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TV Modulation w/o Broadcasting — Parallax Forums

TV Modulation w/o Broadcasting

ggysbersggysbers Posts: 13
edited 2015-03-20 15:53 in Propeller 1
Good day,

Regarding the TV.SPIN object, there must be a means to modulate the NTSC video onto say channel 3 (60_000_000) w/o radiating an RF signal? Eg. Identically to how the VCR, Atari 2600, NES, etcetera put video up on channel 3 yet did not broadcast RF signal off the wire. I just do not yet know enough about the internals of video modulation nor am I very familiar with assembly.

I would appreciate any assistance.

Sincerely, G

Comments

  • RaymanRayman Posts: 14,652
    edited 2015-03-20 12:59
    I imagine you just connect the output to a 75-Ohm coax cable and you could plug it into a TV's NTSC antenna input (if you can find a TV that still takes NTSC)...
  • ggysbersggysbers Posts: 13
    edited 2015-03-20 13:23
    Yes, I have used an RCA-->Coax adapter on my Demo board and run coax the newest LG flatscreen and the older than dirt CRT television set. It does get the job done however I have noticed that neither the new flat screen nor the CRT are really thrilled about doing so. (No, I'm not talking about frequency 61_250_000/60_000_000 on a 5MHz, 6MHz nor 6.25MHz --I don't want to go there).

    Both the cheap RadioShack Channel 3 Modulars and the professional Blonder Tongue NTSC/PAL Video RF Modulator --> Ch. 3 make a signal that won't bleed off the wire. What these two devices do the the "product" is what I would like to do in my Propeller. I should be able to do this, I just don't know modulation theory down to this level nor are my abilities in Assembly all that keen.

    It has something to do with or at the signal Carrier level.
  • ggysbersggysbers Posts: 13
    edited 2015-03-20 13:38
    Okay, now that I've thought about it.... I want Cable TV modulation not Broadcast TV modulation. Both can be on channel 3 yet the are slightly different animals. Not quite identical cousins.
  • Mike GreenMike Green Posts: 23,101
    edited 2015-03-20 14:10
    Although the Propeller can produce a video signal on the lower VHF channels (2, 3, 4), it's not the best quality in my experience. I've tried this with 2 or 3 old TVs and it seems to be the same on all of them. B & W (no color subcarrier) works better than color and external video/RF modulators work much better than the "bare bones" Propeller. I'm sure part of this is a result of the high speed digital switching going on in the Propeller with edges changing on a nanosecond timeframe while the modulators work with slower signals (by a factor of about 100).
  • potatoheadpotatohead Posts: 10,261
    edited 2015-03-20 14:34
    That is a whole lot of the trouble. Props output hard edged signals.

    When these collide with most consumer grade devices, harmonics and ringing in the filters they have, muddy the signal.

    Put a little cap on the P1 output and it can be much nicer.

    Now, you said bleeding off the wire...

    That is just attenuation. You need a resistor to take the signal level down. The TV RF input can be overwhelmed, and or the wire will act as an antenna.

    For the composite signal case, male smoother signals. That is a cap, or higher resolution DAC.

    For the RF case, it is a resistor, or amplifier stage to make the signal strength better aligned to the receiver expectations.

    Edit:

    You might want to do both.

    And... Use a grounded and shielded coax. That can carry even a fairly strong signal without a lot of leakage. Your board will produce some though.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2015-03-20 15:53
    Also, use channel 3. Channels 2 and 4 output from the Prop contain a lot more PLL frequency jitter.

    -Phil
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