signal meters ... true lies?

It has been discussed, within these forums, that some meters indicate "Signal Quality" values, while others indicate "Signal Strength." This is why I prefer CECB's with both (ie. APEX DT502, Sunkey SK-801ATSC, & Zinwell ZAT-970A).

I find the meter on the Insignia NS-DXA1-APT (same as Zenith 901) to be useless. Especially since it has a default dismiss. The APEX and Sunkey need to be manually dismissed.

The correct antenna is always the "Heart" of any DTV system. Generally within 45 miles, most OTA viewers will not notice a large difference between CECB's. It's when you are on the "Fringe," "Deep Fringe," or "Extreme Deep Fringe" that CECB sensitivity is important. It also makes a difference when one has other issues to deal with (ie. Multi-Path, Trees, Edge Reception, etc.).

The meters on these boxes are no different than the typical gauges on a car. You have to calibrate them to reality before they are useful. For antenna pointing, I know what my chances are of locking on a signal, and it doesn't matter that the meters are different. The fact that I use the meters after the antenna is aimed is an indication that I'm still getting useful info.

With my CM7000, I've seen breakups on 100% signal and watchable pictures on a 2% signal. The Zenith has nowhere near that many distinctions, and yet with either CECB, I have a pretty good sense of what the meter is showing me in terms of being able to get a viewable picture, and a lock. My greatest issue is multipath, and it doesn't matter that the meters don't show the same number, the most important information comes from observing signal fluctuation. I'd like more refinement on the Zenith meter, but I still get what I need out of it. For all its fine distinctions, the CM isn't any more useful as a meter, or as a box -- when the CM breaks up at 100%, I know that the Zenith or Zat will handle things perfectly well.

The Zat has two meters, but in nearly all cases, what they call quality is the meter that matters to me. It's nice that there are two, because in other situations the other meter might tell the story. It doesn't aid in troubleshooting, it's more that it explains differences between the CM and Zat in handling a bad signal. If I had a choice, I suppose I'd pick the Zat's two meters because it's interesting to see two indicators, not that it makes a difference in achieving a better signal.

From watching them, I believe that the meters on the Zenith and Digital Stream models are signal quality meters. Is it the case that all CECBs with a single meter type show signal quality, not strength?

I don't know if this is technically correct, but I think of the quality value as something approximating the percent of correctly decoded packets. Can someone clarify what is being measured as "quality"?

I notice that the Zenith's display has more "lag" than the DS. That is, it integrates more numbers before updating the meter. More lag makes it more difficult to do antenna adjustments - especially with indoor antennas in high multipath environments. One can move right past the best point before the meter bar peaks. I have to force myself to move the antenna slooowly while watching the meter.

I don't believe that "ALL" CECB's, with a single meter, are measuring "Quality."

I believe it is bit rate error that some brand signal meters use vs. actual signal strength.

The DTV Pal signal strength meter (that's what DISH calls it) will not display very low non-tunable numbers.
While the Zenith meter will show very weak signals that have no hope of ever being tuned.

So seeing a weak non-tunable signal is only a benefit for antenna tuning if you have no clue which direction the towers are located.

The signal strength meter is only intended to be a tuning guide for that particular receiver. Obviously, various brands will function differently.

I've seen numbers as low as single digits on the DTVPal (can tuner). If the signal is not fluctuating, it will hold picture/sound, without any breakups in the low teens.

What the Zenith meter shows depends on where you are looking at it. If you enter the virtual channel number of a station that's too weak for a lock, it shows "no signal," but you can see the signal when you press "signal" or are viewing signal strength from the manual tuning part of the menu instead.

Perhaps, if you are using a tower-mounted outdoor antenna.

In my case with an indoor antenna and heavy multipath, due to nearby buildings, best reception does not generally come from pointing in the direction of the transmitters. Plus, the location of the antenna in the room matters to within a few inches.

Also, during tropospheric enhancements, the non-tunable signal indication on the meter tells me when DX signals are coming in, allowing me to fish for a spot that will bring them to watchable levels.

