[sdiy] Crumar Bit-1 DCO question

[Neil Johnson]

At the top end it's going to sound crap?

But it's cheap, innit.

Neil


On Wed, 12 Jun 2024 at 14:18, Tom Wiltshire <tom at electricdruid.net> wrote:

> 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
> >
> > ==================
> >       Electric Druid
> > Synth & Stompbox DIY
> > ==================
> >
> >
> >
>
>
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