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[Noise reduction
1/0
characteristics]
LC: Linearity tolerance during recording
(compressor)
±
1 .5 dB
LE: Linearity tolerance during playback
(expander)
± 3.0 dB
5-2. RECORDING SYSTEM
5-2-1 . Microphone Circuit
In order to make the microphones of this deck more
compact, two omnidirectional microphones are used to
provide a stereo effect from the cabinet structure and the
circuit processing. Load resistors, the external microphone
jack and an ON/OFF switch for the wind noise reduction
circuit are on the AD2 circuit board.
The greater part of the microphone circuit is on the AD1
circuit board, with the construction centring around IC291
(LA7471 M). The Leh and Reh signals from the built - in
microphones which are connected to AD2 are respectively
input via Pins @ and @ of C N201 to Pins CD and
IC291 . Moreover, the Leh and Reh signals from the external
@
microphone are respectively input via Pins
C N201 to Pins
and @ of IC291 . Switching between the
built-in microphones and the external microphone is carried
@
out by controlling Pins
Low.
Of the signals input to IC291 , the signals from the built-in
microphones are not automatically stereo signals due to the
fact that two omnidirectional microphones are used.
Therefore, a stereo effect is obtained through signal
processing inside the IC.
®
and @ of
@
and
of IC291 to be High or
(dB)
- 60
LC
Moreover, NTSC models use a fuzzy-logic wind noise
reduction system in which the control signal from Pin @ of
CN202 (from IC927 on the CA1 circuit board)
Pin @ of IC291 , after which the HPF characteristics of the
built - in microphone are changed to cut the low band of the
built - in microphone signal, thus reducing wind noise. A
control signal is generated by IC927 for determining
whether or not there is any wind noise. This determination
is carried out by reading the signal which results after the
signal which has been sent from Pin @ of CN202 on the
@
of
AD1 circuit board to the CP1 circuit board is processed by
the low-band and high - band wave detection circuits which
centre around IC281 .
On the other hand, PAL models monitor the level of
reduction of the built-in microphones inside the IC without
using an external control signal. Because wind noise
consists primarily of low - band noise, if the low - band level is
high, the low band of the built-in microphone signal is cut by
the HPF. Furthermore, by turning SW291 on the AD2 circuit
board OFF, the wind noise reduction circuit can be turned
off so that it doesn't operate. H owever, when an external
microphone is being used, the HPF does not operate
regardless of the position of the switch . The signals from
the built - in microphones and the external microphone are
amplified at IC291 to 24 dB and 28 dB respectively and are
then output from Pin @ (Pin
The standard input level for the external microphone jack is
- 66 dB, so it is amplified to - 38 dB by IC291 .
- 4 - 29 -
Input level
-- 40
- 20
@
for Reh) of IC291 .
0
- 20
Qi
_§!
- 40
o_
s
s
°
- 60
is
input to
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