The most effective audio encoding for a YouTube source file

[SenorKaffee]

With a low-bitrate mono sound the YouTube is not the pinnacle of audio quality.
So I think encoding the most effective audio track is a pretty interesting task, even Director account source files are limited to 100 MB and the day you need to squeeze 30 minutes of video in it may come. ;)

For video there are a lot of guidelines out there - keep it at 320x240, 30fps or less, MPEG4 ASP/AVC. But what about audio?

The YouTube transcoder seems to produce 56kbit MP3 mono sound, so is it best to prepare the audio this way? Does the transcoder decode more advanced formats like HE-AAC correctly? In this case more bandwith could be reserved for the video.

Your opinions?

[Pookie]

I believe the audio is 22050 sampling rate mono MP3 64K bitrate. You can check by downloading Moitah's flv splitter and checking the audio properties.

If anything, I suppose adding a good amount of limiting or dynamic range compression would make the thing stand out. I mean, the quality is immediately destroyed when it becomes Youtubed - you may as well have the sound at a constant volume level. I know this would be counter to all audiophile recommendations for conventional mastering, but flash video/audio is hardly conventional.

Sampling a few different fragments of sound:

Looks like everything is abruptly cut below 80hz and above 9.1K, so forget about earth shaking bass and anything resembling the sound of cymbals. That also means that "S" sounds are attenuated somewhat.

Does the transcoder decode more advanced formats like HE-AAC correctly? In this case more bandwith could be reserved for the video.

Hey, that's a interesting idea. You may as well give them mono audio - providing them with lower bitrate aac as a "payload" and as a result providing you a few more megs of bitrate for the video ... uh, try it and let us know:D If I recall, they're using ffmpeg based transcoders, so they should be able to decode that aac track.

[SenorKaffee]

Okay, I will try - just need to prepare are good audio source.
If I understand HE-AAC correctly it is similar to MP3pro. If the decoder only knows the standard version, it will decode audio of worse quality. Should be noticable.

[Pookie]

I'd give them LC AAC . Try using ffmpeg to transcode that audio as well so you'll have an idea of what it'll sound like:

ffmpeg -i audio.aac -vn -ac 1 -ab 64 -y audio.mp3

[SenorKaffee]

Thanks for the command line.

Hm - I aimed at bitrates even lower than the 64kbit YouTube seems to use, something in the area of 48 or 32 kbit. That´s why I had HE-AAC in mind, I already experimented with 48 kbit HE-AAC audio in a stereo track, although in combination with AVC and for regular distribution on a homepage.

Using the same bitrate makes it a question of how lossy the double encode is. Maybe a 64kbit MP3 mono audio track in the input file sounds the same after passing the YouTube transcoder.

By the way, is there a way to emulate the way YouTube encodes without actually uploading content to the website? That would make testing much easier.

[Pookie]

"By the way, is there a way to emulate the way YouTube encodes without actually uploading content to the website? That would make testing much easier"

Sure, just transcode without any concern at all over the output quality.:D


I'd do a google search on ffmpeg flv youtube

This thread has some interesting tidbits.

http://blogs.chron.com/makingmovies/archives/2006/04/youtube_and_the.html

10 Minute FLV from Youtube opened with GSPOT

http://img2.freeimagehosting.net/uploads/th.c074ccbcb2.png (http://img2.freeimagehosting.net/image.php?c074ccbcb2.png)

[SenorKaffee]

I already stumbled upon that article but quickly dismissed it for telling me to feed FLV to YouTube. Speed isn´t everything. o_O

There are some interesting points in the argument, though.
Using FFMPEG to encode 56kbps ABR MP3 should result in a file that sounds similar to YouTube audio.