Measuring ATSC multipath - rental options - AVS Forum
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post #1 of 26 Old 03-07-2012, 10:54 AM - Thread Starter
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Is it possible to rent a meter or use some other device that can quantify multipath. In the old days we could just look for analog ghosting... but not with digital.

I have a TiVO HD XL DVR. Like many devices it has a signal strength meter, but no indication of quality. It can't even count correction errors or other metrics that help understand how close to the edge a particular channel is.

So
  • Are there multipath meter rentals?
  • What's the best way to infer multipath: signal strength? correction errors in a 24 hour period?

---

In my case I have a strong 12.5 mile path to Sutro tower, but there's a large reinforced concrete gymnasium building across the street. Neighbors report that in the analog days the multipath was horrible. 90 degrees from Sutro, and 35 miles distant at the edge of a hill, is a station I would like to receive. I'm concerned that a better antenna will bring back multipath problems.

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post #2 of 26 Old 03-07-2012, 11:22 AM
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You might get a more specific response if you post over in the Local HDTV info and reception forum. Look for the San Francisco OTA sub-forum. Lots of very knowledgeable and helpful folks there for the Bay Area.
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post #3 of 26 Old 03-07-2012, 11:58 AM
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There's no such thing as a multipath meter... or a Sanity Claus, for that matter.

You can sometimes find a Sencore 1453 or 1453i on eBay for between $100 and $200, and those take long samples - like ten seconds long - and average out the errors incurred by its own tuner. That can help you if you have a lot of patience and are willing to take a few hundred samples, but at this moment, there aren't any listed for under $275.

What works better is any old Avcom spectrum analyzer that was purchased for C-band signal analysis, since those have an input that covers the broadcast TV band. Those are better for your purpose than a digital error counter is because you can observe all of the waveforms simultaneously as you move the antenna around. Once, I found one being sold with a broken digital frequency display but with a good trace on the picture tube that sold for under $100, but at the moment, the cheapest one is selling for $250.

If you cared enough to buy something new, there are a lot of ATTN spectrum analyzers available on eBay from Hong Kong for just $600. I have one and use it often.
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post #4 of 26 Old 03-07-2012, 10:58 PM
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Quote:
Originally Posted by brycenesbitt View Post

Is it possible to rent a meter or use some other device that can quantify multipath. In the old days we could just look for analog ghosting... but not with digital.

I have a TiVO HD XL DVR. Like many devices it has a signal strength meter, but no indication of quality. It can't even count correction errors or other metrics that help understand how close to the edge a particular channel is.

So
  • Are there multipath meter rentals?
  • What's the best way to infer multipath: signal strength? correction errors in a 24 hour period?

---

In my case I have a strong 12.5 mile path to Sutro tower, but there's a large reinforced concrete gymnasium building across the street. Neighbors report that in the analog days the multipath was horrible. 90 degrees from Sutro, and 35 miles distant at the edge of a hill, is a station I would like to receive. I'm concerned that a better antenna will bring back multipath problems.

there is or was a rental place in Livermore that had electronic test equipment for rent.
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post #5 of 26 Old 03-08-2012, 05:06 AM
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The liklihood of someone saving money and accomplishing something constructive by renting a piece of RF test equipment is nil. A decade or so ago, I needed a time domain reflectometer for a one time repair job, and while I don't remember the numbers, the rental company was really looking for someone to lease the equipment long term, and their short term rental was about one tenth of the new, purchase price of the equipment, which was pricey. Most so-called test equipment rentals require the renter to effectively buy the equipment over the length of the lease,
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post #6 of 26 Old 03-08-2012, 08:23 AM - Thread Starter
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What does one look for, on a spectrum analyzer, to distinguish multipath from a direct signal? Again for analog I know just what to do, but for digital I don't know where to start.

