Converting 6 channels to stereo and quality

[swyn]

I have been trying to downmix 6 channels audio files into 2 channel ones (ogg/aac -> wave). The process completes normally, but the resulting file is full of breaks, pops, and crackles. These artifacts are not in the original files. I have tried to convert from AAC and Ogg formats directly but get the poor quality. I have also tried converting ogg files (using oggdec and faad) into multichannel wave files (which still have no artifacts), and then using dmc to convert those into stereo wave files. Artifacts still remain. Other programs such as foobar2000 and faad (with downmix option) can convert these files with no problems at all, but I like dmc and would love to continue to use it. :smile2: Any ideas on fixing this problem is appreciated. Thank you.

[LtData]

Re: Converting 6 channels to stereo and quality

Try the latest dMC beta and AAC codecs. Also, the newest dMC AAC codec uses FAAD to decode AAC files.

[swyn]

Re: Converting 6 channels to stereo and quality

Thank you for the quick reply. I obtained 11.5rc2 and it does indeed downmix to 2 channels correctly when using an aac source file. However, downmixing from a 6 channel wave file still results in artifacts. :(

[Spoon]

Re: Converting 6 channels to stereo and quality

AAC 6 channel >> wave 2 channel is ok
wave 6 >> wave 2 has problems?

[LtData]

Re: Converting 6 channels to stereo and quality

But your WAV 6-channel file plays OK? There aren't any artifacts in it?

[swyn]

Re: Converting 6 channels to stereo and quality

After trying more tracks, it seems to do it converting from both AAC, ogg, and wave. The files it occurs on are pretty low in volume, so I'm not sure if that has anything to do with things or not. Lounder tracks don't seem to have artifacts present (at least I'm not hearing them). Here is how I've been testing things :

aac -> FAAD -> 6 channel wave (plays fine, no problems) -> dbpoweramp -> 2 channel wave (artifacts occur)

ogg -> oggdec -> 6 channel wave (no problems) -> dbpoweram -> 2 channel wave (artifacts occur)

aac -> FAAD (w/-d option) -> 2 channel wave (no artifacts)

ogg -> oggdec -> 6 channel wave (no problems) -> foobar2000 (5.1 downmix) -> 2 channel wave (no artifacts)

I hope this helps clear things up.

[Spoon]

Re: Converting 6 channels to stereo and quality

Perhaps you could email one of the files it does it on, slowly a new dbpoweramp will be ready where the channel mapping (and downmix) percentages can be set:

http://www.dbpoweramp.com/email.htm

[swyn]

Re: Converting 6 channels to stereo and quality

E-mail sent. Thank you very much for taking a look into this matter.

[Spoon]

Re: Converting 6 channels to stereo and quality

I know for 6 ch to 2 I use:

Lt=L+0.707*(C-SL-SR+LFE)
Rt=R+0.707*(C+SL+SR+LFE)

Obviously depending on the audio values it is possible to clip, a later version of dMC will allow the percentages to be set.

[JensPH]

Re: Converting 6 channels to stereo and quality

I know for 6 ch to 2 I use:

Lt=L+0.707*(C-SL-SR+LFE)
Rt=R+0.707*(C+SL+SR+LFE)

Obviously depending on the audio values it is possible to clip, a later version of dMC will allow the percentages to be set.

Hi Spoon
Is the - and the + supposed to mean adding the channels inverted and non-inverted?
Also I don't quite get why you would add the Surround Right to tje Left channel and the Surround Left to the Right channel?

I am downmixing a 5.1 DTS album to stereo by trial and error with regards to the levels because just mapping the channels using ie -50%/-6dB when mapping the LFE to both front channels resulted in massive clipping (even though all channels had already been lowered by 2dB). I have solved that but can't seem to get the levels of the surround and center channels right.

Best regards,
Jens

[Spoon]

Re: Converting 6 channels to stereo and quality

You can use the DSP effect 'Channel Map' and apply the channel exactly as you wish. But remember you are squashing 6 channels into 2 so have to be careful of clipping.

[JensPH]

Re: Converting 6 channels to stereo and quality

You can use the DSP effect 'Channel Map' and apply the channel exactly as you wish. But remember you are squashing 6 channels into 2 so have to be careful of clipping.

Yes and it Works great. But I am wondering if the phase of the surround channels should be inverted. Do have advice regarding that?
BR/Jens

[Spoon]

Re: Converting 6 channels to stereo and quality

It has been over 10 years ago when I worked on it. I used best practices at the time.

[JensPH]

Re: Converting 6 channels to stereo and quality

It has been over 10 years ago when I worked on it. I used best practices at the time.

Yeah I did see that the tread was quite old but hoped for the best

I found the ATSC standard and are experimenting with different settings

http://atsc.org/wp-content/uploads/2015/03/A52-201212-17.pdf

Thanks for the hint about best practices, that lead me straight to the standard

Br//Jens