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HDTV / ATSC signal levels

post #1 of 13
1/1/07 at 3:01pm
Thread Starter 
What is the range of voltage levels and SNR levels that are needed for typical interruption-free HDTV experience? For old analog UHF signals, I seem to recall (20 years ago) that these numbers were around 44 dB SNR and around 1mv (0dBmV) or better for a grade A picture, and about 10dB lower for a grade B picture. As has been noted by many others, digital reception is more of a matter of cliff signal, where it is either coming in or it isn't (no shades of gray or snow).

I assume that the SNR of the of the signal at the transmitter does not matter, and that all of the different kinds of noise seen at the receive antenna (4:1 balun) can be simplified to something like 75ohms at some temperature (sky or ambient?) over a 6 MHz band, resulting in an approximate noise floor number (-53 dBmV or so + 1 dB for the balun)? If this is the case, what is the needed minimum voltage at the balun output to reliably receive an HDTV channel, if the ATSC receiver (often called an ATSC tuner or HDTV tuner) were right at the balun?

Is this oversimplified, in that other forms of distortion (IMD, etc) or other items are likely large contributors? If this is about right, we should be able to determine when it makes sense to buy a bigger antenna or buy a pre-amp, or go home
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post #2 of 13
1/1/07 at 3:07pm
You know more than me, but reading threads, minimum SNR 15 to18, over 22 good, 30 plus is prime. Check out the antenna thread.
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post #3 of 13
1/1/07 at 4:02pm
BUMP, I'm interested in this too!
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post #4 of 13
1/1/07 at 6:13pm
Somewhere, probably in the archived threads, someone posted a bunch of figures for minimum signal levels needed by various popular tuners are various frequencies. A few weeks ago, I took a shot looking for it, but I'm presently on a 28kbs phoneline and gave up after a few unsuccessful forays.

The off-air tuners all have sucky noise figures. This industry is referenced to the dBmV (75 ohm) and so 0dBmV(75ohm) = -48.75dBm. We think that the front end of the receiver needs to see about a 16dB S/N ratio to reliably lock. Most installer-grade signal meters output in dBmV(75ohm), so you would need about -40dBmV measured at the receiver input to not have to worry about glitches.

Very little thought has been given in these forums to the likely effects of intermodulation and harmonic combinations on weak signals. It is not uncommon for there to be 30dB to 40dB difference in signal power in usable signals received from one antenna, and so antenna amplification analysis would likely show that off-air systems develop intermodulation problems before they drive their amplifiers into clipping, whereas cable companies, with large, level analog and digital loads, are often most concerned with either sync compression or different intermodulation byproducts than those most likely to cause problems in broadcast DTV spectrums.

A problem that this forum presently faces is that many members who are technically knowledgeable have found it to have become largely a newbie forum, with the advice given being repititive in nature, so they no longer participate as actively as they once did.
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post #5 of 13
1/1/07 at 6:24pm
In this thread, amps and preamps, you will find a discussion of technical aspects that may be of interest to you, beginning with post #35.
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post #6 of 13
1/1/07 at 6:26pm
Thanks
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post #7 of 13
1/1/07 at 6:41pm
Here's some thinghttp://www.avsforum.com/avs-vb/showt...bu#post9243925
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post #8 of 13
1/1/07 at 6:52pm
http://www.avsforum.com/avs-vb/showt...ght=Davinleeds A little more
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post #9 of 13
1/1/07 at 8:36pm
Thread Starter 
Quote:
Originally Posted by AntAltMike View Post

The off-air tuners all have sucky noise figures. This industry is referenced to the dBmV (75 ohm) and so 0dBmV(75ohm) = -48.75dBm. We think that the front end of the receiver needs to see about a 16dB S/N ratio to reliably lock. Most installer-grade signal meters output in dBmV(75ohm), so you would need about -40dBmV measured at the receiver input to not have to worry about glitches.


This is exactly what I was after, and confirms what I have suspected, in that the DTV needs are far lower than they used to be in the analog UHF world, needing only a SNR of 16dB and signal strength of -40dBmV (10 uV) or better at the receiver. I must admit, however, that I am amazed that such a small signal does the trick. If this is the case, for a reasonable loss of 6 dB of cable, a -34dBmv or stronger signal should suffice without a pre-amp, since the cable will attenuate the signal but not the noise floor.

Quote:
Originally Posted by AntAltMike View Post

Very little thought has been given in these forums to the likely effects of intermodulation and harmonic combinations on weak signals. It is not uncommon for there to be 30dB to 40dB difference in signal power in usable signals received from one antenna, and so antenna amplification analysis would likely show that off-air systems develop intermodulation problems before they drive their amplifiers into clipping, whereas cable companies, with large, level analog and digital loads, are often most concerned with either sync compression or different intermodulation byproducts than those most likely to cause problems in broadcast DTV spectrums.

A problem that this forum presently faces is that many members who are technically knowledgeable have found it to have become largely a newbie forum, with the advice given being repititive in nature, so they no longer participate as actively as they once did.

I wonder about IMD and harmonics as well, for exactly the reason you mention, that there can, and likely are useful signals within 3 harmonics that are 50dB apart in signal level, and adjacent signals like ch 11 and 11.1 (ch 12) here (SF bay area) that are 25dB apart. Somone that rembers more than I, should be able to sort that out.

A problem that this forum presently faces is that many members who are technically knowledgeable have found it to have become largely a newbie forum, with the advice given being repititive in nature, so they no longer participate as actively as they once did.

It appears to be a good forum. The problems you mention are difficult to avoid.

Based on the above analysis, the biggest problem that I am seeing is most likely an issue of the ATI HDTV tuner or the MS Media center products, rather than the SNR or the signal strength or the mutipath or IMD. I will need to buy more gear to test this out. What forum would be best (regional, tuner or OS) to go to with discoveries and questions?
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post #10 of 13
1/1/07 at 8:51pm
Quote:
Originally Posted by larry2007 View Post

What forum would be best (regional, tuner or OS) to go to with discoveries and questions?

I'd try regional next, if you are in a large enough market such that your local forum has sufficient traffic. What is your zip code?
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post #11 of 13
1/1/07 at 8:56pm
Thread Starter 
95128
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post #12 of 13
1/13/07 at 6:02am
http://www.2150.com/broadcast/defaul...=Show+Stations

You can fine tune your exact location here (also confirm the magnetic declination);
http://tiger.census.gov/cgi-bin/mapb...8+(San%20Jose)
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post #13 of 13
1/13/07 at 6:16am
Quote:
A problem that this forum presently faces is that many members who are technically knowledgeable have found it to have become largely a newbie forum, with the advice given being repititive in nature, so they no longer participate as actively as they once did.

Point well made. Too many ask the same question over and over.
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