Atari 7800 Expansion Bay

[Shawn]

 

No, the computer plugged into the right joystick port, not the expansion port.

 

Mitch

 

Yep, and I still want one pretty damn bad too

[Bruce Tomlin]

Here's what I know: LaserDiscs are a technology from the 1950's that stores the video in an analog format. (Digital recordings were invented with the Compact Disc spec, some thirty years later.) This analog encoding means that any linear signal can be encoded via Pulse Code Modulation (PCM). Which means that a standard TV signal should be storable on the disc, presuming that the existing signal was some other format. Given that the players always had composite video with a separate track for audio, it's probably safe to assume that a broadcast signal was the standard generally used. (Although, negatives for movies were often archived on LaserDisc.)

Er, you do know what PCM is, right? It's when you have a group of digital pulses that represent a quantity, like with CD audio. The only PCM in laserdisc is an optional digital audio track.

 

First of all, though there may have been experiments before that, Laserdisc first became a consumer technology in the late '70s. It came after the 2600 but before audio CD. The signal on laserdisc is a broadband FM signal that is overmodulated into a variable frequency square wave, where the timing of the signal reversals contains the information. (Just because it has pits and no-pits doesn't make it digital.)

 

It has separate sub-bands for audio and NTSC video (without the chroma-under crap of videotape), and IIRC, the analog audio has about a 100KHz band per channel, so it's as hi-fi as you can get. In the early '90s, CD digital audio was added in a new sub-band. Then in the mid '90s, Dolby Digital was added as an RF-modulated digital signal in one of the analog audio channels, and DTS digital was added by re-using the CD audio as a digital data stream.

 

LDs have two basic formats: CAV (constant angular velocity) and CLV (constant linear velocity). CAV is synchronized to one revolution per frame (1/30 sec or two fields) for up to 30 minutes of video, and CLV is not synchronized, for up to 60 minutes of video. You can actually see the vertical sync on a CAV disc.

 

CAV is preferred for computer-control applications because it has still/step frame capability. It also uses frame numbers for time marks. (CLV uses minutes and seconds, so there is no way to know exactly which frame you are on without CD audio timing information.)

[jbanes]

Er, you do know what PCM is, right?

Indeed I do. And I have no idea why I said that. I should have said, "Pulse-width modulation".

 

I've been making a lot of stupid little mistakes like that lately. I'm probably burned out. Thankfully, I'm off to vacation on Wednesday. (WhooHoo!)

 

 

First of all, though there may have been experiments before that, Laserdisc first became a consumer technology in the late '70s.

This is true. However, it is still inherently a technology prior to the digitial age, obviously a late-50's/60's technology. The technology did take an extraordinary amount of time to make it to market, meaning that it had a very short market life prior to the introduction of the Compact Disc. I imagine that materials technology had something to do with it, as well as the complete lack of a home video market. And given the attention paid to movie archives at the time (i.e. very little), I suppose it's no suprise that the technology had a hard time finding a niche.

 

It has separate sub-bands for audio and NTSC video (without the chroma-under crap of videotape), and IIRC, the analog audio has about a 100KHz band per channel, so it's as hi-fi as you can get. In the early '90s, CD digital audio was added in a new sub-band. Then in the mid '90s, Dolby Digital was added as an RF-modulated digital signal in one of the analog audio channels, and DTS digital was added by re-using the CD audio as a digital data stream.

Which (if I understand correctly) means that the NTSC signal could easily be recorded to be compatible with the 7800's 228 chroma clocks. Correct?

[supercat]

Which (if I understand correctly) means that the NTSC signal could easily be recorded to be compatible with the 7800's 228 chroma clocks. Correct?

 

If the sync is indeed generated entirely in-band, that would be the case. I would have expected the sync to be generated by the player itself (among other things, having the player generate the horizontal blanking interval would allow the part of the disk that would otherwise store it to be used for something else) but I didn't design such things.

 

BTW, I wonder if any tech-heads could tell me: when Betacam was invented, how was the time-stretching of each scan line handled? To record a YUV image in betacam, the Y signal for each scan line is recorded at 2x speed, followed by the U and V signals at 4x speed. The net effect is that each scan line takes the same amount of time to record as if the signal had been recorded "straight", but the three signals get recorded without crosstalk. I'm curious how one would handle the time stretching and compression, though--an ordinary delay line certainly won't do that.

[Bruce Tomlin]

Which (if I understand correctly) means that the NTSC signal could easily be recorded to be compatible with the 7800's 228 chroma clocks. Correct?

Except that the player has a Time Base Corrector which ensures that the result is proper NTSC. So you'd probably have to have a special LD player with a TBC designed for 7800 overlay use.

