I am having a little trouble understanding how this circuit works. I
got this circuit from one of my microcontroller books in a chapter
describing how to use a PIC16C71 to generate a NTSC-compatible video
signal but I assume that any other microcontroller can use the
attached circuit.

This is out of the text:

"The I/0 line RA2 drives Q1, dropping the video line down to 0 volts
for horizontal and vertical sync. RA0 drives Q2, pulling the video
line down to 0.25 volts for the blanking level. If neither Q1 nor Q2
is on, the video line will be one volt (white video pixel). Resister
networks R3, R4, and R5 generate the proper voltage levels into a
75-ohm load."

I have three questions about this circuit and the text. I understand
why when Q1 is on the video line drops down to 0 volts.

- What I don't understand is then Q2 is on how is the video line is
0.25 Volts.

- Also, why when Q1 and Q2 is off the video line is at 1 volt.

- And my last question is R5 used for impedance matching?

I tried to work out several combinations and I didn't get the
indicated voltage levels described above. Thanks in advance for any
help.

If you have trouble opening the attached file, please visit
http://www.compiledsolutions.com/ntsc.pdf for the same schematic
online

On Mon, 12 Apr 2004 23:31:06 +0000, Rico Rivera wrote:

snip

- What I don't understand is then Q2 is on how is the video line is 0.25
Volts.

- Also, why when Q1 and Q2 is off the video line is at 1 volt.

- And my last question is R5 used for impedance matching?

I tried to work out several combinations and I didn't get the indicated
voltage levels described above. Thanks in advance for any help.



R5 is part of the series chain 220R+75R+external 75R which gives you about
1v when both transistors are off. Note that the external 75R isn't shown
on the drawing, but it forms a voltage divider with the internal 75R!

with Q2 on, you have 33R in parallel with (75R+75R), giving about 27R. The
output voltage, taken across the lower 75R (external) is then half of
0.546v, which is pretty close to 0.25v.

--
Cheers...
Mick
Gave up on viruses & trojans - moved to Linux... :-)
Nascom & Gemini info at http://www.nascom.info