Samsung SV-DVD50 Service Manual page 121

(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
0 1 2
D. FG
SW 25Hz
(V-FF)
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
Circuit Operating Descriptions
7-19