kludge@grissom.larc.nasa.gov (Scott Dorsey) (06/10/91)
Read my ips. JITTER AT THE PHOTOTRANSISTOR IS NOT IMPORTANT!! Jitter in
most points along the audio path is not important. The signal is repeatedly
reclocked before it gets to the D/A. While the jitter on the oscillator that
drives the latch directly before the D/A is absolutely critical, jitter at
any point before then is fairly unimportant unless it is so awful that it
results in errors (something terrible that you could certainly notice).
The signal from the photocell is usually stored in a big FIFO buffer anyway,
so jitter is no big deal. High frequency response is a big deal to make sure
that you get sharp, well-defined square waves which are easy to decode with
a low error rate.
Directionality is also important. In fact, the circuit which drives the servo
to keep the laser on track puts considerably more strain on the design
parameters of the phototransistors than the digital audio stuff does.
Let's all repeat this together: There are two kinds of errors in the bitstream
correctable and noncorrectable. Correctable errors don't matter, because {
once they have been corrected, the data is identical to the original data
on the recording side. Uncorrectable errors occur every few minutes and
cause the player to have to interpolate, but since they are so infrequent
they are not a problem. Jitter before the final D/A stage is unimportant
because when the data is reclocked it will be eliminated and replaced with
new jitter characteristic of the reclocking stage, unless the jitter is so
bad that it causes an error, which doesn't happen in modern logic.
Folks, the D/A stage is the key. Everything before it is just digital
logic; if the bitstream is the same then it doesn't make any difference.
Everything afterward is conventional analogue circuitry that we know how
to build well (even if many manufacturers don't). The D/A stage is the
major source of all our glories and woes and not much else is worth
flaming about.
--scott