HD signal over a coaxial cable - AVS Forum
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post #1 of 7 Old 01-07-2007, 05:04 PM - Thread Starter
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recently I was wondering why a cable company such as comcast can transmit 200+ channels with a couple of HD channels(and their 5.1 sound) signal over a cheap coax. cable and why as a consumer, we have to use an HDMI or component cable to get a HD signal to their T.V. Is it a compressed file they send which is why you need the cable box or if...I can't think of anything else?
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post #2 of 7 Old 01-07-2007, 05:22 PM
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It's all a matter of what signal is being transmitted and how much of the cable's transmit capacity is being used.
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post #3 of 7 Old 01-07-2007, 05:28 PM
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It's compressed via MPEG2 coming into the cable STB, your average HD channel is around 14 or so MB's per second. The signal is uncompressed and passed to the TV, the average HD channel uncompressed is around 800 MB's per second.

Of course if your TV has a cable card or built in QAM tuner, it's capable of doing the uncompressing internally.

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post #4 of 7 Old 01-07-2007, 08:47 PM
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Yes the signal is compressed as it goes over the coax cable, but the reason that so many channels can be sent at the same time is due to modulation. You may already know this, in which case ignore the rest of this post.

if not.... A cable carries an electrical signal (obviously). The signal is composed of many different frequencies, just as sound contains different frequencies. The cable company allocates a certain portion of these frequencies for each channel. Since every channel only needs, say 4 Mhz (i have no idea if this is right, just an example) to be transmitted, many channels will fit within the frequencies that can be sent over the cable. (channel 1 in 1-4mhz, channel 2 in 5-9Mhz and so on). The hardware that turns a certain channel sent over the cable into a picture on your screen must operate at a specific frequency. So you need a tuner (NTSC, ATSC) to demodulate the channel you choose from the signal that arrives from the cable company. Basically, the tuner tunes in to the frequceny you select, filters out all the other channels, and resends the selected channel to the picture hardware at the frequency it needs. (rather like concentrating on your conversation despite being in a crowd of people). That's the general idea.

Also, if the channel is sent digitally, several channels can be multiplexed on the same frequency that would usually hold only one analog channel. Basically, they send channel 1 for a while, then channel 2, then channel 3, and so on, and cylce back to channel 1. If they do this fast enough, then the decoding can take place without any real delay, since tv is broadcast at 25fps (?). With digital channels, the cable company can prefix the data with a tag that says which channel it is. With analog signals, the same technique would be possible in theory (i would think) but you would have no way of telling when channel 1's information stopped and channel 2's began. Keep in mind the cable company still has separate frequencies to on which to broadcast multiplexed signals.

As for using the same coax in your home instead of hdmi... as mjones73 said, you can. You just have to do the tuning with a tuner inside the tv. Incidentally, if you go to radio shack, you can pick up a modulator. These are used for old tv's with no composite + rca audio inputs. you plug the composite and rca audio in and the output is an old analog signal that you can put into the cable/antenna input of the tv. you can usually select channel 3/4 to broadcast it on. also, remember your vhs player has a switch to select channel 3/4 to display it's own output on. this is all the same thing.

i hope this answers your question, though perhaps more long-windedly than you would like.


p.s. if you're sick of expensive hdmi cables and the like. try www.mycablemart.com
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post #5 of 7 Old 01-07-2007, 09:07 PM
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American TV channels are 6MHz wide. Using 256 state quadrature amplitude modulation (256 QAM) a cable company can send about 38 Megabits per second on each channel. American TV has a field frequency of 60/1.001 or about 59.94 fields per second. This does not apply to 720p HDTV which is normally transmitted at 59.94 frames/second. Coaxial cable can carry the very high bandwidth signals that uncompressed HDTV contains but it can not carry it very far (think a few hundred feet maximum).

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post #6 of 7 Old 01-08-2007, 03:26 AM - Thread Starter
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I understand now, thanks. I guess that's why there ar so many cable companies. Thanks for the link, I'll check it out when I get home.
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post #7 of 7 Old 01-08-2007, 06:47 AM
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You could also check out forum sponsor monoprice.com for cables.

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Please don't PM me with technical questions that should be posted in the forum.
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