RGB to S-video/composite using AD-725
- From: NoSp <no-spam2@xxxxxxxxx>
- Date: Thu, 20 Sep 2007 05:18:32 -0700
I need to convert an RGB signal to S-video and composite. Apparently
the Analog Devices AD-725 (http://www.analog.com/UploadedFiles/
Data_Sheets/AD725.pdf) is ideal for this.
There's a schematic in that datasheet which I've extracted as an image
file here:
http://img530.imageshack.us/img530/8/ad725vh7.png
but as a novice there are a few things I don't quite understand:
1) there are pins for digital and analog power supplies. Do I really
need two different power supplies for this, or can I simply connect
pin 4 (analog +5V) and pin 14 (digital +5V) together to a +5V supply,
then pin 2 (analog GND) and pin 13 (digital GND) together to GND of
the power supply?
2) I don't quite understand the oscillator on pin 4. I need a PAL
signal, but I'm confused about the "OSC" as it looks like a simplified
symbol rather than a component.
Do I simply get hold of a 17.734475 MHz oscillator as in one of those
metallic square thingies with 4 pins, or is the schematic referring to
a more complex circuit you have to figure out yourself?
3) Regarding the luma-trap which is supposedly going to improve the
signal, at the end of the circuitry connected to pin 1 (encoding
standard) and pin 12 (luminance trap filter) it says "NTSC/PAL" at the
end. Where do I connect this?
4) I can't see any reason for power down the circuitry in my
application, thus pin 5 (chip enable) isn't needed. However, the
datasheet says that a logic high (TTL) enables the encoding function
while a logic low powers down the chip when not in use. Should I just
connect this to the +5V line from the power supply?
On second thought, I see that it says "..powers down the chip *when
not in use*". Is there some sort of detection circuitry which senses
when a TV etc. is connected to the S-video or composite outputs?
5) my application will "tap" the RGB and sync signals several places.
In other words, in addition to the S-video/composite circuitry the
AD725 will provide I will also be able to connect an RGB monitor, TV
and (S)VGA monitor to it. I suppose I just can't simply tap into the
RGB output of the computer for all these devices, but need to buffer
or recondition the signals first, right?
I believe this is what the AD-8073 (http://www.analog.com/
UploadedFiles/Data_Sheets/AD8072_8073.pdf) is for as it shows an RGB
monitor connected to its outputs:
a) can I use several AD8073 circuits (copying the same connection as
shown in the schematic) connected to the RGB source, then use those
different outputs for all the various display devices mentioned above,
or do I need a different circuit for this?
b) next to the RGB monitor in the schematic it says "from VGA port"
referring to the VSYNC and HSYNC signals. Is this simply the output of
the "VGA connector" on the left in the schematic, or is it via some
buffering circuitry etc?
c) does it make any difference if I connect the AD725 to the outputs
of the AD8073, and the RGB monitor directly to VGA connector, or do I
need to follow the schematic and connect the AD8073 directly to the
VGA connector and have the RGB monitor go to the outputs of the
AD8073?
I guess this question relates a lot to questions 5 and 5a.
Thanks :-)
.
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