Expansion port to A/V RCA jacks?

[Mot]

if the 7800 expansion port

http://cat.asw.cz/~kubecj/7800exp.htm

has:

1 gnd

3 CVIDEO composite video signal

17 AUDIO TIA audio output

 

Would this work for video/audo inputs into the TV?

in 7800 mode at least?

[+Mitch]

if the 7800 expansion port  

http://cat.asw.cz/~kubecj/7800exp.htm

has:

1 gnd

3 CVIDEO composite video signal

17 AUDIO TIA audio output

 

Would this work for video/audo inputs into the TV?  

in 7800 mode at least?

 

Unfortunately, those are imputs not outputs. I don't think the inputs are even active until a game/program enables them.

 

Mitch

[Flipper]

Looking at the schematics (and yes, unreleased documents have a nasty habit of being WILDLY fscking different than reality), the AUDIO pin ties in to the same point as the EAUDIO pin from the cart connector (which we KNOW is a audio input at least). Searching upwards on the schematic, there is nothing that should prevent you from drawing an audio signal from that pin, or injecting one of your own.

 

A crude method of checking this is to measure the resistance between EAUDIO and AUDIO. Less than several megohms and you're probably seeing a true connection.

 

The same appears to hold true for the CVIDEO pin. Injecting your own signal there would be difficult as you have no method of shutting off the internal resistor ladder and color mixer.

 

The MLUM and MCOL pins DEFINATELY appear to be outputs. Building your own resistor ladder should give you SVIDEO out the expansion port.

 

Of course, I have not real hardware, or I'd actually do something about it.

[Zonie]

Funny thing...I was wondering the same damn thing today...

[Flipper]

Well, don't ask me why it works, but I actually tried tying CVIDEO from the expansion port on my 7800 to a video in on my TV, and it was there. Barely, but it was there.

 

Experimentally modifying the video driver circuit from an existing SVIDEO mod, I got a definately viewable picture. Take a look at it here:

http://www.phin.com/gallery/expansionport

 

It's still picking up interference from something. Gives me diagonal lines racing back and forth over the picture. Probably the result of trying to run composite video over a breadboard. But I'm not going to try SVIDEO tonight. Oh well.

 

I cannot, however, find a usable audio signal on the expansion port. It OUGHT to be there, I just can't find a way to make my TV use it.

 

Well, now that we know you CAN draw video off the expansion port, what can we do with it?

[/url]

[Danno]

You could build a video mod that doesn't require soldering.

 

Now that would be nice.

 

Too bad my newly purchased 7800 is one of the "newer" ones with no expansion port.

[Harry_Dodgson]

 

{snip}

 

It's still picking up interference from something.  Gives me diagonal lines racing back and forth over the picture.  Probably the result of trying to run composite video over a breadboard.  But I'm not going to try SVIDEO tonight.  Oh well.

 

{snip}

 

[/url]

 

The interference is coming from the audio mixer. If you aren't going to use RF anymore, disconnect it.

[Danno]

Hey - I have a question.

 

I just bought a 7800, but it's a "newer" one with no expansion port - but upon closer inspection(aka ripping her open) I noticed that the cutout on the metal sheilding is still there, and there are solder points on the mobo labeled Jumper 12.

 

Can I just solder to these points and get the same thing?

 

As much as I may want to look at 8-bit's video mod, I'm not to keen on ripping IC's off of the mobo and doing so much work. getting a decent AV mod for the 7800 should be much easier than all that.

[Flipper]

Again, I ONLY have a breadboard version of this, and I never did understand analog electronics, so your mileage may SERIOUSLY vary...

 

However, if you try and solder to those points, my guess is video should work. Give it a try and let us all know what happens!

 

Rather than disconnecting the audio modulator, couldn't one just put a low pass filter to trim off everything above 5.8 or 5.9 MHz (where the modulated audio sits?)

 

Rather than asking that, I suppose I should try it.

[Danno]

Ok here's a dumb question:

 

I see all these AV and Svideo mods starting from soldering points off of various ICs or diodes...now bear with me as I don't know how to read schematics, and am just hazarding a guess....

 

But would it make more sense to try and find a point to tap into if you removed the RF unit? I mean - somewhere, just before the audio and video is combined and converted to RF in the RF unit, there should be a point where separate pure audio and video signals can be found - right?

 

Since the point of these mods is to replace the inferior RF connection with superior separate composite video and line level audio - wouldn't it make more sense to use a source from the MOBO just before these signals are converted to RF and eliminate the RF unit altogether?

 

Does this make any sense to anyone but me?

[Zonie]

You're on the right track. But those points right before the RF modulator you mention are usually on an IC pin or pullup resistor. Then you have to deal with the signal's ability to produce a picture. There may not be enough voltage to drive the input on the TV. That's where the extra cicruitry comes in. These circuits are usually amplifiers. I dunno, maybe one of those Stereo preamps would work, but these circuots portrayed here are a great way to learn about electronics. Pick up one (or all) of the booklets at Rodeo Shark By Forest Mimms III. "Getting Started in Electronics" is a really good one. The older printings of these are the best..

[Danno]

i understand that the signal needs to be amplified for a standard composite video input, it's just a matter or how much.

 

I just thought removing the RF unti also give you a pre-cut spot for a jack on the console's shell. I figured that would be more convenient.

[Flipper]

That's probably not a bad idea to remove the RF modulator. It seems, at this point, that you have to disable the audio mixer anyway, which would negate the usefulness of RF. Note, I still suspect you can do it without disabling RF, but it will be slow going for me to check it out. Anyone know a GOOD analog electronics book? Not so much a complete one, but the sort that says, "This is a wire. It's made of a substance called metal. First of all, we'll try bending the wire! Wow! See how it bends?" That's about where I am with the analog side of the world.

 

However, that would still only give you access to composite video. And void the warranty on your brand new 7800?

 

You can tie to the MCOL/MLUM0-3 pins on the expansion port and get SVIDEO. That will give you better graphics. But, only in 7800 mode.

 

The only way to get SVIDEO in 2600 and 7800 mode is to tie to the points mentioned in the other hacks.

[Danno]

Ahhh.

 

Now I get it.

 

I didn't realize the difference between 2600 and 7800 output.

 

I learn something new everytime I come here.

[shrackney1]

you should still have the expansion port locations on the motherboard inside. I don't have physical evidence of this, but most of the info I have seen says that it is there, but the hole in the case isn't.