— —
23
Section V
CIRCUIT DESCRIPTIONS
This section details the circuitry of the Inovonics
DAVID-II
. Circuit
descriptions refer to the two pages of Schematic Diagrams contained in
the Appendix, Section VI, Pages 39 and 40.
Component
Annotation
DAVID-II
Schematics may appear to be annotated in a somewhat
haphazard manner, insofar as component reference designations are
concerned. Rather than annotate the
schematic
in a logical sequence,
we have instead chosen to designate the
components on the circuit
board,
top-to-bottom, left-to-right, following the physical placement of
the parts in their neat little rows. It is our expectation that this
practice will aid any required troubleshooting, making it easier to
locate the physical part or test point from an analysis of the circuit
diagram.
The first part of this section covers the general subject of Pulse Width
Modulation (PWM) as it applies to the audio processing circuitry.
Signal path circuitry discussions follow.
A PWM PRIMER
PWM Gain
Control
The
DAVID-II
utilizes Pulse-Width Modulation techniques rather than
voltage-controlled amplifiers or FET attenuators in its three distinct
processing sections. This technology, as applied to broadcast audio
processing, was pioneered by Inovonics in 1982. PWM is a simple,
colorless, quasi-digital method of linear gain control which avoids the
several hassles associated with more expensive, possibly single-sourced
VCA ICs or matched and balanced FET devices. Our implementation
of PWM utilizes the popular 4000-series CMOS parts which are dirt
cheap and available nearly anywhere.
PWM sets the gain of an analog circuit simply by sampling, or
“chopping,” the analog signal; that is, turning it on and off at a rapid
rate. Consider an audio program signal which can be turned on and off
with a toggle switch. When the switch is on, attenuation is zero. When
off, attenuation is infinite. If we satisfy Dr. Nyquist’s requirement and
toggle this switch at a rate at least twice that of the highest audio
frequency, linear signal attenuation becomes directly proportional to
the OFF time as depicted in the chart on the next page.
The
DAVID-II
sampling rate is 152kHz, better than ten times the
highest program audio frequency. Since this is well above the Nyquist
rate, anti-aliasing and reconstruction filters can be simple and gentle.
