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Old 7th January 2006, 05:22 PM   #1
Sandor is offline Sandor  Italy
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Default About CD reading

In an hifi magazine I have read some interesting thoughts about CD reading:

1) When playing a CD, a lot of reading errors occur
2) Depending on the transport and CD quality, error percentage can be up to 50%
3) Ordinary CDplayers apply error correction using interpolation, so the bit stream after correction IS DIFFERENT from the original one.
4) PC CDplayers have a considerably faster speed than ordinary players and apply a protocol with retransmission of wrong bits; the bit stream on the output IS EQUAL to the original one.

These considerations may follow:
A) Points 1) , 2) and 3) can explain why the transport is so important in a digital chain and why some tricks (green markers, green lights etc.) can be effective.
B) Point 4) suggests that a much better transport can be built up using a PC CDplayer with USB output followed by a USB -> I2S/SPDIF converter.

Your opinions are welcome.
Regard..

Paul
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Old 7th January 2006, 06:41 PM   #2
pburke is offline pburke  Germany
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you forget that PC CD formats are also designed for error correction. Redbook audio is not. The only solutions that exist on the PC side are programs like EAC that re-read audio sectors over and over again and compare the reads, moving forward only if there are at least three consecutive matched reads. If not, EAC will read up to 80 times to get the best approximation of what may actually be stored there.

PC drives also don't have to worry about maintaining a steady speed of 150kb/sec.

bottom line: CD transports have to be much more sophisticated than a PC drive to extract the data accurately in realtime. If you use a PC for CDs, rip the data to the hard drive and play it from there.

My CD transport is quickly becoming a backup unit for less than critical listening. My PC hard drive playback via USB-I2S directly to the DAC is dramatically superior to anything else I have ever heard (including PC USB SPDIF solutions - SPDIF is evil, even if well implemented)

Peter
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Old 7th January 2006, 07:49 PM   #3
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In an hifi magazine you could read many other awful things.
You'd better read some technical papers....
For example, in "Handbuch der Audio-Schaltungtechnik" by Paul Skritek (I don't know exact english name of this book, it may be "Audio Schematics Handbook"). There is complete explanation of CD princoples of operation in chapter 17. According to it:

1) Error rate of a CD transport is about 10^-5 (Rheed-Solomon error correction codes is able to handle error rate of 10^-4, 10 times worst)
2) is complete nonsence, or we have unadjusted/broken transport
3) Error rate of 10^-4 means 1 interpolation per 10 hours of playback. As for me, 1 interpolation per 1 hour is acceptable.
4) PC CDplayers have been constructed to read DATA from CD. CD-ROM format itself differs from CD-DA. In CD-ROM there is additional error detection/correction codes in each data sector. Theoretically, CD-ROMs could read data faster and uses buffering, so there is a time to try to re-read sector containing erors.

I built error indicator for my good old Philips CD-160. It shows no errors, unless disk is extremly scratched.

Tried to use CD-ROMs as transports - it sounds awful compared even to AKAI CD-57 or average Pioneer CDs. Maybe some extremely complicated design (microcontroller driven cd-rom + huge buffering) will do better job, but this is not my way to do it.
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Old 8th January 2006, 01:59 AM   #4
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Quote:
Originally posted by pburke
you forget that PC CD formats are also designed for error correction. Redbook audio is not. The only solutions that exist on the PC side are programs like EAC that re-read audio sectors over and over again and compare the reads, moving forward only if there are at least three consecutive matched reads. If not, EAC will read up to 80 times to get the best approximation of what may actually be stored there.

PC drives also don't have to worry about maintaining a steady speed of 150kb/sec.

bottom line: CD transports have to be much more sophisticated than a PC drive to extract the data accurately in realtime. If you use a PC for CDs, rip the data to the hard drive and play it from there.

My CD transport is quickly becoming a backup unit for less than critical listening. My PC hard drive playback via USB-I2S directly to the DAC is dramatically superior to anything else I have ever heard (including PC USB SPDIF solutions - SPDIF is evil, even if well implemented)

Peter
Hi Peter, I'm trying to get together something like you have. I too believe in ripping via EAC and playing back from the HD.

What are you using in terms of gear? Can I assemble a kit like you have relatively easily or cheaply? Or is this something you've soldered together from bitz and pieces lying around the 'scope right next to the particle accelerator?

Seriously, I keep pestering Peter Daniel to make a USB DAC, but he's too busy for the moment.

Any pointers or advice would be really appreciated.

Thanks.

-Matt.
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