Recording; Luminance; Chrominance Pal; Mesecam - Hitachi VTMX900EUK Service Manual

2-14
LP/SP head pair switchover
The switchover between the long play LP head pair and the
standard play SP head pair is made via the HSC signal (pin 25).
4/x scanner in play back:SP head pair: 0V
LP head pair: 1.2V <= HSC <= 2.8V
2/x scanner in play back: always
Head switchover
The video head switchover is made using the HP1 signal (pin 11).
To keep audio linear interference as low as possible, the HP1
polarity should be selected to be inverse and the HP1 level should
be the same as the CROT signal on pin 10.
PB: SP1 / LP1: 1.2V <= HP1 <= 2.8V
SP2 / LP2: 0V
Envelope curve comparator
If the ENVC signal (pin 94) is HIGH, the FM envelope curve on the
LP head is greater than that on the SP head, and vice versa.

6.2 Recording :

6.2.1 Luminance

The input signal (1-scart: pin 38 = scart , pin 36 = front end; 2-scart:
pin 38 = input video selected using STV6401) is connected in the
IC [7004] and is available uncontrolled on pin 32 as VREC
(SECAM; VPS only unit data slicers). It reaches pin 31 via an
electrolytic capacitor [2036]. In the IC [7004] the video signal first
goes through an amplification control process (time constants
determined by C [2035]). After the AGC the video signal reaches
the FBC clamping stage (feed back clamp), then the video signal is
divided onto 3 paths:
•
Loop-through signal path: The video signal is amplified by
6dB following video entry and is available controlled on pin 29
as a VSB signal (OSD entry, data slicer -> I/O, front end,..).
•
Y-REC path: The video signal passes via a 3.5 MHz low pass
filter to vertical emphasis comprising the YNR block (part of
this circuit block is used in REC for vertical emphasis) and a
1H-CCD delay line integrated into the SE IC [7004-C] and an
external emitter follower [7006]. This vertical emphasis can be
switched via IIC and is only active in LP. The Y-signal before
the 1H-CCD can be measured on pins 43 and 45 on the IC
[7004-C] (separated only by a coupling electrolytic capacitor).
The Y-signal after the 1H-CCD is passed back from pin 46 IC
[7004-C] via the E-follower [7006] on pin 41 IC [7004]. After
the vertical emphasis the Y-signal passes via pin 21 [7004],
the E-follower [7008] (the filter, on the base of the emitter
follower is not active in REC mode (due to the low resistance
of the output stage on pin 21 [7004]), via pin 21 [7004] and a
clamping stage to the detail enhancer. The Y-signal is then
passed to the non-linear emphasis, the linear emphasis (time
constant via pin 18, 19 – due to the low resistance of the pin
18 output stage and the transistor [7010] introduced for
impedance decoupling, the FM PB all-pass does not influence
the linear emphasis) and the white/dark clipping stage. The
signal generated in this way then triggers the FM modulator
directly. The FM-Y-signal generated in this way is passed via
the REC-EQ filter and the REC-FM-AGC1 to the Y-C addition
point. The FM-Y-signal can be measured after the REC-EQ
filter on pin 12 [7004].
•
C-REC path: see Chrominance PAL Recording (6.2.2).
GB
<= HSC <= 0.8V
3.2V <= HSC <= 5V
<= HP1 <= 0.8V

6.2.2 Chrominance PAL

The chroma signal is separated from the video signal after the FBC
clamping stage (see "Luminance recording") by the BPF1 band
pass filter and reaches the ACC stage via a delay element (D.E.)
and a low pass filter (LPF). The ACC amplifier stage controls the
chroma amplitude for the subsequent stages (time constant via
capacitor [2038] on pin 14 [7004]). The chroma signal is then
conveyed to the main converter (Main Conv.). The main converter
mixes the 5.06MHz subcarrier with the 4.43 MHz chroma signal to
the 627kHz chroma FM signal. The subcarrier is a mixture of
4.43MHz (the REC APC time constant on pin 65 compares quartz
and burst frequency) and (40+ 1/8) fH = 627kHz (produced by
321fH –VCO corresponds to 8(40+1/8)fH, time constant pin 60/62
and phase rotation in accordance with the VHS standard, 10 [7004]
(CROT)). Via a low pass filter (C_LPF) and the colour killer stage
(KIL), the converted chroma signal reaches pin 72 on the IC [7004],
where it is added directly to the Y FM signal IC internally via a
capacitor [2007]. The colour killer can either identify the incoming
signal itself (PAL yes/no, PAL: chroma signal out, SECAM L:
chroma signal killed) or be set via the I2C bus to PAL MESECAM or
SECAM L. The quartz oscillation (pin 66) is used for chroma
processing, in addition to the reference frequency, and also for
generating the pulse frequency for the combined CCD on pin 49
integrated into the IC [7004].

6.2.3 MESECAM

The signal path is virtually identical to the path for PAL.
The differences are:
No phase rotation.
The filter characteristic for the chroma band passes becomes
wider.
Free-running quartz frequency

6.2.4 SECAM L

The video signal (VREC) from the SE IC pin 32 [7004] passes
through SECAM L SE IC pin 15 [7072] and a band pass filter
(4.3MHz BPF-A) and reaches the cloche filter (CA filter
components pin 21) which reverses the Hf pre-emphasis on the
sender side. The C-signal is then limited (LIM, time constant pin
18) and divided to ¼ of the frequency in the frequency divider. The
C-signal is suppressed in SYNC GATE during the H-sync. period.
The harmonics arising in the division into four and the gating are
suppressed in the band pass filter (1.1MHz BPF) and then pre-
processed in the anti-cloche filter (filter components pin 8) for
standard VHS recording. The amplitude on the REC-chrome signal
on pin 11 [7072] can be set using the setting resistor [3088] on pin
10 [7072]. This REC-chroma signal is passed via transistor [7077]
as a CSRP signal to SE IC pin 72 [7004] following an external drop
(3.9MHz, suppression of the 3
REC-chroma) and added to the FM-Y-signal in the SE IC.
As the SECAM SE IC (LA7339A) has an automatic cloche and anti
cloche comparison, only the REC-chroma signal level is required to
be set.

6.2.5 FM signal

After the addition of the FM-Y-signal and the C-signal, this FM-
signal is adjusted by the REC-FM-AGC2 controlled by the IIC bus
to the preset amplitude (reference: pin 74 [7004] resistor [3009]).
The head pair is selected using the HSC control line.
rd
harmonics of the low frequency