HDTV / ATSC signal levels - AVS Forum
Forum Jump: 
 
Thread Tools
post #1 of 13 Old 01-01-2007, 03:01 PM - Thread Starter
Member
 
larry2007's Avatar
 
Join Date: Jan 2007
Posts: 16
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
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
larry2007 is offline  
Sponsored Links
Advertisement
 
post #2 of 13 Old 01-01-2007, 03:07 PM
AVS Club Gold
 
Davinleeds's Avatar
 
Join Date: Mar 2006
Posts: 5,410
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
You know more than me, but reading threads, minimum SNR 15 to18, over 22 good, 30 plus is prime. Check out the antenna thread.

"The purpose of diplomacy is to prolong a crisis." Spock, Mark of Gideon, TOS
Davinleeds is offline  
post #3 of 13 Old 01-01-2007, 04:02 PM
AVS Club Gold
 
Davinleeds's Avatar
 
Join Date: Mar 2006
Posts: 5,410
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
BUMP, I'm interested in this too!

"The purpose of diplomacy is to prolong a crisis." Spock, Mark of Gideon, TOS
Davinleeds is offline  
post #4 of 13 Old 01-01-2007, 06:13 PM
AVS Special Member
 
AntAltMike's Avatar
 
Join Date: Nov 2001
Location: College Park, MD
Posts: 3,541
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 7 Post(s)
Liked: 20
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.
AntAltMike is online now  
post #5 of 13 Old 01-01-2007, 06:24 PM
AVS Special Member
 
AntAltMike's Avatar
 
Join Date: Nov 2001
Location: College Park, MD
Posts: 3,541
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 7 Post(s)
Liked: 20
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.
AntAltMike is online now  
post #6 of 13 Old 01-01-2007, 06:26 PM
AVS Club Gold
 
Davinleeds's Avatar
 
Join Date: Mar 2006
Posts: 5,410
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Thanks

"The purpose of diplomacy is to prolong a crisis." Spock, Mark of Gideon, TOS
Davinleeds is offline  
post #7 of 13 Old 01-01-2007, 06:41 PM
AVS Club Gold
 
Davinleeds's Avatar
 
Join Date: Mar 2006
Posts: 5,410
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Here's some thinghttp://www.avsforum.com/avs-vb/showt...bu#post9243925

"The purpose of diplomacy is to prolong a crisis." Spock, Mark of Gideon, TOS
Davinleeds is offline  
post #8 of 13 Old 01-01-2007, 06:52 PM
AVS Club Gold
 
Davinleeds's Avatar
 
Join Date: Mar 2006
Posts: 5,410
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
http://www.avsforum.com/avs-vb/showt...ght=Davinleeds A little more

"The purpose of diplomacy is to prolong a crisis." Spock, Mark of Gideon, TOS
Davinleeds is offline  
post #9 of 13 Old 01-01-2007, 08:36 PM - Thread Starter
Member
 
larry2007's Avatar
 
Join Date: Jan 2007
Posts: 16
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
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?
larry2007 is offline  
post #10 of 13 Old 01-01-2007, 08:51 PM
AVS Special Member
 
AntAltMike's Avatar
 
Join Date: Nov 2001
Location: College Park, MD
Posts: 3,541
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 7 Post(s)
Liked: 20
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?
AntAltMike is online now  
post #11 of 13 Old 01-01-2007, 08:56 PM - Thread Starter
Member
 
larry2007's Avatar
 
Join Date: Jan 2007
Posts: 16
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
95128
larry2007 is offline  
post #12 of 13 Old 01-13-2007, 06:02 AM
AVS Addicted Member
 
videobruce's Avatar
 
Join Date: Dec 2002
Location: Buffalo NY
Posts: 14,895
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 50 Post(s)
Liked: 105
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)

Abundant OTA television is what makes this country different from all others. Lets keep it this way.
The Internet is no place for streaming video.
videobruce is online now  
post #13 of 13 Old 01-13-2007, 06:16 AM
AVS Addicted Member
 
videobruce's Avatar
 
Join Date: Dec 2002
Location: Buffalo NY
Posts: 14,895
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 50 Post(s)
Liked: 105
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.

Abundant OTA television is what makes this country different from all others. Lets keep it this way.
The Internet is no place for streaming video.
videobruce is online now  
Reply HDTV Technical

User Tag List

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off