This message from one of the SYSOPS on the CompuServe midiforum is one of
the better explanations of SCMS implementation that I have read. It also
offers an explanation as to why the SCMS on the Hoontech board may be
implemented incorrectly, and why I can copy pre-recorded MD's, but not my
originals.
-- Martin
: 593859 S15/Sound Cards (CIS:MIDIFORUM)
06-Feb-00 21:32:11
Sb: Digital discrepancy
Fm: Dave Tosti-Lane -Sysop 70334,3165
To: Martin Schiff [CONSULT] 76702,1355
Replies: 0 TID: 7735 Par: 593828 Chd: 0 Sib: 0
>> I am told by people who seem to know a lot about SCMS that I should be
able to make a first generation copy (from the original) of a minidisc that
was recorded from an analog source. <<
The operative word is "should". There are several bits in the channel block
data stream that determine SCMS behavior, or at least, that have some
effect. One defines the format as either pro or consumer, that is only read
by "pro" equipment, that is, equipment designed to read either AES3 or
S/PDIF input. If it reads "pro" then the system actually knows to read the
copy protection bit as something else (part of the Emphasis setting). The
Consumer/Pro bit is bit "0" of byte "0", the copyright status bit is bit "2"
of Byte "0", and finally, the _generation_ bit is bit "7" of Byte "1".
The scheme is designed so that you can make a Digital copy of copywrighted
material, but you cannot make a copy of a copy. SCMS first looks at the
Pro/Consumer bit, and if it sees a "1", (for Pro format) it does not operate
at all. (my suspicion is that some consumer gear may not make this check,
simply assuming material is always consumer)
Next, if the material is identified as consumer (Byte zero, bit zero set to
"0"), which it _always_ will be for anything recorded on a consumer level MD
(even a recording made from the analog inputs), then it looks at byte zero
bit 2. If it sees a "0" here, then copyright is enabled, if it sees a "1",
copyright is not enabled, and it stops looking. Again, I strongly suspect
that consumer MD recorders by default set this bit to "0" on _all_
recordings, regardless of their origin.
If the copy bit is set to "0", then the last check is to see whether this is
a first generation or second generation copy. Here is where mistakes are
easily made in setting up receiving systems, because there are two different
schemes to determine the meaning of this bit, and they are exactly the
reverse of each other. For most media, the generation bit (called the "L"
bit) is read as "original" if the bit is a "1" and as a "copy" if the bit is
a "0". BUT, believe it or not, for laser optical products, the reverse is
true, a "0" indicates "original" and a "1" indicates "copy". (For CDs it's
even crazier, and the "L" bit isn't used at all, with the "C" bit being "0"
for "no copyrite", solid "1" for "original with Copyrite" and alternating
back and forth from 1 to 0 at a 4-10Hz rate to indicate "copy of original"
!!)
Byte 1 bits 0-6 are supposed to indicate the type of equipment that is
connected, so that the system can interpret the L bit correctly.
So, what I suspect is happening, is that the card is failing to distinguish
the type of equipment, looking at the L bit, reading a "0" which your MD
encoded since this is an "original" recording, but interpreting it to say
that the MD is already a copy, and not letting you make a digital copy.
Which is scant comfort of course. <g>
The amusing part is, if this is what's happening, then the reason you are
able to make a copy from the pre-recorded MD is precisely because the record
company tried to set it so you couldn't.
Dave Tosti-Lane
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