question about power injector's impact on OTA signal

[AZ_Tivo]

I am using an OTA antenna for my Tivo HD. This (http://www.radioshack.com/product/index.jsp?productId=2062075&cp=2032057.2032187.2032189.2032205&parentPage=family) is an indoor/outdoor antenna but it is sitting on top of my entertainment center. It is totally hidden and does a great job. Most local channels are around 90-95 in strength. This antenna does have a power injector.

I just got an HTPC/Blu ray player and tried to use the same antenna for my two tv tuner cards and there was NO signal. I was able to get some signal after I turned off power injector. But the signal is not as strong. Some channels are around 80 whereas some are in 40-60.

What is going on here? Can someone explain me the logic behind this.

Also, I will end up splitting the antenna input in three and continue to THD with power injector and give other two to PC. But what can I do to improve the signal on PC?

Thanks.

[dwit]

Starting out, by power injector, I guess you mean "amplifier".

What tuner cards are in the pc. Are they atsc, ntsc, or capable of tuning both? Some cards have separate input connectors for digital and analog.

Also, how set up is performed may be a factor. Are you sure the cards are set up for ota, as opposed to set up for cable?

What program are you using with the cards? Media Center, BTv4, MythTV, etc? Just curious on this part.

[AZ_Tivo]

Starting out, by power injector, I guess you mean "amplifier".

What tuner cards are in the pc. Are they atsc, ntsc, or capable of tuning both? Some cards have separate input connectors for digital and analog.

Also, how set up is performed may be a factor. Are you sure the cards are set up for ota, as opposed to set up for cable?

What program are you using with the cards? Media Center, BTv4, MythTV, etc? Just curious on this part.

I guess it's an amplifier. They call it power injector.

The tuner cards are ATSC/NTSC capable of clear QAM. These are OEM version for HP. I bought the HP m9300t. They do have separate inputs for digital and analog and I tried both. Not sure about the cards being 'setup' for OTA or cable. I didn't see any such setting. I am using MC at this time but might try BTV4.

[lrhorer]

Starting out, by power injector, I guess you mean "amplifier".
No, he doesn't. This unit has the amplifier in the antenna, but its power is supplied by what is known in the the CATV industry as a "Power Inserter". In short, this is simply a 3 port device designed to supply power and RF on a coaxial cable. The commercial CATV devices are larger, can deliver much more current, have replaceable fuses, and are more flexible than the consumer devices, but they work the same way. The consumer unit has one combined RF / DC port, one port with a DC block (high pass filter), and one with an RF block (low pass filter). The allows the amplifier to be powered from a remote location, convenient to a 115VAC wall outlet, while the amplifier is placed somewhere more convenient to the antenna, in this case apparently inside the antenna. The residential amplifier inside my house (supplied by Time Warner Cable) has a similar arrangement, and I also have a couple of other Radio Shack RF amplifiers which use a Power Inserter.

You can see the PI in the user's guide here (http://rsk.imageg.net/graphics/uc/rsk/Support/ProductManuals/1501634_PM_EN.pdf).

[lrhorer]

I am using an OTA antenna for my Tivo HD. This (http://www.radioshack.com/product/index.jsp?productId=2062075&cp=2032057.2032187.2032189.2032205&parentPage=family) is an indoor/outdoor antenna but it is sitting on top of my entertainment center. It is totally hidden and does a great job. Most local channels are around 90-95 in strength. This antenna does have a power injector.

I just got an HTPC/Blu ray player and tried to use the same antenna for my two tv tuner cards and there was NO signal. I was able to get some signal after I turned off power injector. But the signal is not as strong. Some channels are around 80 whereas some are in 40-60.
There are a number of possibilities. You may have the PI installed backwards, hitting your PC with power instead of RF and not powering the antenna at all. You may have blown the PC inputs by delivering power (12VDC, in this case) to them. You may have blown the PI by inadvertently shorting it, or your house wiring could be faulty, and accidentally touching the center conductor to your PC case may have blown the PI or the amp, or both.

It's also possible you haven't destroyed anything, but the PI is in backwards. You need to check the installation.

Also, I will end up splitting the antenna input in three and continue to THD with power injector and give other two to PC.
That makes no sense, and the fact it does not makes me feel even more strongly you have your RF plumbing put in incorrectly. The PI has nothing to do with the TiVo. It supplies power to the amplifier in the antenna. Its RF-only output must feed all three devices through a 3-way splitter.

