Rockbox mail archiveSubject: Re: AW: Question about CD-Quality and MP3
Re: AW: Question about CD-Quality and MP3
From: Frej Bjon <fbjon_at_abo.fi>
Date: Thu, 02 Feb 2006 17:30:21 +0200
That's a very good page. *bookmarks*
Now I see, I was under the impression that "joint stereo" = "intensity
stereo", and that "stereo" = the encoder can arbitrarily switch between
stereo and M/S.
On Thu, 02 Feb 2006 16:49:13 +0200, Michael E. DiFebbo <medifebbo_at_rcn.com>
> 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