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Topic: CRC w/o null (Read 442 times) previous topic - next topic
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CRC w/o null

I was wondering if anyone knew how Cuetools knows which samples are null when it calculates a "CRC32 w/o null"?

Basically what I am trying to do is put a wav file in a hex editor, manually remove the null samples, and then do a CRC32 check to see if it matches the one given by Cuetools.

Is this possible at all?

Also, is a null sample always 4 bytes (left and right channel)? Or can it be just 2 bytes (left or right null while the other is not null)?

This is an entirely academic exercise. Im very curious as to how it all works.

Thanks!

Re: CRC w/o null

Reply #1 –
I can't remember the details of this but here's a little info.

This w/o null CRC is emulating an old setting in Exact Audio Copy.
Quote from: EAC Options (Legacy)
No use of null samples for CRC calculations
(Default: enabled, Recommended: disabled)
With this option enabled, null samples (silence) of a track will not be counted for CRC checksums. It has no effect on AccurateRip checksums. When this option is enabled and a track has silence at the beginning/end, this could lead to the same CRC checksums, even with different (or even wrong) drive offset values (see EAC drive options). In general, this option should be disabled to ensure compatibility with other programs such as dBpoweramp.

https://hydrogenaud.io/index.php/topic,57306.0.html
https://hydrogenaud.io/index.php/topic,64481.0.html
korth

Re: CRC w/o null

Reply #2 –
Thank you very much korth. I need to take a detailed look at that info...

Basically all Im trying to do is manually remove the null samples in a track using a hex editor. Its just an intellectual exercise. It seems possible, but VERY time consuming. And I was just wondering if there was an easy way to do that.

Im not looking to rip any CDs, just to analyze the CD rips that I already have.

In a hex editor, it SEEMS like every 4 byte sample starts at an offset location "x", where x MOD 4 = 0. (After deleting the WAV header.) So given an arbitrary sample starting byte location "x", a null sample would be 00s at:

x and x+1,
x, x+1, x+2, and x+3,
x+2 and x+3

If this analysis correct? If it is, then doing this by hand is potentially too time consuming. Id have to go through literally 10s of millions of bytes.

Again, Im not interested in ripping CDs, just analyzing the digital file in a hex editor.

I just want to be able to identify null samples in a track using a hex editor.

Disclaimer: I have ZERO experience with any of this kind of stuff.