GUIDE TO INSTALLATION AND OPERATION
LGK-3901/DSK-3901 |
17
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Loss of timecode
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Loss of closed captions
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Errors in embedded audio
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Errors in Dolby E streams
An SDI data analyzer is needed to monitor the timing of the SDI input sources and the outputs with respect to the
facility reference.
Clean switching between the inputs is made possible by buffering the inputs in-line FIFOs to co-time them before
processing. The reference timing will adjust the active depth of the FIFOs to accommodate small timing differences
between the input signals.
The shortest video processing delay from any input through the LGK-3901/DSK-3901 to the PGM Output occurs when
the input line FIFO depth is at its minimum. By increasing the output reference timing with respect to PGM IN the
FIFO depth is lengthened up to its maximum of 1 line.
4.4.3 HD-SDI Example (1080i)
Timing is dependent on the relative phase between the SDI input and the reference. The following example illustrates
how to set up HD-SDI signals with a tri-level sync reference to achieve the minimum delay through the unit:
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Assume that the HD-SDI input arrives 3µs later than the associated tri level sync (+0 line +3µs)
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Assume that the minimum processing delay is about 1.3µs for HD-SDI standards
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As PGM IN is 3µs later than the tri-level sync and PGM OUT will be at least 1.3µs later than PGM IN, the
minimum starting point to set the reference timing to is 1.3µs + (0 line +3µs) = 0 line +4.3 µs
The reference timing must now be increased so that some data is buffered in the line FIFOs. In the example above
the line FIFO is now almost empty. This is a very undesirable timing point as the FIFO is on the threshold of going
from empty to full if the input timing changes slightly, resulting in a 1 line vertical shift. Furthermore, the whole video
picture may show corruption precisely at the FIFO empty point.
Adding half a video line to the above reference timing will provide enough buffering to accommodate the largest timing
differences when the input is switched to a source that is not correctly co-timed. Co-timed inputs will then have
approximately half of a line buffered in their FIFOs. Any sources that arrive earlier or later will increase or reduce the
FIFO depth respectively.
Line lengths of SDI standards are shown in the following table:
Video Standard
Line Length
Samples
525i / 59.94 Hz
63.5 µs
1716
625i / 50 Hz
64.0 µs
1728
1920 x 1080i / 59.94 Hz
29.6 µs
2200
1920 x 1080i / 50 Hz
35.5 µs
2640
1280 x 720 / 59.94 Hz
22.2 µs
1650
1280 x 720 / 50 Hz
26.7 µs
1980
1920 x 1080p / 59.94 Hz
14.8 µs
2200
1920 x 1080p / 50 Hz
17.7 µs
2640
USO
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