TDA8505 Datasheet, PDF (7/32 Pages) NXP Semiconductors – SECAM encoder

Philips Semiconductors
SECAM encoder
Preliminary specification
TDA8505
VERTICAL IDENTIFICATION
After the low frequency pre-emphasis the signal is
clamped and, if desired the vertical identification sawtooth
waveform can be added. The generation of the vertical
identification is switched on/off by the logic level input at
pin 26.
Figure 4 shows the sawtooth waveform at the input of the
FM modulator with the corresponding frequency values
after modulation.
Vertical identification is only possible if PLL tuning is
selected.
GAIN + LIMITER
The gain of this amplifier is sequentially switched, so that
the amplitude of D'R is 280/230 times the amplitude of D'B
(based on an EBU colour bar). The signal is limited at a
lower and upper level to ensure that the FM modulator
frequencies are always between 3.9 MHz and 4.756 MHz.
A DC offset between D'R and D'B is added which
corresponds with the limiter levels.
FM MODULATOR
The signal of the gain + limiter stage is fed to the FM
modulator.
The modulator control adjusts the DC level at pin 13 to set
the frequency of the FM signal to 4.406 MHz at the black
level of D'R. The modulator control also sets the DC level
at pin 15 to adjust the FM frequency to 4.250 MHz at the
black level of D'B.
At the start of every line the FM modulator is stopped and
is started again by a short duration pulse of the pulse
shaper. These stop/start pulses are operating such that
after two lines starting in the same phase, the start phase
of the third line is shifted 180 degrees. This sequence is
inverted during each vertical blanking.
The FM signal is fed to the internal HF pre-emphasis filter.
HF PRE-EMPHASIS AND BANDPASS FILTER
An HF pre-emphasis filter combined with a bandpass filter
is integrated.
Figures 5 and 6 illustrate the frequency response. Two
resistors in series with a potentiometer at pin 14 adjusts
the frequency to 4.286 MHz with a tolerance of ±20 kHz.
A tuning circuit integrated with an external capacitor
connected to pin 16 guarantees a stable frequency
response for the whole temperature range.
The output of the bandpass filter is connected directly to
the chrominance blanking circuit.
handbook, full pagewidth
July 1994
64 µs
15 µs
5 µs
64 µs
frequency
after
modulation
4.756 MHz
35 kHz
D'R
4.406 MHz
4.250 MHz
D'B
18 µs
6 µs
MLA954
3.90 MHz
35 kHz
Fig.4 Vertical identification sawtooth waveform input.
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