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Rockbox mail archiveSubject: Re: AW: Question about CD-Quality and MP3Re: AW: Question about CD-Quality and MP3
From: Michael E. DiFebbo <medifebbo_at_rcn.com>
Date: Thu, 02 Feb 2006 09:37:29 -0500 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 Page template was last modified "Tue Sep 7 00:00:02 2021" The Rockbox Crew -- Privacy Policy |