To make this piece I started with the standard “T4” design. Then I added two more switches for introducing different types of distortion into the sound. Then I built an LFO circuit into the body of the Speak&Spell. You can achieve lots of different effects with this. It can be used to turn the Speak&Spell into a sort of rhythm/drum machine. View a large scale diagram of the ST505.
The LFO has two functions. It can act as a pitch modulator and it provides rhythmic trigger pulses. These functions can be used individually or in conjunction. The LFO rate can be adjusted with the LFO rate knob and is indicated by a blinking light to the left of the knob. Pitch modulation has two settings, “depth” and “slope.”
“Depth” controls how much the LFO will effect the overall pitch. “Slope” is used to transform the shape of the LFO waveform. The wave can be changed from straight square wave shaped modulation to a smooth triangle modulation. The trigger pulses can be used to regularly trigger the “ON” or “hold” functions of the Speak&Spell. This means that every time the LFO indicator lights, it’s the same as pressing the “ON” button or pressing the “hold” switch. Using the “hold” trigger and pitch modulation at the same time can make some really amazing drumbeat like sequences.