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Discussion Starter · #1 ·
I have a rooftop antenna with a rotor. Most of the local stations here are strong enough that pointing the antenna accurately is not critical, but there are a couple stations that are weak enough that antenna aim is critical. With most of my ATSC tuners pretty much the only way to find the station is to rotate the antenna until the tuner locks in a picture. My Sony Bravia TV, on the other hand, has signal diagnostics that include something called IF-AFC %. This indicator reads 100 when no signal is present, but then as the antenna moves closer to the right direction the IF-AFC slowly drops until it gets down into the 60's when the SNR (signal to noise ratio) starts to flicker with an indication that something is there. Then when the SNR gets up to about 16 the tuner can lock in a signal. With my strong stations the IF-AFC gets down into the 30's. So it seems that unlike other signal indicators, for this one, the lower the number, the better.


The cool thing is that this IF-AFC indicator, whatever it is, indicates the presence of a weak signal long before any of the other signal diagnostics (SNR, the little color coded signal strength bars, and signal level) indicate that anything is present. Anyone know what IF-AFC stands for and what this is measuring? The manual for the TV just says "Select to view diagnosics information for the current signal," with no explanations as to what the different diagnostics are.
 

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I think you mean IF-AGC. This stands for Automatic Gain Control.


The first stage in a receiver has variable gain to adjust for differing signal levels. When the signal is weak, the gain control will be at maximum since the receiver is looking for any signal. Once a signal is received, the gain adjusts to the optimal amount to receive the signal well.


You cannot receive an ATSC signal until the signal-to-noise ratio is above around 16dB, regardless of how strong the received signal is. If there is a strong local noise source, AGC might be all the way down to 40 or 50% and you still get no signal.


Signal to noise is all that matters. Out in the country, far from all interference sources, you might receive a signal so weak that AGC has to crank way up to 80 or 90%, yet the lack of noise means you could get 20 or 25dB S/N, and a perfect, beautiful picture.
 

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Discussion Starter · #3 ·

Quote:
Originally Posted by zorinlynx /forum/post/15732466


I think you mean IF-AGC. This stands for Automatic Gain Control.

You're right. Thank you!
Quote:
You cannot receive an ATSC signal until the signal-to-noise ratio is above around 16dB, regardless of how strong the received signal is. If there is a strong local noise source, AGC might be all the way down to 40 or 50% and you still get no signal.

The cool thing is that in some cases the IF-AGC diagnostic can be used as an aide for antenna pointing, if you know that a signal should be in the air on a certain frequency in a certain direction but you don't have quite enough signal to lock. You're right, usually you need a minimum SNR of 15 or 16 to lock and for most ATSC tuners you get no indication that a signal is present until you have enough signal to lock. What I've noticed though is that this IF-AGC diagnostic will indicate the presence of a signal well before you get enough signal to lock. If you are trying to receive a weak ATSC signal and the antenna is pointed in the wrong direction and you slowly rotate the antenna toward the source of the signal the IF-AGC starts out at 100 and then slowly drops down as more signal is received. So it gives you an indication of where to point the antenna to maximize signal, and in some cases it may indicate that a signal is there but your antenna system doesn't have quite enough gain to lock. So it's a tool that lets you estimate that with maybe 2-3 more dB of gain you could lock if you were to upgrade your antenna or add a preamp.
 

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Very interesting topic! Why doesn't Panasonic and other HDTV's come with these advanced features??? I was searching to find something on IF AGC on google and it landed me in this thread. Not much info anywhere else.
 

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I agree, interesting topic. I didn't even realize my Sony had this feature. It looks like all my locals are coming in at around the low 30's for both AGC% and SNR db

My weakest stations are 43% AGC and 26 SNR. It's also cool I can enter in a digital channel that's not even in my channel listings and it will give all the info, albeit 0 SNR and 100% AGC for a channel with nothing on it.
 

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Sorry to resurrect such an old thread, but it's the only relevant conversation I can find on this topic: Sony's IF-AGC measurement.


