LA3430 데이터 시트보기 (PDF) - SANYO -> Panasonic
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LA3430 Datasheet PDF : 12 Pages
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LA3430
(1) SNC (stereo noise control)
Stereo S/N ratios deteriorate 21.7 dB below monaural but can be improved by varying stereo separation. S/N
improvement becomes apparent, however, only when the separation is 20 dB or worse. In that case, the relation
between separation and S/N improvement is shown in Fig. 5.
SNC in the LA3430 improves S/N ratios in weak radio fields by varying separation. It varies subsignal demodula-
tion level and controls separation. By using the IF stage signal meter level output as the source of the control
signal, S/N ratios in region A of Fig. 2 can be maintained at about 40 dB or better. Ideal S/N enhancements should
provide gradual switching over from stereo to monaural to maintain constant S/N ratios, starting from a point in
region A for 40dB stereo S/N toward a point for 40dB monaural S/N. Methods to set the control level will be
described later.
Fig. 3 shows separation characteristics (SNC characteristics) for voltages applied to pin 8 (SNC terminal) of the
LA3430. Pin 8 is also the base of a PNP transistor, so stereo mode is set when pin 8 is open and monaural mode
is set when it is grounded. SNC terminal control is effective only when locked with pilot signals and when stereo
indicator is lit. External circuit parameters can be chosen in large values that do not affect the IF stage meter
output circuit because SNC control currents are small. This makes designing easy. (See Fig. 6)
(2) Designing external circuits for SNC characteristics (characteristic setting by drawing)
We recommend the following as a way to designate SNC characteristics to have smooth transition of separation
from stereo to monaural in region A of Fig. 2.
Separation vs S/N-enhancement relation ....................................................... Refer to Fig. 5.
SNC terminal voltages vs separation characteristics ..................................... Refer to Fig. 3.
Antenna inputs vs S/N improvement characteristics can be obtained from the drawing if the graph for IF stage
signal meter output vs. antenna iuput and the graph for stereo S/N ratio vs. antenna input are known. From
desired S/N characteristics, SNC terminal voltage characteristics can also obtained. Sample drawings are shown
in Fig. 4, where for simplicity’s sake, SNC, IF meter, and stereo S/N characteristics have been approximated with
straight lines.
For instance :
To obtain stereo S/N improvement characteristics from SNC characteristics, when (a) in the second quadrant of
the chart represents bare SNC characteristics, point 1 projected to the third quadrant shows a 20 dB separation
and a 1dB S/N improvement. When projected from the frist to the fourth quadrant, a point improved by 1dB in
S/N over the stereo S/N line in the fourth quadrant corresponds to point 1. Similarly, point 2 on the SNC
characteristics in the second quadrant corresponds to point 2 in the fourth quadrant. Point 3 in the second
quadrant corresponds to point 3 in the fourth quadrant. Stereo S/N improvement characteristics for each point
are obtainable.
Similarly, (b) characteristics in the second quadrant are projected to form (b) characteristics in the fourth
quadrant, and (c) in the second quadrant to form (c) in the fourth quadrant, thus providing a way to diagram
improvement characteristics.
In the resulting drawings, ideal S/N improvement characteristics are similar to (b) in the fourth quadrant, but
corresponding SNC characteristics have to be (b) characteristics in the second quadrant which are difficult to
realize. Among realistic characteristics, something like (c) appears to be satisfactory. The (c) SNC characteristics
are obtained with a shift by two diodes together with a 1/2 bleeder.
(3) HCC (high-cut control)
In region B where S/N deteriorates to 40 dB or worse even for monaural, the S/N as sensed by the human ear can
be enhanced by suppressing levels at frequencies above approximately 7kHz.
Treble region levels that follow meter voltages can be smoothly attenuated (high-cut control) by impressing IF
stage signal meter output to the HCC pin (pin 7) of the LA3430. Fig. 7 shows MPX output frequency characteris-
tics (monaural) provided by voltages impressed on pin 7. Frequency characteristics for a 100% high cut can be
designated by an external capacitor connected to pin 4. An equivalent circuit is shown below where the designa-
tion is made by the 5kΩ and the C time constant. Approximate values provided by C as expressed in attenuation
at 10kHz are listed in table below : right.
No.1408–6/12
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