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RE: On CRT-type camcorder viewfinders and HMDs

Roberto,

> -----Original Message-----
> From:	Roberto C. [SMTP:]
> Sent:	Saturday, January 06, 2001 9:20 AM
> To:	
> Subject:	On CRT-type camcorder viewfinders and HMDs
> 
> 
> Greetings all:
> 
> I happen to have the guts of an RCA CG690 VHS-C camcorder, and I'm certain
> that the 'corder's CRT-type B&W viewfinder still works.  I'm inclinded to
> put the thing to use as an HMD, but I was wondering if any here has
> experience in adapting CRT viewfinders and could answer a few questions:
> 
> 1.  What few sources I've found to date that mention CRT adaptation state
> that CRT resolution generally exceeds that of most LCDs.  On that note,
> how close do they get to VGA-level quality?
> 
It seems to depend upon the CRT tube, associated circuitry and your scan
convertor.  I suspect that almost any B&W tube is capable of resolution well
beyond 640x480 but that the signal gets degraded by poor driver circuitry,
long wires, crosstalk, cheap scan convertors etc.  My v/f HMD gives an image
comparable with or better than an M1.  This is despite having come from a
camcorder which fell two stories and being connected via a cheap scan
convertor.

> 2.  any special considerations I need to keep in mind while adapting a CRT
> viewfinder?  High voltages and the glass bodies of CRTs immediately come
> to mind...need I keep an eye out on their power consumption with CRTs and
> associated circuitry?
> 
I don't know how much bad weather you're likely to encounter but I made very
sure that my v/f is well sealed against water.  After adjusting it for the
clearest text I could manage, I tried adding coloured filters between the
CRT display and the lens.  This seemed to make the image clearer still -
especially red or blue filters.  I suspect this is because the manufacturer
saved costs by using cheap optics which have slightly different focal
lengths for different frequencies of light.  The downside is that you may
have to boost the brightness of the CRT which tends to "fuzz" the image
anyway.  

Re power consumption, I've never measured it but using the v/f seems to make
little difference in battery life compared with driving a desktop monitor so
I suspect it draws only a few percent of what my SBC does.

> 3.  I suspect that I'll have to add additional circuitry to adapt VGA
> output of a video card to something that can be handled by the
> viewfinder.  Any suggestions on circuits that can allow the CRT to
> handle VGA?  
> 
While you could build a scan convertor from components, it is probably going
to be easier and cheaper to buy.  I use a small (1/4 ISA) cheap board
designed to allow PC games to be shown on TV.

There are also some schematics at
http://www.hut.fi/Misc/Electronics/circuits/vga2tv/composite_adapter.html
for quick 'n' dirty adaptors but you may have to tweak your SGATextMode or
modeline settings with these.

> 4.  I think that the CRT in the CG690 is designated as M01KKX07WB01
> [according to schematics]...anyone have experience in getting this one
> to do HMD work, or comments about its image quality?
> 
> 5.  If the CG690's CRT is not HMD-suitable for some reason, any
> suggestions as to what other CRTs I could try that are known to be usable
> as HMDs?
> 
> Thanks for your time.
> 
> Sincerely,
> RCamama
> Austin, TX
> 
Philip

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