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post #14911 of 16297 Old 05-22-2012, 03:45 PM
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Can you run those tests with your Sony connected?

I would like to try an experiment like that, but I didn't do those tests to reduce urban multipath by putting an antenna in a shielded enclosure. Credit goes to Bill Naivar at Georgia Tech in Atlanta for his novel solution.

Quote:


...but unless that translates into an improved SNR then the shielding is not doing what you think it is doing.

Yes, exactly

The shielding that he used probably did improve the SNR, but more importantly it reduced the errors from multipath so that the FEC capacity was not exceeded. That's why I think that monitoring BER is better than signal strength when aiming an antenna.

http://www.avsforum.com/avs-vb/showt...3#post21754233

If you can not measure it, you can not improve it.
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post #14912 of 16297 Old 05-22-2012, 05:57 PM
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Quote:
Originally Posted by rabbit73 View Post

The shielding that he used probably did improve the SNR, but more importantly it reduced the errors from multipath so that the FEC capacity was not exceeded. That's why I think that monitoring BER is better than signal strength when aiming an antenna.

http://www.avsforum.com/avs-vb/showt...3#post21754233

I read through your post. I think you'd be better off aiming for highest SNR when the strongest signal and highest SNR don't correspond. The reason I say this is because (from what I've read) that the corrected errors go to zero above an SNR of 20 dB but the SNR can keep going up. To put it another way, zero errors doesn't necessarily equal the highest signal quality.

I've run BER tests in the lab on BPSK through 256QAM signals and it only takes a signal increase of 1 dB to go from a non-zero BER to a BER of zero. Of course that non-zero BER might be in the range of 10E-6.

Sounds to me that when you moved your 4221 and saw a large increase in signal quality, that was because you were able to place a major reflection in an antenna null. I don't have any information on how common that situation is. All I can say is that is never the situation here since I have hundreds of reflections coming from a 60 degree swath of terrain east of me. It wouldn't surprise me to find out if most people have to deal with multiple reflections.

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post #14913 of 16297 Old 05-22-2012, 06:58 PM
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I read through your post. I think you'd be better off aiming for highest SNR when the strongest signal and highest SNR don't correspond.

Yes, I agree that SNR is an important figure of merit for an antenna. What I had in mind with my previous post is when there is a multipath problem that challenges the FEC, then BER becomes equally important.

I had a discussion on the TV Fool Discussion Thread about antenna gain, saying:
Quote:
The gain of the antenna is added to the NM because it improves the SNR.

http://www.avsforum.com/avs-vb/showt...1#post15698511 post #393

Nitewatchman answered me saying:
Quote:
Well, the way I look at it, TVfool's predictions are predictions of signal strength, not SNR. The gain of antenna is added to the NM because it increases signal strength.

http://www.avsforum.com/avs-vb/showt...6#post15698876 post #394

An antenna with more gain does indeed increase the signal strength, but so does a preamp.

The difference between the two is that the preamp increases the signal strength and the noise plus adding some noise of its own, but the antenna with more gain increases the signal strength without adding noise resulting in an improved SNR.

The post by Andy Lee about Visual explanation of Noise Margin is interesting:
http://www.avsforum.com/avs-vb/showt...9#post15700679 #397

Quote:
All I can say is that is never the situation here since I have hundreds of reflections coming from a 60 degree swath of terrain east of me.

You do have an unusual situation that requires a very high F to B for your antenna.

If you can not measure it, you can not improve it.
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post #14914 of 16297 Old 05-22-2012, 07:06 PM
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Quote:
Originally Posted by rabbit73 View Post

Thanks for the link to the DHC thread, and in particular to your links in post #716. I have looked at those references and they seem to focus on the SWR between the transmitter PA and antenna which can reduce the SNR/MER below the 27 dB required by the FCC for a transmitted signal.

I think it is a mistake to use the antenna reciprocity theorm to assume that what goes on the in receiver's feedline is equivalent to that in the transmitter's feedline. The signal that comes from the transmitting antenna is sinusoidal (analog.....like NTSC).

