[ID3 Dev] Sync-safe sizes?

[Jim]

This is a little off-topic, but why are you reading in a word for the frame
size?  (Does word in this context mean 16 bits or 32 bits?)

----- 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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