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Re: [Libcdio-devel] CD-TEXT documentation

From: Rocky Bernstein
Subject: Re: [Libcdio-devel] CD-TEXT documentation
Date: Mon, 6 Feb 2012 10:45:53 -0500

On Mon, Feb 6, 2012 at 10:42 AM, Thomas Schmitt <address@hidden> wrote:

> Hi,
> > xor 0xffff is exactly the same things a mod 2**16. You must be talking
> > about something else.
> I understand it like this:
> xor 0xffff inverts the lowest 16 bits.

Ooops -- yes, you are correct!

But again that's the point. Rather than say xor 0xffff  I think it clearer
to say invert the lower 16 bits (i.e. xor 0xffff). If nothing else it would
prevent someone like me from turning this into an "and 0xffff.

> mod 2**16 zeroes the bits above the lowest 16 bits. Like: and 0xffff.
> The CD-TEXT CRC algorithm is ITU-T V.41 with the additional prescription
> of inverting the 16 result bits.
> MMC-3 Annex J states:
> "CRC Field consists of 2 bytes. Initiator system may use these bytes to
>  check errors in the Pack.  The polynomial is X16 + X12 + X5 + 1. All
>  bits shall be inverted."
> My understanding of this prescription is backed by Leon's example data,
> and by the fact that my burner refused to accept the CD-TEXT data when
> i altered the 0x8f packs after computing their CRCs. (Educational mistake.)
> The drive took the data as soon as i corrected those 3 CRCs.
> > I'm just suggesting that rather than describe things bottom
> > up, one might keep a human focus.
> I am watching with high interest how you shape the libcdio documentation
> and will probably make use of it when it does not move any more.
> Have a nice day :)
> Thomas

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