Greetings,
I'm working on a video project and I need to reproduce a composite output that matches an existing device (the TMS-9918A if you want to know.) Let me try to be as brief as possible. I am a little confused by the original device's output, which according to the data sheet is:
Vwhite: 3.0V
Vblack: 2.3V
Vsync: 2.0V
Test conditions: RL = 470ohms
The datasheet also states:
"The TMS9918A composite video output pin (36), is driven by a source-follower MOS transistor that requires an external pull-down resistor to Vss. A 470-ohm resistor is typically used to provide a 1.9 volt peek-to-peek signal on the output. This output will drive most color directly, although in some cases it may be necessary to provide a simple interface circuit to match the monitor's input requirements."
This brings up a few questions. Why would the chip output 2.0 to 3.0 volts in to a 470-ohm load when standard composite video (from what I have found) is 0V to 1V into a 75-ohm load?
Also, the text says that the output into a 470-ohm load will be a 1.9 volt peek to peek that can drive a monitor directly!? This does not match the other specs in the datasheet that show the proper 1V difference, albeit 2V higher than standard, and into a different load.
I'm not looking for color output, but I'd like to do at least black, white, and gray, which I found some info for doing with two digital outputs and two resistors, like this:
Code:
5V-TTL [D1] ---(900 ohm)---+
|
5V-TTL [D0] ---(450 ohm)---+
|
---(75 ohm in monitor)---
|
V (gnd)
This site (
**broken link removed**) shows how this can drive a composite signal and provides the 4 basic voltages:
0V for sync with D1 and D0 at ground
.3V for black with D1 at ground and D0 at +5
.67V for gray with D1 at +5 and D0 at ground
1V for white with D1 and D0 at +5
This is great, except I'm using a 3.3V source (I found another site that used 600 and 300 ohm resistors instead), and my target load is 470 ohms instead of 75. I also need to throw a FET in there and produce voltages from 2.0 to 3.0. I guess?
My current design is something like this:
Code:
+5V
3V-LVTTL [D1] ---(900)---+ |
| |<+
3V-LVTTL [D0] ---(450)---+-------| (FET)
| |<+
(75-ohm) |
| ???
V (gnd)
Sorry for the lousy schematic. Basically I'm using the same input circuit as described on the site I found (above) to make the 0, .3, .67, and 1V signals, which I'm developing via the 75-ohm resistor, and using that as the input to the FET. However, how do I make the output of the FET swing from 2V to 3V with a 470-ohm load, when the gate goes from 0V to 1V?
This is all hypothetical at this point (nothing built or proven), and any insight would be greatly appreciated.
Thanks,
Matthew