Connect with us

Audio/Video to RF. Help!!

Discussion in 'Radio and Wireless' started by Miguel Lopez, Jun 25, 2015.

  1. Miguel Lopez

    Miguel Lopez

    251
    61
    Jan 25, 2012
    Hello

    I need to convert an Audio and Video signal to RF. Is there a way to build a simple circuit to do this. I don't mind if for channel 3 or 4.

    It is to watch the Cuban Digital TV, using a decoder (Audio/Video output) in an old Soviet TV.

    Thanks in advance
     
  2. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Can you please clarify what you want to convert?
    Audio and Video come in many different formats in both digital and analogue variates.
    Additionally, 'RF' comes in many different formats...

    At the very least, you could purchase a composite RF modulator which will take the commonly coloured red/white/yellow plugs and convert them to RF commonly sent over cable and airwaves to channel 3 or 4.
     
  3. Martaine2005

    Martaine2005

    2,436
    648
    May 12, 2015
    Hola Miguel como esta,
    Me gusto mucho su oscilloscope que armaste de soviet partes viejos. Felicitaciones.
    I think what you are describing is the old soviet TV only has RF input and no composite inputs?
    Can you not use an old vcr player and use the inputs through to the TV?
    Martin
     
    Gryd3 likes this.
  4. Miguel Lopez

    Miguel Lopez

    251
    61
    Jan 25, 2012
    NTSC

    Sorry, I can't. The word "purchase" should not be mentioned in your answers please. There is nothing like that here.

    Muchas gracias por sus amables palabras. El osciloscopio sigue funcionando muy bien. Saludos.

    That is the solution that some Cuban are using but sorry, I don't have an old VCR at hand.
     
  5. Martaine2005

    Martaine2005

    2,436
    648
    May 12, 2015
    Hola senor, si usted puede mandarme su dirrecsion, Yo tengo muchos de los viejos macinas. Aqui en Englatera usan 220v. Pero Yo puedo madarse con placer! Y gratis.
    Saludos.
    Martin
     
  6. Miguel Lopez

    Miguel Lopez

    251
    61
    Jan 25, 2012
    My intention is to build it myself (where is the fun otherwise?). Anyway, I will send you a PM soon. Thanks
     
    (*steve*) likes this.
  7. Martaine2005

    Martaine2005

    2,436
    648
    May 12, 2015
    Sorry Miguel. I took the oportunity to practice my spanish.
    Saludos.
     
  8. davenn

    davenn Moderator

    13,347
    1,774
    Sep 5, 2009
  9. hevans1944

    hevans1944 Hop - AC8NS

    4,258
    2,008
    Jun 21, 2012
    I am afraid there is no "simple circuit" to do this. You must take composite video, which includes sync and presumably the color burst signal on the "back porch" of the horizontal sync pulse and apply it as partially suppressed carrier with vestigial sideband modulation on a CH 3 or CH 4 video carrier. On this same carrier you must insert an FM sub-carrier 4.5 MHz from the suppressed video carrier and then frequency modulate the audio on the sub-carrier. It's possible to do all of this with discrete components and transistors of course, but it is much easier if you can find a Motorola MC1374P integrated circuit that does most of the work for you. See attached datasheet. Due to recent embargo and other trade restrictions, this part may not yet be available for import to Cuba.
     

    Attached Files:

  10. davenn

    davenn Moderator

    13,347
    1,774
    Sep 5, 2009
    which is why I gave him the discrete transistor options :)

    Dave
     
  11. hevans1944

    hevans1944 Hop - AC8NS

    4,258
    2,008
    Jun 21, 2012
    Just sayin'... if you can get 'em it's the way to go. Or visit this amazing web site which has a link to your circuit and a few dozen others. Iulian Rosu has done an amazing job of compiling references to just about anything electronic for the experimenter and radio amateur..
     
