This module receives and processes the signals (KOV and gate-pulses) coming from the keyboard. If you press a key on the keyboard, two different signals are created at its two appropriate outputs: The keyboard output voltage (KOV) and the gate pulse. The KOV is proportional to the range of the pressed key which means to the audio frequency relating to that key. The constant gate pulse is used for different controlling purposes and is only present when pressing a key.
The sample and hold stage expects, that the KOV goes back to zero after releasing a key and therefore stores it to make sure that a note keeps on sounding (at the appropriate ADSR setting) after releasing a key.
Indeed the KOV went to zero after releasing a key at the earlier analog-synthesizer keyboards where the KOV was created by selecting one of the 50 or more connection points of a voltage divider made of precision resistors with equal values.
At the digital keyboard I use, this is not the case and the KOV is saved digitally in the keyboard. For safety reasons (e.g. if you want to use an old analog-keyboard instead) the KOV nevertheless is additionally saved in a sample and hold circuit in the keyboard-interface module after releasing a key and the connection to the KOV-output of the keyboard is interrupted.
This happens via capacitor C2, which is located at the input of OpAmp IC4 (CA3140). This amp has an extremely high input impedance. If a key on the keyboard is pressed, C2 is loaded with the KOV voltage. If the key is released again, the connection between C2 and the KOV-input is interrupted via the CMOS switch IC6A. P1 serves as integration resistor for C2. Depending on its setting, C2 is loaded with the new KOV voltage either suddenly without delay or slowly with a gliding change. The resulting effect is already a typical feature of an analog-synthesizer. To make sure that a played note glides to the appropriate value if the key is lost before reaching it, the gliding unit is built by an additional capacitor (C3) and buffer (IC5) following C2.
The two inverting OpAmps IC1 and IC2 are used to amplify the gate pulse because the control input of the CMOS switch doesn’t react on 5 V.
The NAND-Schmitt-Triggers IC7A and IC7B have two tasks in combination with C1 and R5: The gate pulse from the used Doepfer keyboard is high as long as a key is pressed, which is important for a good functioning of the later described ADSR generator. In contrast to this, for saving the KOV in C2 we only need a short pulse to control the appropriate CMOS switch. This short pulse is created by C1 in combination with R5. Due to the threshold of the following NAND Schmitt triggers, this pulse is also delayed a little to guarantee that the switch doesn’t act before a new KOV value is present at the input.
If you play fast on the keyboard, it often happens by accident or by intention that the old key remains pressed after you press a new key. By this you theoretically would have a permanent High state at the gate line which would avoid that the new note (KOV value) is loaded in C2. In addition, the ADSR generator would not be triggered. To make sure that this won’t happen, the keyboard I use sends a very short interruption (zero) pulse at the gate at the moment when a second key is pressed additionally. To retrigger the ADSR generator, this interruption pulse is too short and must be made a little longer by the logical operations of IC8 a, b and c. The output of IC8 c therefore has to be connected with the input of the two ADSR generators for the VCA and the VCF.