[ID3 Dev] Sync-safe sizes?

[Mike van Bokhoven]

Hi Jim,

Thanks for the quick response! OK, that's good - that's exactly what my code
does now. I thought I must be missing something obvious; I thought I'd
checked the frame size specs too, but obviously not! I'll have a read, and
update as required.

Mike.


----- Original Message ----- 
From: "Jim" <jmartin92 at comcast.net>
To: <id3v2 at id3.org>
Sent: Thursday, May 17, 2007 4:14 PM
Subject: Re: [ID3 Dev] Sync-safe sizes?


> Hi Mike.
>
> Welcome to the list.  I think you are confusing the ID3 tag size,
contained
> in the tag header, with the frame size, which is contained in the frame
> header.
>
> In ID3v2.3 and ID3v2.4, the tag size contained in the tag header was
always
> a 32 bit syncsafe integer.  No change there.
>
>
> In ID3v2.3 and before, the frame size was NOT a syncsafe integer.  It was
> just a regular 32 bit integer.  In ID3v2.4, the frame size was changed to
a
> 32 bit syncsafe integer.  So you have to read the frame size differently
for
> ID3v2.4 and later.
>
> Check the frame overview section in the ID3v2.3 and ID3v2.4 specs.
>
> Jim
>
> ----- Original Message ----- 
> From: "Mike van Bokhoven" <mike at ambientdesign.com>
> To: <id3v2 at id3.org>
> Sent: Thursday, May 17, 2007 12:04 AM
> Subject: [ID3 Dev] Sync-safe sizes?
>
>
> > Hi! New list member here.
> >
> > I'm just writing in the hope of a bit of clarification regarding
sync-safe
> > words, mainly the sizes of the tag and frames. The 2.3 spec says:
> >
> > -------------------------------------------------------
> > ID3v2 size              4 * %0xxxxxxx
> > The ID3v2 tag size is encoded with four bytes where the most significant
> bit
> > (bit 7) is set to zero in every byte, making a total of 28 bits. The
> zeroed
> > bits are ignored, so a 257 bytes long tag is represented as $00 00 02
01.
> > -------------------------------------------------------
> >
> > The 2.4 spec says:
> >
> > -------------------------------------------------------
> > ID3v2 size             4 * %0xxxxxxx
> > The ID3v2 tag size is stored as a 32 bit synchsafe integer (section
6.2),
> > making a total of 28 effective bits (representing up to 256MB).
> >
> > -------------------------------------------------------
> >
> > As far as I can see, these two specs are identical with regards to these
> > fields. When I'm reading frames, I read in a word for the size.
Originally
> I
> > was compressing that value to ignore the missing bit. I found that that
> > didn't seem to work for any APIC frame (and I expect any frame with size
> > >127 bytes). On a little investigation, I discovered that the raw value
I
> > read in already represented the correct size, before the adjustment! But
> the
> > specs seem clear - both tag and frame sizes are represented by expanded,
> > sync-safe words.
> >
> > All the tags I'm looking at are version 3 (2.3). Doing some research, I
> > found one person claiming that 2.3 and earlier don't use sync-safe
words,
> > which disagrees with the spec, but agrees with my observations. I
haven't
> > been able to find out much more.
> >
> > My question is - what am I missing here? I can't believe that pretty
much
> > everyone who writes APIC frames is getting the encoding wrong...
> >
> > Thanks,
> >
> > Mike.
> >
> >
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> >
> >
>
>
>
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