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NTSC-PAL TV Signal Identifier(LM1881)

2017-08-03 00:30  
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This article briefly describes the NTSC-PAL TV Signal Identifier (LM1881). Although this circuit is very easy to understand, but also very practical. Circuit elements in depth understanding of the grasp of the principle is very effective. Do you know the circuit, you can see and purchase these components: LM1881 .

This circuit is able to identifyPALandNTSCvideo signals. Its output is high for anNTSCsignal and low if the signal isPAL. This output signal can be used, for example, to automatically switch in a colour subcarrier converter or some other device while anNTSCsignal 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-linePALand 525-lineNTSCprograms. 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 anNTSCsignal, these pulses are 16.66ms apart, corresponding to the 60Hz field rate, while for aPALsignal 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,NTSCsync pulses arriving every 16.66ms will keep its Q output, at pin 13, high.


Figure 1 

HoweverPALsync pulses arriving every 20ms will allow the Q output to go low after 17.9ms, before being triggered high again 2.1ms later. Thus anNTSCsignal will give a constant high output while aPALsignal 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 anNTSCsignal, IC2b is not triggered and its Q output remains low.


However, the pulse train from aPALsignal 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 anNTSCsignal gives a high output, whilePALgives low. For the particular application for which the circuit was developed, diode D1 and the resistor network shown drive the base of anNPNswitching transistor and relay. A dual-colour 3-leadLEDcan also be fitted to indicateNTSC(red) orPAL(green). Note that with no video input, the output signal is high and will indicateNTSC.



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