sinc: continue on lowpass filter with cutoff above Nyquist

[Martin Guy <martinwguy@gmail.com>]

You're welcome. I think it needed it.


Now that we're in minor release phase and can pour as much of the 
fabulous work that people have done on SoX over the last nine years as 
we can into sox_ng, I have a question about SoX itself which I think 
belongs here. The sox_ng (or sox-ng) list  is only for discussion of the 
mountain of cruft I've built on top of and around the code base we know 
and love.


This is about an issue I'm putting in myself for my own use case where I 
process a ripped enormous CD collection with a low-pass brickwall filter 
"sinc" at 15175Hz because a lot of the CDs clip as recorded and all the 
interesting musical information is below that frequency so it removes 
the harshest part of the cllipping.


When it gets to the data CDs with WMAs and stuff recorded at 22050 or 
16kHz, sox dies saying the cutoff frequency is above half the sampling 
rate. But if a brickwall lowpass's cutoff frequency is above the Nyquist 
frequency it should be a no-op, a copy-through.


I've done for myself: where it checks for this case, I just set the 
cutoff frequency to half the sampling rate and it seems to work without 
blowing up but it's not right because if the cutoff is way above half 
the sampling rate, this erroneously fiddles with the top end for the 
edge effects I presume it has around the target frequency. But it *is* 
easy to implement.


My question is, to have the most SoXey result, would it be right to flip 
from sinc to a copy-through when that happens, and how difficult is it 
likely to be to implement?


Thanks, onwards...


    M



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