Thanks to the Zenith's manual tuning feature, it is more feasible to hunt for usable signals/antenna positioning than when you are limited to update scans. I can even sit on an RF channel to wait for the levels to increase until the station ID is displayed - or even until it becomes watchable.

Not everybody cares about pulling in difficult or DX stations, but I find it fun to try.

My DTV Pal signal meter falls off of a cliff at about 50. This is at the Add A New Channel page.

Those were my observations. If the signal drops below about 50 on any of the DTV Pal models I've had, it then goes right to "No signal".
I've never seen any numbers in the 40's or below, even though a Zenith hooked up the same, showed a weak non-tunable signal.

So do I.
I have the Zenith DTT900, Zinwell ZAT-970A, Sansonic FT-300A, and the Apex DT502. My favorite dual-bar box is the Apex.
I have made a side-by-side comparison of the Zenith and the Apex using a splitter. I made changes in the antenna to vary the signal strength and signal quality. The Zenith bar seems to be a combination of signal strength AND signal quality.

Even though signal strength and signal quality are in percent or some other arbitrary unit, it doesn't matter because you are only looking for a comparison----more is better. You will soon get a feel for how much is needed to maintain a lock on a signal.

If you want to know the actual signal strength you will need a signal level meter (SLM) which is calibrated in dBmV. Broadcast engineers use dBm. The conversion factor between them is 48.8: +1.4 dBmV equals -47.4 dBm.
I have calibrated an Apex box using my signal level meter:
http://www.avsforum.com/avs-vb/showt...6#post15414426 post #8424

That's correct.
The signal quality bar is related to BER (bit error rate/ratio). The tuner FEC (forward error correction) is capable of correcting some errors, but it's ability is limited. Once that limit is reached, you will start to see dropout, tiles, and picture freeze.

It is possible to have a strong but poor quality signal.

The factors that reduce signal quality and cause a higher BER are:

1. Improper signal level: A weak signal will cause a poor signal-to-noise ratio; a signal that is too strong can overload a tuner or preamp. A nearby FM transmitter can also cause overload, which would require an FM trap.
2. Reflections from multipath problems, static or dynamic.
3. RF interference or impulse noise in the reception area.

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, 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.

The Zenith appears to combine signal quality and signal strength in one bar. It increased slightly when I rotated the antenna, but it's not as sensitive as the 2-bar box quality bar.

Anyone who is in a weak-signal situation (like worse than about -80 dBm on tvfool) and tries to aim his antenna between two different azimuths to get both stations is going to have a difficult time because off-axis aim causes an increase in BER.

It seems that the signal quality indication is a more sensitive aiming tool than signal strength, because it shows the increase in BER from multipath reflections. In my situation the BER is affected by the weak signal, the fixed multipath reflections, and the changing 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.

The signal-to-noise ratio is the difference in dB between the signal level and the ambient noise level.

A SNR of 15 to 16 dB is needed to maintain lock on an 8vsb signal. Anything more is gravy; anything less and you have problems.

You beat me to it Rabbit.

I too have noticed how slight Off-Axis aiming will get me better reception. This seems to be a normal situation for most "Fringe" and beyond reception.

I've come to realize that having I-81 behind my antenna, is giving me some issues with my high-gain setup. I've been getting CB issues from the truckers running amps and power mic's.

Lazza,

Can you do a comparison test of your SNR option and your CECB?

It would be interesting to see how they inter-relate.

systems2000: I did have you in mind when I wrote that because I remembered your table of stations with low quality numbers for the off-axis ones that you posted after my second calibration run.
CBers with amps probably put out not just a strong fundamental but also harmonics. Most of the amps are not type-accepted. Power mics make it worse because of flat-topping from over-modulation.
Your weak-signal situation is very challenging and requires a lot of ingenuity and experimentation for success.


The signal bars on the Apex DT502 respond much more rapidly, but you can't add a channel after scan which wouldn't be good for a DXer or anyone located between two cities.

