MIDI Drum Machine

  
Simplified Analog Input Schematic This schematic represents the input circuitry for a single input. This circuit is a track and hold. The idea is that the input waveform may reach a peak at any instant as the drummer strikes the pad. When that occurs, the capacitor is charged to the peak input voltage. There is only one A/D converter (ADC0804, see the Main CPU section schematic), and it takes time for it to convert an input voltage to a 8-bit code. No matter when the drummer strikes the pad, the capacitor will `remember` the peak input voltage, so that it can be read later as the A/D convert cycles through the 8 inputs. This plot shows the track and hold cirucit in action. The lower waveform is the raw input from the pad`s transducer, and the upper waveform is the output which will be read by the A/D converter at some later time.
MIDI Drum Machine - schematic

The two switches shown are actually CD4051 8:1 analog MUX chips (see the Complete Analog Schematic. The one labeled Read connects one of the 8 inputs to the A/D converter. After the A/D converter is finished, the CPU briefly closes the Clear switch to dis



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