Description On New Technology - JVC KD-A66 Service Manual

KD-A66A/B/C/E/J/U
Description on New Technology
Computer B.E.S.T. System
Introduction
The B.E.S.T. (Bias Equalizer Sensitivity Tuning) system
which is employed in the KD-A66 is basically the same as
the preceding KD-A8 version and has also been provided
with various improvements for an enhanced performance.
The B.E.S.T. system employs a microcomputer for the
purpose of setting the proper optimum bias current and
sensitivity for tape.
1. Bias
As seen in Fig. 1 (the relationship between distortion
rate and bias current at the 1 kHz to 7 kHz signals), the
output level decreases when a larger bias current is ap-
plied. In this case especially, the 7 kHz signal is sharply
attenuated.
Further, when the bias current is decreased, the three-
dimensional distortion increases. Accordingly, the bias
current is an important factor for determining the
frequency response and the distortion rate.
It is therefore desirable that an optimum bias current
is set for each tape.
— Bias current (uA)
200 300 400 500 600 700 800
o
=z
D 1KHz
hi
2
Qa i]
t=
3 i
O 10F '
T Set bias current value
; T7KHz
a3 |
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2 64 \
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c °F vot Third harmonic distor-
S44 ar tion at 333 Hz
es. Xe (250mW/level)
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2 f----4---¥, Third harmonic distor-
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are Se tion at 1 kHz
Toa S160 mWitevel) Fig. 1
2. Equalizer
As seen in Fig. 2 (with each tape having its optimum
bias current set), this bias current is determined by tak-
ing into account the high-band molecular character-
istic and distortion rate of the particular tape. Equali-
zation level must then be set to obtain a flat molecular
START
16sec
16sec
Far:
l2sec
REC-* Blank Marker
Approx. |.2sec
Point to which the tape is rewound
Marker
Point to which the tape is rewound
Bias sweep spot signal
\
characteristic corresponding to that bias current.
Until now, the equalization level has been determined
by ear and an accurate tuning can only be performed
by experience.
With this tuning system, the equalization level is auto-
matically set by a microcomputer.
Frequency Distribution
o
mo}
Ovseeeee denotes the type of the tape.
3
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ax STD. UD STD. SA STD Duad
3 6 4 | (MAXCE) (10.4) (SONY)
eS
a8
2
3 o : 20
o = 0 8 - Boece to
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eeco00o
oo
ng Ss eo
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es
1 :
Fig. 2
Sensitivity
In tapes, the recording and reproduction levels do not
necessarily coincide with each other. The difference
between the recording and reproduction levels appears
as a beat in the frequency response of a noise reduction
circuit such as the ANRS circuit, the Super ANRS
circuit, etc.
In this tuning system, the difference between these
levels is limited within 1 dB.
Features:
1. Automatic setting of the optimum bias current for
the tape.
2. Automatic setting of the level difference to within
+1 dB, in the case of 10 kHz high-band frequency
which tends to differ between the recording and
reproduction levels.
3. Automatic setting of the tape sensitivity to within
a +1 dB error by the ANRS circuit.
4. For tapes which cannot be automatically adjusted,
its sensitivity is indicated, thus permitting normal
recording in the state in which respective values
have been preset.
Frequency characteristic sensitivity
sweep spot signal
L6sec \1.2sec Marker Approx. |.5sec
Play REC — Blank
~+— REW
STOP (the tape is rewound to the vicinity
ii
WIA
of its starting position.) PLAY —
-
.
END
+ REW
Fig. 3
No. 4193