With the proviso that ones signal has to be of sufficient strength (and most houses receive a signal capable of being split at least twice) I would consider it a general rule.
And would still be wrong!.
I suspect the only installers (or retailers) who would disagree with me are those wanting to sell an amplifier, or more accurately an amplified splitter.
You obviously live in a happy area where everyone gets good signal strength!.
Introduction of a splitter (or an attenuator) should have no effect on the signal to noise ratio of the signal, as opposed to the strength of the signal.
How do you come to that conclusion?, you've reduced the signal by 4dB or so (as you've just said), yet the noise is exactly the same as it was (actually INCREASED slightly - but I'll ignore that) - so the signal to noise ratio has got 4dB worse.
How do you come to that conclusion?, you've reduced the signal by 4dB or so (as you've just said), yet the noise is exactly the same as it was (actually INCREASED slightly - but I'll ignore that) - so the signal to noise ratio has got 4dB worse.
OMG, Einstein is alive and kicking on this forum tonight.
If only he had remembered before logging on that the noise level(within the incoming split signal) is reduced by the same amount (more or less - but I'll ignore that), when compared against zero reference level.
It may be poor practice but it cost nothing and since I am not transmitting through it I can get away with a mismatch without any adverse problems as I am located in a strong signal area with line of sight to the transmitter.
When the time comes to replace the antenna and/or add a downlead for a further TV then i will consider renewing all coax and using a suitable splitter.
You obviously live in a happy area where everyone gets good signal strength!.
How do you come to that conclusion?, you've reduced the signal by 4dB or so (as you've just said), yet the noise is exactly the same as it was (actually INCREASED slightly - but I'll ignore that) - so the signal to noise ratio has got 4dB worse.
I don`t think you`re reading me correctly, I said "with the proviso that the site has sufficient signal" then a splitter is preferable to an amplifier. Obviously if your signal is only (say) 60dBuV, which is 1mV in old money, you shouldn`t really be splitting it without an amplifer.
If your signal is 75dBuV (approx 5mV) then you can split it no problem at all. In fact the resultant signal would still be about 71dBuV, which as about 3.5mV and fine for any TV because they work perfectly well down to around 1mV.
I would say that the great majority of sites receive a signal which is capable of being split twice, and a good number four times or more. We`ve done jobs where it was split six times and still worked perfectly.
On the subject of signal to noise ratio a typical signal might be 75dBuV with a noise floor of 20dBuV, i.e. the signal to noise ratio would be 55dBuV.
If you split this, or use an attenuator, to lose say 10dB then the resultant signal would be 65dBuV but the noise would also have lost 10dB and would therefore now be 10dBuV, i.e. the S/N would still be (65 - 10 55dBuV.
Just to confirm it I`ve just checked it on our signal meter, and the S/N ratio was the same.....
It may be poor practice but it cost nothing and since I am not transmitting through it I can get away with a mismatch without any adverse problems as I am located in a strong signal area with line of sight to the transmitter.
When the time comes to replace the antenna and/or add a downlead for a further TV then i will consider renewing all coax and using a suitable splitter.
Until then... no need.
Well so long as you’re happy then that’s fine, but if you ever get one of those unexplained problems with your system that we see so many times on these forums, and you feel the need to enlist the help of other forum members, please do tell them from the onset that you have a gross impedance mismatch built into you aerial system, it might save them a lot of time and effort.
I'm no TV engineer so I'm not too hot on the theory. I have however tried combining coax cable to feed two TVs off one antenna in the past. Apparently it means that you introduce a mismatch into the 2 feeders causing "standing waves" whatever they are. In my experience it usually means that one of the channels is a lot weaker than it should be. This can be rectified by changing the length of the coax by a few inches, after which you find that another channel is weak. This problem may not be as evident in strong signal areas, but it had me tearing my hair out. In the end I vowed that in future I would use a splitter amplifier every time.
I'm no TV engineer so I'm not too hot on the theory. I have however tried combining coax cable to feed two TVs off one antenna in the past. Apparently it means that you introduce a mismatch into the 2 feeders causing "standing waves" whatever they are. In my experience it usually means that one of the channels is a lot weaker than it should be. This can be rectified by changing the length of the coax by a few inches, after which you find that another channel is weak. This problem may not be as evident in strong signal areas, but it had me tearing my hair out. In the end I vowed that in future I would use a splitter amplifier every time.
