5700ACO
Automatic Changeover
Revision 1.0 Page 25
4. TECHNICAL DESCRIPTION
4.1. AUTO CHANGEOVER SWITCH CRITERIA
Understanding how the ACO makes its changeover switch decisions is vital to designing, monitoring,
and troubleshooting a synchronizing signal distribution system. This section provides information on
how the ACO determines when it is time to switch to the other input.
All inputs, except for analog audio, have signal presence detectors that determine if there is a valid
signal present on that input. The detection logic depends on the type of input signal. For instance
detecting SDI video is different than detecting time code. The results of all of the signal detectors are
fed to one location where the changeover logic is applied.
Some signals may not be “mission critical” but may be included in the sync distribution system. These
signals will be switched during a changeover but will not be a part of the decision making process. The
presence detection of these signals may be excluded from the decision making process by removing
them from the “voting pool” through VistaLINK
®
configuration, or using the com port.
The selected A side and B side signal present indicators are analyzed as follows. For this example,
assume the A side is the currently selected source:
1. If all the present inputs of the A side are also present on the B side, then all is considered
stable. Note that even if the ACO is configured to monitor more inputs than what is present, the
logic performs a comparison of what is currently present on the A side vs. B.
2. Will any signals be lost if switched? If yes, do not switch. There is no point in switching and/or
trading one problem for another.
3. Will any signals be gained if switched? If yes, switch. Note that this criteria covers the case
when the A side loses a signal or the B side gains a signal.
4. Once the changeover is complete, the voting logic is now applied to the B side as the selected
source.
5. Priority may be applied to one of the sides. In this case, if there is the same number of inputs
present on both the A side and the B side, then the ACO will change over to the higher priority
side.
4.2. CHANGE LIMITING OPERATION
Depending on the reference system architecture and the desired exception handling operating
procedures, you may want the ACO to limit the number of switch changes to minimize system
reference disturbances and give someone time to diagnose the problem. When a “change limiting”
condition is detected, all further changeovers are suspended and the Auto LED will flash to indicate a
changeover event. Once the problem has been addressed, re-arm the auto changeover logic by
moving the CHANGEOVER MODE switch to Manual mode and back to Auto mode. Modifying the
“change limiting” mode will also re-arm the ACO. An armed situation is indicated when the Auto LED is
on solid.
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