Question for electrical engineers RE cables.


Something that has confused me for a while. I’ll try my best to explain my question:

Let’s say any type of cable, coax, ethernet, even fiber. They all are capable of transmitting multiple amounts of data on different frequencies.

As an example, let’s say cable TV, using coax. The cable can transmit VHF and UHF frequency spectrums. And I understand that to receive a “channel” (at least in the old analog electronics) it was simply done with a variable capacitor that was tuned to block all the unwanted frequencies, except the one you wanted to see.

So where my question comes in is: what’s actually being transmitted by the cable? If we have 50 channels, all on different parts of the spectrum, is it just one huge waveform with a total frequency and amplitude? And since “frequency” is just the number of voltage changes per second, and “amplitude” is just the strength of the voltages, the “single” waveform necessary to transmit the “bulk” of data is going to be huge!! and super dirty!! (don’t these waves have so many harmonics that interfere with themselves and might damage some of the data they are carrying? So assuming it’s one huge frequency carrying everything, and a “filter” at the end, how the heck does it handle the self-interference problem?

I actually understand the Fiber Cable a bit better. Same concept but with light. We can have DWDM technology to “bundle” a bunch of frequencies of IR light (usually in the 1200nm – 1600nm range), and they all run thru the same cable. But the origin needs to have an SFP for each individual frequency, so you can easily end up with 8 emitters, and your DWDM device at the other side essentially acting like a prism. And light tends to not interfere with itself, so it’s happy to send multiple IR signal on the same fiber, and easily split at the other end.

It’s really just the electrically based cables that baffle me.

In: 8

>is it just one huge waveform with a total frequency and amplitude?

It’s one waveform that is the sum of many frequencies. Using fourier transformation you can split the signal into it’s components each with a different amplitude.

Here is an example of what the waveform looks like when you overlap 2 different sine signals

>don’t these waves have so many harmonics that interfere with themselves and might damage some of the data they are carrying?

The channels are chosen in a way that their harmonics don’t overlap, and they are filtered mathematically, so you can reconstruct the harmonics and subtract them.

>And light tends to not interfere with itself

Light also has harmonics. The two technologies act very similar just with different parameters.

Cable television and fiber optic cables transmit information using multiple “carriers” or frequencies on the cable. Catv actually has 75+ individual channels and frequencies being sent down the cable simultaneously. The set top box or tv knows those frequencies and can lock onto them for the receiver to lock to. Look up us catv chart to find the frequencies being used.

Digital TV is just encrypted digital signals on the analog carriers that the stb then decodes. Each carrier can have several subchannels encoded onto the frequency- two 1080p signals or up to eight 480i signals.

Catv techs have meters that will show the signal strengths of all the carriers on the cable so they can make adjustments to the amplifiers to ensure all signals are at the correct strength in your house.

The federal government requires each catv channel to occupy a set bandwidth and cannot bleed over into adjacent channels. To cause interference is to receive large fines (in the us) from the fcc. The catv operator is responsible for that plus the broadcaster that sends to signal to the catv system.

UHF/VHF use frequency modulation. You start with a carrier wave at a relatively high frequency and then slightly increase/decrease the frequency. If you track the deviation from the carrier frequency over time, you end up with a waveform representing the desired signal.

You don’t get a self-interference problem with harmonics because the frequency range is so small in comparison to the frequencies themselves. If you’re using the numbers 2 through 9, you run afoul of all sorts of harmonic issues (2 with 4, 6, 8; 3 with 6, 9; 4 with 8). If you’re using the numbers 92 through 99, you don’t have any harmonic issues.

Cable TV itself traditionally used a form of amplitude modulation (constant frequency, but the amplitude of the waveform describes the signal). Modern cable television is entirely digital and normally uses Quadrature Amplitude Modulation. QAM takes advantage of the fact that you can vary three features of a waveform (amplitude, frequency, phase) and varies two of them to designate points on a 2d plane, permitting the transmission of multiple bits with two waveforms.

In any case, all of these mechanisms can simply add waves on top of each other because each of those waves has a sufficiently different frequency that they can be separated from one another at the destination.