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Audio CD Error Correction

Started by Tim Wescott January 2, 2013
I vaguely remember reading a statement about audio CDs, to the effect 
that the error correction scheme is designed with some sort of magic so 
that it suffers a certain amount of soft degradation before the audio 
goes out entirely.

This was a long, long time ago, and either I or the article I was reading 
could have it wrong.

Is anyone up on CD audio recording formats?  Is the above statement 
correct, or just a product of faulty memory?

-- 
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
On Wed, 02 Jan 2013 14:53:18 -0600, Tim Wescott <tim@seemywebsite.com>
wrote:

>I vaguely remember reading a statement about audio CDs, to the effect >that the error correction scheme is designed with some sort of magic so >that it suffers a certain amount of soft degradation before the audio >goes out entirely. > >This was a long, long time ago, and either I or the article I was reading >could have it wrong. > >Is anyone up on CD audio recording formats? Is the above statement >correct, or just a product of faulty memory? > >-- >My liberal friends think I'm a conservative kook. >My conservative friends think I'm a liberal kook. >Why am I not happy that they have found common ground? > >Tim Wescott, Communications, Control, Circuits & Software >http://www.wescottdesign.com
CDs use an algebraic code (Reed-Solomon) with heavy interleaving: http://en.wikipedia.org/wiki/Cross-interleaved_Reed%E2%80%93Solomon_coding There's probably a decent tutorial out there somewhere that shows exactly how the interleaving works with the codes, but the idea is that the sequential bits across tracks are heavily interleaved so that a long string of sequential errors gets distributed across many codewords. So each codeword doesn't need huge error correcting capability and can be reasonably high-rate (i.e., low overhead). The downside is that a fair amount of memory is needed for deinterleaving and there is some associated latency. Neither of those is a big deal for the application. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
In article <xb-dnRcbhYSjA3nNnZ2dnUVZ_vEAAAAA@web-ster.com>, 
tim@seemywebsite.com says...
> I vaguely remember reading a statement about audio CDs, to the effect > that the error correction scheme is designed with some sort of magic so > that it suffers a certain amount of soft degradation before the audio > goes out entirely. > > This was a long, long time ago, and either I or the article I was reading > could have it wrong. > > Is anyone up on CD audio recording formats? Is the above statement > correct, or just a product of faulty memory? > >
In the CD, two layers of Reed?Solomon coding separated by a 28-way convolutional interleaver yields a scheme called Cross-Interleaved Reed Solomon Coding (CIRC)... Philippe
On Wed, 02 Jan 2013 21:06:09 +0000, Eric Jacobsen wrote:

> On Wed, 02 Jan 2013 14:53:18 -0600, Tim Wescott <tim@seemywebsite.com> > wrote: > >>I vaguely remember reading a statement about audio CDs, to the effect >>that the error correction scheme is designed with some sort of magic so >>that it suffers a certain amount of soft degradation before the audio >>goes out entirely. >> >>This was a long, long time ago, and either I or the article I was >>reading could have it wrong. >> >>Is anyone up on CD audio recording formats? Is the above statement >>correct, or just a product of faulty memory? >> >>-- >>My liberal friends think I'm a conservative kook. My conservative >>friends think I'm a liberal kook. Why am I not happy that they have >>found common ground? >> >>Tim Wescott, Communications, Control, Circuits & Software >>http://www.wescottdesign.com > > CDs use an algebraic code (Reed-Solomon) with heavy interleaving: > > http://en.wikipedia.org/wiki/Cross-interleaved_Reed%E2%80%
93Solomon_coding
> > There's probably a decent tutorial out there somewhere that shows > exactly how the interleaving works with the codes, but the idea is that > the sequential bits across tracks are heavily interleaved so that a long > string of sequential errors gets distributed across many codewords. So > each codeword doesn't need huge error correcting capability and can be > reasonably high-rate (i.e., low overhead). > > The downside is that a fair amount of memory is needed for > deinterleaving and there is some associated latency. Neither of those > is a big deal for the application.
Thanks Eric and Philippe. Eric, the link was helpful, particularly the part that said: "In case the burst correction capability is exceeded, interpolation may provide concealment by approximation" That makes oodles of sense in the context of a CD recording, particularly if you've not only got the adjacent-in-time samples to interpolate from, but the opposite stereo channel, to boot. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
"Tim Wescott" <tim@seemywebsite.com> wrote in message 
news:xb-dnRcbhYSjA3nNnZ2dnUVZ_vEAAAAA@web-ster.com...
>I vaguely remember reading a statement about audio CDs, to the effect > that the error correction scheme is designed with some sort of magic so > that it suffers a certain amount of soft degradation before the audio > goes out entirely. > > This was a long, long time ago, and either I or the article I was reading > could have it wrong. > > Is anyone up on CD audio recording formats? Is the above statement > correct, or just a product of faulty memory?
CDs and DVDs use table-driven RS coder with interleaving. At the time, there was a lot of talk about error concealment strategies such as polynomial interpolation in case of RS code failure. Error concealment was implemented at chipset level. Looks like those ideas are no longer in fashion; most likely result of the read failure is replay of the previous segment. Those replays could be a PITA; audio softwares have facilities to detect replays. VLV
I think there was once a cd player with blinking lights to indicate when error correction was taking place, and a different color light to indicate concealment. Didn't catch on, it just made people feel nervous. 

The worst CD errors are caused by bad drummers, and these are impossible to correct. 

Bob 
On 1/3/13 12:07 AM, Robert Adams wrote:
> I think there was once a cd player with blinking lights to indicate when error correction was taking place, and a different color light to indicate concealment. Didn't catch on, it just made people feel nervous. > > The worst CD errors are caused by bad drummers, and these are impossible to correct. >
i read that over again and thought about it and i still don't get it, Bob. by "CD error" you mean a bad-sounding CD? -- r b-j rbj@audioimagination.com "Imagination is more important than knowledge."
Umm, that last line was meant to be a joke (obviously not a very good one!). In the musical world I inhabit, drummer jokes are very much in vogue :)


Bob
On 03/01/2013 05:45, Robert Adams wrote:
> Umm, that last line was meant to be a joke (obviously not a very good > one!). In the musical world I inhabit, drummer jokes are very much in > vogue :) > > > Bob >
The classical music equivalent is viola players. Goes back too many years for me to know why. "You are driving fast and reach a fork in the road. On one path a conductor blocks your way, on the other a viola player. Which do you run over?" "The conductor - business before pleasure!" Richard Dobson
Richard Dobson <richarddobson@blueyonder.co.uk> wrote:

(snip)
> The classical music equivalent is viola players. Goes back too many > years for me to know why.
(snip of joke) OK, viola jokes, reminds me of: http://eliseblackwell.com/pages/an-unfinished-score.html Well, it is a pretty good story, but there is one joke in it that I still remember, told by one character in the story to another. Q: How do you keep you violin from getting stolen? A: Keep it in a viola case. -- glen