Difference between revisions of "Nyquist Basics: The Audacity Nyquist Prompt"

Revision as of 13:31, 21 July 2019

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.

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

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:

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

After clicking OK in the "hello" message box, go back to Effect > Nyquist Prompt, and this time, type (print hello) with the parentheses but without the quotes:

Important: This time, click "Debug" instead of "OK".

You should first get a window displaying a "Nyquist did not return audio" or a similar message:

After clicking OK in the window above, a second window appears, displaying the error messages that Nyquist returned:

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 's' variable to reference the current audio file/selection. Thus, you can use basic commands such as 'mult' or 'sum' with 's' 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

Original Signal before command.

Type the following into the Nyquist Prompt (using LISP syntax):

 (sum s 1)

Or type the following equivalent SAL command:

 return s + 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 s (hzosc 19000))

Or the SAL equivalent:

 return s * hzosc(19000)

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

 (mult s (osc-pulse 19000 0))

Or the SAL equivalent:

 return s * 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.

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