Video Amp

JTC

· Has anyone added a video amp to the composit video output ?
I would like to run have the video monitor remote
form the propeller. If so what chip did you use?
Thank You
Jim

Phil Pilgrim (PhiPi)

You probably don't need a video amp to drive a remote monitor. A proper divider network on the Propeller outputs and 75-ohm coax (e.g. RG-59U) should do the trick for you. By a "proper" divider network, I mean one that provides a 2.0V P-P signal with a 75-ohm source impedance. When terminated at the other end by 75 ohms, the signal level will assume a 1V P-P level. The circuit illustrated below will do this for you. It uses 1% resistor values. The divider network suggested in the forum and provided with the Demo Board works fine with short cables, but the impedance mismatch (156 ohms) may cause noticeable ghosting due to reflections over long cable runs.

-Phil

lnielsen

Phil,

Could you expand upon your reply a little? What if I wanted to do 4 or 5 bit video or use a R-2R Ladder, what formula do I use to calculate the right resistors to do a good 75-ohm match and get the right voltage swing? I will be building my own video interface for the ProtoBoard and I want to make it better than the demo or HYDRA boards if possible.

Len

Luis Digital

See: http://cache.national.com/ds/LM/LMH6609.pdf
can be what you seek.

Page 12 (Figure 6. Typical Video Application)

JTC

Thanks Luis,
I ordered a sample and the eval board. I will let you know how it works.
Thanks
Jim

Phil Pilgrim (PhiPi)

Jim,

Unless you're planning to bit-bang the video, three bits is all the resolution you'll get from the Propeller's built-in video generator. And there's no advantage to using an R-2R network for three bits.

As to the formulae, here's what I started with, where R1, R2, and R3 are the DAC resistors, R4 is the resistor to ground, and x || y = 1 / (1/x + 1/y):

····R3 = 2 * R2····;Establish the DAC ratios...
····R2 = 2 * R1
····R1 || R2 || R3 || R4 = 75····;Establish the output impedance.
····3.3 * R4 / ((R1 || R2 || R3) + R4) = 2.0····;Establish the P-P voltage output from the divider.

These are four equations with four unknowns, which can be solved simultaneously to get the right values. Once that's done, the closest values in a table of 1% resistors can be selected. I did this several months ago, so I don't remember whether I solved these by hand or used Derive, which is a math program I often call into play for such tasks. Sometimes, if the complexity is too much for Derive, I'll write a program to solve equations like this numerically, using successive approximations.

Again, I really doubt you'll need a video amp to do what you want to do.

-Phil

Luis Digital

JTC said...
Thanks Luis,
I ordered a sample and the eval board. I will let you know how it works.
Thanks
Jim

I am going to order three from Digikey (8-SOIC), for another use.