Michael Fabian 'Xaymar' Dirks
29 lines
3.8 KiB
HTML
29 lines
3.8 KiB
HTML
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title: "Low Latency Streaming to Twitch"
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category: Blog
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tags: ["Tutorial", "Twitch", "OBS Studio", "StreamFX", "x264", "NVENC", "WordPress Archive"]
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---
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<p class="block">If you've been following my social media for the past few years, or have read my <a rel="noreferrer noopener" href="https://blog.xaymar.com/guides/high-quality-recordings/" target="_blank">Recording</a> or <a rel="noreferrer noopener" href="https://blog.xaymar.com/2020/06/24/the-art-of-encoding-with-nvidia-turing-nvenc/" target="_blank">Streaming on NVIDIA Turing/Ampere</a> guides, I'm always chasing the next higher level of "perfection". And this time I was chasing the lowest possible latency on Twitch - and it appears that I have finally found it, after days of trying.</p><!--more-->
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<p class="block">My search began as usual, with a manual investigation into the problem - I didn't have infinite resources to throw at the problem, and I didn't want to increase Twitch's hosting costs either. Therefore I was stuck with manual testing, which worked out well enough in the end for me, and didn't take much time either. I started my testing with the Key-Frame Interval, going down from 5 Seconds to 1 Second in 1 Second interval. </p>
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<p class="block">Oddly enough, I noticed that certain Intervals would end up "grouped together" into a higher latency. For example, 1 Second and 2 Seconds would end up being 3 Seconds of latency, while 3 Seconds and 4 Seconds would end up being 5 Seconds of latency, and 5 Seconds ended up being 7 Seconds of latency. Unable to make sense of it, I moved on to other settings. </p>
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<p class="block">Look-Ahead was the first one to recieve the axe, and it made no difference - 30 Frames or 0 Frames, both had the same latency. Similar to this was Zero-Latency and B-Frames, neither made a difference. But one option did, and it is what every RTMP+HLS based service recommends against: Adaptive I-Frames, or Scene Cut in x264 terms.</p>
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<p class="block">Both x264 and NVENC use this option to insert complete Key-Frames if they happen to be a better option, for example with a fade to black, or a complete cut to new content. Unfortunately this option results in a measurable and visible increase in latency between a 250 and 500ms - half a Second gone to unfortunate Key-Frame placement.</p>
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<p class="block">This brought the latency ladder down to ~2.4 Seconds for a Key-Frame Interval of 1 and 2 Seconds, ~4.2 Seconds for an Interval of 3 and 4, and ~5.6 Seconds for an Interval of 5. Careful observers may have just noticed the same thing I did, which is that the latency appears to climb in ~1.5 Second intervals - and that's what I tested next. </p>
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<p class="block">I set up the Key-Frame Interval to 1.5 Seconds, and then I watched the Video Stats...</p>
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<figure class="block"><img src="https://cdn.xaymar.com/blog/2021/04/msedge_dRkkOGANII.png" alt="" class="wp-image-2197" width="425" height="409" />
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<figcaption>One Second Transport Latency</figcaption>
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</figure>
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<p class="block"><strong>I finally had it.</strong> Almost a perfectly flat 1 Second latency for the stream itself, though with all the Rendering, Encoding and Muxing buffering, it still ended up being roughly 2 Seconds. Which is still less than the lowest total latency I managed with Adaptive I-Frames being Enabled, and any other Key-Frame Interval. I repeated this test several times, and was able to achieve the same latency over several hours in Firefox, Chrome and Edge. I am a happy Twitch streamer now.</p>
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<p class="block"><strong>Disclaimer:</strong> This discovery is not guaranteed to work everywhere, and may be unique to the Ingest server I used. It may even be different due to changing which GPU is used, which CPU is used, which RAM is used, how your network looks like, etc. <em>It is provided at no guarantee or warranty.</em></p>
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