Public channels still need a digital de-converter box. All other channels must have a box from the provider that has a descrambler in it. They encode their signals so that you can't pirate it without a box.

Cable was (is in some parts of the world) analog in the US and used FDMs. FDM has bandwidth limitations resulting in a limited number of channels -- similar to how radio has a chunk of AM and FM frequencies to broadcast in.

With digital cable, the channels have increased dramatically, since there really isn't a frequency range limitation. The only requirement now is a high speed connection for uninterrupted signal transmissions. Television, phone, and internet all operate on their own frequencies on the line...similar to how DSL and telephones could co-exist. But with television, the box connects to the provider and requests the channel and the provider streams the video over, being decoded by the box.

 

And why did you have to go thru the math.
When we had f(t) which was the sum of all the waves.

You could have just said use the filter on f(t).

Yes, but that would be inefficient, because we would need a filter for every frequency we wished to receive. We would also still need to translate the signal to baseband to process it. It is easier to use a mixer to translate the frequency we want to receive to a common frequency, because then we only need one filter rather than many. In the example above, f(t) represents the signal on the incoming coax line. g(t) is generated by the tuner in your TV or computer. g(t)*f(t) is the output of the mixer that is fed to the filter.

And I guess I don't see why bandwidth matters so much.
What is the point of spacing the channels out more with more bandwidth
have to do with anything. You still have to extract the correct channel from the resulting wave. Witch I can't see being easier with more bandwidth.

Bandwidth has nothing to do with how easy it is to "extract" a signal. It's an important parameter for designing the filter though, as we need to pass the entire bandwidth of the signal we want.

Their are different types of multplexing out their so what does the TV broadcasting companies using frequency division multplexing or time division multplexing.

It depends on what type of TV you're asking about. Analog TV and cable transmit one channel per frequency. Digital TV transmits multiple channels per frequency. Both transmit on many frequencies.

But with frequency multiplexing the all are going at the same time.
Seperation is still to me impossible.
Sorry if I don't get it but thanks for the help.
I would love to know if given a resulting wave if their is some mathematical formula to decompose it back into it's component waves?

The answer to this probably NO. But we can use Fouriers series to approximate it I guess. But all this seems impossible for a filter to compute
having to deal with speed's close to light.

No. This is what the example I gave above showed. We can take a signal that consists of many sinusoidal components and extract any one of them using a signal generator, a mixer, and a filter. Going beyond the mathematics into the real world, the electromagnetic spectrum is composed of many different signals, yet we can still extract any individual signal we wish. Our eyes see different colors, the radio tunes different stations, etc. etc. etc.

Then what is stopping me from watching TV on my computer without a TV tuner card. At least the public channels!

I could just run a wire from the coaxial cable to the audio port of my computer. And write a program to translate it into the correct pixel's.

I guess I would need a tuner to listen in on a channel specific channel.
But even still could I get a mixed picture of a ton of different channels?
Maybe the input voltage's for the audio port and the coaxial cable are way off.

If you just plugged the coax into your audio card it would not work. All you would get on the output was noise. However, if you built a tuner that would tune the channel you wanted to see to baseband and a filter as described above, you would be able to (if the sampling rate of your audio card was high enough) watch TV on your audio card. A high end audio card combined with a cable modem tuner is a popular DIY software radio.

You said the coaxial cable has it's waves as square wave's (i.e digital waves)

But I thought they where still using anolog signals until the end of 2009.
Maybe they are just talking about air wave signals being changed to digital? Because if they had to change it for the cable line then we would need a DAC for are TV's.

Was Coaxial Cable always transmitting digital signal's ??
"Digital Cable" was it ever "Anolog Cable"

Digital TV/cable does not transmit square waves, as they have infinite bandwidth. Instead, we vary the carrier signal to indicate a one or a zero. Digital cable, for example, varies the amplitude. 802.11g varies the frequency. 802.11b varies the phase.

 

welp, i learned something new. thanks for clearing that up! i wasn't aware coax still used sine. it would explain how they get the three services over the cable... i completely missed that in my pondering of the innerworkings! i'm man enough to admit i was waaaaaay wrong on that

 

YOU SAID


I am unclear what bandwidth really is?

just like when you tune across a regular broadcast band radio dial you come to the beginning or lower end of a station, tune across it until it fades out at the upper end, that space from where the signal begins to where it ends is its bandwidth, the space between the stations is called a guard band.






Basically if you are transmitting different waves at the same time you add their height's to get the resulting frequency. But then how do you go from the resulting wave back to a specific wave. Mathematically I don't see how this is possible???

the height of the waveform is its voltage, not its frequency. The frequency is how many cycles it goes through in one second of time.with am the modulation that is impressed on the carrier changes its voltage, while in fm it changes its frequency. If you have an amplitude modulated signal with 100 percent modulation there will be a carrier frequency at its center with voltages called sidebands on either side of the carrier which are 50% of the carriers voltage and removed from the carrier by a frequency distance equal to the frequency or pitch of the modulation or tone. so a wider frequency response will need a wider bandwidth for the total signal (carrier+sidebands). A regular am broadcast band signal can have modulation up to 15khz and needs more bandwidth than say a communications transmitter that might only transmit frequencies up to about 6khz for voice communications.

 

Hi Mathematics,
I think you should look up about Fourier transform in order to understand more about Frequency Division Multiplexing. But here is the basic idea :
Each TV channel is assigned a certain bandwidth for example :
channel 2 from 174MHz-180MHz
channel 3 from 180MHz-186MHz
and so on...
If you look at the signal in TV cable using an oscilloscope (this means you look at them in the time domain), it will look like a mess, all of them are superimposed(added) with one another. I mean you won't be able to tell which signal comes from which channel. However, if you look at the signal using a good spectrum analyzer(this means you look at them in the frequency domain), you will see clearly that the signals can be separated from each other. Hope this helps.

 

Time Division Multiplexing can be thought of like a traffic light at a busy intersection. Just as the lights let groups of cars go through from different streets, one at a time, the multiplexer with a clock signal to syncronize everything, lets different channels each have a slice of time, the signals are then sorted out at the other end of the transmission medium(be it airwaves,a fiber optic cable,or coaxial cable)by a de-multiplexer. Also data can be added to each signal as it gets a green light to identify it from the others at the other end,and data to aid in error correction can also be added.