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Topic: Whipper - Q sub-channels with CRC errors (Read 693 times) previous topic - next topic
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Whipper - Q sub-channels with CRC errors

I have used Whipper on my Arch Linux laptop to rip CDs.  On every single CD I have ripped, I keep getting this errors when the CD is scanned, before the rip starts:

Code: [Select]
Track 1 finished, found 23 Q sub-channels with CRC errors
Track 2 finished, found 183 Q sub-channels with CRC errors
Track 3 finished, found 145 Q sub-channels with CRC errors
Track 4 finished, found 30 Q sub-channels with CRC errors
Track 5 finished, found 25 Q sub-channels with CRC errors
Track 6 finished, found 19 Q sub-channels with CRC errors
Track 7 finished, found 33 Q sub-channels with CRC errors
Track 8 finished, found 111 Q sub-channels with CRC errors
Track 9 finished, found 36 Q sub-channels with CRC errors
Track 10 finished, found 35 Q sub-channels with CRC errors
Track 11 finished, found 122 Q sub-channels with CRC errors

The CDs rip just fine and I get a 100% Rip Quality after every track.  Does anyone know what these CRC errors are?

I tried using Google and most of the links I found just said your CD drive is having difficulty reading the disc.  But I'm getting these on every CD I have tried to rip so far.

Could it be the CD drive itself.  I'm using an external bus powered USB 3 LG DVD drive, that looks like a laptop drive put into an external enclosure.

So, my two big questions are:

  • Do I need to worry about these errors, if the rip quality of every track is 100%?
  • Is there a better CD drive I can use to rip CDs?  Would something internal the PC work better than an external bus powered drive?

Re: Whipper - Q sub-channels with CRC errors

Reply #1
According to wikipedia (hxxps://en.wikipedia.org/wiki/Compact_Disc_subcode):

Channel Q is used for control purposes of more sophisticated players. It has three different modes, but with a common structure for all of them.[1]
Control bits: The first four bits are used for control, each being a flag for a different feature:
Four-channel compact disc digital audio flag: indicates that the track uses four-channel audio (applies only to CD-DA). This is very rarely used on Compact Discs.
Data flag: Indicates that this track contains data (rather than audio). Can be used for muting in audio CD players. Not used in the original CD-DA standard, added in the CD-ROM specifications.
Digital copy permission flag: Used by the Serial Copy Management System to indicate permission to digitally copy the track.
Pre-emphasis flag: The audio track was recorded with pre-emphasis (applies only to CD-DA). Used very rarely on Compact Discs.
Mode bits: The next four bits indicate the mode of the Q channel, which can vary from 1 to 3, and define the structure and contents of the next bits.
Data bits: The next 72 bits contain Q-channel data, and their structure depends on the mode define in the previous bits.
Q Mode 1: In this mode, the data bits contain the Table of Contents of the session (if the Q channel is in the lead-in area), or timing information for the current track (if the Q channel is in the program and lead-out areas of a session).
Q Mode 2: In this mode, the data bits contain the Media Catalog Number (MCN) of the disc.
Q Mode 3: In this mode, the data bits contain an International Standard Recording Code (ISRC) for each track (applicable to CD-DA only). The ISRC is used by the media industry, and contains information about the country of origin, the year of publication, owner of the rights, as well as a serial number.
Cyclic redundancy check bits: The last 16 bits contain an error detection code computed over the previous bits of the channel.

Never heard of the app you used, I would reccomend to use something that can check disks against accuraterip database, to be sure if the rip is error-free.
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Re: Whipper - Q sub-channels with CRC errors

Reply #2
Whipper does use the accuraterip database and it says the cd was ripped successfully.

Re: Whipper - Q sub-channels with CRC errors

Reply #3
Maybe the program reports how many frames of Q subchannel were corrupted on each track. Then a program could use the next correctly read frame to retrieve the barcode or ISRC. Because the subchannels do not have error correction, unlike the main data, some will likely contain errors. The subcodes are not part of the ripped audio anyway.

Re: Whipper - Q sub-channels with CRC errors

Reply #4
I have often wondered if CDs do degrade over time, and the only reason my 30-year-old CDs are playing and ripping just fine is because of the error correction.

Re: Whipper - Q sub-channels with CRC errors

Reply #5
Only the audio data is error corrected (with the help of lots of ECC).  The subchannels are the wild-wild-west, with only the little CRC to indicate some bit(s) is amiss.  But the CRC itself is as safe as the rest of the subchannel bits, not very safe at all.  Every. Single. CD. will do this.  Often the record can be corrected based on history, at least the ones that matter.  Even the ones with a bad CRC can be corrected, usually more often than not.  But none of this affects the audio bits.  That's why everything in the subchannels repeats over and over and over... until the end of the disc.  There's no reason to report these CRC errors to the end-user (you, me, anyone).

 
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