Video signal via coaxial cable (1 Viewer)

[agnerc]

It seems like the coaxial cable is the only video cable coming in from the wall, being able to deliver and type of signal, HD, etc.. so why in the world do we have to go out an buy all these other cables, like HDMI, Component, etc.. why not just have everything hooked up via coaxial?

I mean is coaxial really that good of a cable to whee it can deliver an HD signal?

Chris

 

[Bob McElfresh]

Well all RCA cables are in-fact coaxial. If you cut them - they look just like your CATV coax.

But electronics have to split video into 3 parts, then add in L/R analog audio, or 5.1 digital audio. So thats 4-5 different signals into equipment with separate inputs for each (Video to the TV, audio to the receiver).

This is why we have so many different cables.

While HDMI does have issues - it is a single-cable solution to everything. It's not my favorite, but it works.

 

[Robert_J]

The signal coming over the cable from your wall is compressed and carries your entire cable lineup (or partial lineup if your cable company uses switched digital systems). It must go into a cable box, TV or HD TiVo to tune in the correct frequency to get the channel you want. Then it is decoded and sent over component video or HDMI in an uncompressed format.

Satellite is similar except that the cable from the dish only carries part of the entire lineup. Sat companies aren't constrained by the bandwidth of the cable but by the FCC and the frequencies that are allowed to broadcast from each satellite location. Your receiver, multi-switch and dish all talk to each other and determine which polarized frequency to tune into as well as which satellite location to get the signal from. There is some equipment that will take signals from multiple satellites and 'stack' them onto the same cable. Then you must have a 'de-stacker' at the satellite receiver to convert them back to a signal that it understands.

-Robert

 

[agnerc]

So basically to clarify the signal that is coming in from my Satellite dish is the best the signal will ever be because it is coming in uncompressed? If so, then my question was why not use only coaxial cables like the one from the satellite dish if it carries video and audio. Right now you can simply go directly into the tv, or DVR and out, so why not also have the receivers accept coaxial cables as well? I guess it jsut is not very clear why we have so many different cables when the cable coming in from the satellite works perfect?

Chris

 

[chuckg]

The satellite cable carries radio signals. The tuner then picks out the one signal you want, and separates it into audio and video signals.

The audio signals go on audio cables, sometimes 2 or 5 separates wires. Or it may be digital on one wire or one optical connection. Audio may be combined with other wires inside an HDMI cable.

The video signals could be "composite" which is pretty much the same signal you get from the cable in, but with the "radio" part stripped off so you get only the video signal ( called NTSC ) Video can also be carried on two cables, which carry color and brightness information - we call it S-video. Then there are the three-cable video, which has brightness, and two crazy mixed up signals that convey color information - we call it component video. And, of course there is now digital video carried on an HDMI cable.


Make sense? of course not. We could cram all the signals together and stuff them on RG-6 coax, but then every part of your system would have to agree on how we cram it all together, and be very good at separating the signals. the way things are now we just need to hook up a couple cables, and don't have to fool around combining and separating things at every device.

Fortunately, HDMI might someday replace all these various types of connections with one simple, convenient, high-quality cable. We hope!

Of course, if you want to run video for any real distance (over 50 ft) you will need coax anywho.

 

[Joseph DeMartino]

A coxial cable is just a wire. It carries an electrical signal. "Coax" isn't a technology, it is a transport mechanism. What makes different connections, well, different, is what is being transported.


No, the signal is still compressed because they don't have unlimited bandwidth, either. Your cable company's signal is limited not by the bandwidth of the cable between your wall and your box, but between the head-end and your wall - that is the total bandwith in their system. Because satellites aren't delivering a signal over a piece of wire, they are less restricted, but not unrestricted, in the amount of bandwidth they can. (The satellites themselves had limited capacity, and then there are the FCC regs frequency restrictions that Robert mentioned.)

What is coming out of your wall is a digital signal that can carry voice, data and voice and keep them all separate. Your satellite box decodes and uncompresses this data stream. Then it outputs the result in one of several ways. HDMI lets it send sound and image digitally. Component sends the video in analog form, but divides colors to deliver a purer and more accurate image. Digital audio (coax or optical) sends the audio portion of the signal out as a bitstream while the stereo audio outputs the sound in analog form. The coax output from your cable box transmits both the video and audio signals in analog form and at NTSC resolution. These outputs and inputs are antenna leads designed for a TV system that is nearly 80 years old. They are the lowest quality connection you can have between two a/v devices, not because of the cable, but because of the limitations of the connections and the technology being transmitted through that cable.

A high-quality RCA stereo audio cable and a digital coax audio cable are physically about the same. But one carries only a mono, stereo or matrixed surround signal where the channels inevitably bleed into one another while the other can deliver anywhere from 1.0 to 7.1 discrete channels of digital sound. Same wire, different result.

If what you're suggesting were true everybody would use coax. The problem is with your assumptions.

Regards,

Joe

 

[Stephen Tu]

Uh definitely not, it's highly compressed, encoded, encrypted. Why don't we use simple, single coax for everything? In principle it would be possible, but it makes the electronics a lot more complicated. You'd have to build the satellite tuner circuitry straight into every TV, and since only a minority of people use satellite and people don't like to pay for stuff they aren't using, this doesn't happen often (there used to be a few TVs with built-in DirecTV but they are gone now). It also makes introducing newer better codecs or encryption schemes difficult since it's easier to get people to swap out a STB than to swap out an entire TV.

Basically it's cheaper to use more complicated wires to carry more, simpler signals, than to use a single wire carrying very complex signals but then have to put all the decoding logic in the TV.