15 Dec
2005
15 Dec
'05
2:45 p.m.
On Thursday 15 December 2005 18:06, Phil Frost wrote:
There is truth in what you say, but given the problem
described, this is
likely more confusing than helpful to someone new to signal processing.
If the problem is that the input is a low frequency square wave, from a
Low Frequency Oscilator, the popping has nothing to do with the
"nyquest" limit. If the square wave were coming from an ideal source
(which it isn't) then any higher frequency components likely would be so
small as to be smaller than the quantization error.
The thing is that a LFO implemented as part of a plugin is an ideal source for
all practical purposes, it can and does do a step change in sample value (for
the nieve squarewave case) which if the LFO is running at 20Hz gives the
level of the 1001st harmonic as 1/1001 that of the fundamental, or some 60db
down. Low but hardly buried in the nose, as the mixing products will not be
harmonically related to either signal.
The problem is simply that it contains high frequency
components (under
the limit), or more intuitively, a sudden drop or rise. Drop or rise
more slowly, and the pops go away.
Does hooking the LFO output directly to a speaker click, if so this is the
problem, but if it only acts up when multiplied with another signal then I
still maintain that it is mixer products aliasing that is the problem.
There is no need here for a high order filter here,
and no risk of phase
shifts or overshoot. A simple first order lowpass filter will be quite
effective, as will any other simple techniques such as averaging or
changing the LFO function to put more slope on the rising and falling
edges.
Also the OP probably needs to make the rate of change continuous as that can
also cause a click.
Regards, Dan.