Re: OT: DVD recording/playback...


"Michael A. Terrell" <mike.terrell@xxxxxxxxxxxxx> wrote in message
news:XuadnYpRG-eqo3fVnZ2dnUVZ_qrinZ2d@xxxxxxxxxxxxxxxx

Arfa Daily wrote:

Accepted, but the data rate or spin rate, would be significantly
different
between the start and end of the disk. If the (CAV) drive runs the disk
at a
constant speed, there will be a large difference in the data rate between
the start and end, just as with a CLV drive, there is a large difference
in
rotational speed between the start and end. As far as accuracy of the
writing speed goes, I would suggest that this is pretty tightly
controlled,
and is probably the reason that with high speed writers, the disk is run
at
a constant speed, and the data rate varied to match the writing position
on
the disk between outside and inside edges of the data area. It would be a
lot easier to vary the data readout rate from the record buffer, than to
try
to maintain an accurate varying (high) rotation speed, particularly on
drives that use a DC brushed motor, rather than the brushless DD types
used
by better drives.

As far as CAV drives reading data back, I was being general in assuming
that
the reverse principle was applied in that again, the disc is rotated at a
constant speed, and the data is processed at a constant rate, by varying
the
readout speed of a chunk of buffer memory, rather than the drive
switching
over to CLV for reading.

As to your point about record rates, if the disc is spun at a constant
rate,
then the data must be presented at a variable writing rate, in order to
maintain a constant data pitch along the recorded track. Messy or not, it
is
absolutely essential that this is done to preserve that constant pitch,
which is part of the CD / DVD specification. The difference in the
lengths
of track on each rotation, between the start and end of the data area, is
simply too great for the system to be able to cope with merely as a
percentage error around the nominal data rate. This is why there is a
significant difference between the spin rate at the start and end of a
disc
in a standard CLV home entertainment player, and a sophisticated servo to
take care maintaining a correct spin rate.

Arfa


Arfa, variable sectors per track has been around for a long time. As
your tracks move outward, you change the clock rate of the decoder
circuit, which changes the number of sectors. Commodore was doing it
with floppies over 25 years ago.



Hmmm. I'm not sure how that stacks up with CD / DVD though, as with those
media, there are no sectors as such, just constant length data frames,
recorded to disc at a constant pitch. This being the case, I don't think
that you would be able to decode by simply varying the clock rate of the
decoder. I think, as was suggested by someone else, that you would have to
run the data through a buffer memory, and vary the clocking of the MMU to
arrive at a constant data rate input for the decoder ??

Arfa


.

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