3 ms·

So why would ripping CDs result in less than perfect copies - I thought digital information would almost always be read correctly?

CGamesPlay 4y ago
I read the linked rationale and still don’t understand why this is even a problem.
rlv-dan 4y ago
Transferring from disc to system via a laser is analogue and error prone. What if there is a scratch? What if the cd wobbles?
- asveikau 4y ago
  That's what the CRC is for. Granted, it's not perfect...
  - andy_ppp 4y ago
    But then the CD should skip right or reread the bits - maybe it doesn't maybe it guesses to keep the music playing...
    - asveikau 4y ago
      And I guess that's why cdparanoia exists - to put hyper-focus on those edge cases and get them right. I used to use it to rip in the 90s or early 2000s but never looked deeply into how it works.
ghusbands 4y ago
Some CDs are very scratched. Some CDs are CDRs with dye that hasn't aged well. Some CDs just don't read well (poor manufacturing tolerance, perhaps). Some drives silently return bad data on error. Some drives return a lot of errors towards the last tracks even when there is no problem. Drives vary on where they think tracks start and end (generally a uniform offset per drive). Sometimes, reading audio before the claimed start of the CD takes extra trickery. There are many more issues, besides. By ripping multiple times to verify and comparing to a global database of checksums (and in some cases doing error correction), you can be sure of getting exactly the intended audio.
I_AM_A_SMURF 4y ago
I never understood that either, maybe redbook doesn't have a strict error correction like data CDs have? That's the only thing I can think of but that seems weird though.
- kevin_thibedeau 4y ago
  Audio CDs have weaker error correction. An audio player has to stream data off the disc without any retries. When a block of data can't be corrected it's passed along with the knowledge that the corruption is localized and usually not noticeable.
LeoPanthera 4y ago
Unlike DVDs, CDs don't have sector information. They're just a long continuous spiral of bits, so there is no easy way to tell exactly where on the spiral the head is pointing, especially as many CD players, when you tell them "go to here", will miss slightly. To compensate for this, data CDs include intermittent data on the spiral that says "you are here", but audio CDs don't do this. The only way to ensure that you haven't either skipped over a piece, or duplicated a piece, is to rip the CD in overlapping chunks, and then compare the overlaps.
- LarryMullins 4y ago
  The real question is why you'd ever need a bit-perfect copy of an audio CD when CD players never gave bit-perfect playback in the first place and nobody could tell the difference. Edit: honestly I don't get it. Do you export all raw images from your camera to PNG too? JPG is good enough, and single-pass CD rips are just fine too.
  - Niksko 4y ago
    I mean, that's literally what RAW files from cameras are, and there are uses for them. I don't know if a multi pass CD rip is quite as useful as a RAW image, but your analogy is not as strong as you think it is.
    - LarryMullins 4y ago
      So you archive RAWs, not just JPEGs? I get that professional photographers do, but I never would. And I think they do it to preserve byte depth or whatever, but FLAC from a single pass CD rip has all the bit depth of the original, even if it has a few bits wrong you'll never notice it.
      - zamadatix 4y ago
        I mean I do keep the RAWs but the use case there is a bit different anyways, even the JPEGs aren't a simple matter of "the compressed version" it's also "the preprocessed version". OTOH it takes no more storage to back a CD up correctly vs incorrectly so why be keen to add a generation of loss? You can lower the quality whenever you like but you can never add it back.
PragmaticPulp 4y ago
Audio CDs and data CD-ROMs use different encoding modes. Data CDs have an extra layer of error correction. Audio CDs have less error correction because small bit errors are not a big deal for your listening experience. Most CD players quietly interpolate over small errors in a way that you probably don't even notice.
dirkt 4y ago
As early CD players were intended for audio playback, your CD player can lie to the software. It doesn't tell if it's reading at the exact beginning of a sector (because timing information is multiplexed into the audio stream, and some CD players were not byte-exact when interpreting this), and it also doesn't tell you when it silently corrects some errors. Because for a human listening to the audio won't notice. But of course it makes a difference if you want to make high-fidelity copy. That's why there always have been copy apps (cdparanoia on Linux, EAC on Windows) that do overlapping reads, and then detect and compensate these problems.