An increase in the SWR in the receiving feedline causes an additional lineloss that is of the same nature as the inherent coax loss at that frequency and is added to that loss. Both increase the system NF and cause a reduction in SNR which requires a stronger signal to maintain the minimum required SNR.

A multipath problem that shows on the spectrum analyzer as a ragged signal also requires a stronger signal to maintain the minimum SNR between the dips and the noise floor in order for the equalizer to compensate, if it can.
OK, you've got me hooked, holl_ands.

What kind of a test can I devise to prove Dr. Bendov's theory?

Would it be sufficient to insert a section of coax in the feedline with an impedance much higher or lower than 75 ohms to give a SWR mismatch of 4 to 1, or is there a better way to do it?

Dr Bendov and I believe in Reciprocity...but I understand your
reluctance to blindly accept anything without PROOF....

If you pursue the "wrong" impedance coax, it probably should be the SAME LENGTH
as the "correct" impedance coax to replicate the multipath bouncing back and forth.
I'm not so sure you're going to find coax with the requisite impedance....

You might compare to an antenna with a KNOWN BAD SWR, such as the way too
small U-TUBE 4-Bay Bowtie and see if there is "Excessive Noise Figure", which
would be fairly difficult to PROVE in an Apples vs Pommes de Terre (Potatoes)
antenna comparison: http://imageevent.com/holl_ands/multibay/4bay/utube

Alternatively, you could use a 1:1 Coax Balun instead of the usual 4:1 Balun
[could be Coax] with a known GOOD SWR antenna and see if the Noise Figure
degrades when a 300-ohm antenna is terminated in 75-ohms (4.0 SWR).
This would be a better Apples vs Candy Apples comparison....
But you'll need to use very weak stations to see NF degradation,
and you should conduct the test for multiple channels and perhaps also
perhaps multiple antennas to verify repeatability....
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post #14915 of 16297 Old 05-22-2012, 07:34 PM
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holl_ands:

Thanks for the tips about what would be suitable test.
Quote:
But you'll need to use very weak stations to see NF degradation,
and you should conduct the test for multiple channels and perhaps also
perhaps multiple antennas to verify repeatability....

Yes, I plan to use weak signals and also an attenuator just before a splitter that feeds two tuners and a signal level meter to bring the signal to the cliff and be able to monitor signal strength, SNR, and errors.

This will enable me to see where dropout happens (in dBmV and dBm) with the two antenna systems.

When the NF is greater, then dropout should happen sooner at a higher signal level because the noise floor is now higher. Or, to put it another way, the system that has the largest margin-to-dropout is the better system.
Quote:
If you pursue the "wrong" impedance coax, it probably should be the SAME LENGTH
as the "correct" impedance coax to replicate the multipath bouncing back and forth.
I'm not so sure you're going to find coax with the requisite impedance....

Four pieces of coax in parallel might do it.

Quote:
....but I understand your reluctance to blindly accept anything without PROOF....

Thanks for understanding; I've been burned too many times.

If you can not measure it, you can not improve it.
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post #14916 of 16297 Old 05-22-2012, 07:35 PM
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When trying to align a high gain antenna, it's difficult to find the narrow "sweet spot",
esp. if you rely on a friend to shout out directions....and/or you're poking around the attic
looking for that elusive spot that has barely a sniff of a signal to begin with.....

So I find Laptop PC with an ATSC USB Stick Tuner to be an essential test tool....
Or a CECB Box with a small luggable TV.....at the end of a very long extension cord....

I've found that (FREE) TSReader Lite will display Bit Error Rate statistics when SNR
is well below 15 dB ATSC Threshhold for reception, widening the "search area":
http://www.tsreader.com/tsreader/index.html

NOTE: TSReader only works with a small number of ATSC PCI Cards & USB Sticks:
http://www.tsreader.com/tsreader/hardware.html
Be sure to skip past the DVB-S (Satellite) and DBV-T (Euro TV) sections until
you reach the Terrestrial (ATSC) section....Unfortunately, it appears that the
Hauppauge WinTV HVR-950 and HVR-950Q are the ONLY compatible USB Sticks.