Yes you're about right, good for you. I think if our friend above was watching analogue TV then he might find some ghostly pictures on his screen caused by reflections up and down the length of the cable.
OMG, Einstein is alive and kicking on this forum tonight.
If only he had remembered before logging on that the noise level(within the incoming split signal) is reduced by the same amount (more or less - but I'll ignore that), when compared against zero reference level.
Except the noise isn't coming in with the signal, the limiting noise factor at VHF and above is the frontend noise of the receiver (as I also said earlier) - so you're only reducing the signal, and not the noise at all.
This is basic electronics and radio ham stuff, nothing terribly technical.
If you were talking about low radio frequencies, then your incorrect idea would be right - below 30MHz or so the limiting factor is noise coming in with the signal, and NOT the receiver.
If you split this, or use an attenuator, to lose say 10dB then the resultant signal would be 65dBuV but the noise would also have lost 10dB and would therefore now be 10dBuV, i.e. the S/N would still be (65 - 10 55dBuV.
Just to confirm it I`ve just checked it on our signal meter, and the S/N ratio was the same.....
How can you check the signal to noise ratio?, the noise is generated inside the frontend of the tuner at UHF (and VHF) frequencies - so a meter has no way of checking that.
However, this could explain your incorrect assumption about signal to noise not being reduced by a passive splitter?.
How can you check the signal to noise ratio?, the noise is generated inside the frontend of the tuner at UHF (and VHF) frequencies - so a meter has no way of checking that.
However, this could explain your incorrect assumption about signal to noise not being reduced by a passive splitter?.
The signal to noise ratio is (to the aerial trade) that of the signal coming down the cable. If you look at it on a spectrum analyser you would see this signal as a waveform and then the noise floor as a continuous block of spurious signal at the bottom of the display. This is the background noise from everything out there which gives off RF.
If you use an amplifier this spurious noise goes up by the same amount that you have amplified the "wanted" signal, plus a bit more for the extra noise introduced by the amp.
If you use an attenuator the noise floor goes down, by the same amount that you have reduced the "wanted" signal.
The signal to noise ratio is (to the aerial trade) that of the signal coming down the cable. If you look at it on a spectrum analyser you would see this signal as a waveform and then the noise floor as a continuous block of spurious signal at the bottom of the display. This is the background noise from everything out there which gives off RF.
If you use an amplifier this spurious noise goes up by the same amount that you have amplified the "wanted" signal, plus a bit more for the extra noise introduced by the amp.
If you use an attenuator the noise floor goes down, by the same amount that you have reduced the "wanted" signal.
The incoming noise though is LOWER than that generated in the tuner of the set, at VHF and above the limiting factor is the frontend noise, NOT any possible external noise.
So a passive splitter reduces your signal to noise.
I have to agree with Nigel on this one. Although it is true that a passive splitter will make no difference to the S/N ratio at the aerial plug, that isn't where it matters most for a simple (i.e. non-MATV) installation.
Where the S/N ratio matters is at the IF output from the tuner, where the wanted signal is demodulated. At that point most of the noise comes from the mixer stage in the tuner, and reducing the input signal certainly worsens that S/N ratio. That is why TV AGC systems always act first at IF level, and only after IF gain is minimum do they apply delayed AGC to the RF amp before the mixer.
The incoming noise though is LOWER than that generated in the tuner of the set, at VHF and above the limiting factor is the frontend noise, NOT any possible external noise.
So a passive splitter reduces your signal to noise.
I can`t be bothered to spend any more time on this one. I`d go and look at TV signal on a spectrum analyser if I were you.
I can`t be bothered to spend any more time on this one. I`d go and look at TV signal on a spectrum analyser if I were you.
Try answering a simple question then! - if you have a set in a poor reception area, with a snowy picture - does fitting a masthead amplifier improve the picture?.
Assuming you answer YES (because it does!), how do you explain that based on your idea of how radio waves work?.
However, I'm a little dismayed that anyone who has presumably spent the money on a spectrum analyser can understand so little about how the radio spectrum works?. I'm guessing that you're perhaps an aerial installer?, and not an electronics engineer or anything?.
Let me just see if I can grasp this in simple terms...
A passive splitter reduces the signal by 50% (on each output), yes? Well...by the time the signal reaches the TV then of course the SNR will be reduced, because the amount of noise that will be introduced after the splitter will still be the same.
