TDA9845 Philips Semiconductors, TDA9845 Datasheet - Page 6

• Manufacturer Part Number: TDA9845
• Description: TV and VTR stereo/dual sound processor with digital identification
• Manufacturer: Philips 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.
Dematrixing 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 (V
and for a nominal input signal (AM: m = 0.54) of 500 mV
RMS (V
gain of 6 dB for V
(AM sound)) and 6 dB for V
rail-to-rail operational amplifiers, the clipping level
(THD 1.5%) is 1.60 V RMS for V
for V
minimize 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 C
for pulse shaping and suppression of low level spurious
signal components. This is a measure against
mis-identification.
1995 Mar 20
TV and VTR stereo/dual sound processor
with digital identification
P
= 8 V at outputs V
i 1
), respectively 250 mV RMS (V
i 1
and V
o 1
i 2
, V
i 3
signals (0 dB for V
LP
o 2
signal is built-in. By using
(pin 4) to a Schmitt-trigger
. Care has been taken to
P
= 5 V and 2.65 V RMS
i 3
). A nominal
i 1
i 1
signal
, V
i 2
)
6
The identification signal is amplified and fed through an
AGC low-pass filter with external capacitor C
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 three control input
ports C1, C2 and C3 (TTL-level). With these ports the user
can select between different AF sources according to the
transmitter status (see Table 1). Finally Schmitt-triggers
are added in the input port interfaces to suppress spikes
from the control lines C1, C2 and C3.
After a power-on reset, the logic is reset (mute mode for
the AF channel). After some time ( 1 ms), when the
power-on reset is automatically deactivated, the switch
position of the Main channel is changed according to the
control input port levels C1, C2 and C3.
For standard L, the AM sound is fed via the AF input (V
to the two AF outputs (V
achieved by feeding at AF input V
The logic level combination 111 of the control input ports
(C3, C2 and C1) is not allowed (see Table 1).
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
o 1
,V
FP
o 2
, C
). This can also be
p
Preliminary specification
FP
.
i 3
) has to be adapted
.
TDA9845
L
of around 12,
AGC
(pin 3) to
p
i 1
)