(FREE) VLC Player is used to actually display the video derived from the decoded
MPEG2 data steam. Search to find one of the many download websites.
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post #14917 of 16297 Old 05-22-2012, 10:08 PM
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Unfortunately, it appears that the
Hauppauge WinTV HVR-950 and HVR-950Q are the ONLY compatible USB Sticks.

Not so bad, as those two models are one of the better USB tuner sticks with a 6th gen tuner.
Before I gave mine away as a gift, (I had to test it to make sure it worked OK, ya know) it was very comparable in sensitivity to my Zenith CECB box.
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post #14918 of 16297 Old 05-23-2012, 08:59 AM
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Quote:
Originally Posted by rabbit73 View Post

The post by Andy Lee about Visual explanation of Noise Margin is interesting:
http://www.avsforum.com/avs-vb/showt...9#post15700679 #397

This is very good.

I put together a spreadsheet that I use to calculate System Noise Figure. It's just a fancy way to do a 2nd stage noise figure calculation but it's easy to change the numbers around to optimize your system.

http://images.aa6g.org/System_NF.xls

The graph is helpful because you can determine at-a-glance if your preamp has too much gain or not enough. If the preamp gain is too low then the line will be straight with little or no flattening at the bottom. If the line is flat at the bottom then the preamp has too much gain and you'll increase the possibility of overload. The default numbers entered are the parameters for my existing system at channel 51.

The table let's you experimentally determine noise margin using fixed attenuators or a step attenuator at the TV when you have a preamp installed at the antenna. The accuracy of this is dependent on the accuracy of the numbers you entered.

Chuck
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post #14919 of 16297 Old 05-23-2012, 06:49 PM
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Chuck:

Thanks for the information and link about your System Noise Figure spreadsheet. That looks interesting and useful.

Have you seen the first post in the DHC Signal Amplifiers (Amps, Preamps, Distro Amps) thread? It lists:

1. tczernec's Loss Calculator Spreadsheet
2. majortom's Cascaded Noise Figure Spreadsheet
3. Preamplifier Comparison Chart by holl_ands

http://www.digitalhome.ca/forum/showthread.php?t=42426

When I described my discussion with Nitewatchman in post #14915, you could see that I was aware of the importance of SNR when evaluating an antenna system back in Jan '09. That reminded me of something that I had read on the DTVForum.info, which is the Australian version of AVS:
Quote:


If your antenna installer doesn't have a signal meter that reads BER and MER, get one who does.

The consenus in Australia seems to be that a "certified antenna installer" needs to have a signal meter that reads signal strength, BER, and MER (similar to SNR) to do a proper installaton.
Quote:


.....a step attenuator at the TV when you have a preamp installed at the antenna.....

My step attenuator is one of my favorite tools for making measurements, as you can tell from the link in my signature.

If you can not measure it, you can not improve it.
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post #14920 of 16297 Old 05-23-2012, 06:59 PM
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Quote:
Originally Posted by Calaveras View Post

I don't think you can reliably see multipath on a spectrum analyzer. One of my strongest stations has a very flat analyzer trace but ranks among the lowest for SNR. Variance in amplitude across the analyzer trace does not automatically mean multipath.

There are DTV analyzers that will show multipath.

Chuck

Looking at your animations you posted, I noticed there are ATSC pilot signals at the beginning of the 6 MHz spectrum. The rest of the spectrum is suppose to be noise-like due to the pseudorandom modulation. The pilot is the unmodulated carrier at a reduced value.

