TV Signal Strength - Page 1
Forum section: electronics / beginners

author: apelly
author_rank: Unknown
timestamp: July 11, 2014, 01:31:38 am
content: I'm trying to diagnose a TV reception problem on a media PC. There are many things that can possibly go wrong. The one I'm interested in at the moment is signal strength.I do not have a signal strength meter. I'd like to know if my signal is too strong, or too weak. Is this easy or interesting? If it is boring I will just buy a meter from Jaycar. If it is interesting I will have a play around. If it is very simple I will just do as I'm told.I have a bunch of components, breadboards and test equipment. But no signal analyzer, and little skill and imagination.Can anyone offer some pointers? The interesting signals for me are DVB-S and DVB-T. I do not know the specific frequencies off the top of my head, but I can look them up and let you know if that is important information (and I expect it is)

author: miguelvp
author_rank: Super Contributor
timestamp: July 11, 2014, 01:38:14 am
content: Software Defined Radio uses cheap dongles meant for DVB-T based on RTL2832U+R820T you just need a PC.

author: apelly
author_rank: Unknown
timestamp: July 11, 2014, 01:48:03 am
content: That is true.I was more interested in the actual vs ideal peak voltages on the cable. I'm hoping to see if the signal has been over/under-amplified, in an objective way. That is, without software interpretation.

author: miguelvp
author_rank: Super Contributor
timestamp: July 11, 2014, 01:58:49 am
content: Then you can skip the demodulation and use the R820T digital tunner, but I have not played at all with SDR so this is where my knowledge stops.http://radioaficion.com/cms/r820t-rafael-micro/

author: IanB
author_rank: Super Contributor
timestamp: July 11, 2014, 02:01:08 am
content: Issues with digital TV signals are far more often due to weak or low quality signals. Digital TV generally needs a much better and cleaner signal than analog TV did.Some local history around the problem would help to guess the possible causes. For instance, if it is an off-air signal, has the antenna been upgraded and/or carefully realigned? Has the coax from the antenna to the tuner been upgraded to modern high quality coax if the original was old? An answer of no to those questions probably indicates a weak or poor signal.Even if the signal is coming from a cable in the street, the cable path from street to tuner should have good quality cable and good quality connections. Anything old or crusty is immediate cause for alarm.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 11, 2014, 06:32:54 am
content: Quote from: IanB on July 11, 2014, 02:01:08 amInterestingly,the old type coax commonly used in Australia was less lossy than the "RG6" stuff it is often replaced with.RG6 is,however,much more rugged,& the use of compression crimp type "F" connectors adds a lot of reliability.As you don't need lowband channels,most "Digital" antennas are much more compact than the old analog ones,so they are well worth using.If the old ons have been up for 20 years or so,they are probably in poor condition.If,on the other hand, you are trying to get TV reception with a "dongle" attached to your PC with no real external antenna,you will have the same sort of problems that people do with "rabbits ear" indoor antennas.

author: miguelvp
author_rank: Super Contributor
timestamp: July 11, 2014, 06:48:44 am
content: You can attach the dongle to the same source he is trying to determine, it doesn't have to be the one supplied.Well, as long as it has an BAV99 for ESD protection so I hear since I have not researched SDR much.

author: apelly
author_rank: Unknown
timestamp: July 12, 2014, 12:18:18 am
content: Quote from: IanB on July 11, 2014, 02:01:08 amI can already guess the possible causes. I want to test for the actual cause.Either this was a pointless question, or nobody knows how to build a simple meter for this. I suspect the former.Thanks for your replies guys.

author: Unknown
author_rank: Unknown
timestamp: July 12, 2014, 02:00:37 am
content: Whatever hardware/software you're using should be able to give you at least a basic indication of signal level/quality, even if it's only 0-100% level & quality scales. Most should be capable of showing signal level in dBuV & pre/post correction error rates, & maybe MER.If it only gives rough scales, then generally:If you've got 95-100% for both signal level & quality, then your signal level is fine.If you've got <90~95% signal level, you've got too little signal (even if you've got mostly 100% signal quality).If you've got 95-100% signal level and <90-95% signal quality, it usually means your signal level is too high.If it gives actual signal level & pre &/or post correction error rates, then the acceptable specs are given in AS/NZS 1367 (& available elsewheree.g.):Signal level should be minimum 45dBuV, maximum 80dBuV, optimally between 60-70dBuV. In practice, between ~40-90dBuV can usually considered 'usable'Pre-correction error rates should be better than 8*10-4; the 'digital cliff' is generally considered to be ~4*10-2.Post-correction error rates should be better than ~2 orders of magnitude better than that.MER should be 25dB or better.Apart from that, you may be better searching & maybe asking ondtvforum.info(a lot of good TV-related info buried there, though most of the antenna installers & transmitter techs who used to be regulars have given it up due to the mods tolerating the resident forum idiot alanh), or the TV-relatedAustechsub-forums (where most of the dtvforum refugees have migrated).If you're just interested in building a simple meter, buy a cheap DVB-T dongle & chase up software that'll give you the above info &/or a spectrum plot.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 12, 2014, 02:12:54 am
content: Quote from: apelly on July 11, 2014, 01:31:38 amGoing back to square one, the "S" in DVB-S stands for "Space"--so it is a satellite delivered signal,probably at aroung 15 GHz or so.DVB-T is the plain old "Terrestrial" service delivered on VHF & UHF frequencies up to around 800 MHz.Signal strength meters for each service have very different requirements in both frequency range & sensitivity.If you are in the primary service area of a Terrestrial TV site,you can probably make up a meter with a germanium diode,a 50uA meter & a few other bits,which will give you a reading combining all the signals within the frequency range & angle of vitew of your TV antenna.This will not allow you to discern the signal strength of any of the signals by themselves.Satellite downlink signals are another kettle of fish,however.The signal strength will be very much lower,so your device needs to be much more sensitive,which implies active devices,circuitry designed for microwave frequencies,& so on.Satellite downlinks usually have a Low Noise Converter mounted at the dish (& yes,you will need a dish),which converts the incoming signal to a lower one which will pass through the coax cable with less losses.This IF signal,may,or may not be within the operating frequency range of the simple meter suggested above.