Understanding Feedback and What It's Good For

Why would someone use feedback with loops?

Control over feedback is one of the most fundamental loop techniques, I think. Without it your loops just develop to a certain point, abruptly disappear, and a new loop begins developing. Things only grow, they don't decay. So the loop gets bigger and bigger until you kill it completely. You don't have any continuity, so your loops can't grow and evolve into something else.

Feedback comes from the old universe of delay effects. The feedback setting causes the delayed sound to be reduced in volume by a certain amount each time it repeats. In that sense it is generally used to set how long the delay would last. That concept was applied to looping in some devices, like the Oberheim Echoplex. When dealing with longer loops this becomes a very powerful technique for making your loop evolve into something new over time.

When you've built up a loop, it will have certain elements that dominate and give it a particular character. You turn the feedback down a bit, and those things slowly begin getting quieter. Then you begin adding new elements to the loop, which will be relatively louder since they have not had any feedback applied. With each repetition you add a little bit more to realign the loop in a new direction. Those new elements will then begin dominating the loop, and the character will steadily change. When it has changed to something you like, you set the feedback up to maximum so the level does not reduce with each repeat anymore. It gives a nice evolving effect to the loop, and is a very powerful and expressive tool. By actively controlling the feedback, you can control how quickly this evolution occurs.

Interestingly, this sort of effect is not readily available in the typical sampler, and it would be a bitch to manage in a sequencer. Consequently, I almost never hear people who primarily rely on sequencers and samplers using this evolutionary approach to loops. They tend to have more abrupt changes with their loops, where something immediately disappears and is replaced by something else.

In addition to people used to delays, DJ's and remix artists seem to get the feedback concept, I imagine because they are so used to managing everything with mixer faders. In that case though, it's mostly crossfading. The new loop is gradually mixed in, but from the start the whole thing is there. You rarely have moments in between where you don't notice that it is actually two loops playing at once. With the feedback/evolving approach, you can create the new loop in little bits at a time, so it is not clear what it will eventually be until it gets there. And all along the way, each "evolution" of the loop will likely be an interesting thing itself. It can be very different from crossfading, and very expresssive and creative, which is why people get into it so much!

Understanding the Echoplex's Feedback Implementation

What controls does the Echoplex have for Feedback?

With the echoplex we give 3 ways to control feedback; the knob on the front, a jack on the back for a foot pedal, and midi continuous control. You can set 128 different feedback levels, which is plenty for feedback. Being a guitarist, I use the footpedal method. I couldn't live without it, I vary the feedback constantly to let things disappear at different rates or stay around for a while.


How does Feedback work on the Echoplex? What's going on inside?

The feedback level is applied after the loop audio output, and before the loop audio is mixed back into the delay line. Say you have a 20 second loop and change the feedback. A given bit of audio still has to wait 20 seconds before it is heard again with the feedback setting applied. The feedback structure looks a little bit like this crude ascii drawing:

	                                 _________
	                                |feedback |
	                  |<------------| level   |<---------|
	                  |             |_________|          |
	                  |                                  |
	 input => ------>(x)--->|=========================|------=> output
	                               delay line                
	                                                 
	
		

The idea of cycles and loops (where a cycle is a subset of the loop) is another sort of abstraction, and I'm not even very sure how best to put it into the picture above. This is dumb, but maybe it works:

Think of the path above as a train track that goes around in a circle. Your loop would then be the train following the track, with the front end of the train just reaching the back. Each cycle would be a car in the train. Doing a multiply or insert adds cars to the train and makes the track longer to let it fit. Now, if a given car (cycle) is at the feedback level station when you adjust the feedback, it still has to go all the way around the whole track to get to the output.


So, does the Feedback effect occur after the current cycle, or after the whole loop?

Feedback works over the whole loop, not the cycles. If you have a 20 second loop that consists of a single 20 second cycle, or a 20 second loop that consists of 10 2-second cycles, the effect of feedback is the same. It would be heard 20 seconds after the feedback is changed. It wouldn't really be feedback if it did otherwise.


Once feedback causes the loop to die out, I assume its gone forever?

On the Echoplex, Feedback is undo-able. If you have enough memory available, you can use Undo to go backwards through the feedback reduction, which is a cool effect. So you can bring the loop back that way.

Another option would be to fade your loop out with MIDI Volume, i.e. simply control the output level of the whole unit. That way the loop would still be fully in memory. That wouldn't really be feedback of course.


