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CD Read Errors--Rare?

In the latest issue of The Absolute Sound on p. 131, Robert Harley claims that errors in a CD player's bit stream are rare. His argument is that an error in the first bit in the sample would throw the amplitude off by half, resulting in "a very loud click." Since we don't hear loud clicks during playback, there must not be many first bit errors, which means there must not be many error at all.

Here is my question: Since the sample is only 1/44,100 of a second long, and speaker diaphrams do not react instantaneously to signal changes, would the "click" end up being inaudible?
socprof
eldartford
4,262 posts
 
The error correcting code used for CD data is not a band-aid for the occasional read error. It is an essential part of the data storage and retrieval process. It permits the system to be operated at a much higher bandwidth than the hardware could manage if read errors could not be allowed to occur. Read errors are frequent, but a group of errors extensive enough to get through the error correcting code is rare.
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tarsando
578 posts
 
It wasn't the single bit error he was concerned about if I remember correctly. It was the two bit error, which you can definetely hear, even on LPs.
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panzrwagn's avatar
panzrwagn
552 posts
 

Music is encoded on a CD in digital form as a stream of binary digits or bits. A typical CD may have as many as a million errors.

Here's the best and relatively non-technical explanation of why that's not a problem:

https://www.irishtimes.com/news/science/high-fidelity-how-the-sound-of-cds-stays-error-free-1.2362987

Consider an array of 4 disk drives, a Redundant Array of Independent Drives or RAID.  I write data on the first 3, and on the 4th and calculate the parity (parity is calculated from the sum of the data bits, and is either even, 0, or odd, 1. So, writing 001 sums odd, so the parity is 1, writing 011 sums even, so its parity is 0). When I read the data, if the read sum doesn't match the parity, say 011 is misread as 010, then the parity check algorithm determines the error and corrects it. 

N.B. Modern disk arrays, as used by a streaming service for example, contain hundreds or even thousands of drives and might use striping across as many as twenty or more drives with double parity bit error correction built in. They continue to operate flawlessly even when one or two disks fail completely.  And with a thousand or two thousand drives per array, individual drive failures are common. 

 

 

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