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Old 6-Jan-2012, 4:18 PM   #1
lkruper
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Coorelate performance to TV Fool Analysis?

I have a general question regarding how to interpret TV Fool and make predictions. In looking at the explanations as to what stations are categorized as working with indoor, attic or roof-top antennas it looks like there is a 20db difference between each level. Does this mean that a rule of thumb is that the attic cuts the signal by 20db and that indoors another 20db? I realize that the materials in the house have much to do with this, however, can I quantify this by correlating the signal I have measured at my TV set with this setup?

For example channel 7.1 at real channel 7 is predicted at a NM of 35.8 with power of -55.1 and I measure up to 95% on the signal bar of my Sony TV with an antenna in the attic. There are other diagnostics on the TV but I don't remember what they are. Virtual channel 5.1 at real 31 has NM of 26.8 dB and Pwr of -64.0 and I get 10% signal on the TV. Channel 28.8 (Real 28) has a NM of 16.9 and Pwr of -74.0 and it is very intermittent. Can this data quantify the losses in my system? For example, can I assume that the data for channel 28 quantifies my loss at about 16.9 dB?
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Old 6-Jan-2012, 4:35 PM   #2
ADTech
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Unfortunately, the built-in "signal meters" that display in percentages rarely have a known correlation to actual received signal powers.
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Old 6-Jan-2012, 9:07 PM   #3
lkruper
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Quote:
Originally Posted by ADTech View Post
Unfortunately, the built-in "signal meters" that display in percentages rarely have a known correlation to actual received signal powers.
I called Sony and that is what they said too. Then I went home and looked at the rest of the diagnostics. They also report SNR in dB and so I recorded them:



Callsign, NM (dB), Sony SNR dB
KABC-TV, 35.8, 28.6
KTTV, 35.7, 31
KCOP-TV, 36.8, 26.5
KCAL-DT, 33.9, 24.3
KTLA-TV, 26.8, 14
KDOC-DT, 10.7, 20.1


For the stronger signals it looks I have a loss of 5-10 dB from the TV Fool estimate to my TV. We would need to add the gain of my Radio Shack antenna which is probably not as good as the more modern antennas as well. But if it is 10dB that would make a loss of 20dB from my attic down to the TV.

Does that seem reasonable? Does any of this make sense?

Also, one channel that is intermittent (KTLA) shows a SNR of 14dB but my tuner cannot get a signal today.

I guess in the final analysis I still need to get the most powerful UHF antenna possible. The channel that I really need is Real 43 (Virt 2.1) because this is the channel Rovi delivers it channel guide information through.
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Old 6-Jan-2012, 10:02 PM   #4
mtownsend
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Unfortunately, SNR cannot be directly compared to NM.

SNR is a relative measure between the desired signal and some arbitrary noise floor that is unknown to us. That noise floor is a function of the receiver's circuitry and can be affected by internal components like filters, amplifiers, automatic gain control, and signal processing stages.

NM is measured relative to the thermal noise floor (Johnson-Nyquist noise), which is the natural background noise of a room temperature receiver, and is essentially a constant value derived from the laws of physics.

Since we don't know what the noise reference is in the SNR calculation, we cannot translate it into a corresponding NM value.



Also, if the Sony SNR estimate is accurate, you'll need at least 15.2 dB of SNR to get a watchable DTV signal. That is the theoretical minimum SNR needed to piece together enough good digital data to reconstruct the original video stream. In practice, you'll want to stay well above that so you have some margin for random fluctuations in the signal.
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Old 6-Jan-2012, 10:26 PM   #5
lkruper
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Quote:
Originally Posted by mtownsend View Post
Unfortunately, SNR cannot be directly compared to NM.

SNR is a relative measure between the desired signal and some arbitrary noise floor that is unknown to us. That noise floor is a function of the receiver's circuitry and can be affected by internal components like filters, amplifiers, automatic gain control, and signal processing stages.

NM is measured relative to the thermal noise floor (Johnson-Nyquist noise), which is the natural background noise of a room temperature receiver, and is essentially a constant value derived from the laws of physics.

Since we don't know what the noise reference is in the SNR calculation, we cannot translate it into a corresponding NM value.



Also, if the Sony SNR estimate is accurate, you'll need at least 15.2 dB of SNR to get a watchable DTV signal. That is the theoretical minimum SNR needed to piece together enough good digital data to reconstruct the original video stream. In practice, you'll want to stay well above that so you have some margin for random fluctuations in the signal.
Thank you for the explanation. Since I have a channel which measures 14dB but has no signal and this channel is intermittent, your 15.2 dB makes a great deal of sense.

I am reading your post on the subject at the link http://forum.tvfool.com/showpost.php?p=427&postcount=2

Thanks again.

Last edited by lkruper; 6-Jan-2012 at 10:35 PM.
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