LA3375 Datasheet, PDF (6/11 Pages) Sanyo Semicon Device – Pilot Cancel-Provided PLL FM MPX Demodulator for Car Stereos

LA3375
(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 LA3375. 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.
Fig. 8 shows the relation between noltages impressed on pin 7 and rates (%) of high cut (HCC). When IF meter
output voltage characteristics and region B, S/N characteristics, of Fig. 2 have been obtained, S/N improvement
by HCC can be drawn in a way similar to drawing SNC characteristics.
Fig. 2 shows typical meter outputs of a quadrature detection IF amplifier IC. (Fig. 1 shows data for the LA1140,
LA1230, and LA1231N) HCC characteristics have been designated to permit region B improvements when the IC
is directly connected to HCC (pin 7) terminal of the LA3375. The infinitesmal control currents at pin 7, similar to
pin 8, do not affect meter outputs.
(4) SNC and HCC connection circuits when coupled with the IF stage
Fig. 1 shows sample S/N characteristics vs antenna inputs when SNC and HCC are connected with the IF stage
by an external circuit.
(5) S/N improvements in region C of Fig. 1
Because S/N ratios deteriorate even further in the region C of Fig. 1, it is better to improve the S/N in this region
with IF mutings. The LA1140 is available to linearly vary the IF muting. Employment of the LA3375 together
with the LA1140 further enhances S/N improvement.
(6) Using the HCC terminal for muting
Mutings in the neighborhood of 37 dB are feasible by utilizing HCC functions as muting functions when used in
home stereos and no need exists to suppress treble noises. Fade-in and fade-out of mutings, permitting delightful,
shock-noise-free muting are possible by providing a time constant to the pin 7 control.
(7) VCO damping
VCO oscillations can be damped by applying a voltage not less than 7 V to the HCC terminal (pin 7) to induce a
monaural mode. At this time, both SNC and HCC are in an off mode. Fig. 9 shows flow-in current by voltages
applied to pin 7.
No.901–6/11