Sierra Video Pro XL Series 8 - Manual (Page 55)

GENERIC PROTOCOL
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“F”: Field Delay
The command "F" is used to specify the delay between the time a crosspoint change request is
received by the router and the time the crosspoint switch actually occurs. It must be followed by a
number giving the number of video fields of delay desired. If the number is smaller than the
smallest delay that the router can handle, the smallest delay is used instead. If it is larger than the
largest delay the router can handle, the largest delay is used instead. Note that this command
does not cause a delay in command processing, as the “D” command does.
To understand this command more fully, consider the way that router software will typically
handle a crosspoint command. The last character of the command string, the final “!”
(exclamation) character, is received somewhere in a particular video field, call it video field 7. The
router parses the command string and, for each crosspoint it contains, it puts the crosspoint in a
buffer that is marked to be delivered to the crosspoint hardware on a particular video field.
Suppose that previously, an “F5” command has been received. Then crosspoint commands
whose final “!” command string character was received on video field 7 would be placed in a
buffer that is marked to be delivered to the crosspoint hardware at video field 13 (7+5+1=13).
To understand the reason for adding 1 in the previous sum, consider an “F0” command: it would
ask for output at the very next video field, field 8 in our case. So, it is necessary to add the “F”
argument plus 1 to the field number on which the crosspoint command is received to get the field
number at which the crosspoint will be output.
A typical router will have a minimum delay that is between 1 and 2 fields. Suppose a crosspoint
command is received just before a vertical field mark. The software may be able to prepare the
crosspoint data and send it to the hardware when that vertical field mark occurs, but the hardware
itself typically has a one-field delay in it, so the soonest that such a crosspoint would switch would
be one field (plus a little) from when it was received. If the command were received towards the
beginning of a field rather than the end of a field, the delay would be closer to two fields. Industry
parlance is to call this a one-field delay, because only full fields of delay are counted.
The delayfields argument of this command takes into account the hardware delay. So, if a router
has a minimum delay, including the hardware delay, of one full field, as described in the previous
paragraph, then a delayfields value of 1 causes this minimum delay to be used. A delayfields
value of 0 will also cause this minimum delay, because the router uses its minimum if a smaller
value is specified. A delayfields value of 2, however, will add one more field to the minimum
possible delay. Thus, delayfields specifies the number of full fields of delay between end-of-
crosspoint-command-string-received and crosspoint-switch-occurs. Note that the actual minimum
value of delayfields depends on the particular router model.
Routers typically have a limit to the number of crosspoint commands they can process in one
field. First, there is an inherent delay in sending the command to the router, but beyond that, the
router requires time to parse the command and buffer up the crosspoint data, plus it requires time
to deliver the buffered data to the hardware when the desired video field arrives. Each individual
router has documentation to describe its limitations on how many crosspoints it can process in a
given amount of time.
Larger values for delayfields give the router more time to process commands. Although the long-
term average number of crosspoints that can be processed per unit of time is unchanged, a larger
delayfields
value can improve router performance during a short burst of many crosspoint
commands. For example, suppose a large number of crosspoint commands is sent to the router
in a single large command. If delayfields is small, the router typically wonít have time to parse and
process all these crosspoint commands and place the data in the crosspoint delivery buffer
before the target video field arrives. By making delayfields larger, the user can give the router
more time to process the crosspoint commands.