Let me just see if I can grasp this in simple terms...
A passive splitter reduces the signal by 50% (on each output), yes? Well...by the time the signal reaches the TV then of course the SNR will be reduced, because the amount of noise that will be introduced after the splitter will still be the same.
Is that right?
Yes, pretty well (apart from you lose more than 50%) - the source of noise is the frontend of the tuner. By amplifying the signal at the masthead, you're essentially moving the source of noise there instead (where the signal is stronger), so when it gets to the TV the incoming signal is stronger than it was without the amplifier.
If you fit a set-back amplifier it doesn't do much good, because you've already lost signal down the coaxial cable, so it's nearer to the noise factor of the amplifier.
To get even more 'technical', you might notice that the 'LNB' on a radio telescope is normally cooled using liquid nitrogen, this reduces the noise factor of it's frontend, which helps greatly, as that's the source of the noise. If you nitrogen cooled your TV tuner it would help your poor reception a lot as well, but it's perhaps a little expensive and difficult!
Bear in mind that we are discussing signal-to-noise ratio.
If you mix apple juice and orange juice 50/50 then pour it into two glasses, the ratio of juices will still be 50%.
Exactly - but, in this case, you're pouring part of the apple juice (signal) away before you mix it with the orange juice (noise), so the ratio changes.
Very interesting discussion this. I've certainly seen the quality meter reading on a STB shoot up for a weak signal when adding an amplifier.
But Nigel, why, if they can design preamps to introduce little noise, does the TV become the principal source of noise?
By the time the signal gets to the IF stage wouldn't the signals be strong compared the noise generated within the TV?
And if there is little noise around at UHF external to the TV, then why all the fuss about super-screened TV cable?
Because of the expense of having someone go on the roof, I have hesitated about getting a mast-head amplifier and amplify my signal when it comes in at the window.
I have about 4 metres (max) of sat cable (copper sheath) running down the wall between the TV aerial and the entry point into the buiding. An amplifier with twin outputs is connected there. I've often wondered how much noise is being picked up by the cable.
Very interesting discussion this. I've certainly seen the quality meter reading on a STB shoot up for a weak signal when adding an amplifier.
But Nigel, why, if they can design preamps to introduce little noise, does the TV become the principal source of noise?
At VHF and above, there is no atmospheric noise, what causes the snow on your screen (and the hiss on your FM radio) is noise generated in the receiver, specifically in the front end.
Modern tuners are remarkably good, and have very low noise factors, so mostly a masthead preamp is probably just as noisey as your TV tuner. It's the fact of being where the signal is strongest that makes it work, so the signal is stronger compared to the noise from the preamp.
Notice that if you place a splitter AFTER the masthead amplifer, then both the signal and the noise from it, are reduced by the splitter - but this doesn't happen with a passive splitter, because it adds little noise..
By the time the signal gets to the IF stage wouldn't the signals be strong compared the noise generated within the TV?
The IF output from the tuner is at a much higher level, and noise generated in the IF is much less of a problem
And if there is little noise around at UHF external to the TV, then why all the fuss about super-screened TV cable?
It's not about 'noise' it's about interference, which is a completely different thing.
Because of the expense of having someone go on the roof, I have hesitated about getting a mast-head amplifier and amplify my signal when it comes in at the window.
I have about 4 metres (max) of sat cable (copper sheath) running down the wall between the TV aerial and the entry point into the buiding. An amplifier with twin outputs is connected there. I've often wondered how much noise is being picked up by the cable.
Satellite cable is double screened, so it's a good choice - if your signals are a decent strength, and you're only using an amplifier for the splitter, it's not a problem doing it nearer the TV.
Try answering a simple question then! - if you have a set in a poor reception area, with a snowy picture - does fitting a masthead amplifier improve the picture?.
Assuming you answer YES (because it does!), how do you explain that based on your idea of how radio waves work?.
However, I'm a little dismayed that anyone who has presumably spent the money on a spectrum analyser can understand so little about how the radio spectrum works?. I'm guessing that you're perhaps an aerial installer?, and not an electronics engineer or anything?.
I refer you to answer #41, with the emphasis on experimenting with a spectrum analyser.
I am a TV engineer as well as the owner of an aerial installation business.
