Video test patterns are used to verify the proper operation of video equipment. In this post, we’ll be covering using the most common patterns for SDI verification.
SDI Pathological Pattern
SMPTE recommended practice RP 198-1998 defines a special pattern designed to generate the HD-SDI pathological waveforms. SDI Pathological signal is a special information code to test SDI performance. There are two different pathological waveforms. One pattern stresses the HD-SDI receiver’s cable equalizer and the other stresses the receiver’s PLL. The two pathological waveforms are shown in Figure 1:
The equalizer pathological waveform is poorly DC balanced. It consists of a single High bit followed by 19 Low bits. This basic pattern is repeated continuously across a video line. Note that the opposite polarity of this waveform, a single Low bit followed by 19 High bits, is also possible and both polarities are generated by the RP 198-1998 check field. The PLL pathological waveform is a square wave with 20 Low bits followed by 20 High bits. This basic pattern is repeated continuously across a video line. This pattern is much lower in frequency than the typical bit patterns seen by the receiver’s PLL. Therefore, poorly designed PLLs may not stay locked during this low-frequency waveform.
It is important to distinguish between the RP 198-1998 check field and the pathological waveforms. The pathological waveforms are serial bit patterns that are output by an HD-SDI transmitter after encoding and serialization. The check field is a video pattern that will when encoded by an HD-SDI transmitter, produce the pathological waveforms.
The RP 198-1998 check field is shown in Figure 2. In the top half of each field or frame, the check field consists of repeatedly sending a luma value of 300h and a chroma value of 198h into the encoder to cause it to occasionally generate the equalizer pathological waveform.
These values can be swapped with the chroma channel set to 300h and the luma channel set to 198h; the order does not matter. While encoding half a field or frame of this pattern, the equalizer pathological pattern will usually be produced on several video lines in the frame. To ensure that both polarities of the equalizer pathological waveform are produced by the encoder, the 198h value in the first sample of the first active picture line in the first field of every other frame is changed to a value of 190h. For progressive scan formats, the sample that is changed is the first sample of the first active line of every other frame. It is important to take this into account when testing SDI receivers, as you usually want to compare the received signal with an internally generated pattern. If both signals are not synchronized, reception errors will be generated.
The bottom half of each field or frame in the check field consists of the pattern 200h, 110h. As with the equalizer pattern, it does not matter which value is assigned to which channel. This sequence causes the HD-SDI encoder to produce the PLL pathological waveform during several lines of the frame (on average).
Color Bar Patterns:
SMPTE recommended practice RP 219-2002 specifies a color bar pattern that is compatible with both standard and high-definition equipment. It is similar to the older standard-definition SMPTE EG-1 color bar pattern but has been updated to include a 16:9 aspect ratio pattern and some new features. The SMPTE RP 219-2002 document defines the sizes of the various areas in the color bar pattern and explicitly specifies the digital luma and chroma values for each of the colors. One of the features of this color bar pattern is the Y-Ramp pattern. This pattern ramps linearly from 0% luma (black) to 100% luma (white) from left to right across the pattern. The following figure shows the 16:9 aspect ratio version of this pattern.
The 4:3 aspect ratio version leaves off the two columns (40% gray in the top portion) on either side of the pattern.
The top row of SMPTE RP 219-2002 is a 75% color bar pattern. Sometimes it is useful to remove the lower portions of the SMPTE RP 219-2002 pattern and have the 75% color bars pattern occupying the full height of the frame. In particular, the SMPTE 292M HD-SDI spec requires that the output jitter of an HD-SDI transmitter should only be measured with 75% color bars. Please note that for some generators such as Phabrix Sx Series, the change between columns is not totally clean, as they include a blurring between the different columns, so that there are no abrupt color transitions.
Xilinx has implemented an IP core for generating video patterns that includes these signals. At AI VIDEX, we have used this code to create pattern generators based on FPGA to verify both SDI transmitters and receivers on several occasions, so we recommend using it for the verification of SDI interfaces.