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Minimum signal db required for HDTV


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#1 therat

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Posted 21 March 2006 - 03:11 PM

I just had out TV aerial replaced as well as the cabling. The TV aerial guy used quad-shielded cable and used a spitter so that I could share the signal the family TV in another room.

He tested the db at the antenna cable end where it plugs into my DVICO Dual Digital card on my computer. His meter showed from 51-53db on all channels.

I started fusionHDTV and it showed signal strength to be 88-90 on all channels and db to be 20-21 on all channels.

I asked him about installing an amplifier splitter and he told me that in his opinion I didn't need it. picture quality was good and according to his field instruments the db was excellent.

I have no idea why his db reading was so high and mine on FusionHDTV so low.

With a single HDTV tuner db was ~24.

So simply put, when do you know if the db is high enough? Is there a minimum db recommendation for good digital reception? can the db ever be too high? and lastly, what is the maximum db one could ever expect to register.

cheers

#2 digitaladvisor

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Posted 21 March 2006 - 03:56 PM

Good range. Real test would be in very bad weather - as long as it does not head toward 19db as drop off.

Aerial installers do not understand that above 20db for PC is mandantory using DVICO as guide on db.

DA

#3 charlesc

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Posted 21 March 2006 - 04:53 PM

His meter showed from 51-53db on all channels.
I started fusionHDTV and it showed signal strength to be 88-90 on all channels and db to be 20-21 on all channels.
With a single HDTV tuner db was ~24.
I have no idea why his db reading was so high and mine on FusionHDTV so low.

Because he was probably measuring dBV with the 51-53 dB reading and the dB you are referring to is probably dB S/N (Signal-To-Noise), a different ratio.
And he was hopefully using a calibrated digital field strength meter.

The other reading your card is giving you of 88-90 is probably an indication in percentage of signal strength that most STB/PC cards will show.

(Edited to make it clearer).

#4 des

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Posted 21 March 2006 - 04:58 PM

Thats highest i get on my card is 21db or 23db.

#5 murrayt

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Posted 21 March 2006 - 05:42 PM

Yes as charlesc has said the likelihood is that the field instrument was measuring dBuv, and on my system I get excellent reception at between 50 and 60 dBuV. The SNR is to the oand totally different measure. At 50 to 60 dBuV I get SNR's of about 26 and 27 as measured by the Nebula card(s). My older NXt6000 cards report slightly less SNR.

I recently changed the arangements here and signal levels are now between 70 and 82 dBuV. Not unexpectedly SNR is not really noticeable changed

But here in Sydney D44 was at 39dBuV and SNR 22 dB with quite good reception.

I understand that for a QAM 64 transmission such as is used for DVB transission here an SNR better than 20dB is recommended

Hope that provides some guidance

#6 therat

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Posted 21 March 2006 - 05:44 PM

Good range. Real test would be in very bad weather - as long as it does not head toward 19db as drop off.

Aerial installers do not understand that above 20db for PC is mandantory using DVICO as guide on db.


Thanks DA, that's what I wanted to know.

Because he was probably measuring dBuV (dB micro-volt) with the 51-53 dBuV and the dB you are referring to is probably dB Signal-To-Noise, a different ratio.

And he was using a hopefully calibrated digital field strength meter, and you are measuring a percentage 88-90 strength on an STB/PC card.


Yes that is coprrect charlesc.

Many thanks guys. The aerial guy said he would happily come back and install the amplified splitter if I needed it and to see how things go for a few days. I think I'll do that.

cheers

#7 charlesc

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Posted 21 March 2006 - 08:34 PM

The aerial guy said he would happily come back and install the amplified splitter if I needed it

Have a look over here re info on amplified splitters and distribution amplifiers. Some have lower noise and are better suited to digital reception than others.

#8 mtv

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Posted 21 March 2006 - 08:43 PM

Pro digital meters are the only accurate way of measuring channel power and bit error rates.
The graphs or meters in STB's & cards are only a guide and vary greatly. They can in no way be relied upon to disply "true" measurements.

The channel power (signal level) of 50 - 60dBuV as indicated by the pro digital meter is an ideal level for digital reception.
What's more important though, is the BER (bit error rate) it should also be as low as possible, to ensure error-free reception.

#9 charlesc

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Posted 21 March 2006 - 08:53 PM

I recently changed the arangements here and signal levels are now between 70 and 82 dBuV.

The 82 figure is high murrayt. Have a look over here. dba guide on 'Digital TV Reception - Houses '.

"What is the 'operating window' of a receiver?
Two main factors limit the range of signal levels that should be present at the input to a DTT receiver. The first is the minimum level of the digital signal, which must not fall below the threshold (typically 35-40dBV, but in some cases higher). The second is the maximum level of signal that can be applied before the receiver overloads. The highest signal is usually an analog signal, and this should not exceed 77dBV.
All digital AND analog signals at the input to a digital receiver must fall within this 'operating window'."

