How Does CD Player Anti Skip Work?

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":

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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.

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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

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Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
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Reply to
Jeff Wisnia
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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

Reply to
David Tweed

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.

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Reply to
John Tserkezis

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I hope someone now edits the Wikipedia entry! Cheers, Roger

Reply to
Engineer

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.

Reply to
Joel Koltner

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