generating ntsc video

It's an 8080 with a few added instructions, and usually operates at

2 to 8 Mhz. Minimum 4 clocks per instruction. Last seen in the first Mars lander.

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You're not going to be able to generate NTSC because you can't generate a color signal from 1MHz. You will have to make do with monochrome 60Hz video. If you are using the LM1881then you must already have some external video coming in. You'll need to "genlock" the CPU clock to the external video otherwise the jitter is going to be huge. You have approximately 64 microseconds to put the pixels on each line. Your processor at 1MHz can do no more than 16 instructions in that time so the horizontal resolution will be 16 pixels or less. The poor processor won't have time to do anything else but generate video.

In other words, get a faster processor or an external video generator. The lowest end generators that I know are written for the old 16f84 PIC chip. Google for "picdream.zip". Peter

Are you creating a video output or a video output overlayed onto an existing video picture?

If the second does the output still have to produce an output if the input is missing?

What type of very low resolution information - lines, boxes, characters?

What update rate of information?

Is your information monochrome or colour?

Even for NTSC getting the line frequency accurately from either of those frequencies will be difficult.

You would only use the LM1881 if you were decoding the syncs from a video input, it is not a video sync generator.

There are plenty of CPLD free designs for generating NTSC timing patterns.

Look at Altera and other peoples web sites.

Without knowing answers to the questions above and what space and cost budgets you have it is pretty meaningless to give many pointers, other than I would not start from here.

I would start from knowing I could generate the video signals and knowing what I was trying to output in the way of information. Then consider what micro and other peripherals I would need.

Video data rates can get a lot faster than most micros depending on the application.

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Google on Lancaster "Cheap Video" Don Lancaster wrote a couple of books called "The Cheap Video Cookbook" and "Son of Cheap Video" covering exactly what you are trying to do, with a similar CPU back in the 70s or maybe early 80s. From what I remember, the 808x family was so wasteful of clock cycles that it was not usable for direct video generation for text display.

As another poster mentioned, the LM1881 is useful for decoding the sync signals from an existing video signal. If you wanted to overlay your video on top of some other signal, it's useful. Otherwise, you could probably come up with some discrete counters or a timer chip that would be more useful.

Bob

Anyone remember the Sinclair ZX-80? Good times!

Many thanks to everyone who wrote in; your comments were most helpful. I will post again once I get more details regarding the system and it's capabilities; I am doing this for a class, so I'm pretty much locked in to using the particular microcontroller that comes with the system.

I suppose that I needn't overlay my video onto an existing source, so the LM1881 is out of the picture for me. As far as the type of video I'll be looking to generate goes, some boxes or characters that I store in ROM would be just fine; I'm mostly interested in generating the video as a proof of concept.

I'll be checking out the resources you guys mentioned and hopefully will be able to come up with a tractable problem.

Thanks Again

Jacek

To the best of my knowledge the video was controlled by a Ferranti UCLA (UnCommited Logic Array - FPGA) to drive the timing and shifting out a section of a line's video data. The memory was dual ported with video controller becoming bus master (forcibly..) and grabbing the video data.

The ZX80 did not use the micro to *directly* control timing and data.

As was mentioned in other posts an external controller (CPLD etc..) is one solution to controlling the video and possibly video data.

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You'll need to generate the Line and Frame Sync pulses, and add in the Video information, to create a monochrome VBS ( Video,Blanking,Sync)

-commonly IIRC the composite signal is 1V p-p.

One challenge will be to do this, on an 8085, without also getting time jitter from the software. Start by trying to generate the Sync, and a simple large square box in the centre of the screen. That will establish signal levels, and stability.

-jg

See

for a good explanation of generating video using an 8052.

See

for a cheap, completed solution.