Incoming signal strength- how to measure? - AVS Forum
Cable, Digital Cable - Non-HDTV > Incoming signal strength- how to measure?
amthar's Avatar amthar 07:27 AM 06-12-2007
Hey all -

Just bought my first house and am wiring it to the hilt (which is kind of ironic considering it's 105 years old). There is a coax jack in virtually every room, which gives me a grand total of 20 drops.

I am familar with the insertion drops of splitters, the use of taps to drop the signal to the appropriate level for TVs, the drop in signal strength over specific distances of RG6 (what I'm using), etc.

What I can't figure out is how to determine my incoming signal strength. Knowing that an amp boosts my signal by 34dB or that a splitter drops it by -7dB doesn't really help me if I don't know what my starting point is, does it?

I do NOT want to buy one of thse $400 meters I see floating around.

I did find this: FusionHDTV5 USB Gold. It has a coax input and has a signal strength program. But I don't know if it works for terrestrial cable. I emailed them asking:

"Hello! I am interested in measuring the signal level (dB) at various video outlets in my house, including my main service point. I saw that FusionHDTV5 USB Gold has a signal checker. Is this only for OTA signals / antenna readings? Or can I connect the coax cable line from my cable box on my house and measure the signal levels coming from my cable company? Thank you, this is important for me!"

They are obviously foreign (I think Korean) and replied:

"Hi.
Our signal checker is supporting 8VBS and QAM channel, so you can use it with cable signal.
Thanks."

I'm not really sure what Time Warner Cable uses (QAM, 8VBS, LMNOP, whatever), I'm not up-to-speed on that side of the cable technologies.

Any help would be greatly appreciated. Thanks all.

egnlsn's Avatar egnlsn 07:38 AM 06-12-2007
Typical signal levels at the groundblock are ~10-17dBmV.
phantsam's Avatar phantsam 07:46 AM 06-12-2007
Well for starters QAM is just the digital modulation of the cable channels. A certain number of digital channels are compressed into the frequency on an analog channel and transmited (IE: 256 QAM).

Secondly, as for measuring signal strength to determine what your loss is inside the house. Really, i wouldn't trust anything but a meter. The new ones the field tech's use cost a few grand and the older models are a few hundred dollars.

Thirldy. If you want a good idea of what your signal strength is then you'll need to climb the pole and get tap readings. I don't advise this unless you work for a utility company and have business up that pole to begin with. Power companies and phone companies dont like people on their poles.

Another thing you'll need to know is distance to the Ground Block from the Tap (The Tap is located on the pole or in a pedestal). Readings from both locations will give you an idea of what you should be getting in the house before any splitters are introduced.

Another thing is in your house you have "Additional Outlets or AO's." Tap's are either on the pole or in the ground under a pedestal.

Do you really need to provied a sginal to all of these outlets? Forgive me for being rude but 20 AO's is a bit excessive for any house.

Normally a setup like this would require an in house amp and some intelligent use of splitters. Again this all depends on what the signal looks like coming into the house. Either way i'd imagine it would be a 2 way (modem and amp feeds), then 2 8 ways. Now your signal would be highly degraded at this point. I'd suggest picking a couple of rooms you want the cable in and work from there. Having to Load Balance 20 outlets is not a fun thing to do... either way you may want to get a tech out to check the levels and either advise you on what to do about the feeding the outlets or another plan for the cable in the house.

Quote:
Originally Posted by egnlsn View Post

Typical signal levels at the groundblock are ~10-17dBmV.

You really can't say that. Levels at the GB are dependent on levels at the tap which are dependent on where he is in the amp casacde for his local node. There is no such thing as typical levels. There is "Ideal levels" but not typical.
amthar's Avatar amthar 08:33 AM 06-12-2007
So the answer is, call the cable company and have them come out and give me a reading... ya?
phantsam's Avatar phantsam 09:08 AM 06-12-2007
In a nutshell yes. lol. There's a lot to it, albeit simple onece you get the hang of it. The best bet is to call the company and set up an appointment for a technician.
kenglish's Avatar kenglish 11:11 AM 06-12-2007
Another idea is, to "do the math" on how the losses add up within your system (high-frequency loses due to cable lengths, splitter and tap losses), then select the proper gain amplifier to make everything work out to "zero loss". You probably know the approximate lengths of the cables in the house.

