Genesis RGB signals need 220uf or 470uf caps?

[Paper]

The CXA1145 data sheet lists 470uf caps whereas all the threads I can dig up here mention 220uf for the Genesis RGB cable.

Will it make a difference using one over the other?

[kendrick]

You may not like this answer... An oversimplified way to state it is that the resistors control the amount of color, and the capacitors control the brightness. It's more complicated than that really, but the short answer is the larger capacitors may give you a brighter picture overall. It's really up to you to judge which one is best for your particular TV or picture tube, so you should get a breadboard and build a test circuit so you can switch them out and see.

I use 220 caps myself, if you want to take a poll.

-KKC, watching the UK game groups get unexpectedly hostile all of a sudden.

[viletim!]

Kendrick,
That's not right. These components have nothing to do with bightness/colour adjustment. They are just part of the support circuitry that's reqired for the IC to output standard RGB video.

Paper,
The short answer is it doesn't really matter. And here's the long version. It goes on a bit but it might help demystify how video interconecting works for a few people.

The aim of this whole thing is to get the video from the transmitter to the receiver with minimal distortion along the way. Someone worked out that the best way to do it is to send the video down a length of 75ohm coax (which is cable with a sheith around it that has a 'defined impedance characteristic'). When sending a signal through the coax (or any transmission line in general) to obtain the maximum efficiency and minimum loss the source impedance, load impedance and transmission line impedance should all be matched. So that's how video interconection works and why we don't just use high impedance inputs like we do with audio signals.

To send video to a TV monitor we need a amplifier that has a output inpedance of 75 ohms and an output voltage of 0.7Vpp.

This is how the CXA (and most other chips) do it - they output the video signal at 1.4Vpp with a source impedance close to zero ohms. The video signal also has a volt or two of DC on the output which is there because it's not possible (or not practical) to output a signal really close to the negative (in this case ground) supply rail.

By adding a 75 ohm resistor in series with the output of the CXA then it will form a resistor devider with the 75 ohm termination resistor inside the TV/monitor, reducing the amplitude to 0.7vpp at the input to the TV as well as setting the source inpedance (point B on diagram) to 75 ohms before the transmission line (video cable).

But there's a small problem...there's DC voltage present at point A as well as the signal. Cc is there to block any DC voltage and only let through the AC signal. If it were left out then the DC voltage will flow from point A through Rz, the video cable, and Rl to ground. This wastes power and cause the CXA chip to get hot. Much more current will flow from the CXA's output that it's designed for which will likely degrade the performance of the video amp and distort the video signal.

And now to answer your question... The capacitor value is chosen to allow the passage of the minimum frequency in the video while not being any larger (physicaly) or more expensive than required (I suppose thats the way all coupling caps are chosen). The lowest frequency part of the signal is the field (vertical) blanking which happens 50 times a second (in PAL land). You can look up the formula to calculate Xc (capacitive reactance, that is the series resistance of a capacitor at a certain frequency) if you like. For a 220� cap @ 50hz Xc = ~14ohms and for a value of 470� Xc = ~7ohms. This Xc value is in series with Rz and distorts the signal by reducing the amplitude a bit (because it makes Rz look bigger).

In reality though, the video signal is clamped at every line (15.7 thousand times a second) so slight distortion of the vertical blanking isn't going to matter.

In short, the CXA1145 datasheet recomends 470�, the CXA1645 datasheet recomends 220� (there's no electrical difference between the two chips' outputs), and cheap video game SCART cables often use 100�. Take your pick, I seriously doubt you'll notice any difference between them. I usualy use 220� or whatever's lying around.

If you like you can experement with different values and try some different values, even deliberately tiny caps to see the distortion effects. From memory I think it looks like the brightness varies along the width of the screen (as in it looks ok on the left side and dark on the right). If you use small coupling caps with composite video you might also loose the sync.

Here's the circuit diagram:

Code Select Expand

.   A   (220�)  (75)           C
.  +@----Cc------Rz---@---TL---@+---Cin---->
.  |                  B         |
.  Zo (0)                  (75) Rl
.  |                            |
. ---                          ---
Point A is the output of the CXA chip and point C is the input of the television/monitor.

