Is it possible to use ramp-and-hold as a quasi tap-tempo?

[earthtonesaudio]

I was looking over National's AN-72*, and figures 85 and 86 show a kind of neat ramp-and-hold circuit, one of which looks like it could be made to "hold" indefinitely...

So I was thinking maybe it's possible to use a momentary switch, press it for a certain amount of time, and generate a DC voltage output proportional to the duration of the button press, which could then control delay time, LFO speed, whatever.

Anyone tried this? Is it possible? Would the sample and hold actually "hold" long enough to be useful?

*The appnote is about the LM3900 "Norton" amp, which seem to be rare in stompboxes. And yes, I know it would be a lot easier to just use a microcontroller, but think of the mojo factor you could attain with 100% analog tap-tempo!
Here's the appnote: http://www.national.com/an/AN/AN-72.pdf

[R.G.]

It's not like smart people haven't thought about that one for a long time. Even people on that other forum.

Supose you succeed at obtaning the voltage proportional to the amount of time the switch is hold.
How does that voltage translate to a delay time that corresponds to that amount of time? (I assume you want a tap-tempo delay). That I think is the hardest part...

The problem has never really been how to find the rate of tap which is the only thing you have looked at this point. It is the interface of the rate of tap to the effect's parameter control that is the issue. It is VERY difficult to calibrate the tap time to the parameter.

All the purely analog solutions I can think of, involve ultra low leakage caps and awkward high-precision analog voltage dividers.
(unless you cheat by using a pair of binary counter chips & having one as a "memory"!* but really, it is indeed a problem that screams PIC)

and yours:

The interface shouldn't be too hard. Just PWM an LED.

And mine:

You're right - it should not be hard. But in the face of such wishful expectation, Mother Nature seems to say that in fact, it is.

Here's the dreary reality behind your PWM an LED approach.

1. Presumably the LED is controlling something that then controls the whatever you're doing tap-tempo for. What exactly is that something? How does it then control the whatever? It's probably an oscillator, either LFO (for tremolo) or ultrasonic (for delays). In which case, you have to worry about the next step in conversion - how much circuitry is needed to convert either an LED frequency or a light intensity into just the right LFO or HFO to make the delay come out to be what you tapped in?

2. Given that you can make up that amount of circuitry (its a bit more than a television sweep circuit or a radar range gate, which do similar things, or used to when they were analog) what is its accuracy? That is, when you tap in 0.75 seconds, does the LFO or resulting delay equal 0.75 seconds? Is it instead 0.743S or 0.76S, which are probably OK, or worse 0.62S to 0.95S, which are completely unusable? Linearity matters A LOT in a circuit where you're expecting the circuit to reproduce the rhythm you are tapping in. If you get out, for example, always 0.8 times what you tap in, then you can turn up the internal gain by 1.0/0.8 and have done with it.

But if the "correction factor" needed is different for different times, you cannot apply a correction internally. The human tapping will have to know that short taps aways come out even shorter, mid-side taps come out longer, and long taps come out shorter again, and try to correct for it. Humans are in fact GREAT at doing that kind of thing by long practice; unfortunately, the whole purpose of a tap tempo is to not make the human have to mess with the controls, and now we have them in the middle of it, and a complicated one, not a knob.

3. That nonlinearity thing is a problem at every step. How linear is your tap-to-ramp period? Probably very good indeed. And how good is your ramp period to PWM conversion? Probably OK. How about PWM to light intensity? Probably OK again. Light to LDR? Terrible. Light-to-frequency? Medium to terrible, depending on the method. And for the things that are just OK, moderate or terrible, do the nonlinearities offset one another, or add? Or BOTH offset AND add at different points on the control?

Before microcontrollers, linearity was the Holy Grail of control structures. The controls guys who could linearize stuff down to the point where the remaining nonlinearities didn't matter were the the Big Dudes of the systems world. They made huge salaries for squashing out nonlinearities with a whole toolbox of tricks which got ensconced in weighty textbooks. When the $20, then the $5, then the $1 microcontroller entered the world, the Top Guns were suddenly tap dancing for a living. Microcontrollers let you linearize essentially anything you can describe, even - and especially - if the description can be made in a table of numbers inside the uC. Industry took one look at zillion dollar linearization circuits and uCs and never looked back. Microcontrollers ARE the elegant, genius-like step sideways outside the box that lets these kinds of things work.

So, bottom line time.
1. Can someone design an all-analog tap tempo controller that works OK?
Yes. It's been done backwards with analog radar sets.
2. Can someone design an all analog tap tempo controller that fits inside a stompbox?
Probably. Surface mount lets you get a lot of stuff inside these things. But it's definitely maybe, not "yes, sure".
3. Can YOU design an all analog tap tempo controller that fits inside a stompbox?
I don't know. I think I might be able to, but I also assess it as a "just barely, maybe" not a "yes, sure".

