[sdiy] High frequency VCO as BBD clock

[Tom Wiltshire]

There are some flanger designs using the VCO from the 4046 PLL (as suggested earlier) and then buffering it with some-other-cmos to increase the current drive. This is often just taking something like a hex inverter chip and paralleling three inverters to drive each clock phase output. I've also seen the 4047 multivibrator used as a VCO, with transistors to provide the voltage-controlled current for the VCO. That's handy since the 4047 has Q and !Q outputs. Some designs have to add a 4013 flip-flop to provide the two phases.

My PIC-based FLANGE chips are a minimum-parts solution to the problem of building your own flanger, rather than a maximum-quality solution. The Flangelicious design isn't bad, but it is a bit noisy. Some analogue flangers used a 570/571 compander to limit the noise, and that would be a good idea for my design too. It also doesn't scale especially well to chorus. For a dramatic flange, you want the widest sweep of delay time you can get. 10:1 is not that great, 20:1 is good, 40:1 is very good. Flangelicious manages 20:1. For Chorus, you don't want anything like that at all, since you'll get hideous seasick warbling. Instead, you're trying to keep the pitch modulation subtle, so that the wet signal is different from the dry signal, but still basically the same pitch. You don't want it to go far enough that it's audibly out of tune. So the modulation needs to be far more limited, and also needs to scale down as the LFO gets faster to limit the pitch warble. This is done on some chorus pedals by the simple technique of adding an RC lowpass to the LFO output. This serves to both alter the LFO waveshape from triangle at low frequencies to sine at higher frequencies, and also reduces the depth as the LFO frequency increases. If I'd been writing firmware for a chorus clock chip instead of a flanger clock chip, I'd have included this feature (and I do have a prototype where I tried it).

Tom



> On 8 Feb 2023, at 10:37, Rutger Vlek <rutgervlek at gmail.com> wrote:
> 
> Hi Tom,
> 
> Nice to see you reply! Your website's info on BBD's was a joy to read, and your digital solution to covering both the lfo and clock circuits is my fallback for when it does not work out in the analog domain. For another project I just ordered a stomplfo IC, so will enjoy your work in any case.
> 
> As for flanging, it would be nice if it could be achieved without the need for a separate BBD with fewer stages (that would not work well for chorusing), hence my interest in pushing to higher frequencies, so that I achieve chorus and flanging with the same BBD.
> 
> I already glanced at the DoubleDeka design mentioned earlier in this thread. Great idea, though involving a very expensive part! I was hoping to find a cheaper way. Exponentiating with the 2164, and feeding an integrated linear hf VCO is perhaps my best option? And then buffering the output to obtain sufficient current drive if needed.
> 
> Rutger
> 
> 
> 
> Op di 7 feb. 2023 21:14 schreef Tom Wiltshire <tom at electricdruid.net <mailto:tom at electricdruid.net>>:
> Yes, BBDs are still produced by both CoolAudio and Xvive. There are various types available, covering 1024, 2048, and 4096 stage devices.
> 
> For flangers, getting the delay short enough is usually the problem. It's easy to make a delay of a few msecs, but pushing the clock higher to get really short delays and notches that go way up (0.2msec = 2.5KHz, 180 degrees out of phase, for example) is much harder. 500KHz is good, but only just enough.
> 
> Tom
> 
> 
>> On 7 Feb 2023, at 18:55, rrsounds via Synth-diy <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>> wrote:
>> 
>> Generally speaking, BBD devices have a limited number of “buckets." The faster you cycle through them, the shorter the delay. 500kHz is going to result in a pretty short delay. On the positive side, the S/N is also related to cycling rate, resulting in a much cleaner result at higher rates. 
>> Are BBDs even still produced?
>> David Reaves
>> 
>>> On Feb 7, 2023, at 19:18:08PM, Rutger Vlek via Synth-diy <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>> wrote:
>>> 
>>> Dear list,
>>> 
>>> For a while I've been working on a guitar pedal board for personal use. I'm about to build the second revision of a solid state pre-amp, and was thinking about possible modulation effects too. I would love a nice (stereo) BBD flanger/chorus, but felt a bit fed-up with all the 'classic' designs, particularly the traditional clocking scheme, usually with a limited range, lack of current drive (to overcome capacitance of the BBD clock input) and strictly linear modulation (calling for a hyper-triangle modulation source).
>>> 
>>> Having some experience with designing saw- and tri-core VCO's around the 2164, I was wondering how easy it would be to push the design of a typical tri-core VCO with an 2164-based expo converter into higher frequencies (let's say up to 500kHz) and use it to clock a BBD? Has anyone tried? Or are there fundamental reasons why such frequencies are not possible? I've never (intentionally) worked with circuits outside of the audio bandwidth...
>>> 
>>> Rutger
>> 
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