Whatever the Cable company gives you at the side of your house, should then be what you would get at each outlet. The Cable company is obligated to provide a signal that is optimum for a set or two, so that will be reflected in the levels at all your outlets.
phantsam's Avatar phantsam 11:19 AM 06-12-2007
Quote:
Originally Posted by kenglish View Post

Whatever the Cable company gives you at the side of your house, should then be what you would get at each outlet.

This all depends on a number of factors. For example, condition of the coax, splitters, amp, directional couplers etc. etc. Any number of these things can degrade the signal by the time it gets to the outlet. Don't forget improper fittings either. They're the number one source for egress and line noise.
John Mason's Avatar John Mason 12:04 PM 06-12-2007
There are detailed notes on using diagnostics with various STBs here:
http://www.dslreports.com/faq/sciatl...stic%20screens
--John
prospect60's Avatar prospect60 06:51 PM 06-12-2007
Make friends with a Cable Guy, invite him over to the house for some beers (or just a straight bribe off the books) and have him bring his equipment -- assuming that costs less than a House Call. They could surprise you and come out for a minimum/no charge AND send someone who is really competant.
kenglish's Avatar kenglish 10:59 AM 06-13-2007
Quote:
Originally Posted by phantsam View Post

This all depends on a number of factors. For example, condition of the coax, splitters, amp, directional couplers etc. etc. Any number of these things can degrade the signal by the time it gets to the outlet. Don't forget improper fittings either. They're the number one source for egress and line noise.

That's why I said "...should then be what you would get at each outlet."
He probably knows the approximate lengths of his own cables, and can get the specs on his amp and common splitters. I'm just suggesting that his system be built for "unity gain". Then you get same out as what goes in.
If it's bad, that would indicate the Cable company has problems on their side.
bendog2784's Avatar bendog2784 05:43 PM 06-13-2007
If you have a cable modem you can get find out what your signal levels are on your providers downstream channel for internet (assuming they offer internet service). It could offer you a starting point using equipment you may already have,
egnlsn's Avatar egnlsn 10:15 PM 06-13-2007
<< Quote:
Originally Posted by egnlsn
Typical signal levels at the groundblock are ~10-17dBmV.


You really can't say that. Levels at the GB are dependent on levels at the tap which are dependent on where he is in the amp casacde for his local node. There is no such thing as typical levels. There is "Ideal levels" but not typical. >>

Most CATV systems in the U.S. are designed for signal level of +15 to +20dBmV (high end of the system's bandwidth) at the tap -- regardless of the tap value or location in the cascade.
Shorebreaker's Avatar Shorebreaker 11:10 PM 06-13-2007
Quote:
Originally Posted by egnlsn View Post

<< Quote:
Originally Posted by egnlsn
Typical signal levels at the groundblock are ~10-17dBmV.>>

Most CATV systems in the U.S. are designed for signal level of +15 to +20dBmV (high end of the system's bandwidth) at the tap -- regardless of the tap value or location in the cascade.

At the outlet, what would be ideal signal level from the "+15 to +20dBmV" ?
Thank you
egnlsn's Avatar egnlsn 07:38 AM 06-14-2007
Quote:
Originally Posted by Shorebreaker View Post

At the outlet, what would be ideal signal level from the "+15 to +20dBmV" ?
Thank you

It totally depends on the factors listed previously; distance from tap to groundblock, groundblock to outlet, number of outlets, splitter configuration, amplifier (if present), etc.

The FCC says no less than 0dBmV at the outlet
egnlsn's Avatar egnlsn 07:52 AM 06-14-2007
Quote:
Originally Posted by kenglish View Post

Another idea is, to "do the math" on how the losses add up within your system (high-frequency loses due to cable lengths, splitter and tap losses), then select the proper gain amplifier to make everything work out to "zero loss". You probably know the approximate lengths of the cables in the house.

Whatever the Cable company gives you at the side of your house, should then be what you would get at each outlet. The Cable company is obligated to provide a signal that is optimum for a set or two, so that will be reflected in the levels at all your outlets.