  12. davenn

    davenn Moderator

    13,347
    1,774
    Sep 5, 2009
    its crazy that there are still countries under such restrictions as Miguel has to put up with :(
     
  13. Miguel Lopez

    Miguel Lopez

    251
    61
    Jan 25, 2012
    Thanks to Dave and Hevans1944.

    That would be really nice, but there is nothing like that here. I work with scavenged components mostly, and usually from the Soviet era, so the variant with discerte component will be more suitable for me. I remember back in the 90s, some Cubans had video games stations to play using the TV. Those stations had a circuit to convert A/V to RF which used some transistors, resistors, caps, and inductors only (no ICs) and it seemed to be not so complex (for a electronic pagan like me on those times). I'm searching something like that.

    Could you lead me a bit more? i don't know where to find what I'm looking for on that site. Sorry, RF has never been my stronghold.:(

    Much apretiated. Thanks.

    Really crazy, but that is life (pure and hard), and regret solves nothing, so I have to go ahead with whatever at hand. Hope both governments can work out this situation soon.
     
  14. BobK

    BobK

    7,645
    1,662
    Jan 5, 2010
    Hi Miguel. Hopefully, things will be changing for you and your country in the near future. At least we no longer consider you terrorists!

    Edit to add: I am hoping to visit Cuba soon to really learn how to do the Rumba.

    Bob
     
    Last edited: Jun 26, 2015
  15. ramussons

    ramussons

    342
    61
    Jun 10, 2014
  16. Miguel Lopez

    Miguel Lopez

    251
    61
    Jan 25, 2012
    Nor "terrorism sponsors"!!!!. :D

    You will be welcomed.

    It would but I have two problems with that circuit.
    1-) I would need more data about transformer T1, as I don't know how ro build it.
    2-) It is for PAL format. Soviet TVs in Cuba uses NTSC. Can the circuit be adapted for NTSC?
     
  17. hevans1944

    hevans1944 Hop - AC8NS

    4,258
    2,008
    Jun 21, 2012
    This page under the heading "ATV RECEIVERS/TRANSMITTERS/DEMODULATORS" has many circuits, but unfortunately most are just schematics with few details on actual construction. They all seem to be based on a simple RF oscillator that is amplitude modulated by the composite video signal and simultaneously phase modulated by the audio signal. The ones that will interest you are among the first nine that are listed. Most of the remaining circuits pertain to amateur radio television transmission on amateur radio frequency allocations. None of these circuits produce a broadcast-quality RF signal with NTSC modulation, but they may be good enough for your purposes.

    Perhaps the simplest is this one which uses a single 741 op-amp for audio amplification to modulate the base (and hence the phase) of the single-transistor RF oscillator. The video signal is injected into the emitter circuit to provide amplitude modulation of the RF carrier produced by the RF oscillator:

    upload_2015-6-26_11-32-23.gif

    It is not necessary to use an op-amp for audio amplification. A common-emitter amplifier with a gain of 100 or so, followed by an emitter-follower to provide a low-impedance output, should work. Everything is AC coupled with capacitors between stages.

    Note the very small 10 pF capacitors used to couple audio into the RF oscillator. This is the tricky part. You want enough audio to phase-modulate the oscillator, but not so much that it prevents RF oscillation on CH 3 or CH 4. Just about any high-gain, high-frequency, NPN transistor should work for the oscillator. The 2N3904 transistors that Dave's circuit uses are inexpensive, but I don't know what the Soviet equivalent part number is.

    The adjustable inductor in the RF oscillator is about 10 μH (NOT mH!) at the 50% point of its adjustment range. I think this will be perhaps five or six turns of stiff enameled copper wire on a 10 mm diameter coil form with an adjustable ferrite core. Coil inductance depends on diameter, number of turns, and the ferrite core, so I may be wrong about the number of turns required. It may be difficult to salvage a coil-form with a suitable adjustable ferrite core. I would try using the inter-stage IF coupling transformers from a junk TV, removing all the windings and adding a few turns of wire to see if it will oscillate in the circuit on CH 3 or CH 4.

