Hacker Newsnew | past | comments | ask | show | jobs | submitlogin

There's a CD ripper called Exact Audio Copy that has some pretty heavy EC/misread mitigation that, in its most accurate mode, can easily take an hour for a moderately scuffed-up disc

It's pretty much the gold standard for bitperfect CD rips amongst the torrent community



Sounds strange.

I thought CDs are digital so whatever is received is exact by definition?


When you are playing audio from a CD and there are read errors, you get a worse listening experience if you wait for the CD to spin around a couple more times trying to read the data again causing a pause in the audio than just playing the corrupted data block causing a short glitch in the audio. So that is what CD drives do for audio CDs. The entire format of an audio CD is different from a data CD, the audio tracks are not just files. At least early CD drives did the entire audio processing in the drive and they even used to have a separate audio cable that directly connected the CD drive to the sound card. All this means - or at least meant, I have not ripped CDs in ages - that you have to work pretty hard to get an bit accurate rip of an audio CD because a lot of the processing is done in the drive and the raw bit stream is not easily available to the operating system and applications. Especially error correction - which is best effort for audio CDs and does not guarantee the absence of errors - at least used to happen so early and transparently to the following processing steps that it was very hard to know what happened or did not happen to the data stream.


In addition to the other points made, when errors get through even the error correction that CDs have, for the same reason FLAC can compress audio by about 2:1 and MP3s can do even better, waveforms can be repaired or even outright made up for significant fractions of second without being noticed by a human. (Half a second would be noticed, but you can forge, say, 1/16th of a second fairly successfully with some simple algorithms. If you get really unlucky and you miss one of the really important voice transients like the letter T versus V or B versus D, it might be noticed, but otherwise it's really easy to fill in a gap like that.)

DVDs do not have that characteristic. While the streams will be recoverable, they will also be very visibly broken if the visible data gets corrupted, and that's much harder to repair. We are much more sensitive to visual corruption in general, and the way the compression works can make even small errors loom large on the screen. (As the compression gets more sophisticated, this gets more true, which is why DVDs generally just have fairly confined block errors, but modern codec errors can cause significant corruption on the screen, followed by the corruption on the screen "moving" like the video was supposed to.)


You can distort a digital waveform to the point where a 1 is read as a 0... if you get a perfect read of the disc then sure. But for uncompressed audio, a couple of bits being off here and there won't make much of a difference for most uses. (I'd bet high-speed ripping amplifies this problem)

Of course it's not 'bitperfect' at that point but then thats why EAC does so many retries.


Ah, I'm aware of at least one copy protection system where a bit was set so that if read multiple times, would give multiple answers. If you ripped it, it would be read as a definite 1 or 0, but the loader relied on it being read differently on multiple runs.


CDs are a physical media storing digital data. The data is stored in microscopic pits on the disc surface that, like all physical media, can be subject to read errors, due to various imperfections. It has built-in error correction but it is not perfect, so that program reads the same thing multiple times to get statistical confidence that the data is correct.




Guidelines | FAQ | Lists | API | Security | Legal | Apply to YC | Contact

Search: