How Does CD Player Anti Skip Work?

J

Jeff Wisnia

Guest
Someone asked me to explain to them how the electronic anti skip
function used in portable and car CD players worked and while I knew it
involved buffering the audio, I got thoroughly confused when I tried to
figure out how to explain that in more detail.

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?

Thanks guys,

Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.
 
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?
Click to expand...
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
 
David Tweed wrote:

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.
Click to expand...
May be worth adding, that although a buffer however small, is always being
used, that for the CD players that have the "anti-shock" feature, have a
LARGER buffer than those without.

In addition, they *might* spend a little more on the physical shock
resistance of the tray and such, but not a lot. Since adding ram is by far
easier, smaller and cheaper than the engineering that has to go into a tray to
stop jogging skips, the extra ram wins every time.

So to the OP, it's not really a "feature" in the true sense, it's just more
of what the circuitry already has.
--
Linux Registered User # 302622
<http://counter.li.org>
 
On May 16, 7:33 pm, John Tserkezis
<j...@techniciansyndrome.org.invalid> wrote:
David Tweed wrote:
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.

  May be worth adding, that although a buffer however small, is always being
used, that for the CD players that have the "anti-shock" feature, have a
LARGER buffer than those without.

  In addition, they *might* spend a little more on the physical shock
resistance of the tray and such, but not a lot.  Since adding ram is by far
easier, smaller and cheaper than the engineering that has to go into a tray to
stop jogging skips, the extra ram wins every time.

  So to the OP, it's not really a "feature" in the true sense, it's just more
of what the circuitry already has.
--
Linux Registered User # 302622
http://counter.li.org
Click to expand...
I hope someone now edits the Wikipedia entry!
Cheers,
Roger
 
"John Tserkezis" <jt@techniciansyndrome.org.invalid> wrote in message
news:482e19bb$0$17511$afc38c87@news.optusnet.com.au...
In addition, they *might* spend a little more on the physical shock
resistance of the tray and such, but not a lot. Since adding ram is by far
easier, smaller and cheaper than the engineering that has to go into a tray
to stop jogging skips, the extra ram wins every time.
Click to expand...
Yes, I've heard that the mechanical quality of CD player mechanisms *dropped*
significantly after the ability for the control ICs to read a bit faster than
needed and using lots of RAM buffering became common/cheap. Some of the older
CD changers for automobiles -- prior to "use lots of RAM for anti-skip" --
were quite impressive, with heavy cast frames, spring/viscious dampers, etc.
 
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