    A grid-dip oscillator would be of immense help in roughly tuning the RF coil, using whatever small-value capacitors you can find to make it resonate on CH 3 or CH 4. Note the schematic shows a variable capacitor AND a variable inductor. You only need one or the other, not both. The variable inductor is usually the easiest way to go. Also note that the 20 pF variable capacitor (also usually hard to find) is the main resonating capacitor used with the coil, but the 4.7 pF capacitor from collector to emitter is also important because it provides feedback to make the transistor an RF oscillator. Its value will also have some effect on the oscillator frequency.

    It can be frustrating working with RF circuits without appropriate test equipment, such a 100 MHz bandwidth oscilloscope and a 250 MHz grid-dip meter/oscillator. I am guessing your "test equipment" will be the TV itself. Tune the TV to CH 3 or CH 4 without connecting an antenna, except for a few meters of "twin lead" you will use to couple a signal from your oscillator circuit into the TV tuner. Power up the oscillator, then adjust (tune) the coil to see if you can blank out the "snow" on the TV using just the RF oscillator signal. Use just enough coupling from the oscillator to the TV antenna twin-lead to produce an observable effect. Once you are sure the RF oscillator is producing a signal on CH 3 or CH 4, apply video and audio from your digital converter and then farkle around with gain and tuning adjustments until you get acceptable results. [Note: farkle is American slang term that means to do whatever is required to make it work, without letting anyone know you don't have a clue about what is going on. You must farkle with a very serious face, and perhaps a frown once in awhile, until the circuit bows to your command. This definition has nothing to do with motorcycle accessories or a game involving six dice since I just now made it up.]

    All the above seems like a lot of trouble to go through, just to get audio and video into a working TV that must already have circuits to process audio and video. If you are allowed to "modify" the TV, it should be fairly easy to find the audio and video circuits that are already there and feed those circuits with the output audio and video from your digital converter.

    Best of luck, Señor Lopez.

    73 de AC8NS
    Hop
     
  18. hevans1944

    hevans1944 Hop - AC8NS

    4,258
    2,008
    Jun 21, 2012
    The audio front-end seems to be needlessly complicated, up to and including the transformer T1. And I see nothing that precludes providing video with either PAL or NTSC formatting. This is a simple RF oscillator with amplitude modulation injected in the emitter circuit and audio phase modulation applied to the base of the RF oscillator. This circuit knows squat about what kind of video signal (PAL or NTSC) is modulating the RF output. It is quite similar to dozens of other RF modulator circuits you can find on the Internet.
     
  19. ramussons

    ramussons

    342
    61
    Jun 10, 2014
    PAL or NTSC video is in the Composite Baseband. Sync, Colourburst .... the works.
    It's the audio that needs to be FM'ed to 4.5 or 5.5 MHz based on NTSC or PAL. This is the part that needs some care.
    Invert any composite video and AM it around 52 MHz and you can get it on Ch2 of a TV.
     
  20. hevans1944

    hevans1944 Hop - AC8NS

    4,258
    2,008
    Jun 21, 2012
    Which is why I originally opined that it wasn't simple. However, the audio is FM modulated on a sub-carrier that is (for NTSC) 4.5 MHz higher in frequency than the AM modulated video carrier.

    Apparently the "simple" circuits work because the FM discriminators in old TVs don't care where the FM "sub-carrier" actually is as long as there is an RF signal present with FM modulation in the tuner passband. Well, that's my theory anyway. I've never tried to build an RF modulator, since they were so inexpensive to purchase already made. I just assumed the Motorola MC1374P would do it "right," and after careful reading of its datasheet I conclude that it does.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Similar Threads
There are no similar threads yet.
Loading...
Electronics Point Logo
Continue to site
Quote of the day

-