The effect is created by playing two recordings of the same material on different tape machines running slightly out-of-sync. This leads to a comb filtering effect and that classic 'jet plane swoosh'. The most extreme effect occurs at the point where the two tape players sync exactly, and this leads to the mind-bending 'Thru-Zero' sound.
Most BBD-based flanger stomp boxes only get close to that zero point because your wet signal will always have a finite delay time relative to the dry signal. This patch gives you the ability to pass through the Zero by placing a fixed delay on the dry signal path, too. Using the Chorus module, I set the dry stereo signal to a fixed delay roughly half-way through the range of possible values (4ms to 50ms). The wet signal is split into two mono chains of High-Pass Filter, Low-Pass Filter, variable-time delay (Chorus module again) and finally an Amplifier (utilising both +ve and -ve outputs).
The delay time of the two mono signal paths is voltage-controlled. This can be driven by an LFO for automatic sweeping, but I think you get most fun by 'jogging the tape by hand' with a DC Source knob. I've added a Glide module after the DC knob to make the motion smoother, as MIDI CC resolution is just too coarse.
I've also added a quirk in that the left and right signal paths can be offset from one another so that the 'Zero' point occurs at different times on the left and right. This is bipolar, so you can have the left or right channel go thru zero first, depending on the offset setting and the direction from which you approach the Zero Point.
The most extreme effect is where you mix the inverted output of the 'wet' signals with the 'dry' path. This is because at the Zero Point, the two signals will exactly cancel each other. If instead you mix the positive 'wet' output, you will get an exact doubling of the signal at the Zero Point. It is when you use the inverted wet signal that the filters come into play. It works a little bit unintuitively, as you are filtering the parts of the signal which you want to be PRESERVED when you pass through the Zero Point. So, if you want lots of high-frequency information to be retained, then lower the cut-off frequency of the LOW-PASS FILTER. Conversely, if you want to retain the bottom end, then RAISE the cut-off frequency of the HIGH-PASS FILTER. The filters aren't generally needed when using the positive wet signal, but could lead to some creative effects that I haven't considered yet.
One very important caveat is that this effect introduces a minimum delay which is equal to the static 'dry' (stereo) delay line. This means that it is best suited to processing ENTIRE STEREO MIXES. However, if you want to use it as a real-time effect on a specific track, then you can still achieve Thru-Zero sounds by setting the 'dry' delay to a small value of say 6 to 10ms. This will allow you at least 2 ms wiggle room for the wet signal to 'precede' the dry signal, whilst introducing small, almost acceptable values of latency.
- Thru Zero Tape Flanger.png (23.11 KiB) Viewed 2163 times