Stereo Tracks Tutorial

This tutorial provides a brief introduction to using stereo tracks in Nyquist programming.

If a sound from an Audacity stereo track was given to Nyquist, the *TRACK* variable contains an array of sounds. Because all Nyquist "snd-..." low-level functions only can process mono signals, to use such a function, the *TRACK* array first must be split into single mono signals and afterwards be re-combined into an array before it is given back to Audacity.

In Sal, one could write:

if arrayp(*track*) then
 return vector(snd-function(*track*[0]), snd-function(*track*[1]))
else
 return snd-function(*track*)

Or in LISP, one could write:

(if (arrayp *track*)
    (vector
        (snd-function (aref *track* 0))   ; left stereo channel
        (snd-function (aref *track* 1)))  ; right stereo channel
    (snd-function *track*))               ; mono signal

• (arrayp *track*) - tests if '*track*' is an array

• (vector ... ) - re-combines the two mono signals into a stereo signal. A "vector" is an one-dimensional array

• (aref *track* 0) - the left stereo channel [the 0-th slot of the array]

• (aref *track* 1) - the right stereo channel [the 1-st slot of the array]

Important: The Nyquist interface within Audacity can handle a maximum of two channels simultaneously [Audacity stereo tracks]. If in Audacity more than one audio track were selected, each of the selected tracks will be given sequentially, one after the other, with a maximum of two channels simultaneously [stereo] to Nyquist for processing. It is not possible with Nyquist in Audacity e.g. to copy audio signals from one Audacity track into another track.

multichan-expand

In the "nyquist.lsp" file in the Audacity "nyquist" sub-directory, there is a function "multichan-expand" defined, which simplifies the handling of multi-channel sounds [e.g. stereo tracks]:

(multichan-expand function &rest arguments)

So the "arrayp" constuct from above can also be written:

 return multichan-expand(quote(snd-function), *track*) ;; in SAL
 (multichan-expand #'snd-function *track*) ;; in LISP

This may look a bit more cryptic at first, but it can help to avoid long-winded audio processing functions.