LA3375 Datasheet, PDF (5/11 Pages) Sanyo Semicon Device – Pilot Cancel-Provided PLL FM MPX Demodulator for Car Stereos

LA3375
4. SNC (stereo noise control) and HCC (high-cut control)
The LA3375 has SNC and HCC terminals for improved S/N ratios when operating in weak radio fields. By adjusting
the SNC terminal, noises unique to stereo FM in weak fields can be reduced. The HCC terminals permits further
improvement of effective S/N ratios by lowering treble levels of FM noises in weak fields. (See Fig. 2)
STEREO deteriorates approximately 21.7 dB (compared to MONO) in weak radio feilds (Fig. 2). Generally, when
S/N ratios deteriorate below 30 to 40 dB, noises become quite noticeable. Section (1) shows ways to set SNC and
HCC when radio field strengths are divided into 3 regions, A, B, and C, (Fig. 2). SNC is expected to function in
region A, and HCC in region B. In region C, shallow muting is effected in the IF stage.
(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 LA3375 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 40-dB stereo S/N toward a point for 40-dB monaural S/N. Methods to set the control level will be
described later.
Fig. 3 shows voltages applied to pin 8 (SNC terminal) of the LA3375 versus separation characteristics (SNC
characteristics). 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
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 be 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 1 dB S/N improvement. When projected from the frist to the fourth quadrant, a point improved by 1 dB
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 improve-
ment 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.
No.901–5/11