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:



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.

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.

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

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.

Yes, but the former is more believable, if not proven (yet).

Cheers,
David.

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...

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.