[Shawn]

Which (if I understand correctly) means that the NTSC signal could easily be recorded to be compatible with the 7800's 228 chroma clocks. Correct?

Except that the player has a Time Base Corrector which ensures that the result is proper NTSC. So you'd probably have to have a special LD player with a TBC designed for 7800 overlay use.

 

Do you think it would be possible as well as feasable to make an adaptor cable to use a Laser Disc Player on the 7800 threw the expansion port even if it's just for watching films with? It would be damn cool I think just to go that one step closer that Atari never did.

[Bruce Tomlin]

Do you think it would be possible as well as feasable to make an adaptor cable to use a Laser Disc Player on the 7800 threw the expansion port even if it's just for watching films with? It would be damn cool I think just to go that one step closer that Atari never did.

I'm not even sure it's possible because of this color clocks thing. It's certainly not worth the effort.

[+Allan]

Are there any sources for jacks for the expansion port? It looks kind of non-standard. Has anybody seen these on anything else?

 

Allan

[PacMan]

Hmm, how would one synchronize the units ? Is it possible to synch Maria to an external video source ? That would be kind of cool though, but it's probably not possible.

 

/P

[Shawn]

Do you think it would be possible as well as feasable to make an adaptor cable to use a Laser Disc Player on the 7800 threw the expansion port even if it's just for watching films with? It would be damn cool I think just to go that one step closer that Atari never did.

I'm not even sure it's possible because of this color clocks thing. It's certainly not worth the effort.

 

 

Oh well, it sounded cool A little too cool I guess

[Flipper]

Well, I have thought about doing this, but the color signal has been mentioned as a possible issue. I suppose that could be.

 

The theory is as follows, however.

 

On the expansion port, there is a signal EXTMEN (External Maria Enable). It maps through a couple of gates to MEN on the Maria. Buried in one of the datasheets on the net, is a lttle side note that pulling MEN inactive results in two things:

 

1. 2600 memory mapping is engaged (we knew that one)

2. The sync counters are reset, starting a new frame.

 

So, the theory really OUGHT to be as follows. After a VSYNC is detected (CSYNC is available on the expansion port), count off 260 (ish?) HSYNC pulses. An LM1881 can do the sync seperation for you. That's overkill, but it's cheap enough. At this point, pull RDY low, halting the 6502 gracefully.

 

Using another LM1881 to break down the signal from the LD/DVD player, wait for VSYNC. When you see VSYNC, pull EXTMEN (high?) for 1 cycle, then release it and RDY. The CPU should start over. If the game is using a normal wait for VBLANK loop, it should now pick up with the next frame.

 

Then, when LUM0-LUM3 are all 0, have a video switch select the LD/DVD player (maxim makes switches that should be plenty fast enough).

 

Of course, as someone has suggested, the color signal may not be in phase enough. And there is some RF-design here, too, well beyound my level. On the other hand, many systems have offered genlock capabilities without implementing a full TBC. NTSC color is such crap I suspect you could commit many abuses. Now, that said, why an LD? You can make your own DVD's, and there should be no fudging with the color signal at 227.5. Either way. In order for your TV to decode the signal, the color burst must be pretty much 3.58MHz. The LENGTH of the video data on the line is irrelevant, as long as the sync is still pretty much 15KHz. The mixing and output would be done on the board attached to the expansion port, and the internal RF modulator would be ignored. There is, unfortunately, no way to disable the internal OR drivers used between TIA and MARIA to inject your own signal to the RF.

 

But, yes, the only thing the expansion port is good for is video/audio out of the console. I've got a really crappy SVIDEO out plugged into the side of mine. Of course, it's protoboard stuff, and my protoboard soldering skills are really really scary, so the signal reflects EVERYWHERE, and luma is deplayed by about 2 pixels from chroma, so it's hard to look at.

 

The computer keyboard plugs into the joystick port, needing nothing from the expansion port. What frustrates me about that is I was well on my way to implementing one with an Atmel when two things happened about a year ago:

 

1. A hardware design fault somewhere blew out my 6532. I have not yet checked it out in detail, but I suspect I blew out the output drivers.

2. Curt Vendel annouced that he was intending to make that device REAL SOON NOW. So, I backed off.

 

The schematics are online. The programming docs are online. The communication protocol is simplicity itself. Any microcontroller you choose can easily handle it. I was using an Atmel AVR and had PS/2 to 7800 going just fine, and was typing on the screen. That constitutes approximately one weeks work. And you know, the puttering around after work work, sort of work.

