{"title":"Doug's Blog","link":[{"@attributes":{"rel":"self","type":"application\/atom+xml","href":"https:\/\/doug.lon.dev\/atom.xml"}},{"@attributes":{"rel":"alternate","type":"text\/html","href":"https:\/\/doug.lon.dev"}}],"generator":"Zola","updated":"2024-03-05T00:00:00+00:00","id":"https:\/\/doug.lon.dev\/atom.xml","entry":[{"title":"Fourays: Updating Firmware & Debating Hardware Design","published":"2024-03-05T00:00:00+00:00","updated":"2024-03-05T00:00:00+00:00","author":{"name":"\n \n Unknown\n \n "},"link":{"@attributes":{"rel":"alternate","type":"text\/html","href":"https:\/\/doug.lon.dev\/blog\/2024\/fourays\/firmware-etc\/"}},"id":"https:\/\/doug.lon.dev\/blog\/2024\/fourays\/firmware-etc\/","content":"

Firmware<\/h2>\n

I've done quite a bit more work on the firmware now, and I believe it to be mostly functionally complete for producing music as I now have complete control over the AY chips. There is still some more work to do on the configuration management and screen user interface though.<\/p>\n

AY Control<\/h3>\n

Up to this point not all of the AY's features have been made available via config or MIDI control. We had config control over the noise mixer, but to change this during performance using config is impractical, some other means would be better.<\/p>\n

A common method to enable or disable synth features during a performance is to use MIDI key switches. These switches are normally mapped to the lowest MIDI notes, which are out of range of the instrument. Using this method, we can implement the remaining features of the AY chips in a way which can be controlled during performance. In this instance I'll use a Note On message with velocity > 63 to turn a feature on and a Note On message with a velocity < 64 to turn a feature off.<\/p>\n

Envelope Generator<\/h4>\n

The AY's envelope generator has 3 mix switches, 4 mode switches and a period parameter. When enabled for an oscillator, that oscillator's level will be affected by the envelope generator according to its mode and period.<\/p>\n

All 7 switches are mapped to MIDI key switches:<\/p>\n\n
MIDI Note Number<\/strong><\/th>MIDI Note Name<\/strong><\/th>Switch<\/strong><\/th><\/tr><\/thead>
1<\/td>C#-1<\/td>Enable for oscillator A<\/td><\/tr>\n
2<\/td>D-1<\/td>Enable for oscillator B<\/td><\/tr>\n
3<\/td>D#-1<\/td>Enable for oscillator C<\/td><\/tr>\n
4<\/td>E-1<\/td>Hold mode enable<\/td><\/tr>\n
5<\/td>F-1<\/td>Alternate mode enable<\/td><\/tr>\n
6<\/td>F#-1<\/td>Attack mode enable<\/td><\/tr>\n
7<\/td>G-1<\/td>Continue mode enable<\/td><\/tr>\n<\/tbody><\/table>\n

The period is controlled by CC messages, and the CC can be changed in the configuration. However, the longest period is probably too long for practical purposes, so the CC range is currently hard-coded to be mapped to the lowest 20% of the period range.<\/p>\n

The various modes can be combined to make the envelope generator behave in different ways:<\/p>\n\n\n

\n

From the AY-3-8910 data sheet; Figure 1.<\/p>\n<\/blockquote>\n

Noise Generator<\/h4>\n

The noise generator period is set via CC messages, and the CC for each noise generator can be set in the configuration. The noise mixer configurations have been moved to key switches.<\/p>\n

MIDI key switch mapping:<\/p>\n\n
MIDI Note Number<\/strong><\/th>MIDI Note Name<\/strong><\/th>Switch<\/strong><\/th><\/tr><\/thead>
8<\/td>G#-1<\/td>Mix to output A enable<\/td><\/tr>\n
9<\/td>A-1<\/td>Mix to output B enable<\/td><\/tr>\n
10<\/td>A#-1<\/td>Mix to output C enable<\/td><\/tr>\n<\/tbody><\/table>\n

Pitchbend, Transpose and Detune<\/h4>\n

Each MIDI note is mapped to an oscillator divisor value, which causes the oscillator to sound at something very close to the required note frequency.<\/p>\n

The actual oscillator frequency is calculated as:<\/p>\n

Frequency = Clock \/ 16 \/ Divisor\n<\/span><\/code><\/pre>\n
As such, it is not perfectly possible to match every note frequency across the entire MIDI note range because of the integer division.<\/p>\n
We are driving the Clock<\/code> at 2.0 MHz, which means that the lowest 23 MIDI notes are unplayable. This is because these notes would require a Divisor<\/code> value greater than would fit in the 12-bit divisor control register (4095). The available Divisor<\/code> resolution also decreases as the MIDI note increases, which causes the frequency error to also increase as we go up the scale. We peak at about 4% frequency error in the very highest notes, but nobody wants to listen to 5kHz square waves anyway.<\/p>\n\n\n
To give the performer some control over the exact frequency output of the oscillators we can implement some CC messages to affect the frequency.<\/p>\n