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TechGeek191

join:2009-06-13
Center Valley, PA

[Cable] Cable repair question

The simplified diagrams of how cable is delivered to subscribers does not cut it for me. I have witnessed while cable techs were working on a amplifier that they were able to disconnect the HD lineup but SD channels were unaffected. How can this be?

The same goes for the modems VOIP and internet. Our area experienced a local outage but TV service was unaffected. If the line was damaged wouldn't this affect everything?

How can services be partially affected and how can a cable tech selectively disconnect TV or modem?

Thanks for whoever can answer this technical question


DrDrew
That others may surf
Premium
join:2009-01-28
SoCal
kudos:16
Different services run on different frequencies. Some services using modems need return communication using another set of frequencies.

It's possible that certain problems or damage only affect certain frequencies.

For example, small breaks in the cable or loose connectors will affect low frequencies before they affect higher frequencies. Return or upstream communications use the lower 5-45 Mhz frequency space.

On the other hand, higher frequencies are effected more by corrosion and other things that damage the copper skin of the center conductor of the coax cable. So when the cable equipment gets wet internally, higher frequencies 550 Mhz and up which are used by services such as digital SD and HD channels go out first.

In most of the powered cable equipment (nodes, amps, and line extenders) used for signal distribution in your local neighborhood, the forward (50Mhz and up) and return (5-45Mhz) signals are split and follow different paths within the equipment. The techs can then adjust them separately.

Beyond that, all the different channels are combined in several different spots at the local cable headend and further upstream from there. Depending on where problems happen it can knock out specific services, groups of channels, individual channels, or just parts of channels such as audio, video, closed captioning, etc.

The Cable 101 links listed here should help:
»cable.doit.wisc.edu/cable_resources.html
--
If it's important, back it up... twice. Even 99.999% availability isn't enough sometimes.

TechGeek191

join:2009-06-13
Center Valley, PA
Thanks for your informative reply


grohgreg
Dunno. Ask The Chief

join:2001-07-05
Dawson Springs, KY
reply to DrDrew
said by DrDrew:

Different services run on different frequencies.

To amplify slightly, these different services are often enabled/disabled with filters. Customer doesn't use VoIP or HD? Shunt those segments of the composite signal with the respective filters. Customer upgrading to HD? Disable the HD filter. Et cetera

//greg//
--
HN7000S - 98cm Prodelin/2w "pure" Osiris - ProPlus - G16/1010H - NOC:GTN - NAT 67.142.115.130 - Gateway 66.82.25.10 - DNS 66.82.4.12 and 66.82.4.8 - Firefox 8/MSIE9 - AV/Firewalled by NIS2012


DrDrew
That others may surf
Premium
join:2009-01-28
SoCal
kudos:16
said by grohgreg:

said by DrDrew:

Different services run on different frequencies.

To amplify slightly, these different services are often enabled/disabled with filters. Customer doesn't use VoIP or HD? Shunt those segments of the composite signal with the respective filters. Customer upgrading to HD? Disable the HD filter. Et cetera

//greg//

That really isn't done anymore beyond analog basic and broadcast services. It causes too many problems.

The cheaper drop filters placed at cable taps feeding the drops to homes don't have sharp enough limits to drop certain channels without affecting adjacent channels. It also causes other signal problems, especially if you put multiple filters in series.

It's also expensive to employ filters because it means the cable companies have to roll trucks for every service change. Truck rolls cost $50-100 each due to employee time, truck maintenance, etc.

Service control is done digitally by way of equipment activation (no VOIP? no VOIP modem. no HD? turn off the HD channels in the box.)
--
If it's important, back it up... twice. Even 99.999% availability isn't enough sometimes.


grohgreg
Dunno. Ask The Chief

join:2001-07-05
Dawson Springs, KY

4 edits
Sorry I wasn't a bit more specific. I can see where you applied a literal interpretation and thought analog. I was trying to be generic.

In the digital world, such "filtration' still exists - but it's virtual. That is - and this depends up the medium and the provider - you can "tell" a server (provider end) or a modem (subscriber end) to enable/disable specific services - hence "filtering" them out before they get to the end instruments. And in some cases it's a hybrid action; that is, the server transmits reconfiguration data via the composite/subscriber line to affect an internal change to the subscriber modem.

//greg//
--
HN7000S - 98cm Prodelin/2w "pure" Osiris - ProPlus - G16/1010H - NOC:GTN - NAT 67.142.115.130 - Gateway 66.82.25.10 - DNS 66.82.4.12 and 66.82.4.8 - Firefox 8/MSIE9 - AV/Firewalled by NIS2012