Drop amps come in 3 sizes; 15dB gain is, by far, the most common. Just about all major manufacturers make that size. There are a few that make 10dB gain amps and a couple that make 20dB gain amps.

You could always go to http://www.cencom94.com/Download.html and download their "Cable Attenuation Woeksheet" to calculate the drop losses.

Most cable companies know that there is a pretty good chance that the subscriber will have at least a couple of outlets, and hopefully a cable modem and an eMTA for their phone service. They're certainly going to provide sufficient signal for those. I know that Time Warner does 20dBmV at the tap, as does Comcast (at least around here, anyway).
AntAltMike's Avatar AntAltMike 11:18 AM 06-14-2007
0 dBmV is (or was) the FCC minimum for analog signals. I don't know what threshold level is recommended for digital signals, but it is probably ten dB below that.

If you want to estimate your analog signal strength cheaply, then buy a couple of ten dB attenuator pads, a 6 dB one and a 3dB one, connect your home run line to a standard sized analog TV and start padding down the line until the picture starts to become visibly grainy. The graininess will appear at about -10 dBmV. I've done that when the batteries ran down in my meter and I didn't feel like going back to my truck to get another meter.
egnlsn's Avatar egnlsn 11:05 PM 06-14-2007
Quote:
Originally Posted by AntAltMike View Post

0 dBmV is (or was) the FCC minimum for analog signals. I don;t know what threshold level is recommended for digital signals.ut it is probably ten dB below that.

Depends on the modulation scheme -- 64QAM or 256QAM. 64QAM is 10dBc while 256QAM is 6dBc.

Analog on cable isn't going away anytime soon. Even when programming goes away (probably not for several years), there will still need to be analog carriers for the AGCs in the amps.
AntAltMike's Avatar AntAltMike 08:14 AM 06-15-2007
Quote:
Originally Posted by egnlsn View Post

Depends on the modulation scheme -- 64QAM or 256QAM. 64QAM is 10dBc while 256QAM is 6dBc.

Last time I checked, Comcast in Washington, DC had one analog service channel somewhere between 110 and 120 that was useful for visually confirming the presence and sufficiency of the highest frequency signals. Basically, if the high numbered analog channel is present and the picture is clear, then the digital signals are present and sufficient in strength.

Quote:


Analog on cable isn't going away anytime soon. Even when programming goes away (probably not for several years), there will still need to be analog carriers for the AGCs in the amps

Single channel strip amplifiers and processors use AGC, but I don't think that most cable TV line extender amplifiers do. At least, not the ones I have serviced (I admittedly don't do a lot of franchise cable line work).

I've often wondered how the AGC circuits in these old strip amplifiers are going to respond when they begin inputting digital signals instead of analog ones on February 17, 2009. Unfortunately for me, there are no VHF digital stations presently broadcasting in my markets and I don't have the time to develop my own VHF 8VSB signals to evaluate the strip amplifier's AGC response to varying 8VSB input levels.

AGC in consumer and unsophisticated commercial digital amplifiers is going to give people fits. Because digital transmitters are assigned on adjacent channels in many markets, even amplifiers with somewhat restricted input bandwidths will be called upon to simultaneously amplify signals that vary significantly in signal strength, but an AGC that is set to keep a selected channel at a power level at which a tolerable level of on-channel IMD is developed can still degrade the quality of an adjacent, weaker channel. I have to deal with a strong Washington, DC channel 39 that is between weaker Baltimore channels 38 and 40 and a strong channel 51 that is adjacent to a weak channel 52 in situations in which I am virtually forced to take them off the same antenna, and I would be hamstrung in my efforts to develop a reliable broadband headend carrying those channels if an AGC with a mind of its own kept messing up my plans.