Your 82 is some 5dBV above what is considered the maximum. As we are talking dBV, a 6dBV change is a doubling of the signal.
So you appear to be getting a lot more than is considered ideal. And yet I presume it is working OK. Unless the measurement is not that accurate.

Maybe the Nebula reading is not what you would get from a calibrated digital field strength meter :blink:

#10 digitaladvisor

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Posted 21 March 2006 - 09:24 PM

Pro digital meters are the only accurate way of measuring channel power and bit error rates.
The graphs or meters in STB's & cards are only a guide and vary greatly. They can in no way be relied upon to disply "true" measurements.

The channel power (signal level) of 50 - 60dBuV as indicated by the pro digital meter is an ideal level for digital reception.
What's more important though, is the BER (bit error rate) it should also be as low as possible, to ensure error-free reception.


We know that. But in DAILY practical use I have become accustomed to the lower kind db type rating - SNR. From practical experience when that reading on the card falls below 20db - there are issues.

DA

#11 therat

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Posted 21 March 2006 - 09:36 PM

Interesting comments about the dBV ranges. My analog range here is 73-76 so that looks great and 51-53 for digital signal is good too.

Unfortunately I didn't ask the guy about the BER so I have no idea on that one.

Is it possible to boost the digital signal but not the analog signal?

Edited by therat, 21 March 2006 - 09:38 PM.


#12 charlesc

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Posted 21 March 2006 - 10:04 PM

Is it possible to boost the digital signal but not the analog signal?

Off a single antenna? No. Not practically. The channels are usually interspersed in the frequency range. Often right next to each other.
You would have to selectively take out the ones you wanted, and then boost them, before re-combining them.

Actually a bit like the Master Antenna Television Systems (MATV) used to do in many situations in blocks of flats for analogue reception. Pull out only the wanted analogue channels, level them, then re-combine them before feeding to a distribution amp.

And to do it with two antennas would also be a real problem. Combining the two outputs that have the same channels (after perhaps boosting the antenna for digital) would cause quite a few problems. Both antennas would receive duplicate channels.



My analog range here is 73-76 so that looks great and 51-53 for digital signal is good too.

Same point as for murrayt above re possibly overloading the 'operating window' of the receiver.
The analog if it is really that high is getting close to being too much, as per the dba guide.

The digital seems fine though.

I think oftentimes people don't always take into account the 'old' analogue channels that are still out there. These can still overload the receiving system, whether it is an STB/PC card or an amplifier that is added in.

#13 therat

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Posted 21 March 2006 - 10:18 PM

Thanks charles. I'll just sit back and enjoy it like it is :blink:

cheers

#14 charlesc

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Posted 21 March 2006 - 10:31 PM

Thanks charles. I'll just sit back and enjoy it like it is :blink:

Might as well enjoy it if it is OK. Just answering your question though...



can the db ever be too high?



#15 therat

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Posted 21 March 2006 - 10:46 PM

Might as well enjoy it if it is OK. Just answering your question though...


yep and I appreciate the feedback.

cheers

#16 murrayt

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Posted 22 March 2006 - 05:30 AM

charlesC

I noted your comments and am aware of the recommendations in the standard. Yes the one high figure is SBS as it's transmit power is much higher than the rest. But BER is OK, and reception all around is fine. I'm considering putting a pad in for that just to knock it back a touch.

But I am not measuring using the Nebula as such, I use an Enritsu precision measuring receiver, and a Rohde and Schwarz FS6H analyzer for the measurements

#17 charlesc

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Posted 22 March 2006 - 08:26 AM

But BER is OK, and reception all around is fine.

The max figures given are guidelines, I'm sure. Whether problems are seen will depend a lot on the STB/PC card receiver input characteristics.

It is interesting to note though that it may only take one high level channel to disrupt reception. And that channel may well be one of the old analogue ones, which the STBs would not pick up as such when they are scanning for channels.
So without a field strength meter, you wouldn't even know it was there at that level.

Maybe at that point someone could be guessing that too much signal might be the cause of their problems, and insert an in-line attenuator. This may fix the immediate problem (reduction of the operating window of the receiver) but would also reduce the digital channel reception levels at the same time. Perhaps better to filter out the unwanted channel.
That is where a site survey with the proper field strength meter is invaluable.

#18 Matt P

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Posted 22 March 2006 - 10:00 AM

is there a cheapo tool one can buy to check their various antenna signal strengths?

