All,

I'm having trouble fully understanding tvfool's NM number. They state:

If you take the initial NM value for a given channel, add your antenna gain, subtract all the other system losses, and still end up with a value above 0, then you should be able to detect that channel.
.
.
.
Antenna gain is the only quantity that should ever be ADDed to the NM value.
.
.
.
If an antenna has a built-in amp, the extra gain from the amp SHOULD NOT be included as part of the antenna gain (this actually subtracts from the Noise Margin as we'll see next). Only the raw intrinsic gain of the physical antenna should be added to the Noise Margin.

Be aware that amps and pre-amps will actually cause you to lower your Noise Margin. No matter how much gain an amp or pre-amp claims, it will actually reduce your Noise Margin by the amount listed as the Noise Figure (NF) in its specs


I may be confused, but are they mixing two different concepts here (signal strength [dBm] and signal quality [SNR])? I would think the following should hold:

Factors that affect signal strength [dBm] into receiver:
- antenna gain (add)
- cable/splitter/balun losses (subtract)
- preamp gain (add)

Factors that affect signal quality [SNR] into receiver:
- antenna position (if multipath is present; add/subtract)
- preamp noise figure (subtract)

They seem to use NM as a means to describe a hybrid measure:

If they meant NM to mean signal strength, the following doesn't make sense: They say to add antenna gain and subtract system losses (which should affect signal strength, not SNR) but then they turn around and say to ignore the preamp gain (which should also affect signal strength)

If they meant NM to mean signal quality (SNR), then this doesn't make sense either: They telling us to add ignore preamp gain (makes sense), subtract preamp NF (makes sense), but then they say to add antenna gain and subtract system losses (which should only affect strength, shouldn't it?).

I'm sure I'm missing something so I appreciate anyone who's able to fill in the blanks. In essence, here's my problem:
1) Shouldn't SNR at the receving antenna only be a function of transmitter SNR and any multipath?
2) What the heck is NM really telling us, since to me it seems a hybrid of signal strength and signal quality.
3) Why does antenna gain (according to tvfool) increase the NM (assuming NM is a measure of SNR)? Shouldn't higher gain simply mean that both the noise and signal components of the signal are equally amplified, resulting in no improvement in SNR?
4) Wouldn't the best measure for reception prediction be a combination of SNR based on transmitter SNR and signal strength as specified in the "Pwr" column on tvfool? This should then be matched to the specifications of your particular receiver (or a statistical mean of common receiver specs) to determine how likely you are to receive the channel successfully.

Thanks for all - I hope that I'm just missing something obvious; and if not, perhaps start some dialogue on how this number should really be used!

Tom

If you are referring to digital channels there is no difference in PQ since the signals are all ready MPEG2 encoded as long as the signal strength is enough to receive them and not to stong as to overload your digital receiver.

Hi walford,

I actually was referring to signal quality and not picture quality. In order for a receiver to detect a signal I assume you need both a minimum signal quality level (signal-to-noise ratio, or SNR) and signal strength.

Think of someone speaking to you in public. They need to be loud enough so that you hear them (that's the signal strength portion). Also, they have to be loud enough in your surroundings so that their voice doesn't get drowned out by ambient noises (that's the signal quality, or SNR, part).

You could have a strong signal, like someone yelling at you, but if a fire truck goes by, you're not hearing them. This would be an example of high signal strength and low signal quality. On the other hand, you could be in a quiet room but if someone's whispering from across the room, you might not hear them. This would be an example of low signal strength even with good (transmitter) signal quality.

Interestingly, it seems like you could have low quality with high strength. But it doesn't seem like you could have high quality with low strength, because low strength inherently implies lower quality since the 'signal' portion is attenuated but the ambient noise doesn't fall below the noise floor in the system.

So my original questions still stand..

Tom

And my initial question still stand are you having problems with OTA digital tranmissons or with anallog since therre is not way to alter the PQ of digital content which your tuenr can receive without being overlaoded. You can not compare analog signal strength PQ with digital signal strength since with it digital it is all there or not there.

Nice job Tom! Very practical, applied examples to explain signal strength and s/n ratio! :) I can't answer your questions but Andy Lee, developer of tvfool, frequents this group in the "official tvfool thread". May want to post your questions there (so Andy sees it).

Thanks both!

IDRick - I'll take up your suggestion and post this on the tvfool thread.

walford - I'm referring to digital stations - sorry, forgot to answer earlier :)

The reason why you don't add the preamp gain is because the preamp amplifies the the signal and the noise plus adds a little noise (NF) of its own, making no improvement (even causing harm because of the NF) in the signal to noise ratio. A minimum SNR of about 16dB is needed to maintain lock on the digital signal.

Think of NM as how far above the signal is above ambient noise floor. The smaller the NM, the closer it is to the noise floor, which reduces the SNR.

You can check your own "NM" at your location by inserting an attenuator between your antenna (no amp) and the tuner, as described in the link in my signature. Keep increasing attenuation until the signal just drops out. The amount of attenuation at dropout is your "NM". Just before dropout you will see freeze, tiles, & etc. that are caused by the deterioration of signal quality because the bit errors (BER) are increasing to the point where the ATSC error correction system (FEC) can no longer handle them, and it gives up causing loss of lock.

Don't confuse signal strength (power/level) measured in dBm or dBmV with signal quality. Signal quality is related to BER.

The factors that reduce signal quality and cause a higher BER are:

1. Improper signal level: A weak signal will cause a poor signal-to-noise ratio; a signal that is too strong can overload a tuner or preamp. A nearby FM transmitter can also cause overload, which would require an FM trap.
2. Reflections from multipath problems.
3. Impulse noise in the reception area. (It's worse on VHF-Low. Just ask Trip in VA about PBS at home.)

Keep in mind, with digital, signal quality (BER) is just as important as signal strength, especially with multipath problems.
http://www.wowvision.tv/signal_strength_meters_BER.htm
http://www.wowvision.tv/HDglossary.htm Glossary
http://www.wowvision.tv Home

The importance of signal QUALITY is why I like to use a CECB with two signal bars, like the Apex DT502. The initial aim of the antenna can be done with the signal strength bar (or a SLM--signal level meter), but the final aim of the antenna has to be done with a way to monitor BER using the signal quality bar, to reduce the effect of multipath reflections.

When I was testing the DT502 with my CM4221 antenna I got (for my marginal ch13.1 on RF41):
Signal Quality 60%
Signal Strength 55%

I had aimed the antenna with my SLM, but when I rotated the 4221 slightly to the right I got:
Signal Quality 100%
Signal Strength 56%
Note the BIG change in signal quality with only a slight change in signal strength.

While I certainly agree that there are far better tools for measurement, the CECB's can be very useful for finetuning antenna location. In my case, I have broadcast towers in two directions (200 and 270 degrees). My DIY 4-bay does a nice job of receiving signal from both directions with proper placement in the attic. The APEX 502 was invaluable for optimizing signal strength of my two lowest channels (1 at 270 and 1 at 200 degrees) and deciding final antenna location/orientation.

Signal quality readings from the APEX also had practical application. My antenna is connected to two tvs and a computer capture card. Anything less than 100 on signal quality scale results in audio and video dropouts with the capture card. I get marvelous recordings with proper placement of the antenna (optimized signal strength and signal quality = 100). I'm sure top quality equipment could fine tune my setup even further. I'm satisfied with a $20 tool versus a several hundred dollar tool, especially for a one time use.

Update: I am following Tom to Andy's tvfool thread.