Sometimes the feedback level is unpredictable, ie, it seems to not wait til the end of the loop to reduce in volume, and if I push the pedal up (off), the sound will stop, but sneak back in somewhere in the loop. Also, if I push the pedal off about halfway, and let the loop slowly die out, then push it foward, so it continues to repeat indefinately at this new, lower volume, the volume of the loop jumps erractically-it doesn't stay at a steady volume. Does this sound like a midi, pedal, or user problem?

Paul Dresher helps out:

I suspect that your question/problem with the EDP Feedback control is primarily about understanding what is happening when you are moving the pedal (I use pretty much the same set up but with the Digitech PMC-10 device for generating the MIDI continuous control for Feedback). When you move the pedal you are controlling the level of the looped material being returned to the EDP's memory, so you don't hear the change at that point, you hear the level drop when that point in the loop comes around again. Unless you move the pedal real slow, like a fader on a console, you are going to hear the adjustment as rather abrupt. While its a bit frustrating to not be able to hear the effect of your moves until somewhat later, with practice, its not hard to get the feel of what kind of fade is necessary in order to have a "fade out" at the rate you desire. One strategy I use on my PMC-10 ( I don't know if its possible on the Ground Control) is to have different pedal settings on the floor, i.e. one in which the full pedal movement sends MIDI CC values limited to a range of 110-127, which would generally be a slow fade (depending upon how long the loop is in time), another in which the CC values range from 0-127 (essentially an on-off switch) and others for whatever rate I need for a specific part of a piece. Of course you may have a faulty foot pedal but since I experienced the same question as you when first working with the unit, try the above.

-Paul Dresher

Kim again:

Part of your problem could be as Paul noted, just getting used to how feedback works. It's a bit different from just controlling volume level, and you might just need to practice it a bit to get the hang of it. It is a live, real-time control, and where you use it in the loop will make a difference. It's effect will be heard at the point in the loop where you are moving it.

Feedback Pedal Control

Is the feedback pedal input on the Echoplex Digital Pro audio or CV?

It's a control voltage, set with a potentiometer in the feedback jack. There's no audio there. The audio remains digital for the the feedback path, and the feedback is done by a calculation in the processor. If the audio were being converted to analog for the feedback and back to digital each time through the loop, your loops would start to sound pretty bad after a while.


There is a 100k pot in my pedal, would that work ok for the feedback jack?

A 100k pot will compress the useful range of the pedal into a smaller area, which will make it a bit harder to use. As soon as the wiper of the pot is anywhere past about 18k, you will be at the full feedback level. If you change the pot to 20k linear, you will find it to be much easier to control. Make sure it is an ordinary pot, and not a balance fader or something like that.


Can the feedback range for a pedal be set in the Echoplex?

There's no way to set the feedback range in the echoplex, but its a simple thing to do in the pedal by using different types of potentiometers or putting resistors in series with the potentiometer, between ground and the pot.

If you use midi to control Feedback, many midi devices let you set the range of the continuous controllers, which would be an easy way to set the range for the feedback control.

You could also just stick a wad of tape under the pedal as a stopper.


How do I make my own feedback pedal/controller for the Echoplex?

If you feel compelled to make your own pedal, use a 20K or greater pot. Linear is better, but log works too. The value and taper just change the feel of how the pedal controls the feedback. The circuit is very simple, with the tip of a 1/4" jack connected to the wiper, and the sleeve (ground) connected to the bottom end of the pot. Here's a bad ascii drawing:

		mono 1/4" cable 
		from feedback jack                                   o    this end of the pot
		(not stereo)                                         |    is left unconnected
		                                                    <
		                                                     >
		------                 tip connects to              <
		      |_________         wiper of pot                >
		      |_________||=> ---------------------- o-----> <
		------|                                              >   20k-50k ohm
		           |                                        <    pot
		           |                                         >
		           |                                        <
		           |                                        |
		           |_______________________________________ o
		
		  ground (sleeve) connects 
		                to bottom end of the pot.
		

For more pedal issues, see the Echoplex Footpedal Tutorial

Feedback Trouble Shooting

I understand how feedback works, but things are behaving erratically anyway. Feedback levels seem to be jumping around. Is there anything I can do to fix this?

If you decide it's not a problem with your understanding of feedback, then my guess is that the midi controller data to the echoplex is jumping around and mis-behaving. One possible source of this could be a dirty pot in the midi pedal, causing it sometimes misread it's position and send the wrong data. Or it could be something funny with the midi pedal itself. Similarly, if you are using the feedback jack in the echoplex, it could have it's control voltage jumping around due to a dirty pot or jack contact. Try using a different volume pedal, cleaning it's pot, or clean the jacks. If you are using midi try connecting the pedal directly to the feedback pedal jack on the echoplex and see if that makes a difference.