A 4-voice PSG instrument for Commander X16 retrocomputer

MakeWave is a digital instrument that uses the Programmable Sound Generator (PSG) in the Commander X16 retrocomputer. While the PSG supports 16 voices, this instrument only uses 4 to help develop sounds effects or musical intruments that can be played simultaneously with 3 other 4-voice sounds.

When your desired sound and pitch is achieved, you can save the configuration to a comma-separated-value (CSV) file that has the settings for all 4 voices as a simple table that can be loaded into any text editor or spreadsheet program.

If using a pre-built release, skip to Installation.

Required tools (all executables assumed to be in path):

• git
• GNU make
• cc65

First, clone the repo to your local workspace:

$ git clone https://github.com/SlithyMatt/x16-makewave.git

Next, go into the repo directory and run GNU make:

$ cd x16-makewave
$ make

This will create MAKEWAVE.PRG in the repo directory, along with other build artifacts.

Copy MAKEWAVE.PRG to the directory from which you will be running the X16 emulator, or to an SD card (physical or emulated image) that will be installed (using -sdcard with the emulator, or physically inserted into the X16). No other files are required to run.

From the X16 BASIC prompt, simply load and run the PRG:

LOAD "MAKEWAVE.PRG"
RUN

If loading from a different file device (i.e. not the emulator host file system or the SD card), append the required arguments to the LOAD command, as documented in the X16 user's guide.

MakeWave is driven with a combination of the mouse and keyboard. A mouse is required to select and click on any UI elements. The keyboard is required to enter numerical values.

At start time, all PSG channels are reset to what is shown in the screencap above (even though only channels 0-3 are shown and configurable later). As shown, there are 5 parameters for each channel that represent the full range of configuration available to each PSG channel.

To change a channel parameter, click on it with the mouse cursor. If it is a numerical parameter, a cursor will appear in the cell for you to type in a new value. Once it is fully typed in, hit the Enter key to commit the new value. For other parameters, simply click repeatedly on it until the desired value is shown. The range and meaning of each parameter is described below.

Range: 0 to 65535 (0 to 24.4136 kHz)

This is the unsigned 16-bit integer value passed to the PSG to set the wave frequency for the channel. The sample rate of the PSG is 48.828125 kHz, so the actual frequency output by the channel will be the integer value entered multiplied by half the sample rate (24.4140625 kHz) and divided by the integer range (2^16 or 65536).

This means that the default frequency integer value of 32768 specifies a frequency of 12.207 kHz. For reference, middle C is 262 Hz, which can be best approximated by entering a value of 703.

• Left Range: -,L
• Right Range: -,R

By default, each channel is turned off by having a "-" for both the left and right speaker channels. You can enable output to a speaker channel by clicking on the character to make either "L" or "R" appear, depending on which you click. Clicking "L" or "R" again will turn off output to that speaker channel and show a "-" again.

Note: the PSG does not support different volumes in each speaker channel for a single voice, so a "fade" effect will require using two separate voice channels.

Range: 0 to 63

Controls the volume of the waveform output to all selected speaker channels. A value of zero effectively turns off that voice, and 63 is the maximum possible volume. Actual volumes heard in the speakers will be dependent on amplification and fading of the stereo audio signal coming from the X16 or emulation host.

Range: PULSE, SAWTOOTH, TRIANGLE, NOISE

Defines the shape of the wavefore being output for the channel. PULSE is a square wave, with the duty cycle set by the pulse width. SAWTOOTH only slopes up to the maximum amplitude. TRIANGLE slopes both up and down from the maximum amplitude. NOISE is generated with a dominant frequency at the current setting.

Range: 0 to 63

Defines the duty cycle when a PULSE waveform is selected for the channel. A value of zero will have an extremely small duty cycle. A value of 63 will produce a full 50% duty cycle.

Clicking on the SAVE CSV button will save all current settings to a text file called PSG.CSV. It will have 4 lines of comma-separated values, containing all current settings as 5 integer values for each channel.

The column order is Frequency, Left/Right, Volume, Waveform, Pulse Width.

The integer values used for Left/Right are:

• 0 - L and R off
• 1 - L only on
• 2 - R only on
• 3 - L and R on

The integer values for Waveform are:

• 0 - PULSE
• 1 - SAWTOOTH
• 2 - TRIANGLE
• 3 - NOISE

All of these integer values correspond exactly to the values that need to be written to the PSG registers for each voice channel.

Please note that if PSG.CSV is already in the current directory, it will be overwritten with the current values when this button is clicked. If you need to save it for later, you will need to quit MakeWave and copy the file off to another location before running it again.

To get back to the BASIC prompt, simply click the QUIT button. If you do not click SAVE CSV before then, the voice configurations you created will be lost.