As to whether I know what I`m talking about I refer you to my website : http://www.aerialsandtv.com/
I refer you to answer #41, with the emphasis on experimenting with a spectrum analyser.
I am a TV engineer as well as the owner of an aerial installation business.
As to whether I know what I`m talking about I refer you to my website : http://www.aerialsandtv.com/
If you are a TV engineer you should already know perfectly well that the noise on a TV is generated in it's tuner and as a aerial installer you should already know that installing a masthead amplifier improves the signal because it's increasing the signal ONLY, and not the noise in the TV. By your misguided idea a masthead amplifier would by completely useless as it would amplify the noise along with the signal, which doesn't happen.
You seem to be forgetting the fact that the noise on your spectrum aanlyser is generated by itself, just like a TV!.
Comments
And would still be wrong!.
You obviously live in a happy area where everyone gets good signal strength!.
How do you come to that conclusion?, you've reduced the signal by 4dB or so (as you've just said), yet the noise is exactly the same as it was (actually INCREASED slightly - but I'll ignore that) - so the signal to noise ratio has got 4dB worse.
How can wiring up an antenna be in the same league as an aviation engineer?
Hardly the same level of professionalism now is it!!!!!!
I inserted two coax into one antenna... it works... get over it!
OMG, Einstein is alive and kicking on this forum tonight.
If only he had remembered before logging on that the noise level(within the incoming split signal) is reduced by the same amount (more or less - but I'll ignore that), when compared against zero reference level.
Obviously not to you. If a job is worth doing it’s worth doing properly in my book.
When the time comes to replace the antenna and/or add a downlead for a further TV then i will consider renewing all coax and using a suitable splitter.
Until then... no need.
I don`t think you`re reading me correctly, I said "with the proviso that the site has sufficient signal" then a splitter is preferable to an amplifier. Obviously if your signal is only (say) 60dBuV, which is 1mV in old money, you shouldn`t really be splitting it without an amplifer.
If your signal is 75dBuV (approx 5mV) then you can split it no problem at all. In fact the resultant signal would still be about 71dBuV, which as about 3.5mV and fine for any TV because they work perfectly well down to around 1mV.
I would say that the great majority of sites receive a signal which is capable of being split twice, and a good number four times or more. We`ve done jobs where it was split six times and still worked perfectly.
On the subject of signal to noise ratio a typical signal might be 75dBuV with a noise floor of 20dBuV, i.e. the signal to noise ratio would be 55dBuV.
If you split this, or use an attenuator, to lose say 10dB then the resultant signal would be 65dBuV but the noise would also have lost 10dB and would therefore now be 10dBuV, i.e. the S/N would still be (65 - 10
Just to confirm it I`ve just checked it on our signal meter, and the S/N ratio was the same.....
Thank you and goodnight.
Well so long as you’re happy then that’s fine, but if you ever get one of those unexplained problems with your system that we see so many times on these forums, and you feel the need to enlist the help of other forum members, please do tell them from the onset that you have a gross impedance mismatch built into you aerial system, it might save them a lot of time and effort.
Yes you're about right, good for you. I think if our friend above was watching analogue TV then he might find some ghostly pictures on his screen caused by reflections up and down the length of the cable.
Except the noise isn't coming in with the signal, the limiting noise factor at VHF and above is the frontend noise of the receiver (as I also said earlier) - so you're only reducing the signal, and not the noise at all.
This is basic electronics and radio ham stuff, nothing terribly technical.
If you were talking about low radio frequencies, then your incorrect idea would be right - below 30MHz or so the limiting factor is noise coming in with the signal, and NOT the receiver.
How can you check the signal to noise ratio?, the noise is generated inside the frontend of the tuner at UHF (and VHF) frequencies - so a meter has no way of checking that.
However, this could explain your incorrect assumption about signal to noise not being reduced by a passive splitter?.
The signal to noise ratio is (to the aerial trade) that of the signal coming down the cable. If you look at it on a spectrum analyser you would see this signal as a waveform and then the noise floor as a continuous block of spurious signal at the bottom of the display. This is the background noise from everything out there which gives off RF.
If you use an amplifier this spurious noise goes up by the same amount that you have amplified the "wanted" signal, plus a bit more for the extra noise introduced by the amp.
If you use an attenuator the noise floor goes down, by the same amount that you have reduced the "wanted" signal.
The incoming noise though is LOWER than that generated in the tuner of the set, at VHF and above the limiting factor is the frontend noise, NOT any possible external noise.
