[Audacity-nyquist] sample-at-a-time processing and Nyquist/XLISP Backtrace

[Roger D. <rb...@cs...>]

Responding to some recent messages about sample-at-a-time processing and 
filters, it's a common misconception that the proper way to filter a 
signal is to take FFTs, manipulate the signal in the frequency domain, 
and inverse FFT back to a signal. This often leads to artifacts and poor 
quality. Nyquist has a lot of built-in IIR filters that can be used 
individually or in combination.

You can also process samples in XLISP.  I tried to give some helpful 
guidance in the manual at the end of the "More Examples" chapter in the 
section "DSP in Lisp". If anyone wants to offer better text or tell me 
what was confusing, I'd be happy to expand or correct the text.

NEXT TOPIC:

I agree the XLISP backtrace is kind of confusing, but it's really pretty 
simple: the backtrace is just info from the runtime stack, starting with 
the most recently called function and working backward. Each function is 
printed in two parts: The heading "Function: " precedes the name of the 
function, and the heading "Arguments:" heads a list of arguments passed 
to the function. The function is usually something like 
#<Closure-REVERB-MONO: #45ba50>, but you can probably guess that this 
means a call to the function declared as REVERB-MONO. The arguments are 
printed one per line, but in many cases, lines wrap, so it takes some 
careful reading to parse them.

The backtrace is made uglier by the fact that functions like COND, LET, 
and SETF all appear. The arguments to these functions are lists that get 
interpreted as code. So often, you'll see lots of code in the backtrace. 
This can actually help figure out exactly how you reached the error, but 
again, it can take some careful reading to interpret this.

I find the backtrace facility indispensable, so if you are ignoring it 
because it looks too ugly, probably you should take a closer look.

In any case, you'll be happy to hear that SAL has much prettier 
backtraces with line numbers and argument names; maybe we'll get that 
into Audacity before too long.