Rockbox mail archiveSubject: Re: AW: Question about CD-Quality and MP3
Re: AW: Question about CD-Quality and MP3
From: Michael E. DiFebbo <medifebbo_at_rcn.com>
Date: Thu, 02 Feb 2006 09:49:13 -0500
Sorry for the double-post, but I found this site about joint stereo that
does a far better job of explaining it than I did in my last e-mail:
Michael E. DiFebbo
Michael E. DiFebbo wrote:
> Frej Bjon wrote:
>> There's one thing to watch out for, though: joint stereo. Unless you
>> really, really need the compression, it's better to use real stereo,
>> because joint stereo will "muddy" the stereo field and make it
>> narrower. This type of degradation can easily be heard in even the
>> most noisy situations with earphones.
> This is a common misperception, but at best it is only partially
> true. The term "joint stereo" has two different meanings when used
> with respect to encoding MP3s.
> The joint stereo method that you refer to is "intensity stereo."
> Intensity stereo is only used by modern MP3 encoders for very low
> bitrate files (lower than 96kbps if I remember correctly). For higher
> bitrates, "joint stereo" refers to M/S stereo.
> Intensity stereo encoding functions on the principle of sound
> localization - by removing the stereo component of sounds that humans
> cannot discern the direction of (i.e. the lowest bass frequencies).
> Intensity stereo coding does not perfectly reconstruct the original
> audio because of the loss of data resulting in the simplification of
> the stereo image, and can produce unwanted artifacts with certain
> types of source material.
> "Mid/Side stereo," on the other hand, encodes stereo information by
> using a mid channel , which is the sum of the left and right channels,
> and a side channel, which is the difference of the left and right
> channels. Unlike intensity stereo coding, is NOT in and of itself lossy.
> M/S stereo can be more efficient than left/right stereo (what you
> refer to as "true stereo"). Let's say that you have a recording which
> has a 5 second portion consisting only of a vocal. Vocals tend to be
> panned directly in the center of a mix, and therefore, the left and
> right channels are the same. With left/right stereo encoding,
> identical information would be stored in both the left and the right
> channel. With mid/side stereo, on the other hand, most of the
> information in those frames would be in the mid channel, and very
> little of the information would be in the side channel. Thus, the
> encoder can use fewer bits to capture the information about the side
> channel. Either method produces results that are mathematically
> identical when decoded, but since the M/S method is more efficient in
> circumstances like the one in my example, the use of M/S stereo can
> produce smaller file sizes (or higher quality at an equivalent file
> size) compared to using L/R stereo exclusively. Note that encoders
> can use either M/S stereo or L/R stereo on a frame by frame basis, so
> even with "joint stereo" enabled, the encoder still has L/R stereo as
> an option for frames that require it.
Received on 2006-02-02