I don't know how many of you might have experienced this, but a couple of years ago, DirecTV temporarily sent out 1/2 powered signals (120W) on transponders 30 and 32 (included HBO and Showtime), broadcast from the same satellite slot (101 deg W) that included some strong "spot beam" transponders, and the AGC circuit used in Sonora Design's SAL20-24 "stacker" would develop enough IMD so as to make transponders 30 and 32 unrecoverable. Padding down the input wasn't a viable option, because the AGC would compensate for it. The most practical solution was to swap the SAL20-24s out for Sonora's cheaper and less sophisticated "microstacker", which had a weaker output but no AGC, and so its intermodulation distortion could be better managed.
amthar's Avatar amthar 08:34 AM 06-15-2007
Quote:
Originally Posted by John Mason View Post

There are detailed notes on using diagnostics with various STBs here:
http://www.dslreports.com/faq/sciatl...stic%20screens
--John

Good call on this. Although that site did not describe how to get into my specific model, some additional digging pulled up how to do so. I'll just grab my cable box and a portable TV and hook it up to the service box outside.

Thanks all for the tips.
phantsam's Avatar phantsam 08:47 AM 06-15-2007
Quote:
Originally Posted by amthar View Post

Good call on this. Although that site did not describe how to get into my specific model, some additional digging pulled up how to do so. I'll just grab my cable box and a portable TV and hook it up to the service box outside.

Thanks all for the tips.


By service box do you mean the grey box on the side of your house or the Pedestal under the green box? If it's the green box i'd advise against it. lol. A cable catches you they may get pretty ornery.
sharpjunkie's Avatar sharpjunkie 09:37 PM 06-15-2007
Quick question. I just got comcast internet only. I am using a Samsung LNT 2007 line tv. I was using an OTA antenna to get my cable tv in SD and HD since I don't watch too much tv. Now just for shits and giggles. I plugged the cable input of the tv, not the OTA input, into the wall where my cable modem was plugged in. I used my tv to scan all available channels and now I have cbs, abc, nbc, fox and a few others in HD and SD. I no longer need to use my antenna. Now I am receiving the same channels and shows as I used to with my antenna like 3.1 10.1 40.1 and they come in bright as day. No more moving the antenna. Am I getting free cable or is this going away?
egnlsn's Avatar egnlsn 09:20 AM 06-16-2007
Quote:
Originally Posted by sharpjunkie View Post

Quick question. I just got comcast internet only. I am using a Samsung LNT 2007 line tv. I was using an OTA antenna to get my cable tv in SD and HD since I don't watch too much tv. Now just for shits and giggles. I plugged the cable input of the tv, not the OTA input, into the wall where my cable modem was plugged in. I used my tv to scan all available channels and now I have cbs, abc, nbc, fox and a few others in HD and SD. I no longer need to use my antenna. Now I am receiving the same channels and shows as I used to with my antenna like 3.1 10.1 40.1 and they come in bright as day. No more moving the antenna. Am I getting free cable or is this going away?

I would venture a bet that they you will get to keep watching them. Sounds like the filter they used only traps out up to 550MHz, and the HD channels are way beyond that.

Enjoy!
egnlsn's Avatar egnlsn 10:03 AM 06-16-2007
Comments inline:
Quote:
Originally Posted by AntAltMike View Post

Last time I checked, Comcast in Washington, DC had one analog service channel somewhere between 110 and 120 that was useful for visually confirming the presence and sufficiency of the highest frequency signals. Basically, if the high numbered analog channel is present and the picture is clear, then the digital signals are present and sufficient in strength.

It probably is channel 116 or 117 (750MHz) I forgot that some systems are going all digital. However, there still needs to be analog carriers (even if empty) at either end (still need AGCs. Temperature swings still cause signal level variations). The installers need them to verify their installations.



Single channel strip amplifiers and processors use AGC, but I don't think that most cable TV line extender amplifiers do. At least, not the ones I have serviced (I admittedly don't do a lot of franchise cable line work).

I've often wondered how the AGC circuits in these old strip amplifiers are going to respond when they begin inputting digital signals instead of analog ones on February 17, 2009. Unfortunately for me, there are no VHF digital stations presently broadcasting in my markets and I don't have the time to develop my own VHF 8VSB signals to evaluate the strip amplifier's AGC response to varying 8VSB input levels.

I've been told that Holland Electronics' agile processor does a fine job at passing the 8VSB signal when converting UHF to VHF. Drake also makes a fine solution. Their's is a s piece system that downconverts to i.f. then upconverts to the desired channel. Both pieces are agile, so you can really do what you need. They are in mini-mod cases, which makes them really convenient.

