Anyway, it is not without bugs, but that's why I'm doing it this way in the first place, to test everything and how it works together.
There are a lot of things that work great right from the start. The waveshaper does its job perfectly, giving triangle, bi-directional saw, pulse with PWM and two sub oscillators with selectable square/saw output.
The waveshape mixer CV generator I designed earlier works perfectly, as does the pulse wave amplitude control.
The filter filters, though the range may be a bit lacking. It resonates beautifully too. I have yet to calibrate it and test it fully after moving from +/-15v to +/-12V.
Finally, the output amp and envelope works great, though I had to fix a few rather hard to find bugs in my code.
Now, for each module, here are some things that must be fixed or improved.
Waveshaper
The sine wave amplitude is +/-4V while all the others are 5V. This can be fixed in the voice mixer by changing the input resistor or I can fix it on a new revision of the waveshaper.
The saw and triangle waves have noticeable ringing (noticeable on the scope, not necessarily audible). Adding caps in the feedback of the output will remove most of this. The rest goes away in the filter but I am not sure if it affects anything else.
Pulse width: I don't think the pulse width is quite narrow enough.
When calibrating the symmetry of the sine wave, I can't seem to get a perfect setting that also gives a centered triangle wave. It is not a big issue though, but it means that the triangle is slightly "lower" than the other waveforms, perhaps by 0.5V or so. It is not audible, but it may affect how the circuit clips when mixing multiple oscillators. I'm going to leave it as it is for the time being.
The square wave sub oscillators are not centred. This is because the reference voltage used in centring is derived from -15V, and when changing this to -12V the reference is wrong. Again not audible but nice to fix.
The triangle and sine waves have a very visible notch at the end of the phase, more about that in the DCO paragraph.
Waveshape mixer
As I wrote in my last post, the AS3364 has a sort of dead band in both ends of the CV. Since all waveshape mixer CVs are generated from one input, it means that there is no way to compensate for this without changing the CV generator. In practice, the dead band means that one waveform fades out completely before the next one has reached its maximum. This is especially apparent in the saw to pulse cross fade, where the wave has become a square before the square has reached its top. It is not particularly pronounced so I don't think I'll do anything about it.
DCO
These are not properly wired up yet, so some of these things may go away - right now they run with the wrong calibration circuit and only produce a 0-5V wave when they should give us 0-10V (at least that's what the waveshaper expects). To compensate for this, I'm running the wave through a non-inverting op amp amplifier with 2x gain.
There are a few shortcomings/bugs in the DCO. First of all, some ringing is introduced along the way. I've added a cap to the feedback of the non inverting amp, a 15pF in parallel with the 56k resistor I'm currently using.
It looks like discharging the DCO cap goes too slowly. Either that, or we are limited by the slew rate of the op amps. In any case, this means that the drop from top to bottom of the saw wave is not instantaneous. When generating the triangle wave we invert half of the saw wave to get the "missing" portion of the triangle - but since the falling edge is not perfectly vertical, we get a notch at the end of the triangle phase. I can't say I hear it, but at 8kHz it is very visible on the scope.
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| DCO discharge is not vertical enough, leaving a notch in the saw wave. |
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| Adding a 5p cap across the DCO output op amp takes away much of the ringing at the start of each cycle |
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| 15p cap takes away even more of the ringing. Saw output from the waveshaper at the bottom, some new ringing has been reintroduced. |
Clicking on envelope retriggers: When we play a new note without releasing the previous one, we get a bit of clicking. So far I've been able to identify two probable causes - output amplitude and centring after the filter, and glitches in the DCO.
The DCO glitches manifest themselves as discontinuities in the triangle wave, it suddenly and abruptly changes value. The reasons are:
- When changing from a high to a lower pitch: It looks like the period timer is not reset. Instead, the period is reset when the original frequency would have been reset. This means the amplitude is too low and the start/end matching of the triangle fails.
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| Saw does not reach its maximum as it is reset at the "old" frequency |
- When changing from a low to a higher pitch: now the timer lasts too long, meaning the cap is charged more than it should. It reaches its max and flats out, both distorting the saw/tri and introducing a spike with too high amplitude. That would definitely sound like a click.
The last of these is to be expected. The DCO is supposed to check the amplitude against a known voltage, but without calibration this won't work. The other one on the other hand, is stranger. I need to check the code.
I also got a more serious error while testing the very limits of the DCO. When playing OCT+4 on the MPK-25, switching between e and g makes the DCO drop to a much lower frequency for a single cycle before recovering. It happens consistently. No idea what makes it happen.
Finally, the range of the DCO is currently too narrow. It maxes out at around 8.5kHz but should reach at least the double or ideally above 20kHz. Replacing the integrator cap and regenerating the timer code will probably solve this.
Filter
As with the DCO, the filter cutoff range needs looking into. I have not tried calibrating the filter at all, so it may not be a real problem though, but it looks like the filter is not open enough with CV at 5v.
There is also something going on when changing the wave frequency. Obviously, when the wave frequency reaches the cutoff point, higher frequencies will be attenuated more, so with the filter at a fixed cutoff, switching between two notes will make the amplitude jump up and down. This may be part of the click-sound I'm hearing. Also, the filter has some form of DC filtering, so the balance/centring of the wave changes with waveform/content. This is particularly visible with narrow pulse waves and resonance. Again, a bit of calibration may improve this but I also need to see what other synths do. Also, introducing keyboard tracking means that relative amplitude should not change as much between the notes.
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| Some kind of noise at the start of the envelope (not visible in the CV). This was before I hooked up a probe to the filter output so I don't know where it originated from |
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| Weird glitch in output, again, no idea what this was, especially since it is not at the start of the envelope. |
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| Quick retriggering that changes frequency leads to amplitude changes after filter |
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| Output from filter is not centred. Had a lot of resonance and wave was not symmetric around the X axis. |
Envelopes
These are software, and mostly finished. I still need to hook them up to the GUI to make it easier to test them, but for now they are controlled through 7bit midi. As they have a very large range - 1ms to 30s, I have to use some kind of linear-to-exponential mapping, or we would not get any resolution at the lower parts of the range.
I feel that the mapping I use now still does not have right response to it. Fortunately, I have a script to generate mappings with so it's only a question of tweaking this.
GUI
The GUI should - in theory - output everything needed to control the synth over midi already. But since I want to PLAY the synth using my Akai controller, I'm using up the only available midi port on the voice controller card. I need to find a way to merge the midi output from the GUI with the controller. I THINK that MidiPipe may do the trick.
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