[sdiy] Crumar Bit-1 DCO question

Neil Johnson neil.johnson71 at gmail.com
Wed Jun 12 21:31:07 CEST 2024


Bingo - yes, two 74LS221 generating the 2MHz and modulated in various ways.

Neil

On Wed, 12 Jun 2024 at 19:52, Jonatan Liljedahl <lijon at kymatica.com> wrote:

> IIRC the counters are actually driven by a high frequency VCO, which
> allows smooth pitch modulation (by pitchbend wheel etc).
>
> /Jonatan
> http://kymatica.com
>
>
> ons 12 juni 2024 kl. 15:20 skrev Tom Wiltshire <tom at electricdruid.net>:
>
>> 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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