[ID3 Dev] RVAD -> RVA2

[Jud White]

I've seen "RVAD" and "RVA2" in 2.4, but always implemented as 2.3 (or even, broken 2.3 with 'bits representing adjustment' actually written as 'bytes', sigh).

See the tags archive I posted earlier for examples, it may contain some "TCMP" or identifying COMM/PRIV frames.

Any thoughts on the relationship between RVAD/RVA2?

----- Original Message -----
From: Jim [mailto:jmartin92 at comcast.net]
To: id3v2 at id3.org
Sent: Thu, 18 Jan 2007 15:31:14 -0500
Subject: Re: [ID3 Dev] RVAD -> RVA2

This is slightly off topic but I was wondering if anyone knows of any apps
that use the ID3v2.4 RVA2 frame for volume adjustment?  (I think iTunes
might but I don't have that here to check.  And since their implementation
of ID3v2.4 is broken anyway, I was hoping for something more compliant.)


----- Original Message ----- 
From: "Ben Allison" <benski at winamp.com>
To: <id3v2 at id3.org>
Sent: Thursday, January 18, 2007 2:45 PM
Subject: Re: [ID3 Dev] RVAD -> RVA2


> We use ReplayGain, which standardizes the reference volume, hearing
> threshold filtering and storage mechanism.  For ID3v2.3+, it is stored
> in TXXX frames with descriptions "replaygain_track_gain",
> "replaygain_album_gain", "replaygain_track_peak" and
> "replaygain_track_album".
>
> The old RVAD tag seems to be ill-defined and broken - the meaning of the
> value stored seems to be up to the program that wrote it!  I would be
> surprised if you even ran across a file with this information.
>
> -Ben Allison
>
> Jud White wrote:
> > Turned up some information, but still need the gaps filled in.  Please
help, anybody :)
> >
> > >From the ID3v2.3 spec on RVAD:
> > The 'bits used for volume description' field is normally $10 (16 bits)
for MPEG 2 layer I, II and III and MPEG 2.5.
> >
> > One interpretation is the adjustment data is directly related to the way
16-bit audio is stored (let's assume we're talking about 16-bit for
simplicity).  I personally don't see any other possibilities for
interpretation on this, although I'd be fine with asking any major
implementor what they did.  Nice to see Winamp fixed their Compliance
issues, maybe they could shed some light on RVAD (if they use it).
> >
> > What I turned up (probably most of you know this already) is that each
bit in digital audio is capable of representing roughly 6 dB (specifically,
20*log10(2) dB).  Therefore, 16-bit audio is capable of representing 96 dB.
Initially I was confused - how can 1 bit represent 6dB, and even if it does
how does it increase linearally instead of exponentially?  Quantization, or
in other words, the curve isn't a curve it's a lot of little steps.  As you
increase bits not only does the max dB you can represent go up but the steps
get smaller.  Ok, I'm sure most of you know this, I'm writing to help myself
understand. :)  If someone wants to flesh out this description please do.
> >
> > Here's a good article: http://www.tc.umn.edu/~erick205/Papers/paper.html
> >
> > My take:  RVAD/RVA adjustment is likely implemented the same way as
n-bit audio.  If it is, then dB = value/(2^n - 1) * (6 * n).  e.g., in
16-bit audio: dB = value/65535 * 96.
> >
> > Jud
> >
> >
>
>
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