EMF2024 Cable TV talk, supplementary notes

This page is here to answer questions people have asked about my EMF2024 talk, “Being Elliot Carver: how to build a cable TV network in your living room”. The slides are brief summaries by necessity (it's a 30-minute talk) and it really wasn't practical to talk about everything.

If you have any questions (or just want to geek out about analog cable/satellite TV) then please feel free to email me: philpem@gmail.com

Media links

  • Slides - these are slightly different to the presented ones, to fix a typo and add a missing image.

Most video generation and processing is done at baseband, which means composite video or S-video (separated video, AKA luma-chroma or Y/C).

Generally speaking, most consumer-grade equipment will use RCA jacks for the inputs and outputs, and most broadcast-grade equipment will use BNC jacks.

Standard audio/video cables are fine, and good quality ones can be had for relatively little money on the likes of Amazon. It's worth getting some BNC to RCA adapters (generally BNC plug to RCA socket is all you need). Tee pieces can be useful, but only if your video equipment has switchable termination.

Everything in the video realm is 75 Ohms. Don't mix 75 Ohm BNC connectors with 50 Ohm or you're going to have a bad time. About the only way to tell them apart (if they're unmarked) is by checking the diameter of the centre pin with a go-no-go gauge or other measuring tool.

Domestic modulators will usually (almost always) have a high-impedance line-level input.

Commercial modulators usually accept balanced audio on either screw terminals or XLR. They can often be configured for low impedance (600 Ohms) or high impedance (several k-ohms or more).

If you want to connect an unbalanced source (a jack-plug line-level output) to a balanced input, you'll need to check the manual for the correct wiring. Usually it's some form of connecting the signal line to positive-input, and the ground to both the negative-input and ground. Remember to set the input impedance to “high”.

Daisy chaining: Modulators can be daisy-chained if they have loop-through RF ports. One port is the input, the other is the output; usually they're labelled. You should terminate the unused input with a 75-ohm Type-F termination plug if you have one, but it's not essential.

Combiners: If the modulator only has an output, you can use a combiner to put multiple channel signals on one cable. If you can't get a combiner, a non-amplified (passive) splitter in reverse will work as well. You'll also need one of these if you want to add DataChannel (GI cable box) or NABU signalling to your cable using an SDR or other transmitter. If you need more inputs, you can stack combiners, but at that point you will need a selective level meter or spectrum analyser to get the levels right.

SDR precautions: If you're using a SDR to generate signals, get a minimum-loss pad to convert from 50-ohm to 75-ohm. Couple the SDR onto the network through an RF LNA to protect it. This way, if there's a strong reverse signal, it'll blow the cheaper LNA instead of blowing the SDR's output. This applies especially to the HackRF One, which has a notoriously sensitive RF output switch and amplifier setup.

Ideally you want a good-quality foam-core, low-loss, 75-ohm, CT100 style coaxial cable. Webro WF100 is ideal, if you can find it. Screwfix sell Time GT100 which should be just as good. I've also used the 30-metre packs of B&Q's own-brand cable which is okay but not quite as nice as WF100.

If you have an old reel of CT100 satellite cable kicking around, you can probably still use it, but don't use it for any outdoor or permanent wiring applications. The air core tends to wick moisture down the centre of the cable.

My suggestion is to try and stick to Type-F connectors, and use adapters to convert to other connector types. That applies especially to Belling-Lee (“UHF” or “antenna plugs”). Save yourself the aggravation and buy some Belling-Lee adapters. The most useful ones are:

  • Type-F female/jack to Belling-Lee male/plug: used to connect a Type-F cable to a TV's antenna input.
  • Type-F female/jack to Belling-Lee female/socket: used to connect a Type-F cable to the RF-out of a VCR, or an antenna wall socket.

I also find the following useful on occasion:

  • Type-F coupler: used for joining two Type-F cables end-to-end.
  • DC block: good for protecting SDRs from return DC; mostly useful when connecting satellite receivers.
  • Attenuator: good for protecting SDRs from return RF (impedance mismatches). Especially useful for the HackRF One.
  • Minimum-loss pad: for connecting 50-ohm test equipment (spectrum analysers and so forth) to 75-ohm cable networks. Resolves the impedance mismatch between the two 'worlds'.

Antenna inputs on the back of TVs are almost always Belling-Lee sockets, but antenna wall plates may be male or female.

I use and recommend the PPC snap-seal compression-type plugs. Main reason being they push onto the end of the cable, which chews up the skin on your fingers much less than the screwing motion needed for standard screw-on F-types. The compression gland also makes it harder to pull the cable out and creates a decent seal (which isn't much use for indoor applications).

Starter kits are available from Inbrackets. I have this one), which costs about £16 and includes the cable stripper, compression tool, 20 plugs, and full instructions. If you prefer, they sell the same kit in their ebay store.

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  • Last modified: 2024/06/07 13:54
  • by philpem