Akai GXC-750

1. RECORDING BOLBY N.R. SYSTEM
30
(Refer to Figs. 45 and 46)
1) At recording mode, the input signal passes TR],
then the low pass filter, TR2, TR3, and TR4
and appears at point®)as the output signal.
Also the signal from TR2 passes the high pass
filter and is amplified at TR6 and TR7. At this
time, the signal which passes TR8 is rectified at
D6 and supplied to TRS (FET) gate.
This D.C. bias causes the FET to act as a variable
resistor and change the impedance between the
drain and source, and TR6 input signal is cont-
rolled.
2) When the input signal level is small, TRS (FET)
has a certain impedance value which becomes
like an electronic attenuator. At this time, TR6
input signal e is attenuated by the high pass
filter and TRS and becomes e/k.
(k represents attenuator constant)
This signal is amplified at TR6 and TR7. If we
let A represent the degree of amplification at
this time, output voltage e; becomes:
eg =e(k * A wo. (formula 1)
Here, if we let m represent A/k, formula 1
becomes:
€2 =M* © ww. (formula 2)
Because output signal eg is the composite signal
of input e (signal after it has passed TRI and the
low pass filter) and e2 (the signal controlled by
FET), e) becomes:
&o =e +e, =e+me...... (formula 3)
In formula 3, Dolby recording circuit output
signal eg is recorded at a 10 dB higher level in
relation to a 30 dB lower input level than the
Dolby level. (Refer to Fig. 47)
(Output signal eg, at over 400 Hz signal, becomes
10 dB higher than input signal e)
3) When input signal e in gradually becomes large,
the D.C. voltage which was amplified at TR8
and rectified at D6 also becomes large.
When this happens, TRS5(FET) impedance gradual-
ly approaches zero, For this reason, e signal level
is greatly decreased by FET operation.
That is to say that with regards to signal over
the Dolby level (Fig. 47), the signal supplied to
the base of TR6 is greatly reduced and the situa-
tion develops wherein e> me = ec. For this
reason, formula 3 becomes:
Gp He Homes SE is (formula 4)
Therefore, relative to the signal above the Dolby
level, ratio of input and output becomes an
equal 1:1. (Fig. 47)
INPUT SIGNAL LEVEL
Cin
SIGNAL LEVEL AT POINT(b)
e
SIGNAL
LEVEL
AT
POINT(c)
\
fo
G2
OUTPUT SIGNAL LEVEL
@0=8+ 62
i
ee 2 \
NL? eo \ Xe
Fig. 50 Level and phase of each points
at individual frequency
2. PLAYBACK DOLBY N.R. SYSTEM
(Refer to Figs. 49 and 50).
1) At playback mode, the input signal passes TR1,
then the low pass filter, TR2, TR3, and TR4
and appears at point@®)as the output signal.
TR4 output signal passes the high pass filter and
is amplified at TR6 and TR7. At this time, the
signal which passes TR8 is rectified at D6 and
supplied to TRS (FET) gate. FET functions by
means of this D.C. bias and the signal to TR6 is
controlled.
2) Also TR7 output signal is changed by the charac-
teristic of diodes D3 and D4 and is supplied to
point b) as e, in Fig. 47. This e signal is added
at point (b) as the reverse phase of signal e, and
this signal is dolbyized to become output signal
€9- Expressed by a formula, this beocmes:
€p = e@ + (-e2) (minus indicates
reverse phase)
=e t+ (-me) ..... (formula 5)
3) (Refer to Figs. 49 and 50)
With the playback Dolby N.R. System, signals
recorded above the dolby level are played back
at a 1:1 ratio, and signals recorded at a low level
of under -20 dB on the tape (dolbyized recorded
signals) are reduced by 10 dB and played back.
At this time, because the noise level is also reduced
by 10 dB, the S/N ratio is improved.
4) Therefore, the difference in operation between
Recording Dolby N.R. System and Playback Dolby
N.R. System is the phase relation of the signals
supplied to point (b) in the figure.