The cable lengths for most game consoles setups are pretty short, a meter or two usualy. So you can usualy get away with dodgy stuff like using unsheilded data cable for a transmission line and putting Cc and Rz on the TV/monitor end insted of on the source end.

[kendrick]

Cool Tim, thanks for the correction and the correct explanation. I've seen your website, you know your stuff backwards and forwards. :)

The only thing I'm slightly confused about is the substitution of potentiometers for the 75 ohm resistors for the purpose of controlling color output, like you find on the JROK and other pre-assembled RGB convertors. Or am I remembering wrong, and this on the input on the CXA chip?

-KKC, who's about to negate all his Supergun construction activity this weekend by driving to Orlando and picking up a Neo cabinet whole hog.

[Paper]

Thanks! An answer larger than I ever hoped for.

I'll place an order for both 220uf and 470uf and try it out.

Much more current will flow from the CXA's output that it's designed for which will likely degrade the performance of the video amp and distort the video signal.

I have been using a RGB cable without resistors and caps since 2002.
Is it possible this misuse has caused permanent damage? Or are you only talking about temporary malfunction during improper use (i.e. during overheating)?

This may be a possible source of the vertical lines in the blue signal I've seen on my system.

The cable lengths for most game consoles setups are pretty short, a meter or two usualy. So you can usualy get away with dodgy stuff like using unsheilded data cable for a transmission line and putting Cc and Rz on the TV/monitor end insted of on the source end.

As I've only seen caps and resistors being connected at the tv/scart end I thought this was proper practise. Your explanation corrected this misunderstanding completely however.

Since you say "usually" I have the ask about the exceptions. When do these shortcuts backfire?

-PPR, who used the function "find all posts by this member" to read all of kendrick's one-liners.

[viletim!]

The only thing I'm slightly confused about is the substitution of potentiometers for the 75 ohm resistors for the purpose of controlling color output, like you find on the JROK and other pre-assembled RGB convertors. Or am I remembering wrong, and this on the input on the CXA chip?

The JROK encoder (if I remember right) is meant for arcade boards which don't adhere to the RS-170 video standard that TVs and most monitors do. They use a loose (hence the requirement for adjustments) sort of defacto standard of their own which is based on TTL. They output  analog video at TTL-like voltage levels ie. between 2.5Vpp and 5Vpp with a fixed DC offset. It's done like that because it makes the monitor's video input stage a lot simpler and cheaper make. And it's pointless to build (relatively) complex 75 ohm sending/recieving/clamping circuits when the source and the display are never going to be very far from each other.

BTW. the CXA 1145 and CXA1645 aren't really suitable for encoding standard RGB into standard S-video/Composite because they want 1Vpp video at the input and properly terminated RGB video is only 0.7Vpp. A video pre-amp would fix that though.

Thanks! An answer larger than I ever hoped for.

I have been using a RGB cable without resistors and caps since 2002.
Is it possible this misuse has caused permanent damage? Or are you only talking about temporary malfunction during improper use (i.e. during overheating)?

I suppose it's possible but pretty unlikely. It just makes the video look dodgy (too bright, etc).

As I've only seen caps and resistors being connected at the tv/scart end I thought this was proper practise. Your explanation corrected this misunderstanding completely however.

It's mainly a practical thing - SCART connectors are big and chunky so that's where the componnets go. Much easier than putting them in a cable lump at the other end. They arn't always in the end of the cable. The composite video line (it needs em too) always has it's  resistor/cap in the console and the Neo Geo puts them on the RGB in the console where they rightfully belong.

Since you say "usually" I have the ask about the exceptions. When do these shortcuts backfire?

When you increase the cable length (to, say, 5 meters) or increase the bandwidth (to PC VGA for example). I've got an old VGA (that also does EGA/CGA/MDA) card which doesn't have a proper 75 ohm output. The picture looks bloody awful at 800x600/16 (max it'll go).

At TV scan rates and short cable lengths though, it's not critical at all and signal level is much more important than cable quality/suitability. The main problem with using crappy cable that I've noticed is that the video (low Z) signal gets into the audio signal (hi Z) because there's no sheilding to seperate the two. It resaults in a 50/60 Hz buzz in the audio whenever there's lots of white in the picture.