Vibrato is the slight changing of frequency of a musical note. This can be accomplished by means of a varying time delay in a delay line or by phase shifting and analog signal over time.
The delay approach works fine but creates inherent latency and can sound clownish. So I wanted to create an analog design.
A modulated allpass filter can be used to change phase over time (which is frequency). This analog approach can sound lovely but to sound really good has to linear in its sweep and be modulated in a sinusoidal manner. Both the design objectives are not trivial.
It occurred to me that my PWM phaser design does all this but simply mixes the original signal back in to create the phasing notches. So I modified the circuit and optimized it to simply modulate the filter and it works great. The PWM driven analog switches are very linear and the PIC DDS works great to create the sine wave modulation. I added some pre- emphasis/de-emphasis, adjusted some values and made an improved layout…so the pedal is super quiet. I left in the peaking filter adjustment to allow for some cool modulated filter effects along with the Vibrato. The range of PWM controls the extent(depth) of the effect at a given modulation rate and so the vibrato pitch bend level is easily adjustable. Keep in mind the faster the modulation rate; the pitch shift is also greater because a faster the rate of change of phase over a given time, produces more pitch shift. An alternate analog modulation circuit can be used if desired but the output swing needs to be limited from 0 – 2 volts.
Brainwash Phaser converted into Vibrato pedal:
Coming Shortly Demo:
Circuit Salad 
