Hd example (1080i), Features – Grass Valley Imagestore 750 v.3.0.1 User Manual

Page 40

Features

Reference

Clean switching between the inputs is made possible by buffering the input FIFOs to co-time
them before processing. The reference timing adjusts the active depth of the FIFOs to accom-
modate small timing differences between the input signals.

The shortest video processing delay from any input through the Imagestore 750 to the program
output occurs when the input 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.



To be valid, the input timing value must be negative. If the value is not negative, the refer-
ence offset must be adjusted so that the input timing value is negative.

HD 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 signals with a tri-level sync to achieve the
minimum delay through the unit:

•

Assume that the HD input arrives 3

μs later than the associated tri-level sync (+0 line +3

•

Assume that the minimum processing delay is about 2.6

for HD.

Example: Because PGM IN is 3

later than the tri-level sync and PGM OUT is at least 2.6

later than PGM IN, the minimum starting point at which to set the reference timing is (0 line
+3

) + 2.6

= 0 line + 5.6

The reference timing must now be increased so that some data is buffered in the line FIFOs. In
this example, the line FIFO is almost empty. This is a very undesirable timing point because 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 provides enough buffering to accommo-
date 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 a line buffered in their FIFOs. Any
sources that arrive earlier or later will increase or reduce the FIFO depth respectively.

The following table shows line lengths of the SDI standards:

Half a line at 1080i / 59.94 is approximately 15

. To buffer half a line of 1080i / 59.94, increase

the reference timing (in the example) to 0 line + 5.6

(+ 15

) = 0 line + 20.6



Note: If the path delays of the SDI input sources are such that they are all offset from refer-
ence by several lines then adjust the reference by the same number of lines to align the
output to the unusual input timing. Then add 10 to 20 microseconds to give some buffering
in the line FIFOs.

Video Standard

Line Length

Samples

525i / 59.94

63.5

1716

625i / 50

64.0

1728

1920 x 1080i / 59.94

29.6

2200

1920 x 1080i / 50

35.5

2640

1280 x 720p / 59.94

22.2

1650

1280 x 720p / 50

26.7

1980

Table 4-3: Video Line Duration