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post #7 of 26 Old 03-08-2012, 09:27 AM
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Quote:
Originally Posted by brycenesbitt View Post

What does one look for, on a spectrum analyzer, to distinguish multipath from a direct signal? Again for analog I know just what to do, but for digital I don't know where to start.

Flatness of the "boxcar" wave trace.
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post #8 of 26 Old 03-08-2012, 11:15 AM
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Quote:
Originally Posted by AntAltMike View Post

Flatness of the "boxcar" wave trace.

Unfortunately this pretty much just tells you that there is multipath or not. A spectrum analyzer does not give a quantifiable number for multipath.

It is my understanding that most tuners have readable equalizer parameters. The pattern of the equalizer terms would give a pretty good picture of the multipath since it is essentially "how much of the signal must I delay x amount and subtract from the input signal to cancel the reflection?" A plot of this would give delay and magnitude of the reflections. A lot like seeing ghosts on an analog display.

Tuners seem to have a bunch of useful diagnostics built into them, but I've yet to see a product available to consumers that would allow access to this info. The closest I've seen is the HDHomeRun Tech version, but this still does not give all the info I would want, and it is Windows only.
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post #9 of 26 Old 03-08-2012, 06:53 PM
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brycenesbitt:

A spectrum display doesn't always show the extent of a multipath problem unless it is primarily caused by irregular amplitude across the channel.

Bill Naivar had an urban multipath problem that showed on his spectrum analyzer scan, and was able to solve it with what he calls an anti-ghosting antenna:
www.prism.gatech.edu/~wn17/

at the bottom of page one there is a link to page two which is:
www.prism.gatech.edu/~wn17/Web%20bill%20page%202.htm

On the other hand, Trip in VA had a reception problem that didn't show on a spectrum display of a channel as multipath, so he thought that it wasn't multipath, but it was.
Chattanooga TN: Got Atlanta But Few Locals
http://www.avsforum.com/avs-vb/showthread.php?t=1372416

It wasn't until he was able to borrow an expensive piece of equipment that showed the delay time of the reflections (echos) that he was able to confirm his suspicion of multipath.

My theory is that the reflections didn't show on his original analyzer scans as multipath because they were approx. the same amplitude as the primary signal. The analyzer scan just shows the amplitude across the 6 MHz channel, not the timing of the reflections.

The relative amplitude of the reflections becomes more critical as they increase in time difference from the primary signal.

See the echo table in attachment #1 which comes from page 22 of
ATSC Recommended Practice:
Receiver Performance Guidelines

Document A/74:2010, 7 April 2010
atsc.org/cms/standards/a_74-2010.pdf

Converter boxes were also required to meet equalizer specifications for multipath to be eligible for the coupon program.
NTIA
DTV Converter Box Cupon Program
Information Sheet for Manufacturers
March 2007
The second attachment shows the specs.
Quote:


What's the best way to infer multipath: signal strength? correction errors in a 24 hour period?

No, not signal strength. Trip has strong signals, but poor signal quality. The equalizer has a limit to how much it is able to correct multipath errors, and some tuners are better able to make corrections than others. So, even if you were able to make measurements, it is still your tuner that makes the final decision as to how much multipath error it can handle.

A tool that I have found useful when aiming an antenna for minimum multipath errors is a converter box or tuner that has two signal bars, one for signal strength and one for signal quality. The ones that I have used are the Apex DT502 and the Centronics ZAT502HD. A photo of the dual signal bars is shown in attachment 3. Credit for the photo goes to douglas-b who first posted it on the Centronics ZAT 502 HD / RTC DTA1100HD / Digiwave DTV5000HD thread:
www.avsforum.com/avs-vb/showthread.php?t=1136626
but it is no longer there in his post. My photo of the dual signal bars wasn't as good, so I posted his there as an attachment to one of my posts.

I insert an attenuator between the antenna and the tuner to approach the "digital cliff" so that the tuner is more sensitive to errors (quality). Slight changes in antenna aim produce small changes in signal strength, but can produce large changes in signal quality if you can find a signal path with minimum reflections.

