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Simple crystal (4.43MHz - PAL) oscillator + PLL multiplier circuit wanted.

Discussion in 'Electronic Design' started by Lionel, Mar 5, 2007.

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  1. Lionel

    Lionel Guest

    Per the subject line. I want to experiment with direct PAL video
    generation from a microcontroller, with the minimal number of parts.
    I'm thinking that a good starting place might be to use a CPU clock
    that's based on the PAL subcarrier frequency (4.43MHz) multiplied with
    a PLL+VCO+Divider to give me phase reference points, & directly
    generate the composite colour signal via an R2R ladder.

    So, some questions:

    1) Is this a reasonable concept? (I'm comfortable with per-cycle level
    timing & coding)

    2) Any tips/suggestions/thoughts as to a better approach?

    3) What's a good (simple, cheap, stable) circuit to oscillate a
    standard 4.43MHz PAL sub-carrier xtal? (Ideally just an inverter or
    two)

    4) Suggestions for a cheap + simple PLL frequency multiplier (x8? x4?)
    for the above to give me multiple cycles per PAL subcarrier cycle?

    5) How many analog samples do you actually /need/ to generate per
    subcarrier cycle to get good quality PAL colour?

    6) Any recommended online resources/references that cover direct
    digital synthesis of medium to high quality PAL colour video?

    Thanks in advance for any help or advice.
     
  2. Normally PAL/NTSC is samples at 4 times subcarrier. There are some
    genlock ics around see here:
    http://www.intersil.com/products/pt/parametric_table_10395.asp
    You may have to find out all the joys of the "eight field sequence",
    fortunately, I have forgotten most of it, it's messy.

    Maybe you should think about working in YUV/component/RGB, which has
    no subcarrier, but is sampled at 27M/sec, and is easier than PAL

    List of chips: http://www.epanorama.net/links/videochips.html


    martin
     
  3. A few people have had (limited) success with direct composite PAL
    video generation directly from a micro, like this one:
    http://www.rickard.gunee.com/projects/video/pic/gamesys.php
    Ordinary quality, but shows that the minimum parts count is a PIC and
    two resistors!
    Much faster micros are available would obviously do a bit better than
    the PIC.

    Dave :)
     
  4. Guest

    Note that his _PIC_ solution is BW only, he does color on a SX micro:
    with an about 53MHz clock(12x Fc )
    http://www.rickard.gunee.com/projects/video/sx/gamesys.php

    There are several cheap PAL modulator chips, so a PIC and
    for example a MC1377 modulator chip would be 'low chip component
    count'.
    MC1377 is old, and uses a 4.43MHz xtal, quality is good enough for
    games.

    The alternative is a PIC and a sparate 8.86 MHz (2xfc) oscillator,
    some 4053 switches, some 4013 FF, some inverters and xor gates to make
    a PAL modulator.
    I'd use the MC1377:)
    Or you need a very fast PIC !
     
  5. René

    René Guest

    In the dark recesses of my past, I remember owning a ZX80....

    http://en.wikipedia.org/wiki/Sinclair_ZX80
     
  6. Tam/WB2TT

    Tam/WB2TT Guest

    ......................................that a good starting place might be to
    use a CPU clock
    You might want to consider the converese of that. 20+ years ago it was
    common in cheap computers here to start with an oscillator at 14.318 MHz,
    which was then counted down to the NTSC color subcarrier frequency of 3.579
    MHz. 14.318 is also an integral multiple of the horizontal and vertical sync
    frequencies. I see that 17.734475 is a standard crystal frequency. Is that
    4x the actual PAL frequency?

    Tam
     
  7. I had one too, and it was BW.
    It had a Z80, at 3,58 Mhz or so, and a custom ship with the shift register in it.
    One of the best BASICS ever, the Z80 refresh register was also used for the
    display.
    It was followed by the ZX81 (Timex1000 in the US), and I still have the book
    Timex1000 / ZX81 ROM dissasembly by Dr Ian Logan and Dr Ohara.
    Even controlled an audio switcher with it....
    ZX81 was BW too.
    Later came the Spectrum, never had one, but _that_ one was color I think.
     
  8. Lionel

    Lionel Guest

    Yes, it is. However, I don't know if it's high enough to give me
    enough samples to create a clean Chroma, etc. I'm pretty sure that
    I'll need to sample at a minimum of 8x (35.468950MHz) the base colour
    subcarrier frequency. I /hope/ I won't have to go as high as 16x
    (70.937900MHz). ;)
    That said, using 17.734475MHz means I'd only have to multipy it by 2
    or 4 to get my higher multiples, which may make life easier.
     
  9. http://www.nxp.com/pip/SAA7121H_V2.html
    may be of interest


    martin
     
  10. Lionel

    Lionel Guest

    Unfortunately, it's not accessible:
    ------------
    Forbidden
    You don't have permission to access /pip/SAA7121H_V2.html on this
    server.
    Additionally, a 403 Forbidden error was encountered while trying to
    use an ErrorDocument to handle the request.
    ------------

    Fortunately, Philips have it on their shiny new marketeer-flavoured
    website:
    <http://www.nxp.com/pip/SAA7121H_V2.html>
    ---------
    General description
    The SAA7120H; SAA7121H encodes digital YUV video data to an NTSC or
    PAL CVBS or S-video signal. The circuit accepts CCIR compatible YUV
    data with 720 active pixels per line in 4 : 2 : 2 multiplexed formats,
    for example MPEG decoded data. It includes a sync/clock generator and
    on-chip DACs.
     
  11. I've generated PAL and NTSC CVBS in a Cyclone II (5K) FPGA, $12.90 in
    singles. I load the FPGA with a 0.68 micro. Total parts cost: ~ $16.00
    in singles w/ 8-bit DAC, ~ $17.00 for 10-bit DAC. Parts total in the
    1000+ would be about $8.00. USB is about 1.50 extra if you need
    computer control.
    Do you need genlock? If not, a simple xtal osc at 4x or 8x with
    inverter will work fine.
    Are you familiar with the PAL color subcarrier phase shift wrt the
    field sequence?

    Frank Raffaeli
    http://www.aomwireless.com/
     
  12. Tam/WB2TT

    Tam/WB2TT Guest

    *****************
    That would still simplify things because you could use a frequency doubler,
    or two in series, and not have to worry about phase noise and such of a PLL.
    A 2N2369A, or the like, biased class C will work nice as a doubler at these
    frequencies. You would need a limiter on the output.

    Tam
    *************
     
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