TDA8505 Datasheet, PDF (9/32 Pages) NXP Semiconductors – SECAM encoder

Philips Semiconductors
SECAM encoder
Preliminary specification
TDA8505
CHROMINANCE BLANKING
The chrominance signal is blanked by the internally
generated chrominance blanking pulse. The output of this
blanking stage is connected to the chrominance and
CVBS output circuits.
Y+SYNC, CVBS, AND CHROMA OUTPUTS
The Y output signal of the matrix is added to the composite
sync signal of the sync separator. The output of this adder
at pin 25 is connected to the input of an external delay line
which is necessary for correct timing of the Y+SYNC signal
corresponding with the chrominance signal. The signal
amplitude at pin 25 is 2 V (peak-to-peak value) nominal,
so at the output of the delay line Y+SYNC is
1 V (peak-to-peak value).
The delay line has to be DC-coupled between
pins 25 and 23 to ensure the required DC level at
pin 23. The output resistor of the delay line has to be
connected to pin 17 where (V_ref = 2.5 V).
The output of the delay line is connected to pin 23 which is
the input of a buffer operational amplifier. The output of the
buffer operational amplifier is connected to pin 22 and to
the CVBS adder stage via an internal resistor of 2 kΩ. An
external notch filter can be connected to pin 22. The CVBS
signal amplitude output at pin 21 is 2 V (peak-to-peak
value) nominal. An external emitter follower is used to
provide a 75 Ω output load.
The amplitude of the chrominance output signal which is
connected to pin 18 corresponds with the Y+SYNC signal
at the output of the delay line.
The outputs of the 272 divider are also used for pulse
shaping.
Within the vertical blanking period, another two Phase
Locked Loops (PLLs) synchronizes the FM modulator
during two lines with the 4.406 MHz reference VCO and
during the following 2 lines with the 4.250 MHz reference
VCO. The loop filters are connected to pins 13 and 15
respectively.
It is necessary to use low-leakage capacitors for these
loop filters.
TUNING BY CRYSTAL OR EXTERNAL SIGNAL SOURCE
When the frequency of the sync pulse at pin 29 is not
stable or is incorrect it is possible to tune the FM modulator
using an external 4.250 MHz crystal connected to pin 27.
The 4.25 MHz loop at pin 31 has to be connected to pin 17
(V_ref). A stable line frequency reference is generated by
the 272 divider circuit which is used for the 4.406 MHz
reference loop.
An external signal source, instead of a crystal, can be
connected at pin 27 via a capacitor in series with a resistor.
The minimum AC current of 50 µA is determined by the
resistor values (R_int + R_ext) and the output voltage of the
signal source (see Fig.7).
When crystal tuning is used no vertical identification
is possible.
Crystal tuning is recommended for VTR signals.
Modulator control circuit
The modulator control circuit has two tuning modes which
are controlled by the input at pin 26:
• Tuning by line frequency
• Tuning by crystal or external signal source.
TUNING BY LINE FREQUENCY
Two reference voltage controlled oscillators (VCOs) are
integrated, the 4.4 MHz VCO with an internal capacitor
and the 4.25 MHz VCO with an external capacitor at
pin 27.
A PLL loop with divider circuits directly couples the
frequencies of the two VCOs with the line frequency of the
sync separator sync signal.
The loop filter for the 4.40625 MHz reference is at pin 24
and the loop filter for the 4.250 MHz reference is at pin 31.
handbook, halfpage
TDA8505
OSCILLATOR
R _{int 27}
800 Ω
I 50 µA
R _ext
1 nF
signal
source
V (p-p)
MSA732 - 1
Fig.7 Tuning circuit for external signal source.
July 1994
9