[ID3 Dev] [OT] Sync-safe sizes?

[Jim]

Ok.  I figured that might be the case but just wanted to make sure you
weren't reading only half of it.

Jim

----- Original Message ----- 
From: "Mike van Bokhoven" <mike at ambientdesign.com>
To: <id3v2 at id3.org>
Sent: Thursday, May 17, 2007 1:09 AM
Subject: Re: [ID3 Dev] [OT] Sync-safe sizes?


> I mean word in this sense:
>
> http://www.webopedia.com/TERM/w/word2.html
> http://www.computeruser.com/resources/dictionary/dictionary.html
>
> In this case, as I'm writing for 32-bit platforms only, I mean 32 bits,
but
> I guess it could mean pretty much any size that could be argued to be
native
> to a given platform in some way. e.g. if I were writing for a PDP8, I'd
mean
> 12 bits. So, I should have been more specific!
>
> Mike.
>
>
>
>
> ----- Original Message ----- 
> From: "Jim" <jmartin92 at comcast.net>
> To: <id3v2 at id3.org>
> Sent: Thursday, May 17, 2007 4:50 PM
> Subject: Re: [ID3 Dev] Sync-safe sizes?
>
>
> > 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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> > >
> > >
> >
> >
> >
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>
>
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>



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