[sdiy] Crumar Bit-1 DCO question

[Tom Wiltshire]

Update: Sorry, I was wrong - LFO pitch modulation is *not* possible on the Bit 1. I looks like they introduced that on the Bit 01 and 99 and the DCO design is identical, so the problem remains.


> On 12 Jun 2024, at 14:03, Tom Wiltshire <tom at electricdruid.net> wrote:
> 
> Hi All,
> 
> I've been having a look at the Crumar Bit 1 DCO schematics. It's quite an interesting design. The Bit99/Bit01 synths are an update of this.
> 
> 	https://archive.org/details/synthmanual-crumar-bit-1-schematics-late-revision 
> 
> The gist of it is that there's 8253 counters generating a clock at 16 x the note frequency. This clock then drives a 4520 4-bit counter. The clock itself and the four counter outputs are added together with weighted resistors to make a staircase approximation to a ramp wave. Since the highest square wave is at 16 x the note frequency, the first missing harmonic is the 32nd at -30dB, so it's not too bad as a quick approximation to a ramp. The ramp is fed to a comparator to generate a PWM output in the usual way, and there's a single-transistor fullwave rectifier to create a "triangle" waveform too. It's supposed to be another Mario Maggi design, and it does seem to have his quirky fingerprints.
> 
> However, there's one thing I don't understand. The 8253 counters are clocked at 2MHz. The output is 16 x the note frequency, so a highish C (MIDI Note 84) needs a frequency of 16.744 KHz. For that frequency, the counter is using a division value of 119, but the previous note 83 uses 127, so there are only 8 frequencies per semitone - clearly audible. From there on up, the situation gets even worse.
> 
> Basically, I can't see how there's enough frequency resolution, especially for any sort of modulation (and LFO pitch modulation is possible).
> 
> Any ideas what's going on?
> 
> Thanks,
> Tom
> 
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