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 NXP Semiconductors


  • NTSC-PAL TV Signal Identifier

  • Created: Mar 06, 2014

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Summary



This circuit identifies whether the input is either a Phase Alternating Line (PAL) video signal standard or National Television System Committee (NTSC) video signal standard. The output will be set to HIGH for an NTSC signal and LOW for the PAL signal. This output signal can be used to automatically switch a color subcarrier converter or some other devices while an NTSC or PAL signal is being received. This circuit is suitable for reception of “free-to-air” TV signals, which may contain a mixture of 625-line PAL, and 525-line NTSC programs from satellites.

The TV signal acts as the input for the circuit. The LM1881 IC is a video sync separator that takes the video input signal and generates vertical synchronization pulses. For NTSC signals, sync pulses are 16.66ms apart, corresponding to the 60Hz field rate, while PAL signals have sync pulses that are 20ms apart, corresponding to the 50Hz field rate. The vertical sync pulses are fed into the first monostable vibrator (IC2A) of the 74AHC123ABQ IC. The IC2A has a period very close to 17.9ms set by the 200kΩ resistor and the 0.22µF capacitor. Since the IC2A is retriggerable, NTSC sync pulses arriving at every 16.66ms will keep its Q output at HIGH.

PAL sync signals arriving every 20ms will allow the Q output to go low after 17.9ms, before being triggered HIGH again after 2.1ms later. The 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 the IC2A is fed to the inverting input of the second monostable vibrator (IC2B) of the 74AHC123ABQ IC. The NTSC signal will provide an input that is constantly HIGH for the IC2B; the IC2B will not be triggered and its Q output will remain LOW.

The pulse train from a PAL signal will constantly retrigger the IC2B that causes the Q output to remain HIGH. The period of IC2B, which is about 0.5s, makes it a low pass filter that removes spurious switching due to any input glitches. The final output is derived from the inverted output of IC2B so that the NTSC signal gives HIGH output while PAL gives LOW. The circuit also has LED indicators to display which of the signals are present: red for NTSC and green for PAL.

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