I own a Sony KDL-46V5100 TV and have recently connected a UHF antenna to it: a non-amplified Radio Shack 15-234 bowtie with about 10 feet of cable between it and the TV. The Sony's "Signal Diagnostics" menu shows these measurements for a particular digital channel:

Signal Strength: 90

Phy. Ch.: 19

Freq. (kHz): 503000

Modulation: 8VSB

Status: Lock

Errors: 0
SNR (dB): 32
IF-AGC (%): 105

Interleaving: N/A


It seems to me that Signal Strength and SNR (signal to noise ratio) are good. (Though finding good information on SNR has been difficult, I've gleaned that the higher the number the better, and that the tuner likely won't be able to decode with an SNR below 16 or so. But what's a really good SNR number, or even the maximum possible number?) And the picture looks good, though one thing this weekend bothered me: fast motion on this channel looks better via DirecTV than via my antenna. That got me wondering about the IF-AGC number and if perhaps adding an amplifier and/or a roof-top antenna is necessary even though my other numbers look good.


Posts to this thread from four years ago make sense, but that is not how my TV acts. If I disconnect the antenna, IF-AGC goes to 0.


Questions:

  1. How could IF-AGC be above 100% if 100% represents the AGC circuitry in the TV using all of its power to boost the signal? Or in my case, is the AGC circuitry purposefully weakening a too strong signal?
  2. If IF-AGC is that high, does that mean the Signal Strength number is not an accurate representation of the pre-AGC signal strength?

Bottom line: I'm hoping someone has more information about Sony's IF-AGC measurement.


Thanks!


P.S. My TV Fool output, if it's relevant:
http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d5b94beb64200f6

I'm only interested in the UHF digital signals to the Southwest of me. I live on a hill and have the bowtie antenna facing those signals.
 

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Hello Augie:


The Sony Signal Diagnostics screen is not consistent from model to model. On my KDL22L5000 the Signal Strength and IF-AGC numbers are identical. On other models they are not the same, and both are arbitrary scales. With the signal strength scale, more is better, but the IF-ACG scale can have either a direct or inverse relationship to Signal Strength.


I haven't found a need for the IF-AGC numbers; the signal strength, SNR, and errors are what I use for OTA reception problems. A SONY tech might find some use for the IF-AGC numbers to diagnose tv set problems.


In a good signal area:




In a weak signal area (picture frozen at the "digital cliff" just before dropout):




I did a calibration chart for my KDL22L5000 using my Sadelco signal level meter that reads in dBmV:




When the signal strength gets down to about 55, the AGC is not able to increase the gain any more, but the tv is still able to decode signals that are a little weaker. When the signals are very strong, the signal strength reading never goes above 78, because the AGC is not able to reduce the gain any further. The tv can tolerate signals that are stronger without going into tuner overload.


As you can see, the operating range of the AGC is much less than the signal strength range that the tv can tolerate which is shown by the flat curve at each end. With this set, each increase in signal strength number is conveniently about 1 dB greater than the previous one, in the linear portion of the AGC curve. This is confirmed by the 45 degree slope of the curve.


The signal diagnostics screen for my KDL22BX320 is a little different. I haven't done a calibration curve yet, but IIRC the signal strength goes up to 100, and then says MAX, and goes lower than 55 for weak signals.


You are correct about the SNR; it needs to be above about 16 dB for reliable reception. Any reading above that gives more protection from signals that vary in strength as is often the case with OTA signals.


Your desired signals are quite strong, so I don't think that a preamp and outdoor antenna would give the improvement you hope for, but I never want to discourage antenna experiments because I always learn something from mine. Keep in mind that it is possible to overload a preamp or tuner in a strong signal area.


If you want to satisfy your own curiosity (and maybe even prove me wrong) you can try a temporary setup of a DB4e antenna (no preamp) outside or in a window that faces the transmitters (no metal screen or low-e glass).


If you want to experiment with your antenna, insert a variable attenuator between the antenna and the tv to find out how much you can reduce the signal before the SNR goes below 16 and the errors increase above zero, as is demonstrated in the link in my signature. This will tell you how much margin to dropout you have.




The conversion factor between dBmV and dBm that you see on your tvfool report is 48.8, or roughly 50, so -30 dBmV is about -80 dBm and +20 dBmV is about -30 dBm.
http://www.soontai.com/cal_exunit.html
http://www.jneuhaus.com/volts_to_dBm.html


Best regards,

rabbit73
 
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