tda9847t NXP Semiconductors, tda9847t Datasheet - Page 6

• Manufacturer Part Number: tda9847t
• Description: Tv And Vtr Stereo/dual Sound Processor With Digital Identification
• Manufacturer: NXP Semiconductors

Philips Semiconductors
FUNCTIONAL DESCRIPTION
AF signal handling
The input AF signals, derived from the two sound carriers,
are processed in analog form using operational amplifiers.
De-matrixing uses the technique of two amplifiers
processing the AF signals. Finally, a source selector
provides the facility to route the mono signal through to the
outputs (‘forced mono’).
De-emphasis is performed by two RC low-pass filter
networks with internal resistors and external capacitors.
This provides a frequency response with the tolerances
given in Fig.4.
A source selector, controlled via the control input ports
allows selection of the different modes of operation in
accordance with the transmitted signal. The device was
designed for a nominal input signal (FM: 54% modulation
is equivalent to f = 27 kHz) of 250 mV RMS value (V
and V
500 mV RMS value (V
and V
(0 dB for V
signals is built-in. By using rail-to-rail operational
amplifiers, the clipping level (THD
for V
V
switching plops. Also total harmonic distortion and random
noise are considerably reduced.
Identification
The pilot signal is fed via an external RC high-pass filter
and single tuned LC band-pass filter to the input of a gain
controlled amplifier. The external LC band-pass filter in
combination with the external RC high-pass filter should
have a loaded Q-factor of approximately 40 to 50 to
ensure the highest identification sensitivity. By using a
fixed coil ( 5%) to save the alignment (see Fig.2), a
Q-factor of approximately 12 is proposed. This may cause
a loss in sensitivity of approximately 2 to 3 dB. A digital
PLL circuit generates a reference carrier, which is
synchronized with the pilot carrier. This reference carrier
and the gain controlled pilot signal are fed to the
AM-synchronous demodulator. The demodulator detects
the identification signal, which is fed through a low-pass
filter with external capacitor CLP (pin 5) to a
Schmitt trigger for pulse shaping and suppression of LOW
level spurious signal components. This is a measure
against mis-identification.
The identification signal is amplified and fed through an
AGC low-pass filter with external capacitor CAGC (pin 4)
1995 May 23
o1
TV and VTR stereo/dual sound processor
with digital identification
to V
P
i2
i4
= 5 V and 2.65 V RMS for V
) and for a nominal input signal (AM: m = 0.54) of
). A nominal gain of 6 dB for V
o3
and V
i1
signal (AM sound)) and 6 dB for V
o4
. Care has been taken to minimize
i1
), respectively 250 mV RMS (V
P
1.5%) is 1.60 V RMS
= 8 V at outputs
i1
and V
i2
i3
signals
and V
i3
i4
i1
6
to obtain the AGC voltage for controlling the gain of the
pilot signal amplifier.
The identification stages consist of two digital PLL circuits
with digital synchronous demodulation and digital
integrators to generate the stereo or dual sound
identification bits which can be indicated via LEDs.
A 10 MHz crystal oscillator provides the reference clock
frequency. The corresponding detection bandwidth is
larger than 50 Hz for the pilot carrier signal, so that
f
event of missing synchronization with the horizontal
frequency f
identification signal is made small ( 1 Hz) to reduce
mis-identification.
Figure 2 shows an example of the alignment-free f
band-pass filter. To achieve the required Q
the Q
(effective Q
other Q
accordingly. It is assumed that the loss factor tan of the
resonance capacitor is 0.01 at f
Copper areas under the coil might influence the loaded Q
and have to be taken into account. Care has also to be
taken in environments with strong magnetic fields when
using coils without magnetic shielding.
Control input ports
The complete IC is controlled by the four control input ports
C1, C2, C3 and C4. Which AF output channel pair can be
selected is determined by the control input Port C4 [LOW:
main; HIGH: SCART; 3-state: preset position
(see Section “General information”)]. With the other
control input ports C1, C2 and C3 the user can select
between different AF sources in accordance with the
transmitter status (see Tables 1 and 2). Finally,
Schmitt triggers are added in the input Port interfaces to
suppress spikes on the control lines C1, C2, C3 and C4.
After a Power-On Reset (POR) both registers are reset
(mute mode for both AF channel pairs). After some time
( 1 ms), when the POR is automatically deactivated, the
switch positions of the main channel (C4 = LOW) are
changed in accordance with the other control input Port
levels. If C4 is HIGH after a POR, the switch positions of
the SCART channel cannot change. The reason is, that
the main register is reset (mute mode; see Table 1). Thus,
at first the main register byte has to be changed out of the
mute function, e.g. sound mute.
p
-variations from the transmitter can be tracked in the
0
at f
0
, the RC-network (R
p
H
0
of the coil was chosen to be around 25
. However the detection bandwidth for the
including PCB influence). Using coils with
FP
and C
p
Preliminary specification
.
FP
) has to be adapted
TDA9847
L
of around 12,
p