Is it possible to just measure the RF power of the pilot and try to maximize it by moving the antenna to different locations? Max power would indicate minimum multipath interference. What do you think?
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post #14921 of 16297 Old 05-23-2012, 07:29 PM
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Originally Posted by retiredengineer View Post

Is it possible to just measure the RF power of the pilot and try to maximize it by moving the antenna to different locations? Max power would indication minimum multipath interference. What do you think?

It's easy to measure the pilot with the spectrum analyzer and certainly you can point the antenna by measuring the pilot. In most cases I think you'll find the highest SNR occurs in the peak direction. Occasionally there are exceptions to this.

Chuck
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post #14922 of 16297 Old 05-23-2012, 07:38 PM
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holl_ands & 300ohm:

Thanks for the tips on some useful tools to make my measurements more accurate and consistent.
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.....Or a CECB Box with a small luggable TV....

Got that. I have an Apex DT502 which isn't a very good CECB in general (it locks up on ION stations, you can't add a channel after scan, and ota.dt.man's box died on him), but it does have dual signal bars that give consistent signal quality and strength readings. I can see the reduction in quality as I approach the cliff and dropout happens about the same time as with my SONY KDL22L5000.
Quote:


I find Laptop PC with an ATSC USB Stick Tuner to be an essential test tool.....it appears that the Hauppauge WinTV HVR-950 and HVR-950Q are the ONLY compatible USB Sticks.....those two models are one of the better USB tuner sticks with a 6th gen tuner.....it was very comparable in sensitivity to my Zenith CECB box.

I have resisted the urge to add a computer to my measurement system because I don't like them very much and only tolerate them for what they allow me to learn about DTV.
Quote:


I've found that (FREE) TSReader Lite will display Bit Error Rate statistics when SNR is well below 15 dB ATSC Threshhold for reception, widening the "search area"

You got my attention with that one, because I don't have any equipment that will do that. All of my tuners stop giving me information when they can't decode. My SLM, however, will still continue give me signal strength readings below dropout if I add a preamp to it.

If you can not measure it, you can not improve it.
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post #14923 of 16297 Old 05-23-2012, 07:42 PM
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Quote:
Originally Posted by rabbit73 View Post

Have you seen the first post in the DHC Signal Amplifiers (Amps, Preamps, Distro Amps) thread? It lists:

1. tczernec's Loss Calculator Spreadsheet
2. majortom's Cascaded Noise Figure Spreadsheet
3. Preamplifier Comparison Chart by holl_ands

They look similar to mine except I added the graph and table which I think are helpful.

Quote:


The consenus in Australia seems to be that a "certified antenna installer" needs to have a signal meter that reads signal strength, BER, and MER (similar to SNR) to do a proper installation.

IMO measuring BER is overkill. Uncorrected BER is inversely proportional to SNR and almost every TV will give some version of SNR even if it's only a 0-100% Signal Quality bar.

Quote:


My step attenuator is one of my favorite tools for making measurements, as you can tell from the link in my signature.

I used to hunt for these at the electronic flea market in the south Bay Area. I've got one for 75 ohms and two for 50 ohms and a bunch of fixed attenuators and dummy loads.

Chuck
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post #14924 of 16297 Old 05-24-2012, 01:31 AM
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I'm finding this discussion very interesting, but I'm lost on two abbreviations. What are BER and MER? I've never heard of them before.

Also, Holl_lands wrote:
"NOTE: TSReader only works with a small number of ATSC PCI Cards & USB Sticks:
http://www.tsreader.com/tsreader/hardware.html
... Unfortunately, it appears that the Hauppauge WinTV HVR-950 and HVR-950Q are the ONLY compatible USB Sticks."

It also works well with the Fusion HDTV USB stick.

Larry
SF

My complete SF Bay Area DTV Station Lists: http://www.choisser.com/sfonair.html
Lots of Broadcasting links and information: http://www.choisser.com/broadcast.html

Check out photos and info on my antennas: http://www. larrykenney.com/tvantennas.html

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post #14925 of 16297 Old 05-24-2012, 03:27 PM
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Quote:
Originally Posted by Larry Kenney View Post

I'm finding this discussion very interesting, but I'm lost on two abbreviations. What are BER and MER? I've never heard of them before.