When I was testing the DT502 with my CM4221 antenna I got (for 13.1 on RF41):
Signal Quality 60%
Signal Strength 55%

I had aimed the antenna with my SLM (signal level meter), but when I rotated the 4221 slightly to the right I got:
Signal Quality 100%
Signal Strength 56%

Note the BIG change in signal quality with only a slight change in signal strength.

It seems that the signal quality indication is a more sensitive aiming tool than signal strength, because it shows the increase in BER (bit error rate/ratio) from multipath reflections. In my situation the BER is affected by the weak signal, the fixed multipath reflections, and the changing (dynamic) multipath reflections. My antenna is aimed across a well-traveled road, so I get reflections from cars. (This is an example of the need for the new ATSC M/H standard.) When the quality went up to 100%, the car reflections were less of a problem. My stronger signals maintain a good lock inspite of the cars.

Some of the new SLMs can measure BER and MER (modulation error ratio.....similar to SNR). The less expensive ones, like my Sadelco DisplayMax 5000, only measure emulated BER and MER using noise as a reference. The more expensive SLMs contain an ATSC demodulator that measures true BER and MER, but they cost kilobucks.
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post #10 of 26 Old 03-08-2012, 09:10 PM
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My Sony TV has both a Signal Strength display and a Signal Quality display so the OP could use a Sony TV to aim his antenna for minimum multipath errors per rabbit73's directions.
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post #11 of 26 Old 03-08-2012, 09:19 PM
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Yes, you are correct. My SONY KDL22L5000 has been very useful too.

Good signal:



Bad signal (picture frozen):


If you can not measure it, you can not improve it.
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post #12 of 26 Old 03-09-2012, 01:09 AM
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One problem with trying to improve reception of a single channel using a digital analyzer is that you can't see any incremental degradation that the adjustment of the antenna is making to the signal quality of the other channels. The spectrum analyzer lets you see all the boxcars at once, and the old fashioned, CRT ones have a faster response time than the digital oneswith LED displays do.
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post #13 of 26 Old 03-11-2012, 12:06 AM - Thread Starter
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Clearly the spectrum analysis approach is not for the masses or general user.

Is there an inexpensive/portable converter box that gives good bit error rate calculation? Any way to get the Tivo XL to do that?

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post #14 of 26 Old 03-11-2012, 06:52 AM
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No and no.
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post #15 of 26 Old 03-11-2012, 12:37 PM - Thread Starter
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No and yes:

I found a way to get the data from the TiVO HD XL:


Going here:
Messages & Settings -> Settings -> Channels -> Signal Strength - Antenna
Gives you signal strength only (and you have to pause all recordings).

Going here:
Account & System Information -> DVR Diagnostics
Gives you all the details, including both corrected and uncorrected errors. Strong channels in my area typically get no errors at all (corrected or uncorrected) unless I unscrew the antenna. More fringe stations show corrected errors only. And you don't have to pause recording.

Perfect.

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post #16 of 26 Old 03-11-2012, 03:24 PM
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Looks good!

Thanks for posting the screenshot.

Is the error reading real-time, or does it take some time to make the calculation? You need real-time feedback when adjusting antenna aim.
Quote:


Is there an inexpensive/portable converter box that gives good bit error rate calculation?

This STB doesn't give a reading of errors, it gives a real-time reading of quality, which is the inverse of errors, and just as useful to optimize antenna aim for minimum multipath errors:

www.centronics.com/product.php?id=48
www.summitsource.com/product_info.php?ref=1&products_id=8167

If you can not measure it, you can not improve it.
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post #17 of 26 Old 03-13-2012, 07:22 PM
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brycenesbitt:

Now that you have a way to measure errors, your next decision is about an antenna.
Quote:


In my case I have a strong 12.5 mile path to Sutro tower, but there's a large reinforced concrete gymnasium building across the street. Neighbors report that in the analog days the multipath was horrible. 90 degrees from Sutro, and 35 miles distant at the edge of a hill, is a station I would like to receive.

