Rev. 1.0, 02/00, page 628 of 1141
26.6.6
f
H
Correction in Trick Play Mode
In trick play mode, the tape speed relative to the video head changes. This change alters the
horizontal sync signal (f
H
), causing skew. To correct the skew, the drum motor speed must be
shifted to a different speed in each trick play mode, so as to obtain the normal horizontal sync
frequency. To shift the drum motor speed, software should modify the value written in the
specified DFG speed preset data register in the speed error detector.
This f
H
correction can be expressed in terms of the basic frequency f
F
of the drum as follows.
N
0
f
F
=
×
f
F0
N
0
+
α
H
(1
−
n)
Legend:
n:
Speed multiplier (FWD = positive, REV = negative)
α
H
:
H alignment (1.5H in standard mode, 0.75H in 2x mode, and 0.5H in 3x mode for VHS
and
β
systems; 1H for an 8-mm VCR)
N
0
:
Standard H numbers within field
f
F0
:
Field frequency
NTSC: N
0
= 262.5, f
F0
= 59.94
PAL:
N
0
= 312.5, f
F0
= 50.00