CGamesPlay 4y ago

I read the linked rationale and still don’t understand why this is even a problem.

rlv-dan 4y ago

Transferring from disc to system via a laser is analogue and error prone. What if there is a scratch? What if the cd wobbles?

asveikau 4y ago
That's what the CRC is for. Granted, it's not perfect...
- andy_ppp 4y ago
  But then the CD should skip right or reread the bits - maybe it doesn't maybe it guesses to keep the music playing...
  - asveikau 4y ago
    And I guess that's why cdparanoia exists - to put hyper-focus on those edge cases and get them right. I used to use it to rip in the 90s or early 2000s but never looked deeply into how it works.

ghusbands 4y ago

Some CDs are very scratched. Some CDs are CDRs with dye that hasn't aged well. Some CDs just don't read well (poor manufacturing tolerance, perhaps). Some drives silently return bad data on error. Some drives return a lot of errors towards the last tracks even when there is no problem. Drives vary on where they think tracks start and end (generally a uniform offset per drive). Sometimes, reading audio before the claimed start of the CD takes extra trickery. There are many more issues, besides. By ripping multiple times to verify and comparing to a global database of checksums (and in some cases doing error correction), you can be sure of getting exactly the intended audio.

I_AM_A_SMURF 4y ago

I never understood that either, maybe redbook doesn't have a strict error correction like data CDs have? That's the only thing I can think of but that seems weird though.

kevin_thibedeau 4y ago
Audio CDs have weaker error correction. An audio player has to stream data off the disc without any retries. When a block of data can't be corrected it's passed along with the knowledge that the corruption is localized and usually not noticeable.

LeoPanthera 4y ago

Unlike DVDs, CDs don't have sector information. They're just a long continuous spiral of bits, so there is no easy way to tell exactly where on the spiral the head is pointing, especially as many CD players, when you tell them "go to here", will miss slightly. To compensate for this, data CDs include intermittent data on the spiral that says "you are here", but audio CDs don't do this. The only way to ensure that you haven't either skipped over a piece, or duplicated a piece, is to rip the CD in overlapping chunks, and then compare the overlaps.

LarryMullins 4y ago
The real question is why you'd ever need a bit-perfect copy of an audio CD when CD players never gave bit-perfect playback in the first place and nobody could tell the difference. Edit: honestly I don't get it. Do you export all raw images from your camera to PNG too? JPG is good enough, and single-pass CD rips are just fine too.
- Niksko 4y ago
  I mean, that's literally what RAW files from cameras are, and there are uses for them. I don't know if a multi pass CD rip is quite as useful as a RAW image, but your analogy is not as strong as you think it is.
  - LarryMullins 4y ago
    So you archive RAWs, not just JPEGs? I get that professional photographers do, but I never would. And I think they do it to preserve byte depth or whatever, but FLAC from a single pass CD rip has all the bit depth of the original, even if it has a few bits wrong you'll never notice it.
    - zamadatix 4y ago
      I mean I do keep the RAWs but the use case there is a bit different anyways, even the JPEGs aren't a simple matter of "the compressed version" it's also "the preprocessed version". OTOH it takes no more storage to back a CD up correctly vs incorrectly so why be keen to add a generation of loss? You can lower the quality whenever you like but you can never add it back.

PragmaticPulp 4y ago

Audio CDs and data CD-ROMs use different encoding modes. Data CDs have an extra layer of error correction. Audio CDs have less error correction because small bit errors are not a big deal for your listening experience. Most CD players quietly interpolate over small errors in a way that you probably don't even notice.

dirkt 4y ago

As early CD players were intended for audio playback, your CD player can lie to the software. It doesn't tell if it's reading at the exact beginning of a sector (because timing information is multiplexed into the audio stream, and some CD players were not byte-exact when interpreting this), and it also doesn't tell you when it silently corrects some errors. Because for a human listening to the audio won't notice. But of course it makes a difference if you want to make high-fidelity copy. That's why there always have been copy apps (cdparanoia on Linux, EAC on Windows) that do overlapping reads, and then detect and compensate these problems.