NEW PRODUCT: Taptation Tap Tempo controller

[MoltenVoltage]

I've taken a look at your TapTation PT2399 application note and see that you say the circuit design has an average accuracy of 5ms using a MCP41100 digipot.

This seems impossible given that the 100K ohm MCP41100 digipot has a typical resistance tolerance of +/- 30%.

I understand how you arrived at the 5ms value - dividing the 257 step resolution by the device's target resistance, but unfortunately digipots are extremely variable in their actual resistance and can be expected to have a nearly 30% error.

With the PT2399, a 30% error can amount to an enormous error in accuracy.

For example, at 27,600 ohms, the PT2399 datasheet calls for a 342ms delay time.

70% of that value is 19,320 ohms.

Even, very conservatively, at the next value down on the datasheet (21300 ohms), the stated delay time is 273 ms, with a resulting error of 69ms.

Of course this error is even more dramatic when you get up around 100K ohms (> 1 second of delay time) where you can expect an error of over 150ms!


Have you come up with any way to calibrate the output resistance or otherwise account for this typically huge tolerance error?

sounds like a good pm...

I think the chances of a solution are a little better with 19,000+ brains rather than 2!

[ppatchmods]

i put this on the rebote 2.5 tonight. i omitted the pmw speed and the double time switch. i also used a 2n3904 (i think) and it works great. if you tap in a time and turn the delay pot, it freaks the microcontroller out though. the ratio switch is handy. i managed to cram the rebote with loop, loop on/off switch, modulation knob, ratio switch, and tap switch all in a 1590bb. i'll post a pic in the morning.

[ppatchmods]

sounds like a good pm...

I think the chances of a solution are a little better with 19,000+ brains rather than 2!

didn't mean to sound harsh, your comment came across as more of a criticism than a proposed issue to be solved by the masses. didn't mean to ruffle any feathers

[MoltenVoltage]

Its a very real problem and I'm currently looking for a solution.

For the 100K digipots, they are often around 70K.

Scaling that down to 50K is relatively easy with a resistor in parallel, so the shorter delay time accuracy can be solved with a cermet pot and a multimeter.

The problem is the Microchip digipots only go up to 100K, and, being a programmer and not an EE, I don't know that its possible to "scale up" a pot, (i.e. take it from 70K to 100K).

A series resistor won't do it because your fast delay accuracy goes way off with a 30K resistor in series.

I've also read the Microchip app notes and they don't provide any help.

Maybe 2 100K digipots in series with a parallel resistor?

[MoltenVoltage]

Rather than hijack this thread (sorry!), I started a new one to try to solve the problem of accurate digital control of the PT2399 here:

http://www.diystompboxes.com/smfforum/index.php?topic=87813.0

[Marcvv]

You can use the output of one TapTation to set another TapTation but you have to come up with a pulse feed switching arrangement to do that.

So this is out of my scope of knowledge. Anyone willing to school me on this and help me set something like that up?

Thanks, Marc

Bump

[slacker]

I think what he means is that you can't just hook the output of one up to the tempo input of the other. If you do this the second one won't change tempo because it needs a gap between pulses of at least the maximum tempo time before it changes (from page 5 of the data sheet).
What you need to do is wait until the first one has changed to the new tempo, then send a couple of pulses from it to the second one. Shouldn't be too hard to do this, I'll have a think about it.

[Valoosj]

Any updates on this? Would love to order me a few to put in my Echo Bases.
Will adding a parallel resistor to the pot to make it 50K solve the accuracy problem? I don't need the longer delay times, the 50K standard pot value will do it for me.

[slacker]

No, if you have a problem with accuracy, putting a resistor in parallel with the pot won't solve it. You could use a resistor to make it 50k when it was set to maximum resistance, but at lower settings it would still be wrong.

I'd buy one and give it a try before worrying about an problem that might not exist, I admit I've only got one but it works fine. The Tone God is a good guy, I'm sure if it turns out that the product doesn't do the job it's supposed to then he will be happy to sort the problem out.

[Valoosj]

I'd love to try it, but they're not in the store anymore.

Anyways, why would there be a problem with the pot if you tap in the tempo with the switch? Or am I missing something here?

[ppatchmods]

i used the vero layout and my subdivisions don't match up with the tapped tempo. i didn't add the dbl time switch. i also didn't add the led for the scaled tempo. would either of those matter? this is for a rebote. do i need to disconnect the 1k from pin6 of the 2399 on the rebote? thanks

[ppatchmods]

It seems to be on the faster tapped speeds in the dotted eighth position. It just doesn't match up. Repeats are set pretty low & no modulation. Could the 100k to ground on the tempo scale switch be changed to a different value to make it more accurate?

[Skreddy]

Maybe v2 needs a calibration mode like PTAP    Sounds like this is the only thing its missing, unless it's adapted to use a precision digital resistor instead.  I'm excited about the PWM modulation for phase-shifter application with a double-speed switch.  I wouldn't even care about the tap-tempo part of it if I could just use a pot instead to set the speed and have the 2x speed option with a separate footswitch.

