[INTERFACES]
Build your Own
RGB to Component Transcoder
Tip from Jon Rhees: Jon successfully built
and has been using his own transcoder before any commercial products have been
available. So, if you are handy with the tools and understand
schematics/electronics, then you can probably build one without have to shell out over
$100 for one. Below is Jon's notes from a few years ago. You may want to contact him directly to see if he has since made any
improvements/changes to the design.
"Hello all. I've promised for some time to post my schematic for the VGA->YPrPb
converter I built and have been using for the past 6 months on my HTPC. Finally, with some
time off from work, I got around to drawing up the circuit! (hopefully, Mark R. can post
the schem.)
Here's what it does:
This circuit is designed to convert 60Hz VGA (480p) from a computer's VGA port and drive
progressive-component outputs to an HDTV (or similar display device). In my case, I have
been driving a 56" Panasonic HDTV since May '99 without a hitch. The circuit converts
the RGB colorspace into YUV and translates the HV syncs to the Y signal.
I have *not* made provisions in the circuit for interlaced (1080i), since I'm partly
too cheap and partly too busy to go and buy a G400 which can do the appropriate timing.
Depending on the display device, 1080i *may* work without modification, and more likely
540p will (my set doesn't process interlace timing any differently, but simply detects the
increase in scan rate).
The schematic does not show the power supply, but basically a split supply capable of
about 100mA should do fine. In my case, I use a 'wall' transformer which gave me 9VAC and
generated split supplies using 78L05 and 79L05 regulators.
When finished, I built my circuit into a small case, which has a 1' VGA cord coming out
of it, and 3 RCA jacks to provide the YPrPb.
Yes, it works with 480p and all x480p's. The converter does not distinguish those sorts
of things, but the resolutions supported are a function of the display device. Since
progressive component is primarily designed for HDTV, the supported resolutions (480p,
720p, and 1080i) are the expected formats. The horizontal resolution has nothing to do
with the converter, and the display device doesn't care either -- it is simply a matter of
the 'bandwidth' of the display. For example, I can run a display at 640x480 @ 60Hz @ 31.25
KHz, or 1920x480 @ 60Hz @ 31.25KHz. Note that the scan timings did *not* change, and
therefore the horizontal resolution is a measure of the number of analog pixels smashed
into a single line. The design I am posting has a max bandwidth of 100MHz, and more
realistically, is around 30MHz, depending on construction details. This should be more
than enough for horizontal resolutions past 1024 pixels.
>>> Click here for the Schematic. <<<
Oops! I've spotted my first error on the schematic: Please note that the resistor
coming from the output of the 'Y' op-amp driving out to 'Y' should be 75 ohm, not 1.0K.
As far as 720p, the circuit should work as-is. My Panasonic is the first-generattion
set which doesn't do native 720p, so I can't try it.
If needed, I can supply a power-supply circuit like the one I am using for this, or
someone else can come up with a circuit.
For those thinking of designing a PCB (no, I never designed one):
Take care to keep the design as tight as possible (the op-amp/resistor-section at least).
These are high-frequency signals and the tighter the layout, the better the overall
frequency response. In my case, I ended up using SMT components and mounted the resistors
directly underneath (on back side) the amps.
Some things to improve:
1) As you can see, the SYNC generation circuit is VERY rough. Basically, it worked for me,
so I stopped there. In redesign, I would use one-shots for the sync timing, and possibly
make the circuit auto-adapt to positive and negative syncs. I would also add an extra set
of one-shots with trim-pots to allow rudimentary image centering adjustments. To do this
right, though, I would advise getting hold of the IEEE specs for the component standards
(which I do not have) to get the details just right.
2) I would use a better op-amp. There are several out there which claim better
bandwidth and are better suited to drive the 75 ohm output down a length of cable. In my
case, I am driving about 12' without problem."
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