Dolby Encoder/Decoder - Teac C-1 Service Manual

28
AMPLIFIER SECTION
7. DOLBY ENCODER/DECODER
7-1 ENCODER
The input signal is split, one path going to AMP-1 via R315
while the other is routed to AMP-1 through a HPF (high
pass filter), AMP2, R316.
E2 varies in the value depending on the input signal fre-
quency and level, and is usually at the same phase as El.
Therefore, the input to AMP-1 is the signal obtained by
adding El to E2. The signal passing through RPF is ampli-
fied by AMPs —2 and ~3. The amplified signal is then
rectified by D305 and D306. The DC voltage output of
D305 and D306 is sent to the gate of Q303. Q303 is an
electronic attenuator. The AC impedance between drain
and source lowers and signal attenuation increases as the
gate potential rises. The operating point is determined by
the source potential, and adjusted by R333. When the orig-
AM
(Q30
ORIGINAL
SIGNAL
INPUT
(Gees )
R329,R330
(SsBnt888)
\C332" ae
Fig. 9
7-2 DECODER
The signal which becomes E2 is taken from the output of
AMP-1 (In contrast to the encoder). Input and output
phases of AMP-1 are opposite. As AMP-1 has a gain of 1,
El and E2 are opposite in phase. That is, the input to
AMP-1 is the value obtained by subtracting E2 from El.
Thus, decoder characteristics are opposite those of the
encoder.
AMP-1
(Q109 ,Q110)
ENCODED
SIGNAL
INPUT
CI39 Cl4I
RIS6~RIS9
DIOS, DIO6
Ci42
1 ,Q302)
AMP-2
(Q304 ,Q305)
LIMITER
(Fine cial) _ AMP <2
VS RIO (Qil2,QN3)
inal signal frequency is high (5kHz or higher) and the signal
level is low (—30 dB or lower), signal attenuation in Q303
becomes minimum. (The output is boosted by about
10 dB)
D303 and D304 comprise a limiter. When an input signal
of sufficient level is given, a control signal passes through
HPF, AMP-2 and AMP-3, and the rectified and smoothed
signal is applied to the FET. In the very short time before
FET operates, the input signal is amplifed by AMP-2 to
become E2. The signal is output at a high signal level.
This phenomenon causes distortion in tape recording or
causes an unnatural sound by shifting the operation point
in the subsequent amplifier stage. To prevent this, the
limiter circuit is used to clip high level input signals.
TP.
ENCODED
SIGNAL
OUTPUT
AMP-3 RECTIFIER + p.3
(Q306) (D305,D306) 6
T-1320
Dolby Encoder
DECODED
SIGNAL
OUTPUT
AMP-3 RECTIFIER T.P.3
(Quis) (DIOS, DIO) oO
LIMITER
T~1321
Fig. 10 Dolby Decoder