Playback; Fm Signal; Luminance; Chroma Pal - Hitachi VTMX900EUK Service Manual

6.3. Playback:

6.3.1 FM signal

The FM signal coming from the scanner is amplified by approx.
60dB. Depending on the level of the HSC and HP1 line, the
amplified FM signal is connected to pin 74 [7004]. The envelope
curve signal for the head currently active (TRIV) is output on pin 93
[7004]. In addition, the envelope curves for the SP and the LP
heads which read from the tape are compared and output as the
ENVC signal.
The FM signal (FMPV) on pin 74 [7004] is used internally for Y,
SECAM, MESECAM and NTSC M/N playback and externally for
SECAM playback.

6.3.2 Luminance

The FM playback signal is first adjusted in the AGC stage to a
constant level and filtered in the FM processing (PB-EQ). The
signal exits the IC [7004] on pin 18, passes via an E-follower [7010]
with drop (1.07MHz – only in SECAM units – to suppress additional
chroma remainders externally) to a phase shifter [7003] and enters
the IC once more on pin 17 [7004]. The FM-Y signal limited using
the double limiter is demodulated (FM-DEM) and filtered using a
low pass (SUB_LPF). The demodulated Y signal is also affected by
the recording-side pre-emphasis. This now removes the linear de-
emphasis at the base of the emitter follower [7008].
The filter circuit is effective, as pin 21 [7004] becomes an open
collector output in playback mode, where the load impedance is
determined by the de-emphasis circuit.
The Y signal is then clamped after the E-follower on pin 20 [7004],
filtered using a low pass, and carried by a vertical noise canceller or
dropout compensator (Y.N.R.). To do this the Y-signal exits the IC
[7004] (out: pin 43, in: pin 41) and delayed by 1H in the internal
CCD. The CCD-1H delay line is effective for the Y signal first as a
comb filter (vertical noise suppression) and secondly as a line
storage device for the dropout compensation. The subsequent
switching stages are: The non-linear de-emphasis (NON_LIN
DE_EMP), horizontal noise canceller (N.C.1 / N.C.2) and the
picture control switching to the increase in edge steepness
(PIC_CTL ANR; sharpness). The luminance signal is then added to
the chroma signal (Y/C MIX) and output (pin 29 [7004]) as FBAS
signal via a clamp (FBC), the video input (CHARA INSERT) and a
6dB amplifier (6dB_AMO).

6.3.3 Chroma PAL

This is first adjusted in the AGC stage to a constant level and
filtered in the FM processing (PB-EQ). The signal exits the IC on
pin 18 [7004], and passes via an E-follower [7010] with drop
(1.07MHz ). On pin 17, the FMPV signal is carried from the head
amplifier to the IC [7007] signal electronics.
From the FM playback signal the 627 kHz chroma signal is filtered
using the internal low pass (C_LPF). The ACC amplifier amplifies
and controls the chroma amplitude. In the main converter (MAIN
CONV), the chroma signal is mixed with 5.06 MHz back to the
original 4.43 MHz. The 5.06 MHz are produced in playback from
the free-running quartz oscillator and from the (40+1/8) f
kHz frequency derived from the 321f
converter the chroma signal is freed as far as possible from
crosstalk from additional traces using a 2H comb filter (internal
CCD connections: pin 57 -> 54; pin 59 -> 52 and pin 51 -> 61). The
chroma signal is then filtered using a low pass (LPF), checked by
the colour killer, filtered once again by a band pass, looped through
pins 72 and 71 and then added to the Y signal.

6.3.4 Chroma MESECAM

The signal path is virtually identical to the path for PAL.
The differences are:
No phase rotation.
The comb filter is not active.
= 627
H
-VCO. After the main
H

6.3.5 Chroma SECAM L

During playback the FM signal is passed from the band on pin 74
[7004] after the E-follower [7002] (FMPV) to pin 13 [7072], where
the amplitude is adjusted in the AGC and passed via the same
band pass (1.1MHz BPF) as for recording. The NF pre-emphasis
for the recording is then reversed using a cloche filter (external filter
components on pin 8; the same components as for recording). In
the subsequent stages the frequency of the signal is doubled,
filtered using a band pass (2.2MHz BPF) and doubled once again.
Then follows another band pass (4.3MHz BPF-B), and then the
limiter (LIM) already used for recording. The signal is then
suppressed again during the H-sync. period and passed through a
band pass filter (4.3MHz BPF-A; also used for recording). Before
the SECAM-chroma signal exits the IC on pin 17 [7072], an Hf pre-
emphasis is carried out once more (anti-cloche; external filter
components on pin 21; the same components as for recording).
After pin 17 there is a drop at 2.4MHz which suppresses the 2
harmonic of the chroma from the band, a low pass filter which
improves the harmonics of the high frequency chroma and a
transistor [7073] which has an emitter connected to pin 72 (CSRP)
on the SE IC [7004].

6.3.6 NTSC

During the playback of NTSC signals, the original NTSC chroma is
converted into a PAL chroma signal. This requires an internal
switchover in the IC in the chroma part:
The internal CCD is switched over on a 1H comb filter to
reduce crosstalk.
The NAP switchover is activated and translates the 4.43MHz
NTSC chroma signal into a PAL signal.
Line and picture frequencies remain unchanged in accordance with
the NTSC standard.
The result is a 60Hz NTSC Y-signal with a 4.43MHz PAL C-signal.

6.3.7 PAL M,N

As for chroma PAL (6.3.3).

6.4 General

SECAM: Automatic cloche and anti-cloche comparison: During the
vertical blanking gap the external filter components (pin 21 or pin 8)
on the cloche or anti-cloche are used to create an oscillator and to
divide the resonance frequency produced, and compared with a
frequency derived from the 4.43MHz oscillation (reference signal
from the SE IC [7004]). Depending on the deviation, more or less
internal capacity is connected in parallel to the external cloche and
anti-cloche filter components. This process is carried out during
each vertical blanking gap and thus also improves the temperature
stability.
Chroma selection for REC and PB pin 71 and 72 SE IC [7004]:
Both the PB chroma and the REC chroma in PAL (MESECAM, PAL
M/N) and also in SECAM are passed into the SE IC [7004] via pin
71 [7004]. In all PAL and MESECAM modes the DC voltage is on
the base of the output emitter follower pin 72 [7004] 3.2V and the
both bases of transistors [7077] and [7073] of the SECAM chroma
signals are at 0V -> the PAL/MESECAM chroma signal is added to
the FM-Y signal or to the PB-Y signal, according to REC or PB. In
SECAM PB mode only the transistor [7073] has 2.5V DC voltage
on the base. In SECAM REC mode only the transistor [7075] has
2.5V DC voltage on the base.
2-15
nd
GB