[AZ_Tivo]

There are a number of possibilities. You may have the PI installed backwards, hitting your PC with power instead of RF and not powering the antenna at all. You may have blown the PC inputs by delivering power (12VDC, in this case) to them. You may have blown the PI by inadvertently shorting it, or your house wiring could be faulty, and accidentally touching the center conductor to your PC case may have blown the PI or the amp, or both.

It's also possible you haven't destroyed anything, but the PI is in backwards. You need to check the installation.


That makes no sense, and the fact it does not makes me feel even more strongly you have your RF plumbing put in incorrectly. The PI has nothing to do with the TiVo. It supplies power to the amplifier in the antenna. Its RF-only output must feed all three devices through a 3-way splitter.

The PI is connected right way. It clearly says on it how to connect it. The plumbing is straight forward. Antenna -> PI -> Tivo works fine. Same Antenna -> PI -> Tv tuner does not work. If I power down the PI it starts working with a weak signal. Maybe PI is sending a very strong signal which tv tuner cards can't read.

I can connect the 3 way splitter and try that way. Will let you know what happens.

[AZ_Tivo]

Here is the update. I've put in a 3 way splitter. So now it's Antenna -> PI -> splitter -> THD and 2 inputs to TV tuner. Everyone is happy and I can record nicely on all 4 tuners.

So my theory is that singal with PI was too strong for PC tv tuner cards. When divided 3 ways it was in an okay range for PC tv tuner to tune in. Or am I way off?

[phox_mulder]

So my theory is that singal with PI was too strong for PC tv tuner cards. When divided 3 ways it was in an okay range for PC tv tuner to tune in. Or am I way off?

I saw similar results with my HR10's ATSC tuner.

One splitter feeding the HR10 and the TV, spotty HR10 reception.
Splitting it again to feed the VCR (yes, Video Cassette Recorder),
and the HR10 could tune almost as well as the TV, but still not S3 quality.

Some tuners are more succeptable to multipath, tree leaves reflecting the signal all over the place, than others.
Power inserter not only amplifies the good signal, but the bad signal as well.


phox

[yunlin12]

I would compare digital TV reception with trying to jump through a window. The signal being the jumper and the tuner being the window. The signal strength is the height of the jump. You need to jump at a right height to get through, too low, you get clipped at your knee and fail, too high you bang your head and fail. Now if you have multiple devices each with different tuners, it like comparing different houses each built by a different guy, the windows may not be at the same height, so one signal strength may be right for one, but too high or too low for the other.

Add one more wrinkle on top of this, is that you have a cable going from your antenna (or cable inlet) to your tuner. Think of this as an additional challenge, you are now trying to do a triple jump (or more then triple), and trying to clear the window on the last bounce. If you have to do a lot of hops before you get to the window, your legs may get weaker and weaker, so by the time you get to the window you don't have enough height any more. That's why you need amplifiers. Think of them as trampolines, if they are tuned just right, they help you maintain your height until the last jump. Too strong, they may bounce you too high and you hit your head again. Same principle works with attenuators. If your jumps are too high for the window, maybe you want to jump off of something that don't bounce much, like soft dirt, and then you'll get the right height to clear.

Another issue to consider during amplification is noise. Think of it as lateral aim. You not only have to clear the window height wise, but also don't want to hit the sides. If when you start going towards the window you are off center a little, you have a risk of not clearing it. If you use a trampoline to amplify your jump, it will likely also amplify your mis-aim laterally, and cause you to miss the window.

So my take on this is, you are better off to get your cabling to the right distance, so that you don't need to plug additional amplifiers inline to boost the signal. Anything you add to the line will introduce extra noise. If your cabling is long, you have to use amplification to maintain signal level, at the expense of introducing extra noise. Better amplifier will introduce less. But nothing will increase your signal to noise level to higher than what you can receive at your antenna (or cable inlet).

[lrhorer]

Maybe PI is sending a very strong signal which tv tuner cards can't read.
The PI does not produce any sort of signal. Its only purpose is to provide power for the ammplifier in the antenna. Depending upon your location, it is certainly possible an amplified antenna is inappropriate, and a passive antenna is all that is required.

[lrhorer]

Some tuners are more succeptable to multipath, tree leaves reflecting the signal all over the place, than others.
Multipath distortion in the VHF and UHF band is not genrally caused by tree leaves. The wavelength of the signals is far too long to see major effects from something as small as a tree leaf. Multipath distortion in VHF and UHF signals is usually caused by tall buildings, radio towers, or variable terrain.

