Just bought a new TV! It's "smart" but where my previous dumb TV didn't, my smart TV introduces a delay over HDMI from my Sky+HD box compared to the RF2 output which is wired via coax cable to the other 3 TV's in the house. My research tells me this is likely due to the way the smart TV decrypts the HDMI feed. My 3 dumb peripheral TV's do not have any facility to introduce a delay so I have to ensure the volume is low on the kitchen TV otherwise there is a fiendish echo. I have previously enjoyed electronic construction and am intrigued to know if I have any chance of being able to construct a circuit that could introduce a delay into the RF feed, preferably the powered RF2 output from the Sky box. I believe coax cable can be modelled to the following circuitry: enter image description here

It therefore seems to me that if I was able to calculate values for these components, I might be able to introduce a variable phase shift that would solve my problem. That is, I might fool the feed to think that the cable was a mile long. Perhaps that would result in signal loss/degradation? Can a 200ms delay be done? If so, how? I have found that one can purchase HDMI splitters and modulators. Perhaps that is a more logical approach, for example, that I distribute the split HDMI feed over the coax lines? However, I suspect I might still end up with asynchronous results. I really have done a lot of research and will do more, but would value all opinions and advice. Thank you in anticipation.


closed as off-topic by Chris Stratton, JonRB, Dwayne Reid, Edgar Brown, JRE Dec 26 '18 at 18:57

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  • \$\begingroup\$ Could you explain what a "Sky+HD box" is? \$\endgroup\$ – Hearth Dec 10 '18 at 22:43
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    \$\begingroup\$ I'm voting to close this question as off-topic because it is an XY problem. Delaying a proprietary satellite TV RF signal is absurdly expensive and impractical - you would either need a very high frequency sample, memory, and reconstruct system, or around a hundred million meters of cable which would of course introduce unacceptable loss even if you had the money and space. You must find a different want to solve this end user problem. \$\endgroup\$ – Chris Stratton Dec 10 '18 at 22:56
  • \$\begingroup\$ Ok Chris. I think I have my answer then. Thank you. \$\endgroup\$ – Phil Dec 10 '18 at 23:05
  • \$\begingroup\$ Hearth, it is a proprietary satellite receiver system. \$\endgroup\$ – Phil Dec 10 '18 at 23:06
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    \$\begingroup\$ Transmit the signal to be delayed up to your own geostationary orbital satellite and back. The time delay is about 240 ms if you're on the Equator and 280 ms if you're on the edge of satellite line of sight. \$\endgroup\$ – Transistor Dec 10 '18 at 23:33

Delaying an RF signal by a large amount of time while keeping the distortion low, with a reasonable bandwidth is actually quite difficult. A SAW delay line (Surface Acoustic Wave) may be the solution but 200 ms is quite large, the one I have seen are limited to hundreds of microseconds.

  • \$\begingroup\$ Thanks pserra. I now believe this may be impossible. \$\endgroup\$ – Phil Dec 10 '18 at 23:07
  • \$\begingroup\$ It can be done. Satellite path simulators are a thing, and would create something on the order of magnitude of the needed delay (along with modeling other effects). But they cost far too much for a consumer purpose. You'll have to find a solution at a different point in the overall problem than the RF path. \$\endgroup\$ – Chris Stratton Dec 11 '18 at 3:41

You should be able to achieve this with an off-the shelf RF modulator.

Take the composite video directly from the Sky box and the audio output from your main TV and feed them into the modulator, then drive your peripheral TVs from the output. Depending on the modulator and peripheral TV capabilities you might be able to split the HDMI and use that for the video, which would align the picture and sound better on the peripheral TVs.

I have exactly the same problem but I've not bothered trying to solve it myself other than using the volume controls as you're doing right now.

  • \$\begingroup\$ Thank you Finbarr. I shall investigate. One of the peripheral TVs is not full HD but it sounds like the HDMI splitter/modulator might be the way to go. I'm most grateful for your input. \$\endgroup\$ – Phil Dec 11 '18 at 15:31
  • \$\begingroup\$ The choice of modulator really depends on whether your peripheral TVs can all receive digital TV signals. If any can only receive analog you'll have to use the older style, that in turn will probably dictate what sort of video signal you can feed into it. \$\endgroup\$ – Finbarr Dec 11 '18 at 15:48
  • \$\begingroup\$ So I stick with RF modulator to keep comparability with the older TV. I use the audio from the main TV but the composite video from the source box. Will that mean that the sound is likely to be out of sync with the video on the peripheral TV's? \$\endgroup\$ – Phil Dec 11 '18 at 18:51
  • \$\begingroup\$ Probably yes, but subjectively it'll be a lot better than having the echo. \$\endgroup\$ – Finbarr Dec 12 '18 at 10:05

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