13 Feb
2010
13 Feb
'10
8:15 p.m.
On Sat, Feb 13, 2010 at 6:00 PM, <fons(a)kokkinizita.net> wrote:
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
On Sat, Feb 13, 2010 at 05:37:50PM -0500, Monty
Montgomery wrote:
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.
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.