AutoGK audiosettings

[andre5]

I tried to encode a film with audiosetting: vbr mp3 64kbps.
Instead of encoding to mp3, autogk tries to take the original ac3 and abort.
No problems with 128kbps.

[Mnl]

Hello Andre5

The problem is caused by BeSweet. It is not possible to go under 77 kbps average bitrate Mp3. There is more info about the problem in this thread:

http://forum.doom9.org/showthread.php?s=&threadid=41429

[andre5]

thank you Mnl
do you know if it works with constant bitrate?

[Mnl]

Yes it works with cbr Mp3 at 64 kbps. But still it would be so much better if it worked with vbr at low bitrates, as the audio quality is way better. However the problem is that the thread i referred to in my reply above is quite old, so it doesn't look like low bitrate vbr Mp3 is going to be supported in BeSweet anytime soon :(

Mnl

[KpeX]

VBR vs. CBR at 64 kbps is pointless, the quality will be terrible either way. With most VBR implementations of MP3 the minimum bitrate is 96 kbps.

If you really need to use a bitrate this low, use HE-AAC or possibly Vorbis with MP4/MKV/OGM.

[Mnl]

@KpeX

Thank you for your reply. It is always nice to learn something new. I don't know all that much about audio compression, but I thought variable bitrate was always better than constant bitrate. Am I to understand your answer in the way that vbr loses its advantage over cbr at low bitrates?

I'm quite interested as I have recently done an encode of a movie where I added an extra audio track (comentary by director and main actors) in low bitrate Mp3.

Mnl

[KpeX]

Hi -

First of all, for commentary tracks or other almost-all speech audio, low bitrates won't be quite as bad as music or a full-sound effect track at the same bitrate.

It is true that VBR loses some advantage at low bitrates. VBR audio at 64 kbps must average 64kbps over the entire track. The advantage of higher bitrate VBR tracks is that they can assign lower bitrates to portions of the audio that don't need it, which frees up higher bitrates for sections of the audio that need it more.

For example a 160 kbps VBR encoding might find a speech only section that only needs 96 kbps to have high quality. This brings down the average of the bitrate, and the encoder can spend more bits on an action scene later in the movie with explosions and music that will need 224 kbps for high quality.

However when encoding a 64kbps VBR track, the average bitrate is already very low. It is very unlikely that there will be any areas that need less than 64kbps to have acceptable quality, and not much of the audio will be able to have a much higher bitrate than 64kbps and still maintain a 64kbps average. Additionally when the 64kbps encoder runs into a high action and sound effects scene, it cannot spend much bitrate here because there will be no way to bring the average back down to 64kbps. Therefore at 64 kbps a VBR encoding does not have much freedom to apply different audio bitrates to different parts of the track, and the overall quality will be low, and there will not be much variation or advantage over a 64kbps CBR encoding.

The VBR will still be better than the CBR in quality, but the difference will be low.

Modern codecs such as HE-AAC do a much better job at low bitrates, especially using SBR to replicate a full spectrum signal. I hope my explanation makes sense, feel free to post any other questions on the topic :). hth,

[Mnl]

Thanks a lot for your thorough reply Kpex. I think I get it - I should have been able to figure out for myself that varying the bitrate doesn't help much if the chosen average bitrate is really low. Just never thought about it that way. I see then that it doesn't really matter that BeSweet does not support low vaiable bitrates.

[KpeX]

Originally posted by Mnl
Thanks a lot for your thorough reply Kpex. I think I get it - I should have been able to figure out for myself that varying the bitrate doesn't help much if the chosen average bitrate is really low. Just never thought about it that way. I see then that it doesn't really matter that BeSweet does not support low vaiable bitrates. Well summarized :). Btw, Welcome to the forums!

[manono]

Convert your commentary tracks to Mono. Then 64 kbps is fine.

[Mnl]

Hi Manono

It was my intention to convert the comentary track to mono, but I could not find out how to do it at the time. Reading your reply encouraged me to try to find out how to do it. But after a lot of searching and reading Doom9's audio guides for Mp3, I still can't seem to find out how to do it. I would really like to learn how, as it would often come in handy to be able to make Mp3 mono tracks out of ac3 tracks.

I would really appreciate it if you could maybe post a link to an explanation of how to do it.

[KpeX]

Hi Mnl-

Are you familiar with BeSweet? You should be ;) it's _the_ program for AV audio operations (and used by GKnot). To create a mono MP3 with BeSweet, simply add a '-m m' to the '-lame()' section of your commandline. hope this helps,

[Mnl]

Thanks a lot for helping me out Kpex, I guess it was a bit of a stupid question - if I had read Doom9's guide carefully enough, I would have seen the line saying "For the full list of commandline switches refer to the overview." I just am not all too comfortable with using BeSweet on my own (meaning when Gordian Knot doesn't do all the setting up for me :D), but I'm slowly finding out how to do things on my own.

Just tried adding the commands you gave me and managed to make a perfect cbr mono Mp3 @ 64kbps from an ac3 track in just two attempts :)