Jeff Wisnia wrote:
> Wikipedia gave me this for "Electronic Skip Protection":
> *********
> Technology
>
> When the buffering circuitry is in operation, the compact disc is read
> at a fixed read speed or CAV and the content is buffered (with optional
> ADPCM compression) and fed to RAM within the player. The audio content
> is read from RAM, optionally decompressed, and then sent to the
> amplifier. When the disc reading is interrupted, the player momentarily
> reads the data stored in RAM while the tracking circuitry finds the
> passage prior to the interruption on the CD.
> ********
>
> The word "prior" in the last sentence confuses me, I would have expected
> it to say "after" there.
>
> Could someone please give me a better explanation or a link to one?
Yes, that wording is a bit confusing. What's really happening is that
the audio to the DAC is *always* coming from the RAM, which is being
used as a FIFO buffer. When the CD-to-RAM transfer is interrupted by
a mechanical shock or whatever, the RAM-to-DAC transfer drains down
the FIFO while the CD reading mechanism re-seeks to the point of
interruption. It may seek to a point slightly "prior" to that point
in order to make resuming the reading process a little simpler.
The comment about CAV is a red herring. If the FIFO is full (no skips
are occurring), then the disc is "throttled back" to CLV by the flow
control mechanism, just as it would be without a skip buffer. But when
a skip occurs, the disc will speed up in order to refill the FIFO in
a relatively short time.
Note that none of this helps with a CD that "skips" as a result of
a physical defect in its surface.
Uncompressed CD audio runs about 10 MB per minute, so a CD player
with a "6 second" skip buffer has about 1 MB of RAM.
-- Dave Tweed
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