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TL;DR:

I would like to transfer digital serial data over an existing analog TV cable. How do I do that?

 

Full Question:

In order to transfer some data to a device's control lines remotely, I'm looking into inexpensive, but reliable methods that would use existing in-place system.

Suppose there is a cable running in the client's home that transfers analog RF-band TV signals (e.g. from an RF modulator into an old TV). I'm looking for a DIY solution, or a prebuilt consumer-level product that would be able to send (and receive) some serial data over the standard analog TV coaxial cables.

I would like this system to coexist with the audio/video signals (VHF or UHF) on the line, and don't interfere with the current devices. Additionally, power will be delivered to the devices at each end. I'd also like the transmitters to be able to transmit at different channels, and decode the data back by selecting the channel.

I require both sending and receiving (either full-duplex or half-duplex) serial data (UART, TTL, etc) at a simple baud-rate (e.g. 9600, or if possible 115200 bps), reliably.

Question: Would I be able to build a DIY solution, or design an inexpensive board to handle my request? Are there any solutions that would make this possible?

I was not able to find such a device or any schematics by searching the web.

Since I'm not familiar with the analog RF-band TV, I do not know how I would design something, if anything. I would prefer either an existing schematics, or an existing product that would do this. What route should I go?


Concept schematics:

schematic

simulate this circuit – Schematic created using CircuitLab

(n.b. If possible, I would like to have multiple transmitters and decoders.)

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    \$\begingroup\$ Would I be able to build a DIY solution - I could but i have no idea if you could. Maybe not? \$\endgroup\$
    – Andy aka
    Aug 5 '20 at 21:11
  • \$\begingroup\$ @Andyaka as long as the components are generally available, and I understand what I'm doing, I'm willing to invest in a DIY solution -- especially when I can replicate it in my designs. \$\endgroup\$ Aug 5 '20 at 21:12
  • \$\begingroup\$ Is it possible? Sure, that's what a "cable modem" does. Are you going to be able to implement it? Based on posting the question you did, that is a bit uncertain. You might be able to get a low pass filtered half duplex serial channel through at baseband, or you could look at trying to build some sort of network to adapt telephone modems or low frequency FSK radio systems. But there are going to be a lot of challenging to getting it to work, which are far beyond the realm of an SE Q&A site page. Also questions seeking products to buy are off topic. \$\endgroup\$ Aug 5 '20 at 21:41
  • \$\begingroup\$ Does this answer your question? How would you combine DC power, bi-directional serial data, and an RF signal on a single 50-Ohm coax feedline? You don't need DC power, but the rest fits your requirement exactly. \$\endgroup\$
    – Dave Tweed
    Aug 5 '20 at 21:54
  • \$\begingroup\$ maybe a serial connection using a UART could work \$\endgroup\$
    – jsotola
    Aug 5 '20 at 22:23
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TV coax is terminated in 75 ohms. If we contribute 1milliAmp to that, we get 75 milliVolts signal. With double_ended termination on TV cables (often the case), you would get 37.5 millivolts. So let us use 2 milliAmps, and have at least 75 milliVolts of data signal.

As transmitter, you can use a TTL gate running on +5volts, or a CMOS gate running of +3.3 or +5 volts.

From the TTL output, you need a series/shunt/series RCR. The capacitor is to greatly slow the TTL edge, so the RF TV signals get no interference. The sum of the 2 resistors has the purpose of providing at least 2 milliAmps into the TV coax cable. With 3 volts and 2 milliAmps, the sum of the 2 resistors needs to be 1,500 ohms.

If each is 750 ohms, the equivalent at the middle node is 375 ohms, and 1,000 pF from middle node to Ground produces 375 nanosecond time constant or about 500,000 Hertz bandwidth, This should easily support 100,000 bits per second, yet cause little interference with the TV signals.

For signaling, use a self-clocking PWM signal, with Rising edge the "start", and the pulse width being either "1/3" bit time, or "2/3" bit time.

For your receiver, use a medium speed (100 nanosecond Tpd is fine) analog comparator, with threshold at Vin/2 or 35 milliVolts. Have 5 milliVolts of hysteresis. This uses the Vin+ pin and the Vout.

On the Vin- pin, have a series R and a Shunt C; 3,300 ohm resistor and then 100pF cap right at the Vin- pin. You should add more capacitance there, if you have Bit Error problems. [had been 330 ohm and 1,000pf, but that low R would have caused reflections.]

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As Jason suggests in the comment below, the low_frequency behavior of the cable has strong influence on the signaling method chosen.

I assumed there WAS DC_TERMINATION into 75 ohms.

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  • \$\begingroup\$ yeah,for unidirectional signalling a simple low-pass filter like you describe would work. TV antennas are often not DC terminated the 75 ohm termination is imprecise, and only effective above 20MHz or so. \$\endgroup\$
    – Jasen
    Aug 6 '20 at 3:06
  • \$\begingroup\$ I'd consider manchester or biphase code instead of PWM-per-bit because a balanced signal makes data slicer design easier. \$\endgroup\$
    – Jasen
    Aug 6 '20 at 3:15
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To include VHF+UHF and other signals + power, we would call these Out-of-Band signals (OOB). Since each requirement is unique, there is no generic solution so there are many ways, some which may appear to your skills and tools.

I have personally been involved in R&D in '70's and early 80's with many designs, not for DIY projects though. So your level of learning and commitment may differ from these requirements.

In 1979, Project Ida used DS1 (1.544 Mbps) full-duplex (FDX) between an outdoor pole-mounted repeaters to 100 homes using 6MHz and 12MHz AM carriers with Raised Cosine LC filters. Then VHF for long haul. These are synchronous 24 channel telephone + Data streams not async UART data.

In 1977 I chose biphase over FM using a simple PLL IC for upstream Rx and baseband biphase for downstream @ Mbps data rates.

In 1982 we designed a 4Mbps modem with an inexpensive Mitsui 6MHz BW TV receiver and RF modulator IC. Yours doesn't have to be SSB-RF.

You can look at LC filters and make quick 6th order Bessel designs with tools like Falstad's (>Circuits>larger passive filters). Then find better tools elsewhere.

Then consider FDX chips like the CMX582 200 kbaud chip, but you must master the art of zero inter-symbol-interference (ISI filters ) that ring at the bit boundary instead of group delay distortion in order to get those nice "eye-diagram waveforms"

You could choose Odd Parity and/or CRC or use an Ack/Nak protocol for feedback for reliable communication.

Adding power is like PoE with a suitable choice of PHY or could be step-down 50/60Hz but that might cause baseband noise with load.

This project is not for the faint of heart.

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