Generating composite video on NTSC and PAL TV's

Discussion in 'Analog & Mixed-Signal Design' started by PauloConstantino, Aug 26, 2016.

  1. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
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    Hi All,

    do you know if to generate black and white composite video signals for a TV, the process would be the same for NTSC and PAL tv's? I have an old black and white tv here that I can't figure out whether it is a PAL or NTSC tv.

    I want to construct a circuit to generate composite black and white signals and therefore I need to know if there is any difference between the two systems when it comes to black and white?

    Also, is there a way to tell whether my old tv is NTSC or PAL ?

    Thank you

    Paul
     
  2. Dodgydave

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    Jun 22, 2012
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    Ntsc is a faster Line rate 15.735khz than pal 15.625khz

    and ntsc has a faster Frame rate 60hz, whereas pal is 50hz.

    Biggest clue is the supply voltage, if its 110vac ntsc,, or 220vac pal.

    Also the colour sidebands are at different frequencies, 3.579mhz ntsc,,4.433mhz pal
     
    Last edited: Aug 26, 2016
  3. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
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    The tv is actually portable. it takes 12V DC from an AC 240V adapter. It's also black and white...
     
  4. Dodgydave

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    Jun 22, 2012
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    So what country is it used used in?
     
  5. AnalogKid

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    Aug 1, 2013
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    If the set was purchased in the UK, it is PAL. If it was purchased at a US military base in the UK, it could be either.

    What kind of composite black and white signals do you want to create?

    ak
     
  6. ian field

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    Oct 27, 2012
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    I'd look at the specifications for the likely suspect standards.

    There's parameters such as AM or FM sound, positive or negative video and/or sync etc. A B&W portable can probably be adjusted to the required line and frame rate easily enough.

    A service manual could probably be interpreted - if it didn't tell you straight up anyway.

    When I was in the trade, the shop I was working for got a very grandiose colour set in - it had a 3.58/4.433 switch on the back, setting that right got the colour up, but I had to change the ceramic resonator and tweak the subcarrier IFT to get sound.

    A US set is likely to have a 5.5MHz subcarrier resonator - in most PAL sets it should be 6.0MHz.
     
  7. DickCappels

    Moderator

    Aug 21, 2008
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    The luminance (Black & White) signal for NTSC and PAL are very similar. The differences are in the bandwidth and the timing of the sync signal, so for some black and white applications you can get by with the same video signal, but with different timing and lines per frame for each mode..
     
  8. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
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    The TV was bought here in the UK but it might have come from somewhere else. I think it was made in China many years ago.

    Here is a photo of the TV: [​IMG]
     
  9. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
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    The kind of composite signals I want to generate are pixels from a RAM chip. I still dont know what kind of circuit I will need for this. Is there an easier way of doing it? Because my plan is to use only 74HC series logic chips for the task, as this is what I have been using to build a complete cpu.
     
  10. Dodgydave

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    Jun 22, 2012
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    Does the tv have a scart socket or phono input, you could input a pal video source to test it.
     
  11. ian field

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    Oct 27, 2012
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    With static RAM; it can be as simple as an address counter to sequence the locations and latch the data to a shift register.

    AFAIK: dynamic RAM needs some form of CPU to grab the data between refresh cycles.

    You can do it with "glue logic" but its cumbersome.
     
  12. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Isn't that backwards? Don't you need to refresh between memory access cycles? IOW, you don't really want your CPU stall while refresh is taking place.
     
  13. PauloConstantino

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    Jun 23, 2016
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    Hello Ian. I understand I can do it like that BUT, my concern is how to synch this with the CPU. I dont know if I should use the CPU clock to read RAM and send signals to the TV or should the video circuit have its own clock?
     
  14. PauloConstantino

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    Jun 23, 2016
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    Ian, if you know how to do this, I beg you if we can chat on facebook or via email ?
     
  15. PauloConstantino

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    Jun 23, 2016
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    Should I do it this way: ?

    I have a shift register that takes input in parallel from RAM at every clock cycle of the CPU clock.

    This shift register then outputs this data in serial by means of a secondary clock which is 8 times faster than the CPU clock.

    This 8 times faster signal outputs each bit in series to the video circuit to print it to the TV.


    IS this the way ?
     
  16. ian field

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    A novice might prefer to do refresh in hardware.

    Some very popular CPU types were ready made to handle refresh - I haven't kept up what's available on modern processors.
     
  17. AnalogKid

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    https://en.wikipedia.org/wiki/Memory_refresh

    Or, use all static RAM and there is no refresh. Back in the day that would have cost as much as a BMW. Today, 1 M x 8 bits is a single chip for $7.00 at Digi-Key. Dual port is about $30 to $50, but might be tricky to integrate into a larger non-dual-port array.

    What is the total memory array size (words x bit-width)?

    ak
     
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  18. PauloConstantino

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    Jun 23, 2016
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    I am using static ram of course. Buddy, it's 64K by 8bits.

    I need to know the details of how the circuit will work. As I said, I am planning to use a shift register to pull bytes from RAM, at every CPU clock cycle, then use a faster clock (about 8x) to output the shift register bits in serial onto the monitor
     
  19. joeyd999

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  20. AnalogKid

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    Is 64K the total memory array for the computer? I ask because if it is holding the video bits as you describe, then 25 lines times 80 characters per line (standard format for computer text on a TV) times 35 (a 5x7 character matrix) = over 13% of the total memory space. If you use a character generator chip (or grow an equivalent subsystem), then the memory usage drops down to 2000 bytes, only about 3%.

    In general, your read-and-shift approach is correct. However, there are a couple of different ways to do it, and they affect how much memory is needed and how often it has to be accessed. Also, in the original IBM PC, the CPU clock is derived from the master clock for the NTSC video card. This sorta-kinda established a synchronous relationship between the system memory and the video subsystem, but not really.

    ak
     
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