playing ac3 5.1 in audcatity

[Andabuddi]

how to play a TS file having ac3 5.1 in audacity.

Audio volume seems to bi high. Any preference options recommended for playing ac3 5.1?

[steve]

Audacity does not yet support multi-channel playback.
Mix the tracks down to a single stereo track and use the "Amplify" effect on the mix to bring the level to less than 0 dB.

[Andabuddi]

Thanks Steve!

What mixing algorithm is used in audacity to downmix ac3 5.1 to wav 2 chs?

I know Audacity uses ffmpeg for decoding ac3. But is the downmix 5.1 to 2, different from the ffmpeg?

Because, i see the audio volume is different playing below two in audacity:
- when ac3 5.1 downmixed to wav 2 chs using ffmpeg -- low volume.
- TS ac3 5.1 played using mix and amplify --- high volume.

[Gale Andrews]

What mixing algorithm is used in audacity to downmix ac3 5.1 to wav 2 chs?

I know Audacity uses ffmpeg for decoding ac3. But is the downmix 5.1 to 2, different from the ffmpeg?

Because, i see the audio volume is different playing below two in audacity:
- when ac3 5.1 downmixed to wav 2 chs using ffmpeg -- low volume.
- TS ac3 5.1 played using mix and amplify --- high volume.

FFmpeg doesn't do any mixing as far as I understand it, any more than other codecs do. The mixing is done on the decoded PCM audio. What do you mean by "downmixed to wav 2 chs using ffmpeg" - in Audacity?

If you Mix and Render the tracks their volume will be amplified just the same as when you play them before mixing.

If they were not already clipping (distorting) at this stage by being played together, then amplifying to 0 dB might make the result louder.

If they were already clipping then Amplify would only fix the clipping if your Quality Preferences were set to 32-bit float (which is default). In that case you would have to use the gain sliders on each track before amplifying.


Gale

[Andabuddi]

Yes correct! mixing is not by codec of ffmpeg, but on the PCM audio by the channel mixing block part of the ffmpeg resampler.

What i meant by "when ac3 5.1 downmixed to wav 2 chs using ffmpeg" is, ffmpeg decoding ac3 5.1 audio and later downmixing to stereo by the channel mixing part of the resampler.

My question was the audio volume is higher when i do the 'mix and render' and 'amplify to 0db' for the 5.1 ac3 audio in Audacity.

Whereas in the 2nd case, using ffmpeg for the decoding of ac3 5.1 and downmix to 2 chs(resampler), the resultant 2 chs wav file played in audacity has high volume.

So was trying to find why volume is high in the 1st case i.e with audacity. Does audacity uses different mixing than ffmpeg? Or as you said how do i use the gain sliders for the 5.1 ac3 audio to play the resultant mixed signal with appropriate volume?

thanks! appreciate your response.

[Gale Andrews]

My question was the audio volume is higher when i do the 'mix and render' and 'amplify to 0db' for the 5.1 ac3 audio in Audacity.

It would happen if you mixed and rendered and the result of that was that the peak was below -0 dB (meaning that when you open Amplify, a positive amplification value will be given in the "Amplification (dB)" box). Applying Amplify in that situation without changing its settings would amplify the track so the peak was then 0 dB, which would make it louder.

how do i use the gain sliders for the 5.1 ac3 audio to play the resultant mixed signal with appropriate volume?

The sliders are in the Track Control Panel:
http://manual.audacityteam.org/man/Audio_Tracks#panel

If you want to make the whole mix quieter without altering the balance between the channels, you have to make each track quieter (move the gain slider leftwards) by the same amount. If you only reduce the gain on one channel it will make the whole slightly quieter but the main effect will be to make that channel sound weaker in the mix.

See also:
http://manual.audacityteam.org/man/Mixing



Gale

[steve]

If you want to make the whole mix quieter without altering the balance between the channels, you have to make each track quieter (move the gain slider leftwards) by the same amount.

It's probably easier to make "all tracks quieter by the same amount" by selecting all tracks (shortcut: Ctrl+A), and then applying the "Amplify" effect with a negative amount of amplification.