Also, holl_ands wrote:
"NOTE: TSReader only works with a small number of ATSC PCI Cards & USB Sticks:
http://www.tsreader.com/tsreader/hardware.html
... Unfortunately, it appears that the Hauppauge WinTV HVR-950 and HVR-950Q are the ONLY compatible USB Sticks."

It also works well with the Fusion HDTV USB stick.

Larry
SF

In ATSC, MPEG2 data packets or "messages" are transmitted in 186-byte
data groups, each with a 20-byte Reed-Solomon Parity Check Code. Hence:

DETECTED MER (Message Error Rate) refers to how often these 1488-bit
packets failed the R-S Parity Check.

CORRECTED MER refers to how often packets failed R-S Parity Check but
the R-S Correction Algorithm THINKS it fixed all of the detected bit errors.
[Reed-Solomon typically corrects up to 20-bytes out of each 186-byte "message".]

UNCORRECTED MER can NOT be determined except when a known test data
pattern is transmitted. These are the errors that a viewer actually "sees" (or hears).

ATSC Pass/Fail Criteria has been established as 3 x 10^-6 BER (Bit Error Rate),
which is an average of one bit error in 0.33 Million Bits....or ABOUT one
sound and/or video "glitch" in 222 messages....which is about the point that
ATSC reception become "minimally acceptable" (based on early engineering tests).
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post #14926 of 16297 Old 05-24-2012, 03:43 PM
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Quote:
Originally Posted by Larry Kenney View Post

I'm finding this discussion very interesting, but I'm lost on two abbreviations. What are BER and MER? I've never heard of them before.

BER = Bit Error Rate

When a digitally signal is demodulated you expect to recover the same number that was transmitted. Noise added to or interference to the signal can cause the recovered number to be different from what was transmitted. That's an error and ATSC is designed to be able to fix a certain number of those. In my experience BER is the number of errors per second.

MER = Modulation Error Ratio

This is very similar to SNR but is tailored to a digitally modulated signal. SNR and MER are often used interchangeably even though there are subtle differences. There's somewhat more of an explanation here:

http://broadcastengineering.com/RF/b..._transmission/

Chuck
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post #14927 of 16297 Old 05-24-2012, 03:53 PM
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Quote:
Originally Posted by holl_ands View Post


DETECTED MER (Message Error Rate) refers to how often these 1488-bitpackets failed the R-S Parity Check.

I've never heard of MER meaning that. A Google search on "message error ratio" 8VSB returns no results.

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post #14928 of 16297 Old 05-24-2012, 04:09 PM
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Quote:
Originally Posted by Calaveras View Post

I've never heard of MER meaning that. A Google search on "message error ratio" 8VSB returns no results.

Chuck

That's because the manufacturers don't disclose the algorithm used to determine
their displayed "Signal Quality".....Detected MER is the primary parameter,
because it is readily available from the R-S Decoder...and does the job....

Reed-Solomon Codes (like most Error Detection and Correction Codes) will
provide a Decode Status for each Message, whether a) NO DETECTED ERRORS,
b) DETECTED xx-Symbols in Error and TRIED to FIX them or c) CAN'T FIX ERRORS.
Corrected MER is derived from b). Detected MER is derived from c).
http://en.wikipedia.org/wiki/Reed&#8...ror_correction
http://hscc.cs.nthu.edu.tw/~sheujp/lecture_note/rs.pdf
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post #14929 of 16297 Old 05-24-2012, 05:15 PM
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Quote:
Originally Posted by holl_ands View Post

That's because the manufacturers don't disclose the algorithm used to determine their displayed "Signal Quality".....Detected MER is the primary parameter, because it is readily available from the R-S Decoder...and does the job....

