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How does CD anti skip protection 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 see "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)
The speed of light is 1.8*10^12 furlongs per fortnight.
Reply to
Jeff Wisnia
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To me it sounds like the description is just saying that the CD player hunts for a point before the interruption so that it can sync back up with its buffered data and continue past the place where it skipped. This makes sense if the skip was because of a mechanical jolt to the CD player, less sense if it's because of a scratch or dirt on the disc.

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   Wim Lewis , Seattle, WA, USA. PGP keyID 27F772C1
Reply to
Wim Lewis

Jeff Wisnia wrote: When the disc reading is interrupted, the player momentarily

If it soley focused on finding the passage after the interruption, it won't find the content that was pl.....ay...ing during the interruption to make a seamless join with what was playing before.

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Adrian C
Reply to
Adrian C

The descriptions is a little off.

Data is always being read from the ram that is being used as the buffer. The CD reader is looking ahead on the data and accumulating it in the buffer. This gives enough time to resync the reader from a skip condition and then read ahead again to keep the buffer filled ahead of the reading of it.

The firmware in the reader may implement the methods in their own way for example, Stack Pushing/Poping methods, circular method.

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Reply to
Jamie

If you start re-reading "after" the point of interruption, you'll almost certainly end up missing some data. Audio CD data is organized into blocks (which don't have terribly precise addressing/timing information embedded in them, alas).

In order to continue playing the music without there being some sort of audible skip or glitch, the player has to wait until the disc rotates partway around, start re-reading at a point *before* the block which was interrupted, figure out which blocks it's actually reading for the second time (and discard them), and actually being re-buffering the data starting with the very first block that was not read successfully.

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Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
  I do _not_ wish to receive unsolicited commercial email, and I will
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Reply to
Dave Platt

Thanks for that more detailed explanation. I think I get it now...

Jeff

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Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
The speed of light is 1.98*10^14 fathoms per fortnight.
Reply to
Jeff Wisnia

For car application, anti-skip is done by mechnical cushion in the pass years. Now some car set have MP3 playback so buffer RAM is needed.

Reply to
roy_ccwchan

Dave and others, Jamie is correct. It is a read ahead and buffer refill situation.

Reply to
JosephKK

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