DELTA T

Enhanced Audio Delay

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Meet DELTA T, an audio delay unit that goes beyond usual audio delay functionality. DELTA T is based around the PT2399 delay chip, which serves as a basis for many guitar delay pedals and delay-centered modules. The SYNTHFOX take on this chip aims to use as much of its capabilities as possible, while retaining the trademark lush plus janky sound of the PT2399.

DELTA T features two signal inputs IN1 and IN2. These inputs are AC-coupled, meaning that a slowly changing or constant signal, like a triangle LFO or a DC offset, will get blocked off by the input circuit. First input has a volume control knob, whereas the second input is passed as is.

The OUT signal output of DELTA T is an AC-coupled mixture of the inverted sum of the input signals and the delayed sound from the delay circuit. It is located at the bottom right of the module. The mixture proportion is controlled by a vactrol crossfader, and is set with the BLEND knob and the unattenuated BCV (blend conrol voltage) input. By default, this crossfader fades between 100% dry signal (only input mix) and 100% wet signal (only delay circuit's output), but a jumper on the back can change this behavior so that the mix always has a set volume for dry, while blending in the wet.

DELTA T's TIME setting is set with the big white knob at the top of the module and the TCV (time control voltage) input. TCV has a dedicated attenuverter knob below the initial time control. It allows for either adding or subtracting the TCV input to/from the initial TIME setting. Time range of the delay is a few hundred milliseconds (distinct echo) to a few milliseconds (metallic bucket buzz).

The FBK knob sets the amount of feedback. The more this knob is turned clockwise, the more delay repeats will occur, up to self-oscillation. The feedback of the circuit is inverted, so it is tamer/smoother than in most PT2399-based units. The feedback loop is broken out to the two jacks in the FBK.LOOP box, connected to the FBK knob with a dash-dot line. By default, audio from the feedback loop output (left jack) is normaled to the feedback loop input (right jack): this lets the signal circulate around the delay chip unaltered. Inserting a voltage controlled amplifier into the loop (loop out -> vca in, vca out -> loop in) will break the normalled connection and provide voltage control over feedback amount. Likewise, a bandpass filter in this loop will allow dynamic control of the delay tone.

Last, but not least, there is the CLK output located to the left of the OUT. CLK outputs a pulse that's synchronised to the current actual delay time of the unit, and is indicated with the only LED on the module located to the left of the TIME control. This pulse is bipolar and hot, meaning it is a bit less than 12v when the LED is lit, and a bit more than -12v when the LED is off. It can be used as a tempo clock, bipolar LFO, or even as a sound signal - but in this case, it has to be attenuated to at least 50%, because initially it is very loud.

https://synthfoxmodular.com/module/eurorack/deltat/