Dolby Laboratories 737 - Manual (Page 18)

Model 737 Soundtrack Loudness Meter - Leq(m) Operation and Features
2-11
The selection switches are followed by five ideal rectifiers and squaring circuits. The
five squared signals are added and short term averaged. The resultant varying DC signal
passes to an analog long-term averaging circuit. This requires a logic signal from the
pushbutton logic section to start and stop the average measurement. The analog long-
term averaged DC signal is sent to the DC Out connector on the rear panel for optional
connection to a chart recorder or moving coil meter.
The short-term average DC voltage is converted to12-bit binary and sent to a 6805
processor that also averages, but over a much longer time span. The samples are taken
at 850 msec intervals, summed, divided by the time since the Start button was pressed,
and then converted to a square-root value. This value is then converted to a dB (log)
scale, scaled to a reference level of 85, and sent to the front panel LED display. When
the Stop button is pressed, the averaging circuit and averaging routine are reset while
the last value measured is displayed.
The pushbutton logic is a simple tri-stable gate that drives the LEDs in the pushbuttons
and also generates processor interrupts to indicate mode changes. It is remoted to the
back panel via a single start-stop line.
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