AGC in consumer and unsophisticated commercial digital amplifiers is going to give people fits. Because digital transmitters are assigned on adjacent channels in many markets, even amplifiers with somewhat restricted input bandwidths will be called upon to simultaneously amplify signals that vary significantly in signal strength, but an AGC that is set to keep a selected channel at a power level at which a tolerable level of on-channel IMD is developed can still degrade the quality of an adjacent, weaker channel. I have to deal with a strong Washington, DC channel 39 that is between weaker Baltimore channels 38 and 40 and a strong channel 51 that is adjacent to a weak channel 52 in situations in which I am virtually forced to take them off the same antenna, and I would be hamstrung in my efforts to develop a reliable broadband headend carrying those channels if an AGC with a mind of its own kept messing up my plans.

I don't know how many of you might have experienced this, but a couple of years ago, DirecTV temporarily sent out 1/2 powered signals (120W) on transponders 30 and 32 (included HBO and Showtime), broadcast from the same satellite slot (101 deg W) that included some strong "spot beam" transponders, and the AGC circuit used in Sonora Design's SAL20-24 "stacker" would develop enough IMD so as to make transponders 30 and 32 unrecoverable. Padding down the input wasn't a viable option, because the AGC would compensate for it. The most practical solution was to swap the SAL20-24s out for Sonora's cheaper and less sophisticated "microstacker", which had a weaker output but no AGC, and so its intermodulation distortion could be better managed.

The SAL20-24 is just a uS575 with a PAL20 amplifier in the same case. It is the PAL that has the AGC.


cypherstream's Avatar cypherstream 01:50 PM 06-16-2007
This is the L.E.'s our comcast area uses:
http://www.sciatl.com/customers/Source/590560.pdf

Type 3A amplifier module with AGC. AGC has
445.25 MHz pilot frequency and manual temperature backoff
control.

Even if the Cable company goes 100% digital, as long as they keep that 445.25 MHz pilot frequency running with an analog carrier (Ch 61), I would imagine the AGC in these L.E's would still operate within specs. Techs here record signal levels for Channel 2, 61, and 116. Recently they have been measuring ch 118, to gauge the stability of loading a channel past 750 MHz.

I think you want a 0 dB level for optimum 256QAM input level. It can go from -15 to +15, but at those outer edges you'll start to run into trouble. SNR has a major impact as well. +32dB or higher SNR is required for 256QAM, and I think it's 26 or 27dB for 64QAM.

Of course the higher density modulation scheme, the better the SNR has to be. Which is why it's unrealistic at this point to use 1024QAM over an RF coaxial plant.
Mike Burlingham's Avatar Mike Burlingham 10:22 AM 05-01-2014
Quote:
Originally Posted by phantsam View Post


This all depends on a number of factors. For example, condition of the coax, splitters, amp, directional couplers etc. etc. Any number of these things can degrade the signal by the time it gets to the outlet. Don't forget improper fittings either. They're the number one source for egress and line noise.

 

Yes but no if your signal is 24 going into the on premises box (OPB) or Multi Dweling Unit box (MDU) you have to also think of the run (length of each outlet from where the entrance of the cable is) so if you have a 50 foot run to a outlet from the outside you are losing about 10-15 dB considering if the cable is not damaged in anyway by staples or anything 


Ratman's Avatar Ratman 03:23 PM 05-01-2014
Where were you 7 years ago when this thread was started? biggrin.gif
egnlsn's Avatar egnlsn 08:52 PM 05-02-2014
Quote:
Originally Posted by Mike Burlingham View Post

... so if you have a 50 foot run to a outlet from the outside you are losing about 10-15 dB considering if the cable is not damaged in anyway by staples or anything 
What Ratman said, but 10-15dB loss on a 50' run? Seriously? At 1000MHz, Series 6 cable loses 6.54dB while Series 59 cable loses 8.09dB. Mini-coax does lose 13.82dB@ 1450MHz, so I guess that could be the cable in use.
AntAltMike's Avatar AntAltMike 10:11 PM 05-07-2014
As coincidence would have it, I just learned today what the term "necro" means in th context of internet threads. Next, I'll try to work it into a sentence.
Up
Mobile  Desktop