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

[Monty Montgomery]

On Sat, Feb 13, 2010 at 6:00 PM,  <fons at kokkinizita.net> wrote:
> 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.

Ah, I am perhaps used to seeing... dirtier implementations.  Yes, it's
intended to just be an EQ.  Do you have links to the specified
forward/inverse linear filters?  I may not be thinking of a proper
implementation then.

>> 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.

I didn't say that a digital filter couldn't invert it (at least as
well as it can be inverted).  I said that the typical digital EQ, the
kind you're going to find in any general purpose EQ plugin, is going
to be an acausal linear-phase filter.

Cheers,

Monty