[ID3 Dev] RVAD -> RVA2

[Ben Allison]

This would correspond to value/65536  (-12850/65536 = 0.196, 
-26471/65536 = 0.404)

Mitchell S. Honnert wrote:
>
> For what it's worth...
> When I implemented the RVAD frame for my ID3 library, I found the 
> iTunes implemented it, so I used its implementation as a guide.  (In 
> spite of iTunes' mangling of ID3v4.)  The iTunes UI has a "Volume 
> Adjustment" slider which goes from -100% to +100% with tic marks at 
> every 20%.  A -20% in iTunes results in the Right Volume and the Left 
> Volume fields being "Decrement 12850".  -40% was "Decrement 26471".  
> At the time, I scratched my head a bit, wondering why the frame would 
> need these large values, but I didn't really delve into it too far.
>
> - Mitchell S. Honnert
>
> ----- Original Message ----- From: "Jud White" <jwhite at cdtag.com>
> To: <id3v2 at id3.org>
> Sent: Thursday, January 18, 2007 2:29 PM
> Subject: Re: [ID3 Dev] RVAD -> RVA2
>
>
> 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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