Power inserter not only amplifies the good signal, but the bad signal as well.
While true, it's not really relevant to this discussion.

[lrhorer]

I would compare digital TV reception with trying to jump through a window.
Well, sort of. Digital signals are less susceptible to noise, but more susceptible to distrotion than analog signals. In addition, analog signals degrade more gracefully with a decreasing Signal / Noise ratio than digital signals. An analog video may be judged "unacceptable" sooner than a digital video, but the viewer can still see the picture, and indeed the picture will simply get snowier and snowier with decreasing S/N long after the point of acceptable PQ is passed. Digital, OTOH, will be perfect almost all the way down to the threshold, but very shortly after reaching that threshold will quit altogether.


Add one more wrinkle on top of this, is that you have a cable going from your antenna (or cable inlet) to your tuner. Think of this as an additional challenge, you are now trying to do a triple jump (or more then triple), and trying to clear the window on the last bounce.
No, not really. Unless his downlead is several hundred feet long, and his signal quite weak, the drop from the antenna to the splitter is probably not a large factor. If it is RG-6, one can roughly call it 4 - 5 dB per hundrfed feet at UHF frequencies.

If you have to do a lot of hops before you get to the window, your legs may get weaker and weaker, so by the time you get to the window you don't have enough height any more. That's why you need amplifiers.
The nature of the signal source is a much greater factor. A big problem with OTA is the signal levels may vary wildly, the user being close to one TV tower and far away from another.

Think of them as trampolines, if they are tuned just right
Broadband RF amplifiers are not "tuned".

Another issue to consider during amplification is noise.
'Not in his case.

So my take on this is, you are better off to get your cabling to the right distance, so that you don't need to plug additional amplifiers inline to boost the signal.
Unless his situation is unusual, he doesn't need amplifiers, period. Indeed, I suspect his problem is levels that are too high, at least on some channels. The fact it started working when he split the signal bears that out.

Anything you add to the line will introduce extra noise.
While true, it's not relevant to his situation. In the past, CATV plants have often passed the subscriber signal through as many as 60 amplifier stages before reaching the back of the customer's TV. While the CATV amplifiers use gain stages with noise figures better than those of house amplifiers, they are not that much better. Contemporary CATV systems usually limit their cascades to a handful of amplifiers, but this is not primarily with a goal of reducing noise in mind. Most home amps have a noise fugure of about 12 dBmV, while the CATV amplifiers employ gain stages with noise figures of 9 dBmV. In any case, adding one or two amplifiers to an OTA signal path will not in and of itself significantly increase the noise, especially for digital signals.

Better amplifier will introduce less.
For short cascades, not really. Not significantly, anyway. Assuming a "perfect" (i.e. noiseless) input signal, an amplifier noise figure of 12, and an input level of 0dBmV, the output of a single amplifier will have a S/N of 57dB. That's fine for an analog signal. It's more than fine for a digital signal. If we pass through enough in the way of devices and cable to bring the signal all the way back down to 0dBmV and pass the signal through another identical amplifier, the S/N will be 54dB. Four amplifiers, and the signal is down to 50dB. At this point, the average user will usually notice snow or graniness in the analog picture, but our digital receiver will still be quite happy. What if we don't let the signal drop all the way to 0dBmv, however? If we make sure the signal level does not fall below +4dBmV in the amplifier cascade, then the first amplifier will dominate, and the S/N with 4 amplifiers is still better than 53.6dB, and many viewers will still consider the picture "perfect" if it is analog, and there will be no artifacts if it is digital. While each amplifier does indeed add its own bit of noise, that amount is insignificant for the amps with a +4dBmV input compared to the one amplifier with a 0dBmV input.

The bottom line is, one can't simply infer the perceived PQ will decrease with the number of amplifiers without actually calculating the system noise. That said, most setups don't even need one amplifier, and very few require two.

But nothing will increase your signal to noise level to higher than what you can receive at your antenna (or cable inlet).
True, but his does not sound like a S/N problem.

[yunlin12]

Thanks, lrhorer. I was trying to put the concept in layman's terms by using a few analogies. Most of them are not exactly matches, some of the word selections are misunderstood, like tuning the amplifer, I meant adjusting the amplification level, and not tuning to a frequency.