I am currently flatting in wollstonecraft, we get perfect ABC, 9 and 10 reception but a little fuzzy 7 and SBS ... I am keen to get back into the PVR game but want to make sure I am going to get good pictures before spending my hard earned cashola. Unfortunately due to our circumstances, any kind of antenna upgrades/installations are out of the question.

my kid bro just started as an apprentice electrician, would there be a tool that he can fangle from work to help out?

cheers,

Matt

#19 charlesc

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Posted 22 March 2006 - 10:11 AM

...would there be a tool that he can fangle from work to help out?

The only accurate way of telling what is available is with a digital field strength meter, and they start at a bit under $1k (edit: meant $2k) and go up from there. The more expensive ones have the better features of course.

You could try and get an STB on a sale-or-return basis, like from Dick Smith. If it does no good, you can return it.


... cuurently flatting in wollstonecraft, we get perfect ABC, 9 and 10 reception but a little fuzzy 7 and SBS

Is this analogue TV reception on your TV?

#20 Matt P

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Posted 22 March 2006 - 10:24 AM

The only accurate way of telling what is available is with a digital field strength meter, and they start at a bit under $1k and go up from there. The more expensive ones have the better features of course.

bugger!

You could try and get an STB on a sale-or-return basis, like from Dick Smith. If it does no good, you can return it.
Is this analogue TV reception on your TV?

yes, analogue reception via standard coax

might try a ring around to see if any mates have STBs that i can steal for an hour or so, I know one in chatswood bought a STB and he had horrible reception for 9, but it didn't improve that dramatically from what i remember

if i were to go the DSE option, the signal would not differ from HD to SD streams would it? ultimately i'd be wanting to record things in HD

Edited by Matt P, 22 March 2006 - 10:26 AM.


#21 charlesc

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Posted 22 March 2006 - 10:31 AM

if i were to go the DSE option, the signal would not differ from HD to SD streams would it?

If you receive the stream you get SD, HD , the EPG etc.
You don't get a situation where you get, say, SD well, but HD badly.

#22 murrayt

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Posted 22 March 2006 - 07:15 PM

charlesc

I agree with what you have said. Yes given the differentials in transmit power, it is quite possible that an analog channel at a higher signal strength could cause a problem relative to the lower Ditital channel if it is amplified and saturates the front end of the tuner (STB or PC device

#23 charlesc

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Posted 22 March 2006 - 07:33 PM

I agree with what you have said. Yes given the differentials in transmit power, it is quite possible that an analog channel at a higher signal strength could cause a problem relative to the lower Ditital channel if it is amplified and saturates the front end of the tuner (STB or PC device)


I really believe this is one of the hidden perils of jumping in to use amplification in a digital reception system. The amplifier will be affected by all the channels it has to boost, even if your STB only shows a sub-set (the digital ones) of what is available.
And bearing in mind that analogue signals are usually quite a bit more powerful than their digital counterparts, you really do need to be able to measure what is there ahead of the amplifier.

I also believe use of a good quality Distribution amplifier like the GME Kingray MDA series is handy in these types of situations, as they have a fairly generous amount of headroom that allows them to pass many channels without overloading. And their low noise figures of <1.8 dB is excellent for digital.

But of course, as has been said many times, if the signal quality is not good in the first place (low Bit Error Rate - BER) then no amount of boosting is going to make it better.

#24 murrayt

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Posted 22 March 2006 - 08:30 PM

charlesc

Yes I think there are some who will have fallen foul of this sort of problem.

I've used both Kingray and Johanssen but reckon the that Johanssen has the better noise figure and headroom. But having said that I believe that the input stage of the TV/STB/DVB tuner is probably where the problem occurs.

Your last point, I would put a little differently. If the signal to noise ratio of the received signal is poor then the receiver's ability to recover the signal will result in a poor BER, and amplification will amplify the signal and noise, which cannot improve the ability of the receiver to resolve the signal and achieve a reasonable BER

#25 charlesc

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Posted 22 March 2006 - 08:46 PM

Your last point, I would put a little differently.

If the signal to noise ratio of the received signal is poor then the receiver's ability to recover the signal will result in a poor BER, and amplification will amplify the signal and noise, which cannot improve the ability of the receiver to resolve the signal and achieve a reasonable BER

I agree with that , it is a more detailed and accurate explanation.
However more of a mouthful as well :blink:, and I think the subtlety of it would be lost on most posts. The more general explanation I noted above could suffice, although it is not as accurate.

As a possible re-wording of your note, how about:-

"If the signal to noise ratio of the received signal is poor then the receiver's ability to recover the signal will be compromised/limited. This would result in a poor BER.
Amplification of this signal ahead of the receiver will amplify the noise with the signal, which cannot improve the ability of the receiver to resolve the signal and to achieve a better BER."