So a passive splitter reduces your signal to noise.
Where the S/N ratio matters is at the IF output from the tuner, where the wanted signal is demodulated. At that point most of the noise comes from the mixer stage in the tuner, and reducing the input signal certainly worsens that S/N ratio. That is why TV AGC systems always act first at IF level, and only after IF gain is minimum do they apply delayed AGC to the RF amp before the mixer.
I can`t be bothered to spend any more time on this one. I`d go and look at TV signal on a spectrum analyser if I were you.
Try answering a simple question then! - if you have a set in a poor reception area, with a snowy picture - does fitting a masthead amplifier improve the picture?.
Assuming you answer YES (because it does!), how do you explain that based on your idea of how radio waves work?.
However, I'm a little dismayed that anyone who has presumably spent the money on a spectrum analyser can understand so little about how the radio spectrum works?. I'm guessing that you're perhaps an aerial installer?, and not an electronics engineer or anything?.
A passive splitter reduces the signal by 50% (on each output), yes? Well...by the time the signal reaches the TV then of course the SNR will be reduced, because the amount of noise that will be introduced after the splitter will still be the same.
Is that right?
Yes, pretty well (apart from you lose more than 50%) - the source of noise is the frontend of the tuner. By amplifying the signal at the masthead, you're essentially moving the source of noise there instead (where the signal is stronger), so when it gets to the TV the incoming signal is stronger than it was without the amplifier.
If you fit a set-back amplifier it doesn't do much good, because you've already lost signal down the coaxial cable, so it's nearer to the noise factor of the amplifier.
To get even more 'technical', you might notice that the 'LNB' on a radio telescope is normally cooled using liquid nitrogen, this reduces the noise factor of it's frontend, which helps greatly, as that's the source of the noise. If you nitrogen cooled your TV tuner it would help your poor reception a lot as well, but it's perhaps a little expensive and difficult!
If you mix apple juice and orange juice 50/50 then pour it into two glasses, the ratio of juices will still be 50%.
Exactly - but, in this case, you're pouring part of the apple juice (signal) away before you mix it with the orange juice (noise), so the ratio changes.
But Nigel, why, if they can design preamps to introduce little noise, does the TV become the principal source of noise?
By the time the signal gets to the IF stage wouldn't the signals be strong compared the noise generated within the TV?
And if there is little noise around at UHF external to the TV, then why all the fuss about super-screened TV cable?
Because of the expense of having someone go on the roof, I have hesitated about getting a mast-head amplifier and amplify my signal when it comes in at the window.
I have about 4 metres (max) of sat cable (copper sheath) running down the wall between the TV aerial and the entry point into the buiding. An amplifier with twin outputs is connected there. I've often wondered how much noise is being picked up by the cable.
At VHF and above, there is no atmospheric noise, what causes the snow on your screen (and the hiss on your FM radio) is noise generated in the receiver, specifically in the front end.
Modern tuners are remarkably good, and have very low noise factors, so mostly a masthead preamp is probably just as noisey as your TV tuner. It's the fact of being where the signal is strongest that makes it work, so the signal is stronger compared to the noise from the preamp.
Notice that if you place a splitter AFTER the masthead amplifer, then both the signal and the noise from it, are reduced by the splitter - but this doesn't happen with a passive splitter, because it adds little noise..
The IF output from the tuner is at a much higher level, and noise generated in the IF is much less of a problem
It's not about 'noise' it's about interference, which is a completely different thing.
Satellite cable is double screened, so it's a good choice - if your signals are a decent strength, and you're only using an amplifier for the splitter, it's not a problem doing it nearer the TV.
Henry
I refer you to answer #41, with the emphasis on experimenting with a spectrum analyser.
I am a TV engineer as well as the owner of an aerial installation business.
As to whether I know what I`m talking about I refer you to my website : http://www.aerialsandtv.com/
If you are a TV engineer you should already know perfectly well that the noise on a TV is generated in it's tuner and as a aerial installer you should already know that installing a masthead amplifier improves the signal because it's increasing the signal ONLY, and not the noise in the TV. By your misguided idea a masthead amplifier would by completely useless as it would amplify the noise along with the signal, which doesn't happen.
You seem to be forgetting the fact that the noise on your spectrum aanlyser is generated by itself, just like a TV!.