Panner Waveform Generator


This device is a
microprocessor controlled waveform generator that can be used for
driving a voltage controlled stereo panner for music applications.
Panning is simply the movement of a mono audio signal between the left
and right channels of a stereo sound system.The circuit can also be used
to drive other voltage controlled modules that are used in analog music
synthesis.

The output of the waveform generator is a 0-10V DC control voltage.One of eight waveforms can be selected using an 8 position BCD
switch. Waves include: ramp, triangle, sine, cos(x)*sin(5x), damped 4
cycle sine wave, and pseudo random. Other waveforms may be substituted
by changing the assembly language waveform tables.

The waveform generator runs in an asyncronous manner, int can be
syncronized with other devices by use of an external (5V logic) sync
signal or a manual sync trigger button. The waveform is generator is
clocked by an oscillator circuit, the oscillator has three range
selections, and an LFO rate control for fine
speed adjustments.The output waveform can be smoothed with an adjustable
low pass filter. This can be used to remove the stair steps in the
output waveform for a more rounded waveform.

An array of 8 LEDs is included for
displaying where the current panning position is located, these are
spread evenly across the device front panel for an interesting visual
effect.

Specifications:
Power Requirements:
+5V DC approximately 200mA
+12V DC
approximately 25mA
-12V DC @ approximately 25mA
Output Levels: 0-10V DC
External Sync Input: TTL level (0-5V) DC, triggers on rising edge

Theory:
The NE555 timer is wired as an asyncronous rectangular wave clock
oscillator. It creates pulses that are used by the microprocessor to
step through the selected waveform. Coarse adjustment to the 555 is made
by selection of three timer capacitors. Fine adjustment is performed by
changing a variable resistor.

The LFO clock signal is fed into the
microprocessor’s interrupt pin. Each pulse causes the software to
advance to the next waveform value in a stored table and send the value
to the input side of the DAC, resulting in a new analog value on the output of the DAC.
When the end of the table is reached, the software loops back to the
beginning of the table. The sync input goes to pin PC3. This signal is
either manually generated with a pushbutton, or externally generated
with a TTL level signal. When the sync input goes high, the microprocessor resets the to the beginning of the selected waveform table.

One of eight waveforms is selected from eight tables using the 3 bit BCD select switch that is wired to PC0, PC1 and PC2. If a suitable BCD switch cannot be found, three SPDT toggle switches can be substituted.

The microprocessor is set to use its internal clock, the 15K
resistor on the Xtal pin is part of the oscillator circuit. The internal
oscillator only provides low clock accuracy, this is fine since the
timing is gated by the LFO clock. For stable LFO operation, good quality capacitors should be used for the three LFO range
parts. The microprocessor reset circuit involves a resistor, diode, and
capacitor. This produces a slowly rising reset signal at power-up, and a
quickly falling signal in the event of a brief power outage.

The eight waveform display LEDs are connected to the microprocessor’s PB0 through PB7 outputs via some 330 ohm current limiting resistors.The MC1408P8 DAC receives the waveform level values from the microprocessor’s PA0 through PA7 outputs. The DAC converts the digital values into one of 256 analog levels. The output of the DAC is fed through the LM741 low pass filter stage for waveform smoothing. The LPF value is selected by changing the value of the capacitor across the feedback path.

Power is supplied to the circuit from a triple voltage “wall wart”
or another source of regulated +5, +12, and -12V DC power. Standard
bypass capacitors are connected across the three power supply lines.

Software:
The control program pan.asm is written in 6805 assembly language, it
needs to be assembled and loaded into an MC68705P3 microcontroller IC.

Construction:
The prototype of this circuit was built on a hand-wired perforated
circuit board using wire-wrap technology. The analog components were
installed in DIP headers and were connected
with wire-wrap wire. 8-way resistor networks were used for the 10K and
330 ohm resistors. The controls and input/output connections were
mounted on a rack mount metal chassis, the wiring was connected to the
circuit board with a single dual inline header.

Use:
Connect the waveform generator to the voltage controlled panner circuit,
wire up the mono audio input and stereo audio outputs to the panner.
Select a waveform with the 8 position switch. Tune the LFO
range and rate controls for the desired panning speed. Adjust the low
pass frequency switch for the desired smoothness. Play with the various
controls until a good panning sound is heard.

It is advisable to observe the control voltage output on an
oscilloscope in order to become familiar with the effect of the various
controls on the output signal.

This effect is featured on my Soundtrack for a Low-Budget Sci-Fi Movie audio CD.


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