# A really basic question about the difference between satellite and cable



## Dilerium (Dec 3, 2005)

I am getting one of those free TiVo's installed tomorrow and that inspired a question.

With cable TV, even digital, I can run a single coax cable into a room and split it to two or more TV's, VCR's, etc. with no problem. Of course, there are eventual limits, but the splitting is easy. With DirecTV, however, two seperate coaxs have to come into the room for the two TiVo tuners. Why can't one coax be split? What is it about the way the satellite signal is carried that prevents this? I assume it has something to do with the multiswitch, but I'm not sure.


----------



## SpacemanSpiff (Jan 31, 2004)

http://www.tivocommunity.com/tivo-vb/showthread.php?t=55509

Check this FAQ.


----------



## DavidS (Sep 27, 2000)

The lay answer (the only one I understand) is that the DTV channels are divided into two groups and one cable can only see the channels on one group at a tiime. This, if you tried to have two tuners with one cable, you could only view half of the channels at the same time. With two cables, you can cover all channels.


----------



## Diana Collins (Aug 21, 2002)

In order to use the bandwidth available between the Dish and the Satellite most effectively, alternating transponders (think of them as radio transmitters) use opposite "polarity." Without getting into an explanation of RF polarity, suffice it to say it minimizes crosstalk and other interference between adjacent channels.

When these signals reach your dish, it requires a receiver to be set to the same polarity as the incoming signal. That's why you need "dual LNBs" to receive all the channels on a given satellite on two or more receivers.

Back when satellite started, and you only received from one satellite slot, the designers decided to use the same downlink frequencies (the frequency on the coax) for both polarities. So, when your receiver changes channels, it may "flip" the polarity that is being sent down the coax. If you just "split" the wire to two different tuners, one could ask for one polarity, while the other wants the opposite polarity. Since they use the same frequencies on the wire, they can't BOTH be sent at the same time.

This is solved by using a technology known as "stacking." This puts BOTH the polarities on a single coax by using higher frequencies for one polarity - IOW, stacking one on top of the other. Once stacked, you CAN just split the coax, and then "unstack" it at the receiver. A number of DirecTV receivers used to have built-in "destackers" and these were very popular in multi-family dwelling installations.

The problem today is that we now also have multiple satellite slots. This has made stacking considerably more complicated. Dish Network has a very complex "agile stacking" system, where the multiswitch can put different combinations of satellite slots and polarities on a single coax, allowing a two tuner receiver to be fed with one wire. The multiswitch in this case is VERY complicated.

DirecTV is using a 3 tier stacking system in the new AT9 high definition dish. Here they stack a Ka signal set below AND above the "standard" frequency range, which carries a Ku signal set. In this arrangement, you still need one cable run per tuner, but you only need 4 wires from the dish to the multiswitch, instead of the 8 that would have been required without stacking.

Theoretically, you COULD build a dish that simply put all the signals from multiple satellites onto a single, "split-able" coaxial cable, but you'd need to go all the way down into low VHF carriers to do it, which requires a lot of power. Such a dish would therefore need it own dedicated power supply (you couldn't send it up the coax, as we do now).


----------



## SeattleCarl (Nov 11, 2005)

Dan,

Where can I find the actual frequencies that the LNB outputs - i.e., frequency versus transponder and channel - for DirecTV? I'm referring to specific, as opposed to general (950 to 2150 MHz is general). I have seen countless explanations of 13V versus 18V and 22KHz on/off, and understand how all of that works. But what I have not yet found (in casual searching) is the actual breakout of of transponder and channel versus LNB output frequency list.

(Edited to remove an incorrect assumption on my part. Also, searching on LNB Intermediate Frequency led me to the answers - Thanks Dan for pointing me in the right direction.)

Thanks,
Carl


----------



## Diana Collins (Aug 21, 2002)

To get the downlink frequency (called the IF or Intermediate Frequency) simply subtract 11.300MHz from the transponder frequency.

For example, DBS Transponder 1 operates on 12.224GHz, so the IF is 12.224 - 11.3000 or 0.924GHz, or 924MHz. Transponder 2 is 12.239GHz, so its IF is 939 MHz, which is too close to 924MHz to coexist on the same cable when each frequency is 24MHz wide.


