This circuit is able to identify PAL and NTSC video signals. Its output is high for an NTSC signal and low if the signal is PAL.
This output signal can be used, for example, to automatically switch in
a colour subcarrier converter or some other device while an NTSC
signal is being received. One application is for the reception from
satellites of ‘free-to-air’ TV signals, which in Australia generally
contain a mixture of 625-line PAL and 525-line NTSC
programs. Operation of the circuit is as follows. IC1 is an LM1881
video sync separator which takes the video input signal and generates
vertical synchronisation pulses.
For an NTSC signal, these pulses are 16.66ms apart, corresponding to the 60Hz field rate, while for a PAL
signal they are 20ms apart, corresponding to the 50Hz field rate. The
vertical sync pulses are fed into IC2a, the first of two dual
retriggerable monostable multivibrators in the 74HC123A. IC2a has a
period of very close to 17.9ms, set by the 200kO resistor and 0.22µF
capacitor at pins 14 & 15. Because the monostable is retriggerable, NTSC sync pulses arriving every 16.66ms will keep its Q output, at pin 13, high.
However PAL sync pulses arriving every 20ms
will allow the Q output to go low after 17.9ms, before being triggered
high again 2.1ms later. Thus an NTSC signal will give a constant high output while a PAL
signal will result in a train of pulses 2.1ms wide. The Q output from
IC2a is fed to the inverting input of IC2b, the second monostable, which
has a period of about 0.5s, as set by the 270kO resistor and 4.7µF
tantalum capacitor at pins 6 & 7. With its input constantly high,
resulting from an NTSC signal, IC2b is not triggered and its Q output remains low.
However, the pulse train from a PAL signal
will constantly retrigger it, so its Q output will remain high. The
period of IC2b also effectively makes it a low-pass filter which removes
spurious switching due to any input glitches. The output signal is
taken from the Q-bar (inverted) output, so that an NTSC signal gives a high output, while PAL
gives low. For the particular application for which the circuit was
developed, diode D1 and the resistor network shown drive the base of an NPN switching transistor and relay. A dual-colour 3-lead LED can also be fitted to indicate NTSC (red) or PAL (green). Note that with no video input, the output signal is high and will indicate NTSC.