Grass Valley 2200 - Manual (Page 99)

5-9
Tally
Tally
The switcher subscribes for PID_VIDEO_ON_AIR and PID_KEY_ON_AIR
information from the DPM. The DPM must compute whether a channelÕs image is
on screen or not and update these subscriptions when necessary.
NOTE: The parameter ID used for on screen information is the same as the one
used to communicate on air information from the switcher to the DPM. This can
easily lead to confusion since the same PID is used for different meanings.
In determining on/off screen, the following information should take into
consideration:
¥ location relative to the output raster of the DPM (e.g., off screen)
¥ image size (e.g., reduced to zero size)
¥ rotation (e.g., exactly on edge)
¥ transparency
¥ any other processing or transform which may affect visibility
Additional factors include actual use of the incoming signal.
If the video signal is replaced by a matte Þll, its incoming video is not on screen.
If the key input is not used it is not on screen.
If instead, the video signal is used in a self key, then the video is on screen even if
the video is a matte.
Other factors may exist depending on the functionality of the DPM.
In addition, the DPM may subscribe to the DPMÕs on air status. This information
tells the DPM whether or not the switcher has the DPM video or key on air. The
DPM should
not incorporate this information in the on screen status it sends to the
switcher.
Activity Check
The activity check provides a mechanism for the switcher to detect a link failure
(disconnected cable, etc.) or a patching/routing a different DPM to the switcher.
The switcher periodically tests for link activity by sending out a Subscribe
Immediate command for the DPMÕs ethernet address. If no response is received,
the switcher notes the time-out. When a response is received after a timeout, the
switcher assumes that subscription issues or other updates could have been
missed and reestablishes communications as if the switcher had been reset.
If link activity is present, the switcher looks at the value returned from the DPM. If
it is different than the last response, the switcher assumes a different DPM is now
connected and reestablishes communications as if it had been reset.
The value which the DPM returns is unimportant. However, if multiple DPMs
exist in a facility it is a good idea to report different values from each one to allow
the switcher to detect a change in DPM connections.