Tascam 122 Service Manual page 39

122
3-14-3 Circuits
See Fig. 3-39.
There are three circuits that control the record­
ing bias and equalization characteristics, depend­
ing on the spectrum of the input signal. These
are the control signal circuit, the variable bias
circuit, and the variable equalizer.
1) Control signal circuit
The recording bias and equalization character­
istics must be varied in accordance with high ­
frequency components of the input signal and
their level. For this purpose, it is necessary
to generate a signal which controls them,
which is done by detecting and measuring
high-frequency components in the input sig ­
nal and their level.
In the Dolby B-type processor (as used in the
M 122), the noise reduction control signal is
generated from the same parameters as those
used by the Dolby HX system. This signal is
taken from pin 14 of Dolby N R IC U301
(U401) and supplied to buffer amplifiers
U303-a_and U303-b.
a. U303-a and U303-b select and output the
higher of the !\tVo input signals without
permitting the Dolby processors of the left
and right channels to interfere with each
other.
b. The output of the buffer stage is applied to
pin 12 of high-gain, non-inverting ampli­
fier U303-c. Generally the DC output vol­
tage of this amplifier rises together with the
DC input voltage. A DC bias voltage ad­
justed by semi-fixed resistor R36 or R37 is
applied to pin 13 of U303-c. This voltage
determines the threshold of the control
voltage where the recording bias begins to
decrease due to high-frequency com ­
ponents contained in the recording input
signal. The threshold varies with the type
of tape; R36 adjusts for 'normal' tape while
R37 adjusts for 'Co (Cr02)' tape.
c. The output of DC amplifier U303-c is sent
to the selector circuit which generates
conditions for Dolby HX system operation.
The output of U303-c is fed to the variable
bias circuit and variable equalizer circuit
only in the following conditions.
NR SYSTEM switch: NR+HX
SPEED switch:-STANDARD
BIAS switch: Co (Cr02) or NORMAL
2) Variable bias circuit
The output of the selector circuit is applied
to pin 2 of U303-d.
a. U303-d is an inverting amplifier with a gain
of 1 using Vcc/2 as its reference voltage.
The DC output voltage falls (rises) as the
DC input voltage rises (falls).
The output of U303-d controls the base
voltage of transistor 0303 which is located
in series with the power supply circuit of
bias oscillator U 1 03.
b. When the level of high-frequency compo ­
nents contained in the recording signal is
high, the DC voltage appfied to pin 2 of
U303-d by the control signal circuit rises
and, therefore, the DC output voltage from
pin 1 of U303-d falls. When the voltage at
pin 1 of U303-d falls, the base voltage of
0303 falls and its base current decreases.
As a result, the collector current of 0303
decreases, the voltage drop across the col­
lector and emitter increases, and the volt ­
age supplied to bias oscillator U1 03 falls.
When the supply voltage to U103 falls, the
output of the oscillator falls, that is, bias is
lowered. The minimum and fixed (maxi ­
mum) bias levels are determined by R328
and R329.
3) Variable equalizer
The output of the control signal circuit is
supplied to the variable bias circuit and the
variable equalizer simultaneously.
a. The variable equalizer is basically a low­
pass filter. A pair of variable conductance
amplifiers U302s are used to provide vari­
able cutoff time constant Tc, which is a
linear function of the current flowing
through resistor R335.
b. When the level of high-frequency compo­
nents in the recording signal is high, the DC
output voltage of the control signal circuit
rises and current flowing through R335
increases. As a result, variable cutoff time
constant Tc becomes large and the tumover
frequency falls.
c. Since the variable equalizer is basically a
low-pass filter, reduction in turnover fre­
quency results in increased attenuation
at high frequencies. Because of this, the re­
cording signal passing through the variable
equalizer varies as curves
®
and
@
shown in Fig. 3-37.
39