DT or other electronic nerds

[Utah]

A couple of questions about OTA HD broadcasts...

What causes the signal strength of OTA HD broadcasts to fluctuate so much? For instance, According to the Diagnostics screen on my TV, the current station I am watching has the following:

*Signal Strength: bounces between 41 - 73

*Errors: 0 (when the station is locked which means the strength is above 50 I believe). Or 60 or 255 when it is not locked

SNR (db): 0 - 24 (doesn't seem to be any rhyme or reason when this changes. It is not changing based on the signal strength.

AGC (%): 25 - 28


And lastly, what is everythinig I just listed? Errors? SNR? AGC?

[DT]

I saw the Nerd Symbol flash up in the sky, took a hit off my asthma inhaler, put on the glasses and jumped on my Segway (aka, Seg-gay).

SNR is Signal to Noise Ratio - basically there is the signal (i.e., the part you want) and everything else (the noise). You want a high SNR though that high is different on analog vs. digital. What you want is a high likelihood of signal recovery (getting the part you want) - in a digital signal, this is easier to do, so your SNR can be lower. I believe in analog TV you wanted something like 50dB+ and for digital anything in the 20-30 is considered excellent. Your equipment itself adds to the noise element of SNR, length of cable runs, external sources too. On old equipment it would be a uniform kind of quality loss/snow, on digital systems it looks like weird pixel blocking, frame dropouts, etc.

The lower the SNR the higher your AGC will be - Automatic Gain Control is a circuit designed to artifically create more dynamic range (to improve isolation between the good and the bad). SNR goes down, AGC will go up - though obviously it's not magic and you're going to lose more than you gain.

Strength is a little harder as it's kind of arbitrary- normally it would be measured in dBm, but what manufacturers do is to pick some dBm as the artificial "perfect" and call that 100.

The reason your strength might fluctuate but your SNR stay the same, is as long as there's enough strength to reach your receiver intact, it's good - the noise present (and ratio of signal to noise) is the same. That being said a higher "strength" signal might overcome more noise, but that starts getting complicated. It's like if you're at a concert, band isn't playing, talking in a normal voice (strength), band comes on (noise) now you can't be heard. If you increase your strength and yell, you *still* can't be heard.

[Utah]

hmmm, thanks DT, that answered alot of the definitions for me... But still confused as to what would cause the signal to fluctuate so dramatically. It will literally be in the 80's, then drop down to the 40's for a few seconds, then back up to the 80's. It makes watching a show nearly impossible...