I also know that I will not do so. I would (and have done) take one look at the requirements, and reach over into the cabinet for a $1.50 PIC or AVR and have the code running tomorrow.

I do not mean to be harsh, and I do not mean to be picking on you. I'm trying to save you a lot of trouble, expense, and frustration that I think would go along with a serious effort to design one of these. I could be full of it, and sometimes am. But this is really meant as friendly advice explaining what I think is a deeper topic than you may have dug so far.

This is one of those seductive ideas where there just ought to be a simple, direct, analog way - but that none has presented itself over decades of efforts. Another few decades of work might solve it. Or it can be running in a few days with a microcontroller.

[earthtonesaudio]

I'm not really looking for quick and easy, or even accurate. If I wanted a quick and easy tap-tempo delay I'd just buy a used DD-5.

So I've done some more thinking on how to do it.
I want to ramp-up to a certain voltage in a certain time, while holding the footswitch down. Once released, I want the output voltage to be translated to PWM to control brightness of the LED which is currently aimed at an LDR in my modded Fab Echo delay pedal.

Okay, that's not too hard, and it doesn't need to be super-accurate, because it's a delay pedal, and the repeats will fade away eventually. You'd need more accuracy for a tremolo or something else that was constantly modulating.
But that just sets the tempo once. What if you want to set the tempo again?

That's probably going to require some clever switching. I'm going to want to ramp-up the voltage every time (rather than ramp up and then ramp down), and when the button is released I want the ramp voltage to be held, and the next button press should start the ramp over from zero again. To do that with a momentary pushbutton switch is going to require some extra stuff. The solution I'm thinking of at the moment is to build two separate ramp-and-hold circuits, so that stepping on the footswitch sets the ramp voltage on one and resets the other, and releasing the footswitch allows the ramped-up voltage to be held constant and applied to the following interface circuitry.

So my new question is, what type of switching arrangement would suffice to perform all these actions? I'd like to use a SPST momentary, but I think it might require a DPDT momentary. If all else fails I can just build a little actuator and press an array of SPST switches, but I'd rather keep the complexity in the electronic realm, not in the mechanical.


So basically I'm stubbornly refusing to use a microcontroller. I think the hard way is fun!

[R.G.]

Bless you and good luck. Let us know when you get there.

[earthtonesaudio]

Thanks!

[earthtonesaudio]

Not quite there yet, but I've made a little headway. I've solved (in theory anyway) the switching/resetting problem. I've got it down to a SPST switch, a flip-flop circuit, and two separate ramp-hold circuits. The SPST sends the voltage to the ramp-up input, and the flipflop sends voltage to the ramp-down input. The ramp-down gets the full voltage, but the ramp-up gets a divided down voltage, so while one circuit is reset, the other is slowly charging up.

Next to-do is the summing amp and PWM. I'm up to two LM3900 chips, but I have two amps still unused on one of them, and I might be able to free up another. The hardest thing so far is their crazy pinout. I think I fried one of them by mistaking the +V pin with the ground pin. Good thing I have some spares.

[Hobie]

i don't think it would be that hard to make a tap tempo delay. The only problem would be to calibrate the variable resistor/whatever! (lm13700, ldr, fet, transistor, etc?) to drive the delay time of the chip your using.
Using a Uc it's really easy to sample a switch and create a VC to control the said variable resistor. add a synth style DC mixer maybe a LIN/LOG converter and let the calibration begin.

look at the beavis audio project with the pwm phaser. See RG's ASMOP page for further Inspiration. Also take a look at the ARDUINO page http://www.arduino.cc for probably the most accessible Uc i've seen ever...... (my first uC actually) and find out that there's a wealth of users that more than happy to share source code, schematics, hints and tips, etc, whatever...

I've only made a tap tempo LFO, and a tap tempo baby10/vanishing point/zvex ooh wah. But i can see a diy tap tempo delay in the near future ....

or should we just go to the dark side Buy some ad/da chips, get a dsp and sitll have to learn C or even worse Assembly?

toedeloe

hobie

[earthtonesaudio]

The only problem would be to calibrate the variable resistor/whatever!

Yup. I'm trying not to worry about that part yet.

Using a Uc it's really easy to sample a switch and create a VC to control the said variable resistor.

Yes, and anyone wanting an "easy DIY tap tempo" should go that route. Personally, I am going for a certain aesthetic (op-amps, mostly analog, parts I already have on hand, etc) which means I'm okay with more difficulty and less accuracy if the aesthetics are right.


Anyway, an update:
I've been working my way linearly through the stages of the circuit, trying to figure out what works and what doesn't. I've finished the input triggering and reset parts, now I'm fine-tuning the ramp-up sections. They need a regulated low voltage input so that the ramp is consistent regardless of battery voltage. Maybe I'll use an LED. It might be kind of cool to have a LED light up while you're holding the button down anyway. Useless, but cool.

I still have no idea if my idea for voltage-controlled PWM will work, but I don't want to get ahead of myself.