[au_loki]

i used the vero layout and my subdivisions don't match up with the tapped tempo. i didn't add the dbl time switch. i also didn't add the led for the scaled tempo. would either of those matter? this is for a rebote. do i need to disconnect the 1k from pin6 of the 2399 on the rebote? thanks

Hey there - I built the vero too (funny that!) and did find an issue with timing as well.
One thing you will need to do is drop off the 1k resistor as it will "add" onto the digital pot resistance.
Also remember that the 'double time' switch will bring the digital pot resistance from a 'max' of 100k to a 'max' of 50k.
Remember that the 'double time' switch will also result in the 'tempo scale output' LED run at double the speed. (I have a MCP41050 waiting to be put in).

One thing I have been meaning to do is set it up in dotted eighth mode and run the output of the 'tempo scale output' to a transistor to actually give me a dotted eighth input to my digital delay pedal.
There's something to report back on later.

Later au_loki

[The Tone God]

Sorry for the late replies. I'll try to cover what I can here.

The good news is TapTation will be back in the store shortly. Aron and I have worked something out that should remedy the shortage problems we have been having.

The +/-30% accuracy on the digital pots: One has to learn the politics involved in the way technical datasheets are written. The only specs that a company is held to with datasheets are the specs that they guarantee. You basically have to read in between the lines for the rest. They obviously do not want to guarantee the resistance accuracy so they quote a huge number to cover themselves of liability. I have worked with alot of digital pots and I have never seen one vary by 30%.

The 5% accuracy: This is the max variation of delay time between the positions of the digital pot. When a time is tapped in the TapTation will set the pot to the closest position to the that time under ideal conditions. Thus the largest gap between time entered and delay time will be the 5%.

Now I say "ideal conditions". There is alot that can make the circumstances less the ideal and most of them the TapTation itself cannot deal with. Variation in the Pt2399's clock, temperature stability, power supply drift, circuit layout, etc. will all contribute to the accuracy issue. Lets face facts about the PT23* series, it was never meant to be accurate. It is an IC family that aims to make it cheap and easy for manufactures to add audio delay be it toys, karaoke machines, and other cheap devices. We just happen to be able to use it of our delay needs and there is nothing wrong with that as it is cheap and easy to use which are important factors for DIY needs. They also IMHO sounds fairly good but to expect any level accuracy out these ICs is quite frankly unrealistic.

I do have some theories in how to make the TapTation more accurate but some of those ideas might end up creating more bulky circuits increase cost and error for the DIYer. If you need that kind of accuracy which would require more circuitry you might as well switch to a different technology. The TapTation is a "best shot" added feature for a device that was never meant to be accurate. Its fairly simple and fairly accurate as my testing showed. Mind you who really needs that kind of accuracy ? Its not like the note will ring long enough for the time discrepancy to matter.

i used the vero layout and my subdivisions don't match up with the tapped tempo. i didn't add the dbl time switch. i also didn't add the led for the scaled tempo. would either of those matter? this is for a rebote. do i need to disconnect the 1k from pin6 of the 2399 on the rebote? thanks

Yes remove the 1K. It would throw off the calibration.

I do have to stress that the TapTation is only setup for the 100K digital pot. Replacing it with the 50K will throw all the calibration off.

A calibration mode...hmmm. What would I be compensating against with a calibration mode ? The variation in the PT2399 ? The circuit layout ? I would have to do more testing to see how adjusting the time to resistance would affect the internal calibration that is already in place. I'll look into it later.

Andrew

[MoltenVoltage]

I received a shipment of ten 100K digipots (MCP4151) that are all around 75K, so a large variance can't be that uncommon.

Feel free to use the self-calibration technique I came up with here:
http://www.diystompboxes.com/smfforum/index.php?topic=87813.0

The system self-calibrates each time you power up so you can use 50K, 100K, or even a couple of digipots in series.

[R.G.]

At the risk of being repetitive, long term accuracy is the Achilles heel of any continuously running tap tempo LFO.

It's different for delays, where the inaccuracy will always be for only a few cycles of time delay, and restart on each note.

This is a fundamental difference in tap tempos.

[bonkdav]

I read through this and I didn't see any posts about phasers, flanges, or trems.  I do not have the ICs right now but I am very interested in expanding the use of this circuit to more applications. If anyone has experimented with the Taptation in a circuit other than delay, please post about it......if no one has....will somebody please, so i can rest easy until i get the parts. (not that im not going to use it in delay im just wishing for broader horizons.

[Beo]

Anyone have any opinions / preference between the Echobase modulation and the Taptation modulation? I plan on implementing just one, and I'm leaning towards the echobase modulation, using the wave shape mod. If I want a modkill switch on the taptation modulation, is it sufficient to switch pin 7 of the digipot to ground?

[slacker]

I like both of the modulations, the taptation one sounds very nice, the modulation speed is tied to the delay time or a multiple of it though.
With the Echo Base the modulation speed is independent of the delay time so you can do chorus effects with a short delay time and a longer modulation speed, or you can run a fast modulation speed with long delay times.

Yes, kill the taptation modulation you can just short out the mosfet. If you kill the modulation on the Echo Base there will still be some resistance in series with the digipot which will affect the accuracy of the tapped delay a bit. If you want maximum accuracy I would add a switch to short out the transistor.