Questions about Audio Bitrates for YouTube

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The "Merging Cellar" is the place where you can share your tasting experiences and discuss everything from technique, artistic matters or even business practices, but not necessarily about Pyramix. Feel free to pick the brains of the talented Merging forum users. Enjoy.

Questions about Audio Bitrates for YouTube

Postby fl » Tue May 10, 2016 15:25

I've got a client who wishes to be able to easily upload parts of the concert recording we did on Sunday, to YouTube. There was no video shot, so I'm guessing that he intends to have it play under a static image (in other words, not a high amount of variable video information displayed - will this free up more processing "space" for audio?).

I found YouTube's requirements for upload here - https://support.google.com/youtube/answer/1722171?hl=en

First off, they require MP4 for the container/wrapper. I know that it is fairly common practice for the audio within an MP4 to be either MP3 or AAC encoded, although just about any audio file format can be accommodated, with the attendant trade-off in bitrate speed.

They request that audio be AAC-LC (is this the same as what Pyramix Publishing calls LC-AAC?) encoded, at either 48 or 96 kHz. - no information is given about wordlength, so I'm guessing either 16 or 24 bit would be fine, although I'm unclear whether wordlength even has any meaning at all within the context of data-compressed audio such as MP3 or AAC. The more meaningful metric is the bitrate.

Further down the YouTube requirements webpage, there is a section dealing with Bitrate, which confuses me. They state that for 2 channel audio, the bitrate should be 384 kbps, but the maximum encoding rate available for either AAC or MP3 is 320 kbps - what gives?

I'm guessing (again) that the 384 kbps rate refers to audio encoded within an MPEG1 Layer 2 or MPEG2 Layer 2 codec (the only difference being the maximum number of audio playback channels allowed - 2 for MPEG1, more than 2 for MPEG2). I'm guessing that many video cameras encode their files within the MPEG1 or MPEG2 standard, and that this is where that bitrate setting becomes meaningful.

Yet another guess brings me to the conclusion that MPEG1 Layer 3 - what the world knows as MP3 - gives more efficient compression, and (guessing yet again) that a 192 kbps MP3 would give the equivalent audio quality as that provided by 384 kbps MPEG1 or 2 Layer 2. I'm also guessing that this would hold true for 192 kbps AAC encoding.

Am I anywhere close to understanding this correctly? Could someone with more knowledge and experience with encoding audio for video please help me resolve my questions, and clarify whether my guesses are anywhere close to the mark? Isn't YouTube (Google) likely to simply re-encode uploaded videos anyway, as part of their presumably automated intake procedure?

In any event, I'd like to be able to provide my client with audio files that will integrate seamlessly with YouTube's requirements, and maybe learn a little bit about producing audio tracks for film and video. Thanks in advance.
Frank Lockwood, Toronto, ON, Canada
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