We seem to be talking across each other. My comment refers to the meaning of MER that everyone uses, not how manufacturers determine Signal Quality. Message Error Ratio for MER is used by nobody while Modulation Error Ratio is used by everybody. Larry was asking for the meaning of MER in the context of the current discussion.

Chuck
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post #14930 of 16297 Old 05-24-2012, 05:32 PM
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Quote:
Originally Posted by holl_ands View Post

That's because the manufacturers don't disclose the algorithm used to determine their displayed "Signal Quality".....Detected MER is the primary parameter, because it is readily available from the R-S Decoder...and does the job....

It would be very difficult to construct a meaningful "Signal Quality" from R-S performance. The difference between 0 errors and totally unwatchable is only about 2 dB in signal to noise ratio. Here's a post I made a few years ago with some measurements made with a precision step attenuator and a demodulator evaluation board.

**********************************************************

I've found a brand new evaluation board for the ATI/AMD T316 8VSB and QAM demodulator chip. It come with a Windows User Interface that interacts with the board over USB. Here's what it looks like on my PC:



The UI controls the tuner and show if there is MPEG-2 frame lock. Separate windows show the demodulator status and AGC status. I'm also running a DVB-ASI capture program (Enensys DiviCatch) and VLC to decode the MPEG-2 Transport Stream from the eval card.

I took some measurements on my strongest signal, KNTV-DT on RF channel 12. Here's the results:
Code:
Attenuation   Equalizer S/N         AGC%       # of Pre RS errors/sec
0  dB             32.0              40            0
10 dB             32.0              40            0
20 dB             32.0              43            0
30 dB             30.5              52            0
40 dB             24.3              61            0
43 dB             22.1              64            0
46 dB             19.7              67           <1
47 dB             18.5              68           <10
48 dB             17.6              70           <1000
49 dB             16.6              72           <10000
50 dB             15.7              74           <100000 heavily impaired
51 dB         loss of lock
KNTV-DT delivers a rock crushing signal to my lab in Milpitas, CA. It's at least 43 dB more than necessary for perfect reception. Also note that the difference between a watchable signal (at 49 dB of attenuation) and no signal (at 51 dB of attenuation) is only 2 dB.

Here's the KNTV-DT 8VSB constellation diagram with 10 dB of attenuation:



The eval board works better than my old Samsung SIR-T165 receiver. I had actually thought KCNS-DT was off the air, but in reality, it's just too weak for the Samsung. Here's the constellation for KCNS-DT on RF channel 39 when it was pretty impaired on the eval card (and non-existent on the Samsung):



Ron

HD MPEG-2 Test Patterns http://www.w6rz.net
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post #14931 of 16297 Old 05-24-2012, 06:46 PM
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Quote:
Originally Posted by Calaveras View Post

BER = Bit Error Rate

When a digitally signal is demodulated you expect to recover the same number that was transmitted. Noise added to or interference to the signal can cause the recovered number to be different from what was transmitted. That's an error and ATSC is designed to be able to fix a certain number of those. In my experience BER is the number of errors per second.

MER = Modulation Error Ratio

This is very similar to SNR but is tailored to a digitally modulated signal. SNR and MER are often used interchangeably even though there are subtle differences. There's somewhat more of an explanation here:

http://broadcastengineering.com/RF/b..._transmission/

Chuck

My understanding of BER is the same as yours, but BER can mean Bit Error Rate or Bit Error Ratio. The terms are often used interchangeably, but that is not proper. Bit error rate is, as you said, the number of errors in a given period of time. Bit error ratio is the number of bit errors in a given total number of bits, and is usually expressed in scientific notation as in ATSC A74:
Quote:


A DTV receiver should achieve a bit error rate in the transport stream of no worse than 3x10E-6 (i.e., the FCC Advisory Committee on Advanced Television Service, ACATS, Threshold of Visibility, TOV) for input RF signal levels directly to the tuner from –83 dBm to –5 dBm for both the VHF and UHF bands.

As you can see, they are using a number that expresses bit error ratio, but they are calling it bit error rate.

