Delay Pedal with Infinite Repeats with no tail / volume build up / oscillation

[tenser75]

I'm trying to build a simple delay with just the function of holding a sample, think like the boss DD hold mode...

I saw some videos about a simple mod to create the infinite delays but they are traditional feedback that becomes bigger and bigger...

i basically just need a sampler that repeats one sample over and over as long as you keep pressed the momentary switch...

any idea of what schematic should I look for?

thanks

[balkanizeyou]

if you're planning to keep it simple and work with pt2399, I don't think that's doable. You can take some standard pt2399 circuit and fine tune the feedback resistance to the point where it's on the verge of oscillation (but it doesn't oscillate yet), but even if you achieve the "infinite" repeats, they're going to degrade with each repeat to the point where they're not really similar to the sample you started with.
If you want both clean and infinite, you need to get to the programming side of things (or maybe FV-1 already has some presets like this one? I've never played with this chip so I have no idea).

[idiot savant]

I did a simple delay like this. I based it off of rring's simple pt2399 delay. Though you should look at slacker's echobase as well.

Getting infinite repeats is a challenge! The echobase uses a diode clipper in the repeats path to limit how loud the repeats can get. I dunno if you can tune it to get infinite repeats though. Rring's delay has a boatload of post delay filtering; an 8 pole filter! He has a little video on his website that shows some nearly infinite clean-ish repeats.

check it out: circuitsalad.com

As far as the switching logic, I used electronic switching, and a AVR to control it. One switch on the input, one on the output. You leave the input connected in bypass, then disconnect the input, and connect the output to play the "sample".

I wasn't really satisfied with my efforts, and I haven't gotten back to it, but it's doable. It should work much better on a spin fv-1 than a pt2399.

[ashcat_lt]

Boss uses both diode limiting and compansion to keep things from escalating out of control.  It works in my software emulations, but I can't help you with how to do it in meatspace except to say look at the schematic for one of the Boss delays that do what you want.

[Transmogrifox]

Seems some kind of automatic gain control circuit is wanted for preventing excessive build-up.  Opto compressor circuit block could be a starting point.

[PRR]

You put the sample in memory, and play it back over and over.

Getting JUST the segment you want will be tricky.

Traditional "reverbs" and "echos" are the wrong architecture.

[midwayfair]

You can use a diode on pin 7 (and/or in the PT2399 to get a distortion-free signal clamp (look up my Hamlet delay), but unless your playing is perfectly consistent it doesn't matter how you manage the signal level -- compression, distortion, current limiting in the chip -- this isn't a job for analog circuitry.

I'm not sure how difficult it would be of a program for the FV-1. You need to have it record a sample of length x set by a knob with a rolling re-write, and to stop recording and replay the sample when the switch is held. There is probably a simple looper program you can Git for the rolling rewrite, and the second half is just a momentary switch. The wrapper is:

Guitar > signal splitter > signal A goes to recorder, signal B direct to output > when switch is held signal A also goes to output.

[slacker]

I'm not sure how difficult it would be of a program for the FV-1.

It's pretty straightforward, here's one version from the Spin forums http://www.spinsemi.com/forum/viewtopic.php?t=161&postdays=0&postorder=asc&start=36 this might need a little tweaking because it kills the dry when playing back but that would be simple to change, maybe using the spare pot as  a wet level/mix control.

[tenser75]

i don't mind actually the degradation... now that i'm thinking it could be cool

if you're planning to keep it simple and work with pt2399, I don't think that's doable. You can take some standard pt2399 circuit and fine tune the feedback resistance to the point where it's on the verge of oscillation (but it doesn't oscillate yet), but even if you achieve the "infinite" repeats, they're going to degrade with each repeat to the point where they're not really similar to the sample you started with.
If you want both clean and infinite, you need to get to the programming side of things (or maybe FV-1 already has some presets like this one? I've never played with this chip so I have no idea).

[ashcat_lt]

Perfect unity feedback should repeat infinitely without escalating.  It may not be easy to achieve with real world parts...

I'm pretty sure that the Boss momentary "Hold" function simultaneously sets feedback to unity and disconnects the input from feeding the loop.  At least, it doesn't have to be much more difficult than that.  If you actually want the tails to cut off when you lift your foot, things get a bit more complicated.  You'll have to build some kind of logic into the switch sensing mechanism...

