Ladder filters in the feedback loop

Put a low-pass filter on a delay's wet output and all the echoes get darker. Each repeat passes through it once, so the filter applies the same cut to each one.

Drift Delay puts its two filters inside the feedback loop, between the delay line and the point where the signal is fed back in:

input → drive → soft clip → delay line → ladder 1 → ladder 2 → character → wet
                     ↑                                                   │
                     └───────────────────── feedback ────────────────────┘

The dry signal bypasses the filters. The wet signal passes through both of them again on each trip round the loop, losing more of the filtered frequencies each time. That changes how quickly those frequencies decay.

The arithmetic

Take a low-pass at 1 kHz with a 6 dB/octave slope, and a signal component at 4 kHz. That's two octaves above the cutoff, so roughly 12 dB down as it passes.

On the output, the filter takes 12 dB off that component in every repeat. The feedback control sets the drop in level between repeats.

Inside the loop, the filter's contribution adds up: roughly 12 dB off the first repeat, 24 dB off the second, 36 dB off the third. By the fifth repeat, that component is 60 dB below where it started, on top of whatever the feedback setting has already taken off.

You can hear this on drums. The early repeats still have some of the attack; later ones lose their high end and sound duller. A fixed output filter gives every repeat the same tone.

Choosing a slope

The slope sets how steeply the filter cuts beyond the cutoff. Inside the loop, a steeper low-pass removes the top end in fewer repeats.

Drift Delay offers 6, 12 and 24 dB/octave on both the low-pass and the high-pass. New instances default to 6 dB/octave because the gentler cut lets filtered repeats ring out longer. Try 24 dB/octave when you want the filtered frequencies to disappear quickly.

Resonance

Both filters are resonant transistor ladders. Resonance adds a peak around the cutoff. At the top of the range, the filters self-oscillate, producing a tone without an input signal.

That tone feeds back into the delay line too. It returns a delay time later and passes through the filters again. Try setting a resonant peak ringing, then moving the delay time to bend its pitch.

Each stage of the ladder saturates. This lets the resonant tone remain audible as the loop level rises at high feedback settings.

Band-pass, notch and all-pass

Band-pass keeps a band around the cutoff. Repeated passes narrow the tail until it can sound almost like a sine wave. Try it for a ringing dub echo.

Notch cuts a narrow band at the cutoff. At high loop levels, saturation adds harmonics that can partly fill the cut.

All-pass shifts phase while leaving the level response flat. Those shifts accumulate on each repeat, giving a phaser-like effect when the echoes mix with other signal.

Two filters, series or parallel

Series sends both channels through Filter 1 and then Filter 2. A low-pass and a high-pass in series let you set the upper and lower edges independently, useful for a telephone-style echo.

Parallel sends left through Filter 1 and right through Filter 2. Each side can then lose different frequencies as it repeats. Try a low-pass on the left and a high-pass on the right: the left tail gets darker while the right gets thinner.

Using an output filter

If you want echoes that keep the same tone as they fade, use a filter on the wet output and leave the filters inside the loop off. In-loop filtering is useful when you want the tone to keep changing through the tail.

You can also switch Drift Delay's Delay control off to use drive and the filters without echoes. The modulation controls still work, so you can use the same LFO filter sweeps on the input signal.

The manual lists the filter controls and routing options. There are more audio examples on the Drift Delay page.