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[OTL (Over Trap Limiter) recording system]
Previously, when the FM luminance signal which has been
FM modulated and the low band conversion carrier chroma
signal were mixed and recorded, a frequency trap which
matched the low band conversion carrier chroma signal
which was mixed with the FM lumi nance signal was inserted
and then removed . However, when the signal which was
added passed through areas which had limiter character
istics, such as the heads and the tape, the upper sideband
of the FM luminance signal was bent back in the recording
(playback) system and the sideband range which should
have been removed by the trap was restored and bead
interference occurred in signals such as the low band
conversion carrier chroma signal.
OTL is a recording system which prevents such interference
which is caused by limiter characteristics from occurring.
The basic system configuration is shown in Fig. 4-1 .
4-5. RECORDING AMPLIFIER
At IC1 91 , the FM luminance signal, low band converted
chroma signal, FM audio signal and the ATF pilot signal are
mixed, divided into two lines based on the RF switch pulse
and then amplified by the recording amplifier so that the
specified current flows to the respective head.
Furthermore, negative feedback damping is carried out from
output to input at the recording amplifier in order to improve
frequency characteristics.
4-6. PLAYBACK AMPLIFIER
The extremely weak signals which are induced in the head
are input to Pins @ and @ of IC1 91 respectively via rotary
transformers. The signals which are amplified to approxi
mately 60 dB inside the IC are the converted i nto contin
uous waves by the switching circuit, after which they are
output from Pin ® .
For the purpose of Hi8 compatibility, positive feedback is
also carried out at the input stage in addition to negative
feedback damping so as to reduce the input levels by
equivalent amounts and to improve frequency character
istics.
FM
Limiter
modulation
The FM-modulated signal is first input to the phase
inversion circuit. The inversion frequency from this phase
inversion circuit is set to the frequency which was initially
required to be removed. After this, the limiter is again
applied to make the amplitude constant, and the lower
sideband is removed. The principle of this utilises the fact
that the phase of the odd-numbered upper and lower
sidebands is exactly opposite in phase in reference to the
carrier.
That is to say, by inverting the phase, an AM component
occurs in the FM-modulated wave, but the limiter removes
this AM component so that the odd-numbered lower
sideband is fully removed, and because there is no AM
component in the signal output by the limiter, no sideband
wave will be generated after this, even when the signal
passes through a recording system with limiter character
istics.
Phase
inversion
Fig. 4-1 .
4-7. LUMINANCE SIGNAL PLAYBACK SYSTEM
The RF signal which is output from Pin ® of IC1 91 is tuned
slightly upwards to 7 MHz. Then the unnecessary compo
nents are removed by the AFM, ATF and chroma TRAP
circuits and the remaining signal is input to Pin @ of IC1 01 .
After the amplification by the RF AGC is controlled to a
uniform level at IC1 01 , the signal is output from Pin @ to
the RF signal processing IC (IC1 81 block).
At IC1 81 , inversion is prevented by the feedback - type soft
limiter circuit. After this, the band is switched and amplified
according to whether the mode is Hi8 or normal, and then
the signal is output from Pin @ and input to Pin
and demodulated.
The demodulated signal passes through Pin @ . If it is a Hi8
mode signal, the carrier is suppressed by the external LPF
and the frequency characteristics and the delay time are
compensated for by the equaliser, after which the signal is
input again to Pin @ . If the signal is a normal mode signal,
then the wave characteristics of the signal are improved by
the clip compensation circuit, after which the wave is
restored to its original form by the main de - emphasis and
sub de-emphasis circuits, i n the reverse to the procedure
during recording. After this, the edge signal is improved by
<J)
the line noise canceller, and then the signal is output from
Pin
and input via the LPF and equaliser to Pin ® . The
signal then passes through the noise canceller and Y/C mix
to the external terminal. Moreover, because the 75 Q d river
which is output has a sag compensation function, the
combination capacity can be made smaller.
- 4 -26 -
Limiter
@
of IC1 01
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