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APPENDIX B - Troubleshooting
Problem: The video image of the ALIF receiver shows horizontal lines across
the screen.
This issue is known as Blinding because the resulting video image looks as though you're
viewing it through a venetian blind.
When video is transmitted by ALIF units, the various lines of each screen are divided up
and transmitted as separate data packets. If the reception of those packets is disturbed,
then blinding is caused. The lines are displayed in place of the missing video data packets.
There are several possible causes for the loss of data packets:
• Incorrect switch configuration. The problem could be caused by multicast flooding,
which causes unnecessary network traffic. This is what IGMP snooping is designed to
combat, however, there can be numerous causes of the flooding.
• Speed/memory bandwidth issues within one or more switches. The speed and
capabilities of different switch models varies greatly. If a switch cannot maintain pace
with the quantity of data being sent through it, then it will inevitably start dropping
packets.
• One or more ALIF units may be outputting Jumbo frames due to the video resolution
(2048 horizontal pixels) being used. If jumbo frames are output by an ALIF unit, but
the network switches have not been configured to use jumbo frames, the switches
will attempt to break the large packets down into standard packets. This process
introduces a certain latency and could be a cause for dropped packets.
• One or more ALIF units may be using an old firmware version. Firmware versions
prior to v2.1 exhibited an issue with the timing of IGMP join and leave commands that
caused multicast flooding in certain configurations.
Remedies:
• Ensure that
IGMP snooping
is enabled on all switches within the subnet.
• Where each ALIF unit is connected as the sole device on a port connection to
a switch, enable
IGMP Fast-Leave (aka Immediate Leave)
processing on each switch.
• Check the video resolution(s) being fed into the ALIF transmitters. If resolutions using
2048 horizontal pixels are unavoidable then ensure that
all switches.
• Check the
forwarding mode
on the switches. If Store and forward is being used, try
selecting Cut-through as this mode causes reduced latency on lesser switch designs.
• Ensure that one device within the subnet is correctly configured as an
usually a multicast router.
• Ensure that the firmware in every ALIF unit is version 2.1 or greater.
• Try adjusting the transmitter settings on each ALIF to make the output data stream as
efficient as possible. See
Alter ALIF transmitter video settings if necessary
Problem: The audio output of the ALIF receiver sounds like a scratched re-
cord.
This issue is called Audio crackle and is a symptom of the same problem that produces
blinding (see left). The issue is related to missing data packets.
Remedies:
As per blinding discussed previously.
to reduce unnecessary
Jumbo frames
are enabled on
IGMP
Querier,
for details.
continued
46
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