The effect of jitter is displayed as faint lines before and after the ideal waveform on the eye waveform, as shown in Figure 3. The greater the amount of jitter, the higher the risk of incorrect data samples being made. The result of incorrect data samples on the program’s video and audio content may be dramatic because individual incorrect bit samples could corrupt whole data words or, even worse, the serial-parallel frame words that define each video line. The loss of individual video or audio samples may be acceptable, but the corruption of metadata may not be because it could affect everything downstream.
The eye pattern can be used to display a range of different jitter artifacts that can be isolated using filters. Jitter occurs when a signal varies in time or amplitude outside of the ideal waveform shape.
The jitter measurement is quantified as a proportion of the Unit Interval from -0.5 UI to +0.5 UI. The dashed lines in Figure 3 show where this would appear on the eye-pattern display. Testing needs to focus on the amount of Alignment Jitter and Timing Jitter. The worst-case combination is 0.5 UI, which is right on the sampling edge. The SMPTE 424M standard requires that 3G-SDI Alignment Jitter is < 0.3 UI and that Timing Jitter is < 2.0 UI. It can be difficult to make exact measurement of jitter directly from the eye pattern, so instruments that automatically measure jitter are especially useful.
To make sense of the displayed jitter, and to help identify the source of the jitter, it is useful to apply frequency range filters to isolate the different types of jitter that are present. Test equipment can provide a range of filter options that can be applied to the signal before measurement.
Jitter can be caused by a range of different sources, and the artifacts displayed in the eye waveform are typically due to combinations of these at different frequencies and amplitudes. Here are some examples:
- Transmission jitter caused by the equipment generating the video signal. This could include clock phase jitter, amplitude jitter and wander.
- Transfer jitter caused by repeated processing though equipment such as distribution amplifiers and cable equalizers that can introduce additional internal jitter to that already present.
- Receiver phase-lock loop errors in equipment decoding an SDI data stream.
- Data-dependent jitter due to data content that, even after randomization, contains sequences of zeros or ones long enough for the clock recovery/regeneration to drift or lose phase lock with respect to the data.
- Thermal and shot noise caused by the integrated circuits used within equipment that can cause random errors.
- Electromagnetic interference in long cable runs that may be susceptible to the power grid and power switching.
- Waveform distortion due to equipment within the broadcast chain, long cable runs, poorly terminated or un-equalized cables, poor frequency response of cables, or poor equipment return-loss.
Engineers and manufacturers alike face the challenge of dealing with the myriad of video formats that are now possible with 3G-SDI. Testing is needed to ensure that equipment is correctly designed and that all formats are correctly supported.