Edited August 1, 2006 by Flipper

[shadow460]

Do you think it would be possible as well as feasable to make an adaptor cable to use a Laser Disc Player on the 7800 threw the expansion port even if it's just for watching films with? It would be damn cool I think just to go that one step closer that Atari never did.

I'm not even sure it's possible because of this color clocks thing. It's certainly not worth the effort.

 

 

Oh well, it sounded cool A little too cool I guess

 

One thing that might be useful for, though, is to make a scrolling background like the ones in Raiden or R Type. That would mean only having to render sptires for enemies, the player, and all the bullets on screen with the rest played back from some kind of media.

[carmel_andrews]

Re: the 7800 'expansion' bus/port...Was the SIO interface/port (from the A8), incorporated into this bus/port or was the 7800's 'expansion' bus/port just an extended version of the Atari SIO port as i seem to remember reading that you could attach 1050's/810's 1010's/410's etc to the warners version of the 7800

[+Mitch]

Re: the 7800 'expansion' bus/port...Was the SIO interface/port (from the A8), incorporated into this bus/port or was the 7800's 'expansion' bus/port just an extended version of the Atari SIO port as i seem to remember reading that you could attach 1050's/810's 1010's/410's etc to the warners version of the 7800

 

The 7800 expansion port was for the 7800 laserdisc addon only. The 7800 computer SIO bus was on the keyboard module.

You may be thinking of the 5200 expansion port, it has most of the standard SIO signals on it.

 

Mitch

[A_Username]

Re: the 7800 'expansion' bus/port...Was the SIO interface/port (from the A8), incorporated into this bus/port or was the 7800's 'expansion' bus/port just an extended version of the Atari SIO port as i seem to remember reading that you could attach 1050's/810's 1010's/410's etc to the warners version of the 7800

 

The 7800 expansion port was for the 7800 laserdisc addon only. The 7800 computer SIO bus was on the keyboard module.

You may be thinking of the 5200 expansion port, it has most of the standard SIO signals on it.

 

Mitch

 

So technically you could use the 5200 as a computer throught it's expansion port?

[Classic Pac]

With a contact I had at Atari in it's heyday I knew for a fact Atari did plan a lot of stuff for that expansion port. But as far as I know no working models were ever made for anything ever planned.

[Bruce Tomlin]

So technically you could use the 5200 as a computer throught it's expansion port?

You could, but it would be a mutant, mixed-up variation of the standard A8 architecture that wouldn't run anything without conversion but BASIC games, and not even some of those. If you thought the Atari 1200 XL was bad...

[A_Username]

So technically you could use the 5200 as a computer throught it's expansion port?

You could, but it would be a mutant, mixed-up variation of the standard A8 architecture that wouldn't run anything without conversion but BASIC games, and not even some of those. If you thought the Atari 1200 XL was bad...

 

I remember their being a tidbit of information about this in the did you know... section on the main page saying the 5200 was basically an A800 without a keyboard. I didn't know it would be THAT uncompatible. I thought that with a crapload of work and a whole bunch of electronic knowledge, you could add on all features of an A800 that were missing from the 5200 and turn it into a full featured computer.

[carmel_andrews]

I can't remember which one but i think either Antic or Analog (American Atari magazines ran 1 or 2 issue series on transporting/converting A8 games to the 5200 and Visa Versa

 

I didn't know the 5200 had an sio port, so does that mean you can use A8 devices like 1010/xc11/12/1050's etc with a 5200 (i did'nt think it could as the software doesn't exist, unless you were a 5200 software programmer that had this software)

[+Allan]

I can't remember which one but i think either Antic or Analog (American Atari magazines ran 1 or 2 issue series on transporting/converting A8 games to the 5200 and Visa Versa

 

I didn't know the 5200 had an sio port, so does that mean you can use A8 devices like 1010/xc11/12/1050's etc with a 5200 (i did'nt think it could as the software doesn't exist, unless you were a 5200 software programmer that had this software)

 

There is no system software for it. You would have to do everything with your program. The port is not even set up like the 8-bit sio ports, it uses the 5200 cartridge port type. I'm not sure what you could do with it. Maybe with a bank-switched cart you could write something to take advantage of it. You could maybe write a simple DOS and load programs off of a 1050, but it would be a lot of work. Maybe you could do a high-score device.

 

Allan

[Rik]

This expansion bay thing,ive read that some 7800's had the capability,and some didnt,how do ypu tell which one you have?

[Video]

The 7800's that don't have it, well, don't have an expansion port. Some still have the hole in the case, but it's empty (no connectors) And some have just a solid plastic case, with no port hole or anything. Though I imagine it wouldn't be to hard to build a new expansion port onto a 7800 that's missing it, unless something internally was vastly changed too.