Video buffer required

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I'm trying to design a neat VGA selector that will allow one of two VGA
outputs to be driven from the one VDAC. I should be able to do it with a
couple of video buffers for the RGB signals but despite lots of trawling on
the Web, I can't find a suitable buffer device. Does anyone have any
experience of this kind of thing and if so, could you suggest a device that
could do the job please?

TIA,

Rog.



Re: Video buffer required
On Saturday, in article

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Search for "Field Memory" or "Frame Memory" or "Frame store"

Logic Devices were going to bring out a configurable frame store device.

If you find field memory remember it most likely is designed for interlaced
video and may need higher speed for non-interlaced VGA video stream, or
possibly doubled up. Most field memory chips are fairly cheap as they
are designed for VCRs and TVs.

Failing that high speed RAM and a PLD/FPGA to control the addressing and
dual porting.

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Re: Video buffer required
Try
http://para.maxim-ic.com/compare.asp?Fam=Vid_Mux_Buff&Tree=Video&HP=Video.cfm&ln =
Maybe more than you need, but something there should do the trick.

Robert

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Re: Video buffer required
Thanks for the suggestion but I've had a look at Maxim parts. The supply
voltage is a bit of a problem in that I only have a maximum of 3.3V
available. I've found a few muxes by other manufacturers (Intersil, AD) e.g.
a triple 2:1, but I need a 1:2 selector which don't seem to be around or
separate video buffers that can drive one output each and either select one
or have both running all the time. Again I haven't found a suitable one yet.
Thanks again.

Rog.

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http://para.maxim-ic.com/compare.asp?Fam=Vid_Mux_Buff&Tree=Video&HP=Video.cfm&ln =
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Re: Video buffer required
On Sunday, in article

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Last time I used any RGB muxes for 2:1 was a maxim part at +/-5V the
MAX463 which suited the application as the opamps were using the same rails
and got me better full swing on video signals for digitisation. Normally used
AD8052 op-amps for the video buffering.

I am not sure I understand the rest of your requirements about "both [outputs]
running all the time".

The MAX463 is a triple 2:1 video mux, last I used I got direct from
Maxim and Farnell.

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Re: Video buffer required
Thanks but a mux isn't what I need. I need either a selector or have 2 amps
running from the DAC and drive both outputs all the time.

Thanks,

Rog.

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http://para.maxim-ic.com/compare.asp?Fam=Vid_Mux_Buff&Tree=Video&HP=Video.cfm &
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Re: Video buffer required
On Sunday, in article

Please reply AFTER the trimmed quote as this is getting difficult to follow.
 
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If you have two outputs I would normally put two sets of amps in to
drive each as these types of monitors cannot be chained like normal video
monitors. This is assuming you want the SAME image on two monitors.

If you need to turn an output off or blank it, consider either using
a small crosspoint switch or extending clamping controls to blank signal.

Otherwise your choice is either use video amp chips (aka Maxim MAX496)
or your own amps. Watch the MIN output current on video drivers as these
assume a permanent DC path not always shown in datasheet example circuits.
Especially when no cable is connected.

Considering a 75R back terminated video feed should be driving at a max
of 2/150 = 13mA, anything requiring a minimum output current more than that
needs extra items on the output. Been caught by that one before.

As this is taking a VGA type output to (assumedly) drive two VGA type
monitors, get high bandwidth opamps, bearing in mind the dot clock
speed of the resolution you are driving at. For straight VGA (640 x 480
non-interlaced) anything over 100MHz bandwidth will do for short cable runs.

Unless you have long cables and have to drive and compensate for cables
opamps will do, needs some external resistors for gain and back termination
to do the job quite easily.

If you only have 0V and 3.3V as your rails ensure your video is offset from
0V, otherwise you will have problems. Remember also the video drive should
be 2V at the amp output stage before any back termination resistors.

Opamps like AD8052 work on 3.3V perfectly OK, and are speced on single 3.3V
rail as well.

I generally run video circuits where possible from +/-5V to allow for easy
ground referencing, clamping and allow for any amplification factor likely
to be required (one ADC required the input to be offset by about 1.4V and
have a 2V swing, from 1.4 to 3.4V).

If handheld/battery device you should put AC coupling on outputs. The norm
for video is mains driven equipment DC couple outputs and AC couple inputs,
to avoid too much drift and coupling effects.

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