[LAU] 1980's cds: analog to digital conversion

[fons at kokkinizita.net]

On Sat, Feb 13, 2010 at 05:37:50PM -0500, Monty Montgomery wrote:

> The RIAA preemphasis is not a biorthogonal filter.  There will always
> be at a minimum some phase/group delay as the 'perfect' inverse filter
> is unstable.  You can only approximate it.  Another example of an
> analog horror we no longer need to put up with in the digital era :-)
 
The RIAA filter applied (normally) in the cutter is
(in Bode-plot form)

+6dB/oct up to 50 Hz
flat     up to 500 Hz
+6dB/oct up to 2122 Hz
flat     above that

It is the combination of 3 simple first order filters.

The first can't be inverted completely, as it would
lead to infinit gain at DC. It can be inverted with
any precision you care for within the audio range.
But that one is not the problem here.

The two others can be inverted exactly. 
 
> A digital EQ alone isn't even close, as digital EQ is nearly always
> phase-linear (acausal).  It's a completely different style of filter.

Sorry but that is nonsense. The pole/zero at 500 and 2122 Hz can
be inverted by about 2 lines of C code, and the phase response
will match the analog one. As long as you don't approach half
the sample rate the 'simple' digital filters are the same as the
corresponding analog ones. And the highest pole/zero frequency
is this case well below that limit.

Ciao,

-- 
FA

O tu, che porte, correndo si ?
E guerra e morte !