Tascam 122 Service Manual page 38

122
3-14-2 System outline
To improve the frequency response at high fre ­
quencies, especially for high-level, high-frequen­
cy signals, the Dolby HX system automatically
controls bias and recording equalization for
optimum levels with respect to the level of re ­
cording input signal and the relative amount of
high-f, r equency components.
1) Bias
a. When bass - and mid-range components
dominate in the input signal or the overall
l evel of the input signal is low, a fixed high
bias is applied. (See
®
in Fig. 3-36. ) In
this case, frequency response at low fre­
quencies can be improved by selecting a
bias more suitable for lower frequency
content than the conventional fixed bias.
b. When the input signal involves high-level,
high-frequency components, bias is decreas ­
ed instantly depending on the frequency
components and level, increasing the sensi­
tivity at high frequencies. (See @ in Fig_
3-36.)
2) Record equalizer
The Dolby HX system not only controls bias
as described above but also recording equali ­
zation. This is because tape sensitivity varies
at high frequencies, accompanying the change
in bias.
a. When bass and mid-range components
dominate the input signal and bias is select­
ed as
®
in Fig. 3-36 , tape sensitivity is
low at high frequencies and, therefore, the
recording equalization is that of curve
®
shown in Fig. 3-37. With curve
®,
the
amount of boost at high frequencies is
very large but the level of high-frequency
components of the input signal is low.
Therefore, distortion will not occur during
recording.
b. With high-level, high-frequency signals, bias
is lowered. (See @ in Fig. 3-36.) With
lower bias, tape sens itivity increases at
high frequencies and the amount of record ­
ing equalization may be reduced, as shown
by curve
@
in Fig. 3-37.
Reduction in the amount of recording
equalization, i.e. boost, results in expand ­
ing the headroom of the tape saturation
level of high-frequency signals. (This is the
origin of the name of Dolby Headroom
Extension system.) As a resul,t, frequency
response at high frequencies is greatly
improved (see Fig. 3-38.) and at the same
time, distortion at high frequencies is
minimized.
38
, - - - ®
BIAS
C ~NT
AFTER C!<ANGE
i - - - -
I CONVENTIONAL CO MPRO MIS E FIXED BIAS CU RRt:N T J
: ®
STATIONARY BIAS , HIGHER THAN BE FO RE
REC ORD ING COMPENSATIO N
AF~ER BIAS C HANG E
SENSITIVITY CHAN G E
ACC OMPANYING BIAS vARIATION
L .~£
,. ( fi IGH 8 .ASJ
Fig. 3-36 Tape bias characteristic (2)
TAPE SATURAT ION LEvEL
WHEN BASS AND MID- RANGE
COMPONENTS DOMINATE SIGNAL
MARGIN LARGE
"HEN HIGH-FREQUENCY C OM PONENTS
DO MINATE SIGNAL
AMOUNT OF C OMPENSATION NHDED
LOW _.
---,-,FR~EQc:::UE,,-,NY
_ _ •
Hoe H
Fig. 3-37 Recording equalization characteristic (2)
BIAS : VARIABLE
G
PARAMETER : RECO RDING LE vEL
lj
i
&~ + 5 dB
o
dB
~~
II!'~
-10 dB
~~
§o
-20 dB
'"
~
FlIEQ UE NCY
l OW • • WIG M
Fig. 3-38 Recording frequency response (2)