 EdrickI aspire to tell the story of a lifetimePremium
join:2004-09-11
Woburn, MA | [Cable] Internal Channels Inside of Building Always looking to learn something new,
So one thing I've been curious about;
Say you have a building or a home and you want to have an internal channel whether it be digital or analog. If you're injecting a signal into the system do you need to put a filter on to prevent it from going back onto the rest of the CATV system on that node?
If you're injecting said signal would that not prevent a channel on that same frequency from coming through and how is this affected whether it's digital or analog?
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 panth1The Coyote
join:2000-12-11
Boca Raton, FL | If the channels you want to place your own channel on already contains a signal then you would need a channel elimination filter. Having two channels on the same frequency, whether analog or digital would cause reception problems.
Imagine one person yelling in one ear and another in your other and trying to understand what they are saying.
Most amplifiers have high return loss. Assuming no incoming signal is on the channel, having a house amp and injecting the signal afterwards will act as a return filter.
It was a lot easier in the analog only days to use a cheap modulator above the cable companies channels. Today everything is just about digital so you won't really know what frequencies are used and not used without a CATV signal meter or know how to use a cable box's diagnostic screens to figure it out. Digital also allow cable companies to switch frequencies fairly easily only to mess up your channel configuration. |
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 DarkLogixTexan and ProudPremium
join:2008-10-23
Baytown, TX
kudos:3 | If its cable then just use a frequency used by cable internet, if you make sure you're not back feeding and that you're doing this after your cable modem you might be good. (then again your cable box might use it too.)
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said by Edrick:If you're injecting a signal into the system do you need to put a filter on to prevent it from going back onto the rest of the CATV system on that node?
Yes. If you don't, the CATV company can easily track this type of interference and disconnect your service until you fix it.
said by Edrick:If you're injecting said signal would that not prevent a channel on that same frequency from coming through and how is this affected whether it's digital or analog?
No, the channel modulator cannot filter the cable channel. On analog you will see cross-hatching or "zebra" pattern interference. I don't know what you get on digital - probably just a blank screen.
When you inject your own channel to replace a channel on your CATV feed, you place a notch filter on the incoming cable. You place multiple notch filters to remove multiple channels. You would commonly use a directional coupler (DC) to inject the channel although there are more sophisticated ways. It's usually a good idea to select the DC value based on signal strength measurements and add attenuators as needed. You do all of this before your amp.
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 DrDrewSo that others may surf.
join:2009-01-28
SoCal
kudos:10 | reply to DarkLogix
said by DarkLogix:If its cable then just use a frequency used by cable internet, if you make sure you're not back feeding and that you're doing this after your cable modem you might be good. (then again your cable box might use it too.) You still have to filter out the frequencies used by the modem. They'll interfere just like any other RF channel.
No matter what channel is picked, it has to be empty of other signals.
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EdrickI aspire to tell the story of a lifetimePremium
join:2004-09-11
Woburn, MA | reply to Edrick
So I suppose the big question is, what's the best method of picking a frequency? |
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plat2on1
join:2002-08-21
Hopewell Junction, NY | said by Edrick:So I suppose the big question is, what's the best method of picking a frequency?
I think something in the FM band(95 96 97) would be a good choice as cable companys generally don't use those anyway |
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The quick and dirty would be if you could find a QAM tuner that would tune & give you a list of all in-use RF channels, encrypted or clear, and then hook up your DOCSIS 3 cable modem and see what it tunes to. You can eliminate all of those.
Remaining would be open spectrum plus anything used for backchannels (STB signaling, I don't know the industry nomenclature) and any other services such as premium music if anybody still offers that.
(I don't know if anybody has a good online spectrum analysis or channel list. That would be a good resource. For example this list shows Clear QAM channels ... »www.silicondust.com/hdhomerun/li···_9886451)
Now choose a reasonable range from what is not in use. Get a notch filter and an agile modulator for that range and experiment.
Or, you could just pick a range of channels that you do not mind losing and get a notch filter and modulator in that range.
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