S312 Phase Locked Loop

Product Description

 

(This module is included in the System B)

 

A phase locked loop (PLL) is an oscillator device that TRIES to follow the frequency and phase of an input signal. You can think of it as a frequency follower. PLLs are rather famous devices that has been used in the field of communication systems and generally in electronics since the 1930s. Yet, their usage for audio has been usually experimental. Through out history, the device evolved from being primitive and unreliable to being robust and accurate. However, for audio usage. The most interesting results stem from all the wackiness and in-accuracy.

 

A PLL is composed of three components. A phase comparator (PC) that receives the audio input and produces a bi-state signal (0,+1) or a tri-state signal (-1,0,+1). The output of the phase comparator is then fed to a low pass filter (or a slew limiter) that smooths it. Which is then fed to a VCO. The output of the PLL is the output of the VCO. This output is also fed back (Looped) to the phase comparator. In layman’s terms, the job of the phase comparator is to “tell” the VCO to increase or decrease it’s frequency in-order to match the input audio frequency and phase. The phase comparator continuously monitors the input and the VCO to make sure they match in both phase and frequency. Have a look here for more information about PLLs.

 

The S312 features 7 different phase comparator modes that range from very bad to good. As a result, the sounds that can be achieved vary from mildly tuned to complete chaos and randomness. First, the XOR mode is an old and unreliable method. JK uses JK flip-flops which produces weird results at times. RS uses RS flip-flops which are a bit better at following the input frequency. RSc is RS with a bad counter. PFD (phase frequency detector) is the best method of all the 7. It is closely related the method used in the well known 4046B chip. The PC (phase counting) which we developed here at SoloStuff. And last the Ring which uses a ring modulator which is also an idea we had at SoloStuff but never saw any one use!!.

 

Each of the seven modes are sensitive to the cutoff of the low pass filter. Which can be dialed using a knob and CV input. The tuning of the VCO can also be adjusted with Knob/CV. The Comp Amount knob sets the depth at which the comparator/filter affects the VCO. The Comp Out provides the direct output of the phase comparator for you to directly process using your desired filter or slew limiter. You can then feedback the result to the Tune CV to create your own custom PLL. In this case, you may like to set the Comp Amount to zero to block the internal loop.

 

Usage Ideas:

-Typically, you would want to put a PLL immediately after an oscillator. Remember that a PLL is not an effect. It’s doesn’t process the input. It merely detects the pitch of the input and uses that to drive it’s own square VCO.

-Try FM or ring modulated sound for the input. Or any thing that crosses zero voltage rapidly. This would confuse the PLL and can produce interesting results.