Circuit Operating Descriptions
7-19
(5) Drum Phase Error Detector
Drum phase control must start operating after the drum motor is brought to the correct rotational speed by the
speed control system . Drum speed control works as follows in record and playback.
◆
Record : Phase is controlled so that the vertical blanking intervals of the recorded video
signal will line up along the edge of the tape.
◆
Playback : Phase is controlled so as to trace the recorded tracks accurately.
A digital counter detects the phase deviation from a preset value. The phase error data is added to speed error
data in a digital filter. this filter controls a pulse-width modulated (PWM) output, which controls the rotation
phase and speed of the drum.When the error is zero, the PWM circuit outputs a waveform with a 50% duty cycle.
The phase counter error detector compares the phase of the DPG pulse (tach pulse),which contains video head
phase information, with a reference signal. In the actual circuit , the comparison is carried out by comparing the
head-switching (HSW) signal,which is delayed by a counter that is reseted by DPG, with a reference signal. The
reference signal is the REF 25Hz signal,which differs between record and playback as follows.
◆
Record : V sync signal extracted from the video signal to be recorded (frame rate signal,
actually 1/2 V sync).
◆
Playback : 25Hz signal divided from the system clock.
(6) SW 25Hz Pulse Generation
The SW25Hz pulse is generated from IC601.
D.PG
D. FG
SW 25Hz
(V-FF)
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 0
Fig. 7-19 SW 25Hz TIMING CHART