My understanding of MER is also the same as yours and agrees with wiki:
http://en.wikipedia.org/wiki/Modulation_error_ratio

Thanks for the interesting link from Broadcast Engineering. There is a section in that paper that expands on your answer to retiredengineer about measuring the pilot:
Quote:


One might be tempted to use the pilot level as a reference and infer the total average power from that point. Pilot level measured at the transmitter output can be an indicator of power level, but it is not a substitute for accurate power measurement. In the field, the pilot level can be affected by selective fading or cancellation and is therefore not a reliable indicator of received signal strength.


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post #14932 of 16297 Old 05-24-2012, 07:31 PM
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dr1394:

Thanks for your interesting post with the VLC Media Player screen shot and about using an attenuator at the cliff.
Quote:


The difference between 0 errors and totally unwatchable is only about 2 dB in signal to noise ratio. Here's a post I made a few years ago with some measurements made with a precision step attenuator and a demodulator evaluation board.

With my equipment I'm not able to monitor pre RS errors, only post, but with my 1 dB per step attenuator I also notice that it takes about 2 dB to go from nice picture to terrible. If we didn't have the FEC, we would notice the gradual loss of signal quality, but with the FEC we see a sudden loss of quality when it reaches its error limit.

My OTA signals vary in strength because there are trees in the path, so it is a bit difficult to manage the attenuation at the cliff in a smooth manner, making it necessary to repeat the test a few times for consistent results. I am, however, able to set up in my car at a remote area for LOS signals that come across the water with less variation in strength (except when a boat goes by!).

I have dreams about making an in-line attenuator before the step attenuator that would keep the signal a little more constant perhaps using an AGC circuit that drives a lamp in front of a light sensitive resistor that is in the shunt resistor of a 75 ohm T attenuator.

An 8VSB signal generator is not in my price class.

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post #14933 of 16297 Old 05-24-2012, 09:13 PM
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Quote:
Originally Posted by rabbit73 View Post

dr1394:

Thanks for your interesting post with the VLC Media Player screen shot and about using an attenuator at the cliff.

Here's the original thread from 2009, with pictures of the evaluation board.

http://www.avsforum.com/avs-vb/showthread.php?t=1122271

The setup was antenna (and USB control) -> T316 evaluation board -> DVB-ASI -> Enensys DVB-ASI capture device -> USB to VLC player. I was playing the KNTV stream with VLC at the time, but when you take a screenshot of VLC, the video gets blacked out.

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post #14934 of 16297 Old 05-25-2012, 07:45 AM
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Quote:


but when you take a screenshot of VLC, the video gets blacked out.

???

From the Video Lan documentation:
Quote:


Snapshot

This option is useful if you want to capture a portion of the video as an image.
1.Select Advanced File Open from the Media menu. The Open dialog box is displayed.
2.Select a file and click VLC - play button.png Play.
3.To capture an image from the video, select Snapshot from the Video menu.

The image is captured in the .png picture format and is saved in the C:\\My Pictures folder by default (C:\\Users\\Username\\Pictures).

Then use a picture resizer to reduce the picture so it will fit on these bulletin boards.
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post #14935 of 16297 Old 05-25-2012, 08:32 AM
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Quote:


A DTV receiver should achieve a bit error rate in the transport stream of no worse than 3x10E-6
Quote:
Originally Posted by rabbit73 View Post

As you can see, they are using a number that expresses bit error ratio, but they are calling it bit error rate.

Actually that's not clear to me at all. When I was performing these measurements in the lab we'd just give the number as a kind of shorthand. The "/sec" part was always left out so I can't tell from above what they really mean. I just assumed what was meant was 3x10E-6/sec.

I also see a problem using R-S errors to determine SNR. If at 20 dB SNR (or whatever the number is) the errors go to zero, how do you calculate higher SNRs? It must be done another way.

