Generating composite video using only basic chips

Discussion in 'Analog & Mixed-Signal Design' started by PauloConstantino, Aug 30, 2016.

  1. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
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    Hi all,

    I am looking for a way of generating composite black and white NTSC or PAL video signals using only basic chips like the CMOS 74HC series, clocks and resistors. Can anyone give me some idea of how to start doing this? I don't want to use a microcontroller for this.

    Help would be very much appreciated!

    Thanks

    Paul
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    First, what kind of signals, true analog video or test patterns that usually are switching between 0% and 100%?
    Second, do you want to grow the sync signals from scratch or use a sync generator chip?
    Third, this is not a trivial project. How much about NTSC do you already know?

    ak
     
  3. DickCappels

    Moderator

    Aug 21, 2008
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    If you are not going to need color you can generate the signal very simply by making (for North America) two oscillators -one at 15.734 kHz for horizontal and another at 59.994 Hz for vertical -plus or minus a percent or two. If the oscillators are not locked together, then you will have a random interlace signal.

    If you want to go full-blown you can do it with the normal counters, gates, and one-shorts. I used to test a RS-343 (mono version of RS-170) that used RTL, Think about it.

    RS-170
     
  4. Papabravo

    Expert

    Feb 24, 2006
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  5. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
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    I have that book! I forgot about it. Thank you!
     
  6. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
    121
    5
    I know almost nothing of tv signals. I am building a computer from scratch using just the 74HC series of chips, and I want to build a video output for it, hence why I need to learn this!

    It will display pixels from memory and that's it. Help!
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The Cookbook you already have is an excellent place to start. There are frequency and timing differences between NTSC and PAL, but for monochrome video the concepts and basic circuit functions are almost identical.

    An important starting point is the memory. The video memory can be independent of the computer's main memory as was common when the book was written, or it can be a slice of the main memory, which is much more common with today's graphics chipsets.

    If the video system is sharing main memory, then you need to manage memory access so the computer writing to the memory doesn't upset the video system reading it for display. One way to do this is to buy specialized dual-port memory chips. Another is to grow dual-port management around standard chips. The good news is that today's chips are so freaking fast that the conflict time is a very small fraction of what it once was.

    ak
     
  8. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
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    If I have a chunk of RAM that represents VRAM, how do I read this chunk and send it to the TV in a nutshell ? The way I thought about it was whenever the CPU needs to read/write to RAM, the video output will have to stop reading RAM. Will I need to have a TV buffer and so on ?My main concern is how to read ram and send it to the TV.
     
    Last edited: Aug 30, 2016
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If the only video is text, then it is far more efficient to store the ASCII code for each letter in memory (an ASCII character is 7 or 8 bits, while a displayed character can be up to 35 or 63 dots), and decode it into the correct raster dots after it is read by the display circuits. Been a long time since I've roamed around in those old circuits, but as I recall the video reader gets priority over the computer writer, because if the video system misses a read there is blank space on the screen.

    ak
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Many years ago; Elektor magazine published a TV pattern generator project constructed entirely with TTL chips.

    There were several pages of logic formulas explaining how the fully compliant CCIR sync was generated.

    Manor supplies in London used to sell a pattern generator kit, but it used a couple of 555 chips to generate the sync - this was completely useless, so I built a TTL divider chain board. It wasn't as genuine as the Elector project, but it worked most of the time.

    There used to be all in one sync timebase generator chips for TV cameras and such, but I'm not sure if any are still in production.
     
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