The term called ENVOLOPE is one of the very important factor for the emitted sound characteristic of the GUITAR or a particular musical instrument. It relates to the way of a musical note rises to its peak amplitude with respect to its time and maintain its value.
The beginning period is called ATTACK and the retaining period is called SUSTAIN. When the sound producing stimulation is removed the sound decaying started and reached to its minimum within a finite time is called DECAY TIME. Different musical instruments have their own characteristic ATTACVK, SUSTAIN and DECAY times. Alteration of the envelope of musical waveforms could be made easy in steps of different musical instruments.
The average waveform of the envelope of ATTACK, SUSTAIN and DECAY is shown¬ in the diagram that relates to Time v/s Amplitude.
Finally the schematic diagram of the envelope control circuit has been posted in the link below…
Detail Circuit Description:
FET TR1 is used as a voltage controlled amplifier where the gain of TR1is inversely proportional to its bias voltage. This bias voltage is directly depends on the charge applied to capacitor C5. Please note that, C5 has been connected as shown in the schematic and a separate negative supply has been provided through relay contact to charge the capacitor.
The manual trigger has been provided for the Envelope Generator and is built around transistor TR3 and the first stage a transistor TR2 (UJT) is used as a conventional relaxation oscillator is used for the automatic mode. A mode selector switch (SW) is used to select the Envelope Generator either in “AUTO” mode or “MANUAL” mode. When the system is in the MANUAL mode pressing of FOOT SWITCH is necessary to press momentarily so that, C7 is charged properly. This charging Cuts off TR3 pulling the relay ON. Initially capacitor C5 is kept charged to a negative voltage with respect to ground through the N/C contact of the relay.
As a result the any signal applied to the GATE of TR1 through C6 would not pass through. When the FOOT SWITCH press momentarily capacitor C5 is shorted through resistor R6 via relay N/O contact. As a result the negative bias voltage at FET (TR1) and it begins to allow the Input Signal to build gradually. Therefore, the potentiometer R6 started the ATTACK time of the system.
After the FOOT SWITCH would be released capacitor C7 hold its charge for a very short period depending upon the setting of the potentiometer R2 and the capacitor C3. However, the potentiometer determines the SUSTAIN Time of the unit might be typically between 10 to 20 seconds.
Once the charge is lost on capacitor C7 the relay would be released and capacitor C5 would again get charged to the negative voltage through R7. The potentiometer R7 therefore determines the DECAY Time. Once the capacitor is fully charged no signal will be pass through the system.
In the AUTO mode the trigger produces pulse at some time span interval over a wide range of frequencies. These pulse CLOSE and OPEN the relay as per the pulse rate. This pulse rate has been determines with the potentiometer R15. As well as the setted value of the potentiometer with guide the time span of two pulses.
Additionally another one switch (SW2) has been provided to add another one capacitor and by adding the capacitor C9 with the existing C8 the time gap would be much longer as per the choice the user.