----------



## Dilerium (Dec 3, 2005)

Thanks, all, for great explanations. I now feel that I have a much better handle on how the whole thing works. The thing I was missing is that the multiswitch is really the key to the whole scheme. Without that, the DirecTV doesn't work.

So, how does cable cram hundreds of channels on one cable and satellite can't?

On a related note, when I first got D*, I was getting internet service from the cable co. I told the D* installer that I would prefer not to run another cable to the room with the computer (and a TV). The installer installed a device (I believe called a diplexer) between the cable in the ground and the multiswitch. Then, the signal from the cable and the multiswitch entered the house on one coax. Then, he installed another diplexer at the other end and sent one leg to the D* receiver and the other to the cable modem. This worked perfectly. (I ended up dropping cable for DSL due to other issues with the cable company). I still have the diplexers. Would a setup similar to this avoid my having to run another cable from the multiswitch to the new DirecTiVo? In envision this: 2 outputs from multiswitch --> diplexer --> one cable into house --> diplexer behind TV --> two inputs into DirecTiVo. Would this work?


----------



## Jonathan_S (Oct 23, 2001)

Dilerium said:


> In envision this: 2 outputs from multiswitch --> diplexer --> one cable into house --> diplexer behind TV --> two inputs into DirecTiVo. Would this work?


Unfortunately it wouldn't work. A diplexer works because the cable (or antenna) signal uses a totaly different frequency range that the satellite signal. So it is trivial to run them together on the same coax, and separating them is simply a matter of placing frequency specific filters on the correct outputs of the diplexer.

But the two satellite polarities (or 4 or 6 satellite signals from the larger dishes) run on almost identical frequencies (in fact in overlapping frequency ranges), so if you tried to combine them with a diplexer you just get interference, and not a usable signal.


----------



## Dilerium (Dec 3, 2005)

Jonathan_S said:


> Unfortunately it wouldn't work. A diplexer works because the cable (or antenna) signal uses a totaly different frequency range that the satellite signal. So it is trivial to run them together on the same coax, and separating them is simply a matter of placing frequency specific filters on the correct outputs of the diplexer.
> 
> But the two satellite polarities (or 4 or 6 satellite signals from the larger dishes) run on almost identical frequencies (in fact in overlapping frequency ranges), so if you tried to combine them with a diplexer you just get interference, and not a usable signal.


Thanks. I thought that might be the case.


----------



## Diana Collins (Aug 21, 2002)

Dilerium said:


> ...So, how does cable cram hundreds of channels on one cable and satellite can't...


Cable uses MUCH more signal power, and a MUCH wider frequency band. That allows then to use narrower, more closely spaced, carriers, resulting in the ability to cram more TV channels into the same space.

Look at it this way, a satellite transponder generates only about 200 watts - about the same a backyard floodlight - but is "seen" by your dish from a distance of 23,000 miles.


----------



## goony (Nov 20, 2003)

Dilerium said:


> So, how does cable cram hundreds of channels on one cable and satellite can't?


Another aspect is that your local cableco only has to make room for a handful of locals... the DBS company (Echostar, DirecTV) has to use part of their total bandwidth to supply locals to dozens of cities, thus they have less channels to offer any one subscriber - e.g. no "Extreme Underwater Competitive Knitting" channel via DBS, but the cableco may have it.


----------



## JaserLet (Dec 13, 2005)

goony said:


> no "Extreme Underwater Competitive Knitting" channel via DBS, but the cableco may have it.


This is also why smaller cablecos have fewer channels than the big cable conglomerates. To add a channel to analog cable, the cableco has to tune in that station with a dedicated satellite receiver, modulate that a/v stream into a television channel, and combine that channel in with the rest of their feed. Various stations come from various satellites. This is why some of the better smaller cablecos have 7 or 8 satellite dishes and racks upon racks of equipment just for their analog cable feed alone. A 70 channel analog cableco would literally need 70 satellite receivers, 70 modulators, and an array of signal combiners. Plus spares. Newer equipment combines several receivers and several modulators into a single box, but a great deal of equipment is still needed.

Digital cable is slightly easier to manage, the receiver and modulator concept is very similar, however with digital a monster cableco like Comcast is able to have one satellite dish / receiver farm to collect and combine the stations and then just pipe all of these channels via high speed fiber optic connections to the various cities it serves. To add a new channel to their digital offering they just need to add a single new receiver/encoder unit at the home office. No more need to install new eqipment at each and every city.


----------