Chuck
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post #14936 of 16297 Old 05-25-2012, 08:53 AM
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Quote:
Originally Posted by dr1394 View Post

It would be very difficult to construct a meaningful "Signal Quality" from R-S performance. The difference between 0 errors and totally unwatchable is only about 2 dB in signal to noise ratio. Here's a post I made a few years ago with some measurements made with a precision step attenuator and a demodulator evaluation board.

Just so I have this correct in my head once and for all.......

From your table above, the non-corrected R-S errors are essentially zero at an SNR of 20 dB, meaning the error correction system isn't doing anything. Between 20 dB and 15.2 dB the errors rise but the error correction system can handle those errors and produce what appears to be an error-free picture and sound. At 15.2 dB and lower SNRs there are too many errors to be corrected and the system collapses, or falls off the digital cliff as we like to call it. So the whole purpose of the error correction system is to extend the useable SNR from 20 dB to 15.2 dB.

Please correct me if I have anything wrong in my summary.

Chuck
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post #14937 of 16297 Old 05-25-2012, 11:52 AM
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Quote:
Originally Posted by 300ohm View Post

???

From the Video Lan documentation:

Snapshot...

Then use a picture resizer to reduce the picture so it will fit on these bulletin boards.

I use the VLC "snapshot" function all the time. When I said "screenshot", I meant just hitting the PrtSc key to capture the entire desktop. I was trying to show all the demodulator windows in one picture. The other open windows (like VLC and the Enensys capture device) were just incidental. Also, that desktop is Windows XP in "Classic" theme.

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post #14938 of 16297 Old 05-25-2012, 12:12 PM
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Quote:
Originally Posted by Calaveras View Post

Just so I have this correct in my head once and for all.......

From your table above, the non-corrected R-S errors are essentially zero at an SNR of 20 dB, meaning the error correction system isn't doing anything. Between 20 dB and 15.2 dB the errors rise but the error correction system can handle those errors and produce what appears to be an error-free picture and sound. At 15.2 dB and lower SNRs there are too many errors to be corrected and the system collapses, or falls off the digital cliff as we like to call it. So the whole purpose of the error correction system is to extend the useable SNR from 20 dB to 15.2 dB.

Please correct me if I have anything wrong in my summary.

Chuck

You're close, but there's one more thing to know about. There are two levels of error correction. The first level is the Viterbi convolutional code, and the second level is the Reed-Solomon code. The demodulator application is not showing the amount of errors being corrected by the Viterbi code, only the RS code. Bit errors are starting to creep in an some higher S/N, but it's not being shown.

So your synopsis is correct, except that it's happening at two levels. First the Viterbi code extends the system S/N and then the RS code extends it even further.

In general, the Viterbi code does well with single bit errors, while the RS code is better for error bursts. At the same time, the bits going into the RS decoder are interleaved. That is, the bits that make up one 207 byte packet were not sent sequentially, but interleaved in time. This helps when there are noise bursts on the channel. Instead of the noise burst ending up in one or two packets, it's spread out into many packets.

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post #14939 of 16297 Old 05-25-2012, 03:12 PM
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http://www.linuxtv.org/wiki/index.ph...our_DVB_device for anyone interested in Linux related DVB and hardware
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post #14940 of 16297 Old 05-25-2012, 03:36 PM
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FYI: EVM (Error Vector Magnitude, a percentage) and MER (Magnitude Error Ratio, in dB)
definitions can be found on pages A-5 and A-6:
http://128.238.9.201/~kurt/manuals/m...00A%20User.pdf

I've only seen EVM used in some (early) technical white papers, such as Tektronix's
"Signal to Noise Relationships for 8-VSB" and Dr. Bendov's re VSWR degradation:
http://e-sites2.tek.com/cmswpt/tidow...&ci=2238&lc=EN

Sorry if I jumped to confusing MER....should have used R-S SER (Segment Error Rate),
although I try to reserve the use of SER for Symbol Error Rate. I also avoid using
Block Error Rate to avoid confusion with BER....
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