TDA7514 STMicroelectronics, TDA7514 Datasheet - Page 68

• Manufacturer Part Number: TDA7514
• Description: SINGLE-CHIP FM/AM TUNER
• Manufacturer: STMicroelectronics

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TDA7514
from stereo to mono is performed. The signal strength thresholds for the automatic stereo blend functions
can be set by programming the gain of the Smeter contribution to the SNC signal (compared to a fixed
threshold sets the full stereo point) and subsequently programming the voltage threshold against which
the SNC signal is compared to set the full mono threshold.
In case the transmission is stereo, it is possible if so desired to set it to forced mono to improve the re-
ceived SNR.
The TDA7514 stereo demodulator additionally performs the functions of roll-off compensation and pilot
cancelling. The amount of the former is programmable. The latter function is activated when an FM MPX
signal with pilot tone is detected; the function is disabled in AM and in FM if no pilot tone is detected.
5.4.4
The L and R signals are low-passed filtered by the high cut and, subsequently, by the de-emphasis filter.
The high cut filter consists of a fixed resistor and a 5 bit digitally-controlled binary-weighted capacitor
(whose value therefore changes between Cmin and Cmin + 32 x Cstep). The digital control is done by
converting the filtered Smeter into a 5 bit word. Programming the internal resistor value, three possible
corner frequency ranges (800 Hz - 4 kHz, 2 kHz - 10 kHz, 4 kHz - 20 kHz) are available. It is possible to
force the actual range to be smaller than one of the above mentioned ones by setting:
The controlling signal for this filter is chiefly the filtered Smeter. It is also possible to program the high cut
control input selector to use the SNC instead of the Smeter. The contributions to the SNC signal from the
Smeter, adjacent channel and multipath detectors is the same as for the stereo blend function. The level
"0" of the converted signal (corresponding to the maximum filter corner frequency) is obtained when the
control signal is equal to or greater than the programmable threshold VHCCH. The level "31" (correspond-
ing to the minimum filter corner frequency) is obtained when the control signal is equal to or smaller than
the programmable threshold VHCCL. Noise blanking is performed at this stage. "Corners" in the waveform
due to the holding action of the noise blanking circuit are smoothed by the successive de-emphasis filter.
The fixed de-emphasis filter can be programmed to 75 us and 50 us; in AM mode it is shifted to a seven
times higher corner frequency.
5.4.5
In AM mode the PLL is on though no pilot tone is present; the stereo demodulator is forced to work in the
mono configuration and the pilot canceller is off.
5.4.6
The only difference between the AM and the FM configuration of the input stage lies in the input imped-
ance only (30 kOhm for AM, 100 kOhm for FM). The functions are identical.
5.4.7
The input stage is followed by a fourth order low-pass filter with a cut-off frequency of approximately 4 kHz
and a delay of approximately 130 us whose main purpose is to generate the delay in the signal path nec-
essary for audio noise blanking. The filter contributes to the low-pass filtering of the AM signal as well.
5.4.8
The high cut filter is AM can be used both statically by programming a fixed corner frequency (the 800 Hz
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– the maximum capacitor value (4 position programmable control);
– a 5 bit word to be used as limit (32 possible values), in conjunction with a bit that sets whether this
– it is additionally possible, if so desired, to keep the high cut filter to a fixed position by sending the
– the high cut filter can also be defeated via a dedicated bit.
– 4 kHz range has been specifically designed for AM although it is available also in FM) or dynamically,
limit is to be used as a maximum or as a minimum;
position code in the previously mentioned 5 bit register and further setting a dedicated bit;
exactly like the FM high cut filter. See the FM high cut filter section for further details.
HIGH CUT AND DE-EMPHASIS FILTERS
AM MODE
INPUT STAGE
DELAY FILTER
HIGH CUT FILTER