[linux-audio-dev] Re: applying RIAA curves in software

[fons adriaensen]

On Tue, Oct 25, 2005 at 04:10:12PM -0500, Richard Smith wrote:
> >
> > > The RIAA record curve reduces bass and increases treble, and the reverse
> > > RIAA curve for playback does the opposite.
> >
> > Sorry, but this is plain wrong. The RIAA filter used when cutting a disk
> > master will boost bass (below 50 Hz), and reduce high frequencies. This
> > actually leads to a worse S/N ratio on playback. It looks like this:
> 
> I'm confused then.
> 
> This page:
> 
> http://www.bonavolta.ch/hobby/en/audio/riaa.htm
> 
> Has a spread sheet that runs the math on the equation presented. 
> Unless I'm just backwards for RIAA reproduction it yeilds roughly 20dB
> of gain for 20Hz and -21dB for 21kHz.  Which seems backward from what
> you are saying.

That curve, the same as (3) in my previous post, is often called
'the RIAA curve' but it isn't. It is the combination two things:

*  a 1/F (or -6dB/oct) filter that is required to compensate for the
   +6dB/oct response of a magnetic cartridge,

*  and the real RIAA playback curve, (2) in my previous post, which
   boosts high frequencies.

In other words, the general downward slope of the filter you refer to
has nothing to do with RIAA equalisation, it's there only because
that filter is designed for use in a preamp for a magnetic cartridge.
If you would have a flat frequency response from the cartridge, then 
the filtering required needs to boost high frequencies, as in (2).

If you take (2), and turn it 45 degrees clockwise (that adds the
-6dB/oct slope for the transducer), you get the curve you refer to.

So the idea that the RIAA curve was introduced to improve the S/N
ratio at high frequencies is just wrong, it does the opposite.

-- 
FA