My Boss GigaDelay does infinite repeat when you turn the feedback knob all the way up.  If I play something, it will repeat over and over again for days (yes, I sometimes leave my stuff plugged in and powered up).  If I play something else over the top of it, it will repeat that mix over and over again.  If I play some more, it turns into either a lush, complex, layered symphony or (much more often) a horrible mess.  But it never actually gets much louder.

Now, inside a computer, we have no "environmental" constraints on our dynamic range.  Our "power rail" is literally billions of volts.  Pull up the most basic of plugin delays and set feedback to unity.  You play a little bit, and it will repeat for days.  You play a little more, and it repeats that mix.  But if you played exactly the same thing, it'll be close to 6db louder.  Keep playing the same thing and it gets louder and louder every time the loop comes around.  It'll quickly get so loud that the your converters can't follow (0dbfs), and what you hear will clip, all the red lights will come on, but the numerical readout will keep going up and up and up...

The most basic pedal delays do essentially the same thing, except of course that they do eventually have a hard limit.  It'll keep pushing further and further into that ceiling, though, and even 5V square waves are REALLY LOUD by guitar standards.

That's where Boss's compander chip comes in.  You mix the input with the delay buffer output, compress the crap out of it, feed it back to the buffer, then expand what's coming out of the buffer before sending it the output and back around to the mix/compress step.  The expander of course can't really put the dynamics back where they were, it just does what it does.  It's all helped by some judicious diode limiting and works amazingly well both in hardware and in software.

[anotherjim]

There are those designs using voice memo chips. The ISD1820 I think it is. However, I tried one a very - very - long time ago and seem to remember it was very lo-fi and couldn't repeat without a short mute period. But back then I didn't have a lot to go on apart from a photo-copy of the data-sheet.
Anybody have any love for these?

[Kipper4]

^ Didn't Smallbear have a project with those chips?

[Transmogrifox]

The voice memo type of chip might be a go if you can find something like that with acceptable audio quality (or maybe lo-fi is ok).  If you put 2 in parallel then you can use an out-of-phase LFO to stagger the start of playback from each chip.  The LFO also modulates a cross-fade circuit to pan between the two so chip B is playing the middle of the clip during chip A glitch/mute period and vice versa.  Both chips record when you stomp the sample button, so you end up with  a relatively smooth transition around the loop.

The trick is synchronizing the LFO with start & end times and then maybe the compromise is to let the LFO time the recording stop so the loops are mostly synchronized with the LFO.

Then there is always the Raspberry Pi.  I just succeeded in getting latency down to < 2ms with a USB audio interface and ran a chorus and delay on it.

In that platform the implementation of something like this is much more straight-forward.  You store samples in the circular buffer when the button is depressed and stop storing when let off.  Apply a fade-in and fade-out function to the start and tail of the snip and then you loop that back infinitely without application of any feedback.  The head and tail parts overlap and add so you get a gradual crossfade between the start and end of the snipped segment to minimize the "blip" on every repeat.  This is something that would be a very complex analog circuit to emulate, in addition to attempting to get unity feedback.

If you get a Raspberry pi and a USB audio interface you might convince me to code it.  I'm in the process of building a pi-based DSP multi-FX and this is one effect I could use so I have incentive to code it at some point.  Then I can upload the full flash card image and if you had the same USB interface it would be plug 'n' play.

[anotherjim]

Madbean have a memo chip looper - it's using the ISD1020.
www.madbeanpedals.com/projects/Loophole/Loophole.pdf
Freppo (Parisit studio) posted a project not long ago -  but I don't think he got around to producing a commercial pcb/instruction set.
http://www.diystompboxes.com/smfforum/index.php?topic=113628.0

[tenser75]

I just realized that yes, I basically was asking to build a looper (duh)
thanks for the links..very useful!!

basically what i wanna try it almost the hold mode of a boss dd7...

[J0K3RX]

BOSS RC‑1 Loop Station.. $79.00 or Mooer Micro Looper $58.00 and if you want to get a spooky vibe going try throwin down some reverse loops with a RC-30 or RC-50..  Reverse loops are hypnotic!! you start messin around with one and then you realize it's morning and time to go to work and you been sittin around for like 12 hours in another dimension.