Encore

Ooh shiny

Well if there was any kind of schedule, I'm definitely behind it. Interesting things keep appearing and distracting me. Like that thing over there. Back in a minute...

The latest shiny is that development snapshots of xroar now have Game Master Cartridge support. This is exciting stuff: I've been thinking about the GMC for some time as a vehicle for a 'deluxe' version, and this was just the excuse I needed to have a play. The banked ROM would allow for a greater variety of sprites and a larger background map, plus the SN76489 sound chip could give more options for in-game sound. More on that in a bit.

Lost in music

I felt I was on a roll with the music, and it seems to be a hot topic at the moment, so I stuck with it and have now recorded my changes to CyD as a fork on GitHub.

I'd previously written about how I tweaked CyD to play variable duty cycle square waves and added simple support for drum samples. Since then I've improved the duty cycle control a little and added a couple more example tunes.

I originally had a bit of code like this to control the pulse duty over time:

        lda duty
rate    equ *+1
        adda #0
cond    bmi skip
        sta duty
skip

The bmi instruction prevents the duty cycle being negative, so we could start with a low value and a positive rate to give a waveform that changes from narrow pulses to square, or go the other way by starting with a large positive value and have a negative rate.

As an added bonus, the bmi instruction can be modified into a brn to make the duty keep wrapping round and cycle endlessly. That sounds really good, but I wondered how much better it would be if I could have the duty ping-pong between two arbitrary limits.

The difficulty with that is there are then two conditions to check for, and which one we check depends on which direction the rate is heading. For quite a while I didn't bother because it sounded like too much complexity for the gain. I didn't want to add too much code because the time taken reduces the quality of the sound.

Then I thought of a way. Not a pretty way, but a way nonetheless. The following only adds one immediate instruction to the critical path. The other instructions only come into play briefly when one of the limits is reached:

        lda   duty

rate    equ   *+1

        adda  #0

cond1   equ   *+1

        cmpa  #$e0

        bls   store

        neg   rate

cond2   equ   *+1

        ldd   #$2024  ; bhs=$24 bls=$23

        ldx   cond1

        stx   cond2

        std   cond1

        bra   skip

store   sta   duty

skip

This exploits the fact that the limit value and the branch opcode are sitting right next to each other in memory and can both be written at the same time with a 16-bit store. Each time the limit is hit, the limit value and branch condition are updated and the sign of the rate is reversed. Ugly, but worth it: This gives much more variety to the sound.

Bend it like Beckham

To help with manually entered music I've added a macro to set up portamento effects. This is where one note transitions into another by smoothly sliding the frequency between two values and is a familiar element in many chiptunes. The way CyD does it is by adding a constant value to the frequency value each 'tick', or 1/50th of a second. So to slide from one note to another we need to calculate how much to add per tick by dividing the desired frequency change by the number of ticks over which it should happen. This could be done by the assembler if the frequency values are defined in the source code.

Among the CyD source files there was a nice Perl script that calculated the note frequency lookup table. I say 'was' because I've butchered this script most 'orridly (Perl just doesn't understand me) to enable the assembly source to calculate its own timings based on how the channels have been configured.

This script now also outputs the note frequency lookup values as symbols which in turn allowed me to add the helper macro to set up a portamento based on the start and end notes and the time interval. This makes it really easy to add guitar-like bends.

After much experimentation I've discovered that a snare drum sample can be lengthened and enhanced by switching to white noise near the end of the sample. The white noise then just continues until cut off by another sound. This sounds a bit odd on its own, but when mixed with other channels, gives the impression of a longer sample with reverb. I think it sounds pretty effective anyway:

All your bass are belong to us

As I've invested a lot of time in CyD and am familiar with the way it works, I hacked together a GMC version to get a feel for how things might sound. What quickly became apparent is that the lowest notes are not particularly low. It goes down to about a B2, which is nowhere near the lowest note on a guitar, let alone a bass.

One way to get lower notes is to use a lower clock frequency than the standard 4MHz. In fact Time Pilot '84, the arcade game that my game is at least trying to be loosely based on, runs three similar chips at half the NTSC subcarrier frequency, or approximately 1.79MHz. That would get you a mains-hum-like A1, the second lowest string on a standard bass, and a bit closer to what we like.

Another way is to make use of the 'periodic noise' capability. This is a function of the noise channel, but isn't really noise, just a low duty square wave with a frequency that is (I think) 1/15 of the usual frequency. It has a thin, buzzy kind of sound.

Yet another route to lower notes is to modulate the attenuation registers to superimpose lower frequencies onto the normal output. This requires CPU time so wouldn't be very suitable for in-game music, but might sound interesting enough to be good for title music, especially if the duty cycle varies over time. I'm very tempted to give it a go.

Play it again, GMC

I reworked my example tune above to play more nicely on the SN76489. I had to transpose it up 4 semitones and throw away the really low notes, which makes it sound completely different. I also ended up mashing in some other ideas just to hear what it would sound like and this is the unholy result:



The echo effect at the start is easily achieved by playing the same thing on two channels with a moderate delay between the two. In this case the delay is something like 300ms. When the notes bend it really comes alive. I've also snuck in some cheeky vibrato as well to spice things up a bit. This is just a rapid series of bends above and below the start note to make the frequency wobble around in a pleasing way.

The drum sounds are simply bursts of noise with the volume and frequency changing over time. The kick drum has a really rapid drop in frequency to make it sound like a thud while the snare has a short period of rapid frequency drop followed by a longer period with more gentle change. It works quite well but it leaves me wondering how good a result could be achieved with a more sophisticated shape to the frequency envelope.

I've put my GMC version of CyD on github, just in case anyone wants to have a play...