Prompt Basics

Setting up

The Audacity Nyquist prompt appears in Audacity's "Tools" menu.

Load a Sound File

Sound files are imported into Audacity via: File > Import > Audio or the shortcut CTRL + SHIFT + I.

If you have no pre-existing sound files to work with, you can create your own mono or stereo tracks via the Audacity "Generate" menu.

The Nyquist Prompt

Select the track(s) and click Tools > Nyquist Prompt.

The Nyquist Prompt appears like this:

Hello, world!

Simply enter the code below and press Apply to receive a "hello world" prompt:

(print "hello world")

The programming language used above is Lisp. The Nyquist prompt automatically assumes you're using Lisp if the first character is an (. If it isn't, the prompt assumes you are writing SAL, which is more similar to C-like syntax. If you prefer SAL, a "hello world" program looks like this:

return "hello world"

The Nyquist Debugger

The prompt also comes with a debug button. It shows you various warnings and non-fatal errors after the program finished. For example, if you forget the quotation marks around "hello world", just hitting apply will just do nothing, whereas hitting debug will tell you where you went wrong:

;Lisp input: (print hello world)
error: unbound variable - HELLO
if continued: try evaluating symbol again
1> 

;Sal input: retun hello world
>>> parse error: Syntax error.
>>> in NIL, line 1, col 14.

return hello world 
             ^

Basic Nyquist Commands

The current Nyquist manual is here: Nyquist Reference Manual.

A useful index of Nyquist commands is here: Nyquist Language reference

Audacity uses the *TRACK* variable to reference the current audio file/selection. Thus, you can use basic commands such as mult or sum with *track* and the Nyquist prompt will replace the file/selection with the result (or as Audacity calls it, "returned audio").

Simple Examples

Applying a DC offset to a signal

Type the following into the Nyquist Prompt (using LISP syntax): (sum *track* 1) ``Or type the following equivalent SAL command: return *track* + 1

The whole signal has now moved up to above zero.

Modulating with a carrier frequency

To multiply a signal with a generated carrier signal, you can use the following commands:

(mult *track* (hzosc 19000))  ; Lisp
return *track* * hzosc(19000) ; Sal

The (hzosc 19000) produces 19kHz sine wave carrier.

(mult *track* (osc-pulse 19000 0)) ; Lisp
return *track* * osc-pulse(19000, 0) ; Sal

The (osc-pulse 19000 0) produces 19kHz square wave carrier (note the 0 is the bias or 50/50 duty cycle, -1 to 1 = 0%-100% pulse-width ). Applying the 19kHz square wave carrier obtains this result.

The top and bottoms of the signal can then be clipped using the Hard Limiter option from the effects menu (0dB limit and Wet level 1) if required.

The above examples show how you can use the many Nyquist commands to perform basic signal processing without using scripts.