> Date: 21 Oct 2001 23:30:04 -0400
> From: Stainless Steel Rat <[EMAIL PROTECTED]>
> * wolfgang buresch <[EMAIL PROTECTED]> on Sun, 21 Oct 2001
> | Q#10: "What bitrates are used?"
> | A:".....In LP2 and LP4 modes 20 bytes of dummy data per
> | 212 byte soundgroup......" --> those 20 bytes -- are they
> | a fact or just a wild guess?
>
> They are a fact. The dummy data is required so that MiniDiscs recorded in
> LP modes will not damage older players. That is, those 20 bytes per sound
> group are the equivalent of "no sound" for older players instead of loud
> noise. That is why you get slightly less than 160 minutes in LP2 mode out
> of an MD-80 disc.
Correct.
>
> Sony had to do it that way becuase they left no room for expansion in the
> MD storage algorithm. Admittedly, they never forsaw a time where anyone
> would want to *reduce* recording quality.
Actually, they left bits in the TOC for this purpose, but they never
required manufacturers to observe those bits. Too bad.
> | We know from ATRAC1 that it records at (exact) 292162.5
> bits/sec. I could
> | not find an exact number for ATRAC3 (LP2) -- is it 132300?
>
> Looking at ATRAC in "bits per second" is somewhat inaccurate because it
> doesn't work that way. An ATRAC encoder gets a 16 bit wide block of data
> (Linear PCM, same as CD-DA). Standard ATRAC removes 4 bits out of every 5
> from the signal resulting in a 5:1 reduction ratio. LP2 is 10:1, removing
> 9 out of every 10 bits. LP4 is not 20:1 bitwise reduction, but ~15:1
> reduction (I would have to check for the exact figure) combined with
> partial stereo channel merging (joint stereo).
>
> | I also read somewhere that LP uses huffman coding (fact?) -- only on
> | computers or on MD decks as well?
>
> Fiction. There is no data compression in any MD ATRAC algorithm. It is
> strictly bitwise reduction via psychoacoustic modeling.
Wrong. Both LP2 and LP4 use Huffman coding of the spectral coefficients;
this is the main reason LP2 sounds nearly as good as plain ATRAC while using
50% fewer bits. If you really want to compute the "bits per second" rate
just
compare with original ATRAC:
ATRAC: 424 bytes per sound group (stereo)
ATRAC3 LP2: 192 bytes per sound group (stereo) 132kbps
ATRAC3 LP4: 96 bytes per sound group (joint stereo) 66kbps
The ATRAC3 codec also has an intermediate 105kbps mode but it is not
used on actual MDs because the block size does not fit into an integral
number
of 192 byte sound groups. I think it is used on MemoryStick audio devices
though, since they don't care about sector sizes.
-- Howard Chu
Chief Architect, Symas Corp. Director, Highland Sun
http://www.symas.com http://highlandsun.com/hyc
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