Texas Instruments SN76477

Overview

• SW1 is a five position rotary switch that selects which capacitor controls the one-shot circuit that sets the envelope timing.
• SW24 is the one-shot trigger switch, closed position is active.
• R1 a one megohm linear potentiometer sets the actual one-shot timing within the range selected by SW1.
• SW2 is a five position rotary switch which selects which capacitor sets the range for the low frequency oscillator.
• SW3 is the switch used to turn the low frequency oscillator on or off.
• R3 a one meagohm linear potentiometer sets the actual frequency for the SLF oscillator.
• SW4 is the switch that takes control of the voltage-controlled oscillator's duty cycle. If the switch is off the VCO operates at 50% duty cycle by default.
• R6 a 50k ohm linear potentiometer is used form a voltage divider used to set the VCO duty cycle, the on time vs off time of the waveform. The source voltage for this voltage divider is the five volt busbar.
• SW6 is a four position rotary switch which selects which one of three resistors sets the range of frequency for the VCO.
• SW5 is the switch that turns the VCO on or off.
• R8 is a one megohm linear potentiometer which sets the actual frequency of the VCO.
• SW7 is a six position rotary switch which selects which of six capacitors also sets the VCO frequency range.
• SW8 is the switch that selects either an internal voltage control or an external voltage control for the VCO.
• R12 a 50k ohm linear potentiometer is used form a voltage divider to set an internal voltage to mimic the action of an external VCO input. The source for this voltage divider is the five volt busbar.
• SW9 is a five position rotary switch which selects which resistor to set the audio output level.
• SW10 is the switch that enables attack timing. If this switch is open, there is no attack slope. If this switch is closed the attack slope is set by R24.
• R24 is a one megohm linear potentiometer which sets the actual attack timing.
• SW11 is a six position rotary switch which selects which of six capacitors also sets the limits of the timing of the attack and decay envelope generator.
• SW12 is the switch that enables delay timing. If this switch is open, there is no decay delay slope. If this switch is closed the decay slope is set by R24.
• SW13 is a four position rotary switch which selects which one of four capacitors affects the noise filter.
• SW14 is the switch that enables the noise filter.
• R26 is a one megohm linear potentiometer which sets the actual response of the noise filter timing these two switches are tied to pin 5 of the 76477 sound generator. Most demonstration circuits show these two switches in parallel from pin 5 to ground. Pin 5 is the ‘system enable’ pin for the 76477, if open the sound generator is off. If pin 5 is connected to ground, the sound generator is running. Both switches are included for that reason.
• SW17 is the switch that enables the internal noise generator clock. If this switch is closed then the noise generator operates on its internal clock.
• R29 A 47k fixed resistor. This resistor is in series with SW17 and is the current reference for the internal noise generator clock. In the 76477 spec sheet this is called out as 47k. This value could be tweaked but straying too far from 47k might make the internal noise clock unstable. To control the clock frequency then the next Item, ‘J1' is a better choice.
• J1 External noise clock input. If this input is used SW17 should be open. Waveform can be most any wave shape and most any frequency from a few Hz to about 2 mHz and can range from a half a volt to a maximum of five volts. Some very strange tones can be produced with a rather low frequency input to this jack.
• SW18 and SW19 are switches that together control the logic for the envelope generator. If both switches are open then the VCO only is selected. If SW18 is closed and SW19 is open the envelope generator is controlled by the one-shot generator. If SW18 is open and SW19 is closed the mixer only is selected. If both SW18 and SW19 are closed the VCO is selected with alternating polarity.
• SW20 – SW21 and SW22 are switches that together control the logic for the mixer; SW20, 21 and 22 open selects the VCO, SW20 on and SW21 and 22 open selects the super low frequency oscillator, with noise filter. SW21 on and SW20 and SW22 off selects the noise generator output only, SW20 and 21 on and SW22 off selects the SLF oscillator and the VCO. SW20 and 21 off and SW22 on selects the SLF oscillator only. SW20 and 22 on and SW21 off selects the SLF and the VCO and the noise generator. SW20 off and SW21 and 22 on selects the VCO and the noise filter. SW20 and 21 and 22 on inhibits the mixer.
• SW23 is the switch that selects what voltage input the VCO will follow. If SW23 is open the VCO will be controlled by the SLF oscillator. If SW23 is closed then the VCO will be modulated by the voltage input to pin 16.
• J2 External VCO input voltage (selected or disabled by SW8. It can be most any wave shape and most any frequency from a few Hz to about several kHz and can range from a half a volt to a maximum of five volts.

Uses

• Arcade games:
• * Space Invaders
• * Stratovox
• * Vanguard
• * Sheriff/Bandido
• Educational toys:
• * Gakken EX-System synthesiser
• Electronic musical instruments:
• * Axesynth
• Home computers:
• * ABC 80