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Theory of Operation
3–24
AM700 Audio Measurement Set Service Manual
Phase Offset and Jitter Generator (diagram 4)
Refer to schematic diagram 4 of the A3 Digital Audio circuit board for the
following discussion.
The Jitter DAC, U103, converts a jitter data byte into an analog signal for
insertion into the Phase-Locked Loop. The effect of the insertion produces a high
speed phase modulation of the digital signal that carries the digitized audio
information.
The phase of the digital signal is another parameter of the digital signal that may
also be adjusted by the user. A PAL, U104, supplies the data and clocks to the
generator encoder PAL, U108. When a phase shift is added, U104 adjust the
amount of phase shift that is applied to the digital signal. This is a fixed phase
shift as opposed to the rapid shifting that is applied to jitter the signal.
The phase-locked loop is composed of a phase comparator, loop filter, inte-
grating amplifier, and a voltage-controlled oscillator. The output of the oscillator,
U64, is converted to a TTL signal by the circuit formed by Q12 and Q13, and
applied back to the Clock 0 input of the phase shift PAL, U104.
Generator (diagram 5)
A simplified block diagram of the Digital Audio Generator was shown
previously in Figure 3–5. Refer to that illustration and to schematic diagram 5 of
the A3 circuit board for the following generator circuit description.
The Encoder PAL, U108, generates the digital audio word and the VUCP
(validity, user, channel status, and parity) bits that make the AES format signal.
The AES formatted signal is used directly to drive the optical output circuit, but
additional processing of the signal is needed to drive the XLR and Unbalanced
output connectors. The Left and Right frames are interlaced in the serial digital
stream. When jitter is applied, the Encoder PAL outputs the jitter signal data to
the Jitter DAC, U104 (diagram 4), to add the programmed amount of shifting to
the signal to produce the jitter effect.
Jitter DAC
Phase Shift PAL
Phase-Locked Loop
Encoder PAL