PDF1. Video Signal DC Restoration
NI 5122/5124 digitizers support DC restoration, which adjusts the back porch level of the video signal to 0 V. DC restore is performed on the digital data, rather than with an analog clamping circuit. When using DC restore, set the input voltage range to approximately twice the expected signal amplitude to prevent signal clipping as the amplitude and offset of the video signal change. DC restore measures the voltage of the back porch during the interval between the end of the color burst and before the start of the active line, then subtracts this value from the signal for the remainder of the active video line. Excess noise on the back porch portion of the video signal may cause inconsistent level restoration between lines.
2. Signal Conditioning
To provide proper termination to the video signal, use a 75 Ω terminator on the input of the digitizer in 1 MΩ input mode.
3. Copy Protection System Implications
Many video signal sources, particularly DVD players, produce a copy protection signal. DVDs can be encoded to enable this function, which inhibits copying the signal by injecting pulses that violate video signal standards, and disrupts input circuitry on VCRs. These signals often include false synchronization pulses and signals that exceed the voltage range of a standard signal. Any false pulses may cause erroneous line triggering on the NI 5122/5124. To avoid this error in your application, try one of the following solutions:
- Deactivate or remove the copy protection signal from the signal source.
- Recover triggering information in software by taking an untriggered waveform with the NI 5122/5124 and searching for valid trigger points in the waveform.
Related Links:
Fundamentals of High-Speed Digitizers
Video Signal Fundamentals
