single pin video generated output

Peter Verkaik

I did some calculations on the following schematic to see if
video level signals could be generated with one pin.

+5V o--[noparse][[/noparse] R1 ]----+
pin o--[noparse][[/noparse] R2 ]----+· level
·················|
···············[noparse]/noparse] 75[color=white]·[/color
················ |
················ V

pin·· signal·· level
·L··· sync·····Vs
·I··· black··· Vb = 0.3 + Vs
·H··· white··· Vw = 1.0 + Vs

white:
···· 75

---

* 5 = 1.0 + Vs
75 + (R1//R2)

black:
··75

---

* 5 = 0.3 + Vs
75 + R1

sync:
·· (75//R2)

---

* 5 = Vs
(75//R2) + R1

If you work this out, the results are (exactly !):
R1 = 175 ohm
R2 = 210 ohm
Vs = 1.2V
Vb = 1.5V
Vw = 2.2V

Has anyone tried this, and does the 1.2V dc offset bother the video input
(I believe not because these are mostly ac coupled).

regards peter

T&E Engineer

Bean creates NTSC video with 2 pins for his Mere Mortal Video Game Driver (MMVGD) (and probably the SX Video Module too).

 HARDWARE CONNECTIONS:
'----------------------
'          1K
' RA.0----\/\/\----+
'          470     |
' RC.7----\/\/\----+---------------------------------------- Video Out
'
'          1K
' GND ----\/\/\----+
'          22K     |
' RA.1----\/\/\--------------------------------------------- Audio Out
'
' RB.0 --- JStk1Up Button --- GND
' RB.1 --- JStk1Down Button --- GND
' RB.2 --- JStk1Left Button --- GND
' RB.3 --- JStk1Right Button --- GND
' RA.2 --- JStk1Button Button --- GND
'
' RB.4 --- JStk2Up Button --- GND
' RB.5 --- JStk2Down Button --- GND
' RB.6 --- JStk2Left Button --- GND
' RB.7 --- JStk2Right Button --- GND
' RA.3 --- JStk2Button Button --- GND
 
 
' -------------------------------------------
' Video generation details
' -------------------------------------------
' NTSC:
'   Each line = 1/15734 seconds
'   3 lines of vsync
'   21 lines of blank lines (wait for retrace)
'   238 lines of visable video

http://forums.parallax.com/showthread.php?p=598779

www.sxvm.com

Peter Verkaik

I know that. I integrated this two-pin setup in some of
my own boards. Just wandering if someone did a one-pin
setup, as the timing code needs to be altered quite a bit due
to the fact that changing pin direction takes a lot more
cycles than changing pin output level.

regards peter

Phil Pilgrim (PhiPi)

Peter,

That should work. But I'd be more inclined to run it through a transistor driver. The reason is that, by relying on the pin tri-stating, your source impedance is constantly changing. This could have an impact with long cable runs. With a transistor driver, you can get a 75-ohm source impedance, without having to fiddle with the resistor divider values. Higher-valued divider resistors also put less stress on the microcontroller pin, and the transistor isolates it somewhat from transmission line issues.

Attached is a sample schematic. It's an emitter follower whose source impedance is determined largely by the collector resistor. This circuit will output a 1V p-p signal regardless of whether the monitor it's feeding is properly terminated. Due to the interposed B-E junction, the sync tips will be at 0.6V instead of 1.2V.

-Phil

James Newton

Peter, that is really interesting, but I'm a little confused...

What does the [noparse][[/noparse] 75 ] represent?



▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
---
James Newton, Host of SXList.com
james at sxlist,com 1-619-652-0593 fax:1-208-279-8767
SX FAQ / Code / Tutorials / Documentation:
http://www.sxlist.com Pick faster!

Chris Savage

James...should be ohms...I have noticed some people use R when the value is in ohms rather than not having any symbol.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Chris Savage
Parallax Tech Support

Peter Verkaik

James, it is indeed 75 ohms.

Phil, thanks for the driver schematic. It made me realise that
if a video input is indeed ac coupled, without the driver I
possibly would never get a stable·1.2V dc offset because the cap would
charge/discharge dependant on video frame content. The driver
does provide a stable 1.2V sync tip level at the transistor base.
(I have seen video inputs where the cap is the first component blocking all dc current,
although most video inputs·have a 75ohm resistor to gnd before the cap that allows dc current
to flow).

Still want to know if someone has done such a one-pin setup
and has some working code in either asm or sxb.

regards peter
·

Phil Pilgrim (PhiPi)

Peter,

Most devices that receive video signals have a DC-restorer (clamp) after their AC coupling stage, which firmly plants the sync tips at a known level. This is necessary for the very reason you mention: that the average DC level depends on the video content — but regardless of how its driven. The main benefit from the driver is presenting a constant impedance to the transmission line.

-Phil

Bean

I think it would be doable at 50Mhz.
20MHz if you could make a circuit that generated sync when the pin was an input. I don't know how you would do that without a negative voltage though.

Bean.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Did you know that 111,111,111 multiplied by 111,111,111 equals 12345678987654321 ?

www.iElectronicDesigns.com

·