Dear Jim, Stefan and the group,
Jim asked:
I imagine Spectrum Lab must be doing the clipping in the time domain,
i.e. limiting the maximum amplitude of any particular sample that is
above the average level - perhaps Wolf could describe how this
function works?
Yes, the 'hard limiter' / clipper in each DSP box works entirely in the
time domain (unlike the frequency-selective limiter in the FFT-based
filter unit):
Each input sample in the time domain, X, is full-wave rectified and run
through a lowpass. That low-pass filtered average, scaled by the dB
value for the relative clipping level, gives the actual comparator value
Y. In other words, Y follows the rectified input average 'very slowly'.
Then, for each original sample X, the program just does this in pseudo-code:
if X > Y then X = Y;
if X < -Y then X = -Y;
There is a second, absolute, threshold value for this limiter which is
just "dB below full scale", which would do nothing more than two
anti-parallel diodes at the input.
In contrast to a classic 'noise blanker' X is not set to zero if the
threshold is exceeded.
Cheers,
Wolf .
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