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Multichannel audio quality

Hi there guys,

When I was suggesting 128kbps Windows Media Audio 9 Professional would be enough for "CD Quality" to convert an AC3 file (which is extracted from DVD), one of my friends said:

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I don't think 128kbps is good enough for encoding 5.1 sound. Think about it. 128 / 6 is 21.3. That's 21.3 kbps for each channel. That's the equivalent of encoding stereo sound at 42 kbps. I really don't think that's enough. I would recommend 384kbps for 5.1 sound. That's more like 128kbps for stereo and will sound much better.


My question: Is it how it works?

If that's correct we will need 384kbps at least in order to deliver 64kbps for each channel.

Thank you,
McoreD


Edit: Thanks guys for all your replied. Always learning something new.

Multichannel audio quality

Reply #1
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My question: Is it how it works?

No. Your friend ignored the existance of Multichannel Decorrelation.

This encoding technique means that the encoder encodes the redundancy among the channels only once, since it's a waste of space encoding this information on each channel separately.

That means something like: if you encode a stereo stream at 128kbps, the encoder encodes one stream "alone" at 56kbps, the other at 56 too, and the redundancy between them at 16 (56+56+16=128), giving each channel an actual bitrate of 72 (56+16) and not 64.  (I hope I made myself clear)

AAC supports this technique, and WMA Pro probably does as well. Ogg vorbis does support, but it isn't implemented in the encoder yet.

I think a bitrate around 256-320kbps would be better for good quality.

Multichannel audio quality

Reply #2
Also LFE (Low Frequency Effect) channel is the supporting bass effect channel which probably doesn't take lots of kbps at all.
Juha Laaksonheimo




Multichannel audio quality

Reply #6
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I don't think 128kbps is good enough for encoding 5.1 sound. Think about it. 128 / 6 is 21.3. That's 21.3 kbps for each channel. That's the equivalent of encoding stereo sound at 42 kbps. I really don't think that's enough. I would recommend 384kbps for 5.1 sound. That's more like 128kbps for stereo and will sound much better.


In 5.1 channel config you have following situation:

2 channel pairs (L, R  and  L', R')
1 single channel ( C )
1 bass, or, LFE channel

In a coding system which does not use so-called inter-channel "coupling" you can figure it out by:

bit rate = 2 x stereo bit rate for same quality  +  1 mono bit rate for same q + X, where X is only few kb/s  for LFE...

So, for, say AAC - to achieve quality of 128 kb/s, stereo:
2 x 128 + 64 + 1 Kb/s = ~ 320 kb/s  very close to, actually 321

For MP3 you would need:
2 x 160 + 80 + x... = ~400 Kb/s for the same quality

For HE-AAC:
2 x 48 + 32 (because of coupling channels need less) = 128 Kb/s

For MP2:
2 x 256 + 128 = 640 Kb/s


Etc...

Multichannel audio quality

Reply #7
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Microsoft claims(http://www.microsoft.com/windows/windowsmedia/9series/codecs.aspx) that wma pro @ 192 - 256 would be similar to 384kbps ac3.

Microsoft also claims WMA std has the same quality of MP3 at half bitrate.

The page, which I am referring to, isn't ment to be read by some lamers who believe that xing mp3 is listenable at 96 kbps. Its target audience is most likely to be big-scale broadcasters and tv companies because it compares wm9 to mpeg2 at broadcast bitrates instead of saying something like "mpeg-4 quality at half bitrate".
This page might give even some correct information because most of their claims are believable even when thinking mpeg-4.

 
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