Message timing – Grass Valley Switcher Products User Manual
Page 107
Switcher Products — Protocols Manual
107
Message Timing
The interface is not designed as a pure controlling device/controlled device
relationship; both the switcher and DPM have responsibility for part of the
communications and function more as peers: The switcher forwards source
selection button presses to the DPM and orchestrates running effects; the
DPM sends video, key, and front/back information to the switcher to
control aux buses.
This protocol is derived from one used for internal communications
between the Grass Valley Group Krystal™ Digital Picture Manipulator
control panel and effect manager via an ethernet connection. Much of the
message structure is to support communications between multiple panels
and multiple effect managers but was retained for consistency. The Krystal
panel to effect manager communications is not described in this document.
The format of messages is described in Message Format
Message specifics are described in Parameters
Data communications specifics are described in Transmission Media and Pro-
tocols
Communications examples are contained in Examples
.
Message Timing
The timing of some messages is critical in order for the switcher and DPM
to perform in lock step. These are considered real time messages.
The run control messages sent by the switcher to the DPM must be acted on
in a fixed number of video fields from when they are sent by the switcher.
A delay adjustment is provided to compensate for variations in devices.
Source changes sent by the DPM to the switcher will be processed so that
the video will change at the beginning of the next field.
All other messages do not have a deterministic requirement and are pro-
cessed differently within the switcher.
Transmission of real time messages should be given priority over the non-
deterministic ones. The maximum packet size was chosen to allow for
interleaving high priority messages when several low priority messages are
awaiting transmission.
Note
The last packet containing real time messages sent by the DPM in a field must
complete transmission 3 milliseconds before the end of the field. (13.6 mS
from the start of the field for 525/60 or 22 mS for 625/50.) No real time mes-
sages should be sent to the switcher during this end-of-field period. Trans-
missions during this time can disturb critical processes within the switcher
and cause the program video output of the switcher to glitch randomly.