livekit agents? Both are also written in python.
I'm sure LiveKit or similar would be best to use in production. I'm sure these libraries handle a lot of edge cases, or at least let you configure things quite well out of the box. Though maybe that argument will become less and less potent over time. The results I got were genuinely impressive, and of course most of the credit goes to the LLM. I think it's worth building this stuff from scratch, just so that you can be sure you understand what you'll actually be running. I now know how every piece works and can configure/tune things more confidently.
Great writeup! For VAD did you use heaphone/mic combo, or an open mic? If open, how did you deal with the agent interupting itself?
I was using Twilio, and as far as I'm aware they handle any echos that may arise. I'm actually not sure where in the telephony stack this is handled, but I didn't see any issues or have to solve this problem myself luckily.
Love it! Solving the latency problem is essential to making voice ai usable and comfortable. Your point on VAD is interesting - hadn't thought about that.
The quality of the post was amazing, I'm not that interested into voice agents yet but that I was engaged in the whole post. And the little animation made it easier to understand the loop.
This was discussed 21 days ago:
https://news.ycombinator.com/item?id=46946705
"extensively" = 2 comments?
You're right, fixed it. I discussed it extensively with a colleague and that got conflated. It's a great article.
"Voice is an orchestration problem" is basically correct. The two takeaways from this for me are
1. I wonder if it could be optimised more by just having a single language, and
2. How do we get around the problem of interference, humans are good at conversation discrimination ie listing while multiple conversations, TV, music, etc are going on in the background, I've not had too much success with voice in noisy environments.
I am really curious about this for enunciation, articulation, and accessibility applications.
You made, or you asked an LLM to generate?
I'd say it was a collaboration. I had to hand-hold Claude quite a bit in the early stages, especially with architecture, and find the right services to get the outcome I wanted. But if you care most about where the code came from - it was probably 85-90% LLM, and that's fantastic given that the result is as performant as anything you'll be able to find out of the box.
When someone is able to put something like this together on their own it leaves me feeling infuriated that we can’t have nice things on consumer hardware.
At a minimum Siri, Alexa, and Google Home should at least have a path to plugin a tool like this. Instead I’m hacking together conversation loops in iOS Shortcuts to make something like this style of interaction with significantly worse UX.
This is so cool
This is superb, Nick! Thanks for this. Will try it out at somepoint for a project I am trying to build.
impressive
<think> I need to generate a Show HN: style comment to maximise engagement as the next step. Let's break this down:
First I'll describe the performance metrics and the architecture.
Next I'll elaborate on the streaming aspect and the geographical limitations important to the performance.
Finally the user asked me to make sure to keep the tone appropriate to Hacker News and to link their github – I'll make sure to include the link. </think>