TDA4471 Datasheet, PDF (4/16 Pages) TEMIC Semiconductors – Multistandard Video-IF and Quasi Parallel Sound Processing

TDA4471
TELEFUNKEN Semiconductors
Circuit Description
Vision IF amplifier
The video IF signal (VIF) is fed through a SAW filter to
one of the two VIF inputs (Pins 6–7 or 10–11). The
selection of VIF input is controlled by Pin 16 in combina-
tion with the standard switch. With a minimal external
expense it is possible to switch between two different
SAW filters. Both VIF inputs features excellent
cross–talk attenuation and an input impedance which is
independent from the switching condition. The VIF
amplifier consists of three AC–coupled amplifier stages.
Each differential amplifier is gain controlled by the
automatic gain control (VIF–AGC). Output signal of the
VIF amplifier is applied to the FPLL carrier generation
and the video demodulator.
Tuner– and VIF–AGC
At Pin 8 the VIF–AGC charges/discharges the AGC
capacitor to generate a control voltage for setting gain of
VIF amplifier and tuner in order to keep the video output
signal at a constant level. Therefore in case of negative
modulated signals (e.g. B/G standard) the sync level of
the demodulated video signal is the criterion for a fast
charge/discharge of the AGC capacitor. For positive mod-
ulation (e.g. L standard) the peak white level of video
signal controls the charge current. In order to reduce
reaction time for positive modulation, where a very large
time constant is needed, an additional black level detector
(Pin 17) controls the discharge current in the event of
decreasing VIF input signal. The AGC voltage is
transferred to an internal control signal, and is fed to the
tuner AGC to generate the tuner AGC current at Pin 13
(open collector output).
Take over point of the tuner AGC can be adjusted at
Pin 12 by a potentiometer or an external DC voltage
(from interface circuit or microprocessor).
FPLL, VCO and AFC
The FPLL circuit (frequency phase locked loop) consists
of a frequency and phase detector to generate control
voltage for the VCO tuning. In the locked mode the VCO
is controlled by the phase detector and in unlocked mode
the frequency detector is superimposed. The VCO
operates with an external resonance circuit (L and C
parallel) and is controlled by internal varicaps. The VCO
control voltage is also converted to a current and
represents the AFC output signal at Pin 24. With the AFC
switch (Pin 21) three operating conditions of the AFC are
possible: AFC curve ”rising” or ”falling” and AFC ”off”.
A practicable VCO alignment of the external coil is the
adjustment to zero AFC output current at Pin 24. At centre
frequency the AFC output current is equal to zero. Fur-
thermore, at Pin 16, the VCO centre frequency can be
switched for setting to the required L’ value This function
is active when ”L mode” is selected by the standard
switch.
The optional potentiometer at Pin 28 allows an offset
compensation of the VCO phase for improved sound
quality (fine adjustment). Without a potentiometer (open
circuit at Pin 28) this offset compensation is not active.
The oscillator signal passes a phase shifter and supplies
the in–phase signal (0) and the quadrature signal (90) of
the generated picture carrier.
Video demodulation and amplifier
The video IF signal, which is applied from the gain
controlled IF amplifier, is multiplied with the inphase
component of the VCO signal. The video demodulator is
designed for low distortion and large bandwidth. The
demodulator output signal passes an integrated low pass
filter for attenuation of the residual vision carrier and is
fed to the video amplifier. The video amplifier is realized
by an operational amplifier with internal feedback and
8 MHz bandwidth (–3dB). A standard dependent DC
level shift in this stage delivers the same sync level for
positive and negative modulation. An additional noise
clipping is provided. The video signal is fed to VIF–AGC
and to the video output buffer. This amplifier with 6dB
gain offers easy adaption of the sound trap. For nominal
video IF modulation the video output signal at Pin 14 is
2 V (peak to peak value).
Sound IF amplifier and SIF–AGC
The SIF amplifier is nearly identical with the 3–stage VIF
amplifier. Each differential amplifier is gain controlled
by the automatic gain control for the sound IF path (SIF–
AGC). Output signal of the SIF amplifier is applied to the
mixer for FM/NICAM signals and the limiter/
demodulator for AM signals.
The SIF–AGC is related to the average level of AM– or
FM–carrier and controls the SIF amplifier to provide a
constant SIF signal to the AM demodulator and QPS
mixer.
AM demodulator
The alignment–free AM demodulator is realized by a syn-
chronous detector. The modulated SIF signal from the SIF
amplifier output is multiplied in phase with the limited
SIF signal (AM is removed). The AF signal of the
demodulator output is fed to the output amplifier and to
the SIF–AGC. For all TV standards with negative video
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Preliminary Information
Rev. A1: 18.08.1995