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Servo Block
Overview
The servo block in a D8 camcorder must control the mechanism for two
different modes - analog and digital. In order to do this, the D8 uses two
separate mechanism control ICs. The inputs of these two ICs are wired
ORed together. This means that each of these ICs can control the mecha-
nism. When one Mecha Control IC is operating the mechanism, the lines
on the other IC are at a high impedance state. Communication between
IC401 DV Mecha Control, IC902 8mm Mecha Control (Analog) and IC801
Mode Control determine who is controlling the mechanism.
Analog Mode
When the camcorder is in analog playback mode, the servos and all
switches and sensors are used for the same purpose they would be in a
normal analog camcorder. IC902 8mm Mecha Control controls these
operations.
D8 Mode
When the camcorder is in D8 mode, the capstan operates at twice the
normal speed and the drum runs at 4500 rpm instead of 1800 rpm. You
can tell if you are in this mode by the sound of the drum. Due to the
speed, the drum makes a distinct sound that is higher in pitch than the
standard analog drum. The speed of the tape is also a good indicator
since it travelling twice as fast.
IC401 DV Mecha Control is controlling the mechanism while the servo is
in digital mode. This IC checks for proper information coming from all of
the sensors and switches. These sensors and switches have the same
functions that they normally would on an analog 8mm camera. This IC
controls the speed and phase of the drum and capstan motors.
Drum
The drum FG pulses are used to determine the drum speed in record
mode. Comparing the incoming V sync signal to an Xtal oscillator creates
a reference signal. This reference is compared to the drum PG signal to
maintain phase. The drum PG signal is a 75 Hz signal.
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The drum FG pulses are used to determine the drum speed in play mode.
The drum FG signal should be 450Hz. Drum phase is controlled by com-
paring the Xtal oscillator signal to the Drum PG pulses.
Capstan
The capstan FG pulses determine the capstan speed in record mode.
There is no capstan phase control in record.
The capstan FG pulses determine the capstan speed in playback mode.
The phase of the capstan circuit is controlled by the ATF signal, which is
read from the tape.
ATF
As mentioned earlier in the Video path section, the D8 system employs
ATF. A process called 24-25 Modulation is used to create pilot signals.
24-25 Modulation means that an extra bit is added to every three bytes of
information. By using certain patterns for the extra-bit, different frequen-
cies are obtained during playback.
24-25 Modulation
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