How do I get rid of the horrible screen that flashes up when I switch inputs to my projector?

my projector puts up a big
blue(black/yellow/whatever) screen every time my switcher switches – can I stop
this happening as it looks very amateurish?

You are experiencing the BSOS (Blue screen of
sync)!

Maybe. Depends on projector, switch
and what you are switching.

The basic
problem is that the projector is loosing sync with the incoming video. This is
probably happening because you are using a mechanical switcher that is
monentarily disconnecting the signal while it switches over from one input to
the other – this is very common with consumer AV switchboxes. Another reason may
be that you are switching video sources that are not themselves synchronised,
that is they do not start each video frame at the same time. Unless the sources
are sync’d or ‘genlocked’ or have both been put through a timebase corrector
(TBC) this would be the case. The projector is expecting a sync pulse at a
certain time to start displaying each frame of video. If it starts missing a
few, it decides there may be no signal present and displays a helpful message to
this effect. Unfortunately if it displays this message to the 500 people in your
audience just as your speaker is getting to a serious point, then its not that
helpful.

There is really only one way
to ensure that this never happens, and that is to never stop sending video sync
pulses to the projector. This means that something has to keep sending them
even when you are switching.

This can be
either be a pro switcher that uses electronic switching rather than mechanical
switching, or a video mixer that outputs stable sync’d video regardless of what
is happening on the inputs.

There is
another way to avoid this, which is to see whether your projector can be set up
to NOT display this message when it loses sync – some projectors can turn this
option off (or just display a blank black screen instead) which is much
preferrable.

One other option, if your
switcher does switch OK without dropping sync, but the problem is that you are
needing to go to a blank screen and are switching to an unused input to achieve
this, then you need to have SOMETHING on that input driving sync pulses to keep
everything happy. It could be a computer scan converter or TV out displaying a
black screen, an old camera with its iris closed, a VCR playing a blacked tape
etc. etc.

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