Quote:
Originally Posted by brycenesbitt View Post

Does anyone have experience with channel 54 (RF 50 UHF) reception, in the Berkeley flats or beyond?

My goal is to receive that channel OTA... is this realistic? Right now I get about 20% signal strength on the TiVO using a generic rabbit ear/loop antenna, which is not enough for a picture

Device: TiVO HD XL
Location: Acton at Ada, Berkeley CA 94702
Desired Reception: KQEH/KQED Plus 54.1, without messing up KQED or KRCB which I get now at 85% and 80% signal strength respectively.
Available antenna mounting location: first floor chimney 12 feet up from TV, or 2nd floor chimney with 360 degree sky view 30 feet away from TV.

I looked at some of the answers given to you on page 279 of the San Francisco, CA - OTA thread. I don't think I can give you much better advice than what was given to you by:
SFischer1, post 8352
Larry Kenney, 8359 and
Calaveras, 8361

Quote:


.....In my case everything I watch is first recorded by TiVO, so rotors and switches are out. The TiVo does have two RF inputs, so if I need a special purpose antenna for KQED Plus that's OK.

Quote:
Originally Posted by brycenesbitt View Post

I'm at a middle height in the North Berkeley hills, theoretically line of sight to Sutro tower except for the trees. I get too many episodes of blocky digital breakup with rabbit ears......

Quote:


I'm concerned that a better antenna will bring back multipath problems.

It depends upon how you define a "better" antenna. To me, most any antenna would be better than your rabbit ears and loop for your multipath problem.

I didn't see a tvfool report for you, so based on your hints I made a guess as to what it might be:
www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d0b86240707b55b

Your screen shot was of 22.1 which is KRCB RF 23 at 320 degrees magnetic. You show many errors; is that the channel you would like to receive?

If so, you are going to need an outdoor directional antenna for KRCB. The concrete gymnasium building across the street is at about 90 degrees, so you would need an antenna that rejects signals from the side if you aimed the antenna at KRCB to reject the reflection from the gym or aimed the antenna at the gym to use a good reflection and reject the direct signal from KRCB.

If neither of those work, you might be a good candidate for an "anti-ghosting" antenna.

If you can not measure it, you can not improve it.
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post #18 of 26 Old 03-14-2012, 10:55 AM
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If you put an antenna outside, it will need to have the mast and coax shield grounded to meet the requirements of the NEC and your local electrical inspector. Before you do that, there are a few things you might want to try indoors in the "anti-ghosting" antenna category.

You could put a UHF antenna (for 22.1, CH23) in a cardboard box lined or covered with aluminum foil and bring the coax out the bottom with an F81 coupler. This could be placed inside with its open end facing a window (no metal screen or low-e glass) that faces north on the second floor. You could also put the box in the attic (if you have one in your Spanish-style home) or just have the antenna in the attic with a large piece of cardboard covered with aluminum foil on the east side of the antenna to block reflections from the gym.

To get a good signal for KRCB, you might even need to do what Bill Naivar did in page 2 of his link in post #9 with an antenna in a can on the roof of Casa Nesbitt!

It is not realistic of you to expect to get everything you want from all directions with a simple antenna and no rotator or switches (because of the DVR) with strong reflections coming from the gym.

But, it's always a good idea to try the simple way first, you might get lucky!

If one particular channel is very important to you, it would be necessary to dedicate an antenna and DVR just to that task. An alternate solution, in addition to the JoinTenna/filter approach mentioned on the SF thread, would be to have a timer and a coax relay switch antennas at the time the DVR starts recording.

I don't think you need a preamp because your signals are quite strong, and it might be overloaded by the strong signals from Sutro unless the the CH23 antenna was shielded from them.

This ends my analysis of your reception situation.

