Dolby Hx Circuit - Tascam 122 Service Manual

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1122
3-14. DOLBY HX CIRCUIT
NOTE: The Dolby Headroom Extension system
(Dolby HX system) is provided in the
record circuit of the deck and operates
only in the following conditions.
NR SYSTEM switch: NR
+
HX
SPEED switch: STANDARD
BIAS switch: Co (Cr02) or NORMAL
EQ switch: Co (Cr02 ) or NORMAL
Mode: record/reproduce mode
3-14-1 Bias characteristics
Generally tape has the bias characteristic as
shown in Fig. 3-33. In the bias characteristic,
(relationship of bias current to record/reproduce
level) of a record signal of 315 Hz, for example,
the record/reproduce output level rises with bias
current and it reaches maximum at the maxi ­
mum sensitivity bias current (peak bias). If bias
current increases further the record/reproduce
output level falls gradually. On the other hand,
distortion is diminished as bias current increases.
The variation of output level with bias current
follows a relatively gentle curve at low frequen ­
cies but is steep at high frequencies. The output
level at peak bias falls as the frequency becomes
higher.
For tape having specific bias characteristics,
major factors determining bias current are sensi­
tivity, distortion, and frequency response.
Normally bias is determined in the following
way. In the frequency spectrum of music signals
which are most commonly recorded low and
middle frequency components dominate and the
spectrum drops steeply at high frequencies.
Conventionally bias was determined by taking
mainly sensitivity and the distortion of bass and
mid-range components into consideration, sacri ­
ficing frequency response at high frequencies.
However, for maximum sensitivity at bass and
mid-range frequencies, bias should be much
higher than at high frequencies. Therefore,
strong equalization is required for recording high
frequencies. (Fig. 3-33 and 3-34 show recording
equa lization for 10k Hz at peak bias of 315 kHz.)
When equalization during recording is too
great, tape tends to be saturated by high­
frequency components when the recording level
is high or when the signals include many high ­
frequency components. This results in an in­
crease in distortion and a degradation of fre­
quency response (see Fig . 3-35). When bias is
too low, the recording magnetic field becomes
wider and, therefore , demagnetization increases
at high frequencies . This phenomenon becomes
more noticeable with higher signal levels. There­
fore, demagnetization causes the frequency
response at high frequencies to diminish as the
recording level increases.
From the above facts, we determined that bias
must be relatively small in order to improve
frequency response at high frequencies, especial­
ly for high-level, high-frequency signals.
~__
1 0 ' " ' ]
BIAS CURRfNT FOR
I S. l IIMr
MA XIMUM SENSITIVITY
3L5 HI
,
DISTO RTION AT ll5 Hz
"
S IroIAlL
BIAS CURRENT
L ARGE
( SHA LLO W B I AS I "til ... C HIGH BI AS)
Fig.3-33 Tape bias characteristics (1)
TA PE SATURATI ON LEVEL
MARGIN
LARG E
AMOU NT OF
RECORDING C OMI't:NSATION
AT 1 0 kHz
ll5 Hz
1 0 kHl
LOW ....
_.;..;FR=EO=UE=NC:..;.Y_~'>
HIGH
Fig. 3-34- Recording_compensation characteristics (1)
%
BIAS : FlX£D
~
i' PARAMETER : RECORDING LEVEL
"'~
. 5 dB
0,.
o~
o
dB if--'
~-
"~I
"''''
,, ­ '
0 _
- 10 dB
3
"''''
0
II!
3
-20 dB
•
ll 5 Hz 10 kHI
FREOUENCY
LO W
':>
H I GH
Fig. 3-35 Recording frequency response (1)
37