User manual

THEORY OF OPERATION
Overall Functional Description
The balanced inputs to the unit are fed to the input cards which process the signals into
unbalanced form, these signals are then fed through low pass filtering to prevent
aliasing components appearing at the outputs of the unit. Buffered drive signals are then
fed to the input level controls on the front panel and the outputs from these front panel
controls are fed to preset gain stages on the input cards, the outputs of which are fed to
the stereo sampler card.
The stereo sampler card accepts the outputs from the input cards together with the
feedback signals and mixes them into balanced stages to avoid common mode
interference pickup. Each channel is fed to a monolithic sample and hold device. These
devices are strobed in antiphase at a sampling rate of 40kHz with the crystal switched in,
but variable on vco. The outputs of these are fed to a comparator/gain switching section
alternately by means of a FET switch. The signal is then scaled by 1, 2, 4 or 8 to optimise
the signal to noise ratio, and a two bit exponent number is generated to note the scaling
factor used. The signals for driving the ‘traffic signal’ LEDs for channel ‘b’ are also
derived from this board.
Control and timing signals for this board are derived from the memory control board,
the clock inputs being generated by either the crystal oscillator or the voltage controlled
oscillator (vco). The ADC conversion command is then decoded, and the resulting
response, ‘conversion complete’ is used to control the latching of the correct exponent
for that sampling period.
The sampled analogue output from the sampler card is fed into the ADC via a buffer
amplifier. Timing signals from the sampler card cause a conversion to be initiated. The
twelve-bit result is latched into a set of registers/data highway drivers for storage in
memory. The level indicating drivers for channel ‘a’ are also located on the ADC card.
The vco is also generated on the ADC card by first generating a triangular waveform
whose frequency is varied by the vco speed control on the front panel. This triangular
waveform besides feeding the drive circuitry for the vco speed LED, also feeds a shaping
chip which generates a sine wave. The sine wave is buffered and fed to the vco depth
control on the front panel; the output from this is then mixed with a preset d.c. voltage
and is fed to the voltage controlled oscillator. The d.c. voltage thus gives the nominal
output frequency of the vco, the amplitude of the sine wave gives the range of swing
(depth) from the nominal frequency, and the frequency of the sine wave increases and
decreases the rapidity of the swing (speed). The vco has the effect of altering the
sampling rate.
The memory control card provides timing signals for the random access memory (RAM)
array and also controls which addresses in the in the array are written to and read from.
These addresses are loaded into the memory control board by the pitch changers via
ribbon cables.