Please let us know how it works out for you, Bryce.

Best regards,
rabbit

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post #19 of 26 Old 03-16-2012, 01:03 AM - Thread Starter
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I have a solution, thanks! My TV fool by the way is:
http://www.tvfool.com/?option=com_wr...0b86c5853dfcf5

I borrowed an Antennas Direct DB2, and to my surprise it worked great. I can get KQEH (50), KRCB (23), all the San Francisco stations including KGO (7 - VHF), and a great FM signal to boot. This is despite the fact the TV stations are each 90 degrees apart, and the DB2 is a UHF bowtie antenna that's not supposed to work for High-VHF.

Frustratingly Antennas Direct does not publish a reception pattern for the DB2... but it definitely is directional, perhaps with nulls. I initially had bit errors on KQED (30), but rotating the antenna drops that noise right out.

The signal is not robust enough to split. It works great on the TIVO, but is too weak if split for the FM radio also. I've ordered a Weingard CA-8800 in hopes of getting it all, with no amplifier.

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post #20 of 26 Old 03-16-2012, 08:57 AM
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Quote:


Frustratingly Antennas Direct does not publish a reception pattern for the DB2...

We did a formal data sheet for the DB2 a while back, but we never got around to attaching it to the product page on the web site. Since the DB2 is now discontinued and we don't have an archive for legacy product information, we really don't have a place to put it.

I was going to attach a copy of the datasheet to this post, but I'm limited to 500K for a .pdf file and the datasheet is about 700K. Instead of splitting the file, I'll be happy to email a copy specifically to you if you send us an email to info@antennasdirect.com . I would ask anyone else to refrain from requesting the file for the moment until the following request is resolved.

If a moderator can override the default file size limit and post the file on my behalf, I'll be happy to get it posted if for all to see here on the forum. Alternatively, if my file size attachment privilege can be expanded, I'll add it to this post later.

We have formal data sheets for many, but not all, of our products (notable exceptions are C4 and DB8) that are available under the "Documents" tab of each products web page. We're working on it as resources permit...

Cheers!

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post #21 of 26 Old 03-16-2012, 11:06 AM
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Quote:
Originally Posted by ADTech View Post

If a moderator can override the default file size limit and post the file on my behalf, I'll be happy to get it posted if for all to see here on the forum. Alternatively, if my file size attachment privilege can be expanded, I'll add it to this post later.

Checking, but next time contact me directly with an AVS Private Message for prompt attention on any issue that requires administrative action.

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post #22 of 26 Old 03-16-2012, 11:20 AM
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Your best bet is to host it on a site like box.com and link to it here.

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post #23 of 26 Old 03-16-2012, 06:42 PM
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Ken,
Will do next time.

Mike,
Thanks for that tip. It reminded me that we have Dropbox account and that files in the Public folder can be shared via a link.

Therefore, here's the link to the DB2 Tech Data Sheet.

http://dl.dropbox.com/u/23201192/DB2-TDS.pdf

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post #24 of 26 Old 03-16-2012, 08:20 PM
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brycenesbitt:

Thanks for the report with the good news! It sounds like a big improvement.

Your tvfool report shows KRCB a lot weaker than in mine for your location.
Quote:


......the DB2 is a UHF bowtie antenna that's not supposed to work for High-VHF.

As you found out, it will work if the VHF signals are strong enough.

Glad that we were able to help.

If you can not measure it, you can not improve it.
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post #25 of 26 Old 03-16-2012, 10:55 PM
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Winegard didn't ever have a "file size" problem with its initial plots for its Square Shooter. When blown up, it looked like a checkers board.
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post #26 of 26 Old 06-18-2012, 11:26 PM - Thread Starter
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Here's how it all worked out:
using the TiVo and the kindly posted DB-2 specs, I was able to rotate the antenna to avoid multipath.
I added a Winegard CA8800 FM Band Separator, and got FM out of the deal also.
Thanks.

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