IN THE DIGITAL GENERATION
Back in the ‘good old analog’ days, two video signals had to be aligned horizontally, vertically and color phased. This allows an image to be switched without a noticeable change in position or color. A black burst signal is used to time the entire system.
Horizontal alignment is based on a horizontal reference point. This is defined as the 50 % point of the leading (falling) edge of the horizontal sync signal (see Figure 1)
In switching digital signals similar requirements apply. The video must be aligned horizontally and vertically.
A standard definition digital data stream is divided into 1716 samples (words) per line of video. Synchronization is accomplished by allocating certain word groups for this purpose. These groups are known as the End of Active Video (EAV) and the Start of Active Video (SAV). These word groups occur on every line of video. In SD the ‘Horizontal Reference Point’ is represented by word 1473 (see Figure 2).
This provides for H-sync continuity during analog/digital and digital/analog transitions. Therefore the EAV and SAV become timing reference signals.
The EAV and SAV are displayed as a large pulse on an SDI or HDSDI scope (see Figure 3). There is usually a menu setting to display or not display the pulses.
These pulses can be used to align two digital video streams even if they are of different resolutions. Aligning the EAV/SAV insures that the video starts in the same place for each line.
This procedure uses the Leader LV5700 SD/HD scope, LT425D SD Signal Generator and the LT443d HD signal Generator. The SD output of the LT425D is connected to one input of the LV5700. The Black Burst output of the LT425D is connected to the reference-in on the LV5700. The HDSDI output of the LT442d is connected to the second input of the LV5700.
1) Set the LV5700 to view channel A (SDI source)
2) Set the LV5700 to External Reference.
3) Turn on Line Select and view line 50 in the active video area.
4) Set the scope to have the EAV and SAV displayed.
5) Position the trace to have the SAV visible.
6) Set the sweep to ‘Times Ten’.
7) Set the Cursors to ON and place the Reference Cursor (Blue Line) at the SAV pulse.
8) Switch the LV5700 to Channel B (HDSDI signal).
9) Select Line 50 in the active video area.
10) Place the Delta Cursor (Green Line) over the SAV pulse. This will show the timing difference between the two sources.
11) Adjust the horizontal timing of the LT443d to move the SAV under the Cursor Reference (Blue Line).
This method can be used to adjust or verify proper horizontal alignment of digital sources. Proper monitoring and adjustment of the signal path will insure the best quality possible.
Jim Alfonse, owner of Tri-Sys Designs, is a Systems Integrator with twenty-five years experience in the Broadcast Industry. He's designed, built and commissioned installations from Satellite News Vehicles to Production Suites to OB vans. Jim has been involved with several equipment manufacturers performing video standards compliance and signal integrity testing.
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