[sdiy] High frequency VCO designs

[Tom Wiltshire]

Good summary, Phil.

For the chips I've been looking at, 2MHz or 4MHz clock frequencies are the most common. For the typical 9-bit set of divider ratios (/239 to /478), this gives a top frequency of C = 8368Hz and the octave below it. So, yeah, KHz range.
Some chips used a slower clock and only 8-bit division, for still-worse accuracy. Others ranged the "top" octave from 4186Hz.

Depending on the chip used, the output was either 30% or 50% square waves. One thing I don't understand is the 30% pulse choice - or how it was generated. Doing 25% seems easier to me, and has the benefit (IMHO) of emphasising the second harmonic rather than the odd ones.

Using a 16F1xxx PIC gives me a 8MHz instruction rate. It isn't too difficult to do cycle-accurate delays between incrementing a count and outputting that count to an IO port, so it should be possible to put one TOG output and it's associated divider onto a single PIC. The 8MHz gives a "Top" octave of probably 16KHz rather than 8, so there's an improvement to be had too. I'm still thinking about how to do 25% or 30% pulses.

Tom



On 23 Sep 2014, at 18:37, Phil Macphail <phil.macphail at liivatera.com> wrote:

> Top-octave generators are (were) a set of dividers clocked from a crystal. Typically the colour-burst frequency for NTSC was used (around 3.5MHz), and the outputs were square-waves for each note in the top octave of a keyboard (so 4-8KHz-ish). These outputs could then be divided by flip-flops to produce lower octaves, switched by the keys of a keyboard.
> As you can imagine, in processes available in the 70's (PMOS, 1-layer metal if you were lucky) there was a shortage of isolation between the counters, so there was every opportunity for unwanted tones appearing anywhere and everywhere. A really simple idea for polyphony, but the implementation had many drawbacks, if a certain character,
> 
> 
> Phil.
> 
> From: Richie Burnett <rburnett at richieburnett.co.uk>
> Date: Tuesday, 23 September 2014 20:22
> To: Phil Macphail <phil.macphail at liivatera.com>
> Cc: <Alan.Needham at centrica.com>, "tom at electricdruid.net" <tom at electricdruid.net>, "synth-diy at dropmix.xs4all.nl" <synth-diy at dropmix.xs4all.nl>
> Subject: Re: [sdiy] High frequency VCO designs
> 
> If not pulses or sawtooth waveforms then what waveforms does a top octave generator output? And at what frequencies? "Top octave' makes me think of frequencies in the kHz range. 
> 
> -Richie,
> 
> Sent from my Xperia SP on O2
> 
>