I am trying to write a simple measuring plugin for Audacity, and it is about as much fun as dropping stones on my skull. All I want to do is take a piece of sound and find the average of all samples (fragment DC offset ) so that I can represent this as a number for the user, and so I can subtract the DC offset from the samples for further processing. I know and understand the math I want to do, but I don’t understand how to do this in Lisp / XLisp / Nyquist / whatever.
Background information in this stream
As far as I know, there is no function for this. For some reason, the snd-avg function does not actually calculate the average sound value, as you might expect. It first executes the absolute value, and then calculates the average value , calculates the average value and then performs the absolute value. Although there is a separate snd-abs function that can do this. >: (
So I have to write my own? Does this mean converting sound to an array and then calculating its average value?
(snd-fetch-array sound len step)
Reads sequential arrays of samples from sound, returning either an FLONUM or NIL array when the sound ends.
(snd-samples sound limit)
Converts patterns to a lisp array.
Nyquist Functions
And there’s not even an average function, so will I have to do the amount too? But do math functions only work on lists? So, do I need to convert an array to a list?
And it will also use a huge amount of memory for longer signals (18 bytes per sample), so it would be better to process it in pieces and make an aggregate average . But I don’t even know how to make an unoptimized version.
No, (hp s 0.1) will not work because:
- I want to remove only DC and keep arbitrarily low frequencies. 0.01 Hz should pass unchanged, DC should be removed.
- The high-pass filter is causal, and the first waveform samples remain unchanged, regardless of which knee frequency you use, making it useless for measuring peak samples, etc.