Nyquist Basics: The Audacity Nyquist Prompt

This page explains how to use the Audacity Nyquist Prompt to test-run Nyquist code snippets. The "Nyquist Basics" tutorials start with very simple examples at the top of each page with increasing complexity towards the end. You do not necessarily need to work through to the end of each page to be able to understand the other tutorials. If you feel confused, try any of the other Nyquist Basics tutorials, in any order. Every "Basics" page contains lots of notes and hints where you can find extra information.

Related article(s): Digital Audio Technology Nyquist Audio Programming Nyquist 3.02 Reference Manual - the latest Nyquist Manual (mostly using SAL syntax) Nyquist 2.37 Reference Manual - the latest version entirely using LISP syntax

Please note: Nyquist supports both a LISP syntax and a more conventional syntax called SAL. You can choose a manual from a link above based on which syntax you are using.

Steve 07Apr18 P1 Images need updating

• Peter 21Jul19: Done - by trimming off the TCPs (to aid future generalization against future TCP changes). And added the indentations that Connie likes.

Setting up

This article uses LISP syntax for Nyquist commands. For an introduction using SAL syntax, see Nyquist Basics: The Audacity Nyquist Prompt With SAL.

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

For testing generate commands in Nyquist, you can install the Nyquist Generate Prompt plug-In, or simulate the behavior of a generate type plug-in in the Nyquist Prompt effect.

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.

Create a Mono Track

1. Click Tracks > Add New > Mono Track. An empty mono track will appear in the Audacity window:

2. Click Generate > Tone. Leave everything as-is and just click OK. The mono tracks will become filled with the generated sound.

Create a Stereo Track

1. Create an empty stereo track.

• click Tracks > Add New > Stereo Track.

An empty stereo track will appear in the Audacity window.

2. Now click Generate > Tone. Leave everything as-is and just click "OK". The stereo tracks will become filled with the generated sound.

The Nyquist Prompt

Recent versions of Audacity have an option to "Use legacy (version 3) syntax" in the Nyquist Prompt. Ensure that option is NOT enabled for these examples.

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

The Nyquist Prompt appears like this:

The Nyquist Interpreter

In the Nyquist Prompt window, type (print "hello") into the text field with the parentheses and quotes, as follows:

NOTE: The Nyquist Prompt must see an open parenthesis as the first non-space character; otherwise it will assume this is written using SAL syntax. Otherwise, we might have just typed "hello", which worked in earlier versions of Audacity.

When you click OK, you should get a message box displaying the word hello:

The message box disappears when you click OK.

This, as you have learned, is how you can send messages to the user.

The Nyquist Debugger

Go back to Tools > Nyquist Prompt, and this time, type (print hello) with the parentheses but without the quotes:

Important: This time, click Debug instead of OK.

A "debug window" appears, displaying the error messages that Nyquist returned:

NOTE: The Nyquist debug window only appears if you click Debug button. If you click OK button, then errors are printed to the Audacity log.

Basic Nyquist Commands

The Nyquist manual entirely using LISP syntax is here: Nyquist Reference Manual.

The current Nyquist manual introducing SAL syntax is here: Nyquist 3.02 Reference Manual.

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").

Prior to version 4 syntax, the variable S was used instead of *TRACK*. You may encounter this in some old plug-ins or old documentation. The old syntax is obsolete and should not be used in new code.

Simple Examples

NOTE: These examples are focused upon using Audacity to manipulate digital signals (clearly Audacity is better suited to audio, but features such as Nyquist can open many other uses). For those interested, the signal used is an infra red (IR) sample from a remote control.

NOTE: Each code example will be shown once using LISP syntax, and again using SAL syntax. Either form can be used, but never try to mix them in a single program or prompt! Prior to Audacity Version 1.3.13 (Beta), only LISP syntax works.

Applying a DC offset to a signal

Original Signal before command.

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))

Or the SAL equivalent:

return *track* * hzosc(19000)

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

(mult *track* (osc-pulse 19000 0))

Or the SAL equivalent:

return *track* * osc-pulse(19000, 0)

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.

NOTE: Unfortunately, this isn't the end of the road for this sample; it is near, but the curved "head/tail" of the signal causes a problem for the digital signal being produced [and it was also upside-down, too...]. This will hopefully form the basis of some more complex examples, since I shall need to use Nyquist to:

1. Find the zero crossing points
2. Then only apply the carrier frequency to those regions above zero.

Or find another suitable command...

Thanks to Forum moderators stevethefiddle & kozikowski for their help with Audacity & Nyquist.