[sdiy] Tap Tempo LFO

[Jerry Gray-Eskue]

<< the high-frequency
resolution is probably not adequate to support decent timing & frequency
control at the rates needed to clock a BBD.>>

Keep in mind the concept:

<<I'm wondering how easy it would be to reprogram it to have a
 1:4096 tap:clock ratio? >>

How fast do think the BBD needs to run? and can you double tap at that rate?
These devices are so small, 1024 samples and less for most parts that very
high rates would not yield enough delay to be very interesting, the lower
rates are where the fun is.

- Jerry


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Eric Brombaugh
Sent: Tuesday, December 08, 2009 7:35 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Tap Tempo LFO


Interesting discussion.

Using a general-purpose MCU to create a clock for a BBD is probably out
of the question for most garden-variety processors. While a lot of them
have divider-based timing generators built-in, the high-frequency
resolution is probably not adequate to support decent timing & frequency
control at the rates needed to clock a BBD.

There is one part I can think of that might work for this:

http://www.analog.com/en/analog-microcontrollers/analog-microcontrollers/adu
c7128/products/product.html

A little ARM processor with a built-in DDS that can run up to 21MHz with
0.005Hz resolution. Costs a bit more than you probably want to spend
though, and might need some fairly careful layout. OTOH, it's got decent
ADC on-chip and could make a very interesting Digital VCO besides its
application as a BBD clock.

Might be able to build a home-made DDS with an external CPLD though -
that would be a bit cheaper if you picked the right parts. Alternatively
you could use a cheap RF PLL chip (usually programmed via SPI) - those
would give great jitter-free clocks in a single external chip. They're
also doing some amazing things with programmable CMOS oscillators these
days that give wide frequency ranges in very small & inexpensive parts -
check out Silicon Labs 'Any-Rate' line for example.

Eric

On 12/08/2009 05:56 PM, Tom Wiltshire wrote:
> Mike,
>
> Thanks for the kind words.
>
> There are two reasons why what you suggest isn't possible.
>
> One is that the output sample rate simply isn't high enough. It's only
> 19.5KHz, which rules out any frequency above almost 10K even in theory.
> In practice, you wouldn't get much above 2 or 3KHz. The sort of output
> rates you're talking about are 100's of KHz.
>
> Secondly, the part of this that is 'tap tempo' is the LFO frequency,
> e.g. the rate of change of some parameter. This is not the same as the
> delay time being tap tempo. For this, you need to do a different sum
> involving the clock rate of the delay and the length.
>
> It's a damn pity that it isn't do-able though, because this is something
> I've wondered about enough times myself. If you could get a uP to
> generate a clock signal for a BBD directly, you be able to do tap-tempo
> delays, flangers, chorus and the like, and you'd only need the uP and
> the delay line. None of that messing about with clock chips or VCOs
> driven by LFOs.
>
> Still, BBDs suck, so it doesn't matter, right? ;)
>
> T.
>
>
> On 8 Dec 2009, at 20:44, Mike Beauchamp wrote:
>
>> Tom, First of all this is absolutely brilliant.
>>
>> Secondly, I'm wondering how easy it would be to reprogram it to have a
>> 1:4096 tap:clock ratio? (if that makes any sense).
>>
>> The reason I ask is that I'd really like to use this to add a
>> tap-tempo to an analog delay...
>>
>> The actual wave output could remain 1:1, just the clock output needs
>> to be 1:4096. Then I could use the wave output to drive an indicator
>> LED for example.
>>
>> --
>> Mike Beauchamp
>> mobile: +64 21 0 AS I BIKE (2742453)
>> home: +64 4 801 8304
>> work: +64 4 915 4068
>> http://mikebeauchamp.com
>> http://therevox.com
>>
>>
>> On Wed, Dec 9, 2009 at 12:40 AM, Tom Wiltshire <tom at electricdruid.net>
>> wrote:
>>> Hi All,
>>>
>>> I've mentioned my Tap Tempo LFO project here before. The difference this
>>> time is that I've actually finished it!
>>>
>>> Details are online:
>>>
>>> http://www.electricdruid.com/index.php?page=projects.taplfo
>>>
>>> There are example circuit diagrams for a tap tempo LFO, tap tempo
>>> sequencer
>>> clock, and tap tempo tremolo, plus, a datasheet, code, etc. Any
>>> questions,
>>> please ask.
>>>
>>> If you're into PIC programming or know someone who is, you can get
>>> code from
>>> the site. If not, I'll be trying to sort chips out soon, and if I can
>>> persuade Steve Daniels that this is a good idea, you might be able to
>>> buy
>>> programmed chips from Small Bear. He did show some interest, but
>>> we'll see
>>> whether it's really worth his while. I hope so.
>>>
>>> Thanks,
>>> Tom
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at dropmix.xs4all.nl
>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>>
>
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