I, too, have noticed that in order for the Pal Plus to show a picture, the signal strength indicator number has to be in the high 50's, low 60's, or there is no picture. On a windy night a few days ago, the signal meter displayed numbers all over the place, going from 60 to 70 to 0 within seconds!

I realized that with the wind, trees blowing outside my residence, that such disruption in picture quality and strength/retention should be expected. I fantasized having a Channel Master box which is supposed to be strongest as far as reception goes. Then .... I thought of all the times when I was watching analog tv and the transmission FROM the station itself gets pixillated or frozen or lost. (that is, the station, itself, s receiving from another source its signal, all done digitally) I thought, "if a tv station with all its high tech equipment can't prevent the occassional disruption of a digitally transmitted picture, then we as consumers with these relatively cheaply constructed CECBs should also expect disruption caused by various factors."

Until a newer technology comes along (and cable tv from what I gather keeps a signal online better than satellite transmission), the converter box picture quality will consist of occassional picture disruption. I find that the signal meter of the Insignia shows a more stable holding of the picture vs. the greater fluctuation I have seen in the meters of the Zinwell 950-A and the Pal Plus. (with the Pal Plus, as long as the meter reading is at least in the high 60's, the picture holds most of the time.) The Zinwell's meter reading I consider highly unreliable - most stations that come in fairly strong indicate a 100% level on the top bar (intensity) and about 60 to 90% on the bottom bar (quality). The intensity level can be very high yet at times the picture can break up.

It concerned me that another person had the experience of his Channel Master indicating a 100% strength reading and the picture broke up. If a box that is rated the best for reception quality can't hold the signal at this level, what hope is there for other boxes to do any better? Then again, the breakup of that signal could have been caused by uncontrolled external factors, such as nearby interference from a neighbor running his vacuum, trees blowing, gust of wind, etc.

Anyone know the physics behind the disruptive effect caused by wind? Could the force applied to nearby materials and surfaces, such as building exteriors and vegetation cause some sort of electrostatic charge in the atmosphere which eventually disrupts the waves being sent in its midst to the converter boxes?

The wind that moves the branches of the trees causes a constantly changing disruption of the signal that increases the errors in the data stream carried by the signal. It is often categorized as a form of dynamic multipath interference.
Dynamic multipath interference is much more difficult to correct than static multipath (such as a secondary signal reflected from a building).
The new ATSC M/H standard is designed to meet the challenge of the dynamic multipath problem so that the lock on the signal can be maintained when the receiver and its antenna are moving.

It's my opinion that the problem should have been considered in the original ATSC standard instead of adding a "patch" afterward. If the Committee had initially realized that some receiving antennas might be in motion when in use we might have ended up with a more robust standard better able to deal with some of the fixed antenna dynamic multipath problems that we are now stuck with like moving traffic, moving tree branches, and people moving near their indoor antenna.

IMO, the ability to add channels to the ones found in the initial scan should have been a mandatory CECB requirement.

If everyone, who has an APEX DT502, would contact APEX Digital about producing updated firmware to solve the "ADD" channel issue, maybe they would understand how much of a problem this is. It can be done because the Sunkey SK-801ATSC, Zentech EZTV DF2000, GE (22729 & 22730), Goodmind DTA (900 & 1000), and probably others have the same firmware and HAVE the channel "ADD" capability.

EPG's - http://www.avsforum.com/avs-vb/showthread.php?t=1022201

Sounds like a "dedicated" thread created just for FIRMWARE suggestions with detailed info /contact info/ some type of form letter to use as a "guideline" for the APEX series units would be a better place to post the above- than here in this thread....

BTW, Another much needed feature on the APEX units is a guide from which to select programs- for "reoccurring" events.
(EG- such as done on the DTVPal series units- it's point and click within the guide- and then at the appointed time the box tunes to whatever was selected)

The DT502's self-destructing "one-time reminder function while better than nothing- is useless for those of us that want to set the box (once) to tune to same station for their favorite program at a certain time...

Quick show of hands- who here would buy a vcr, dvr recorder, Tivo, etc that lost it's programing and had to be set-up each day for reoccurring events?... that's how the APEX DT502 self-destructing timer function basically operates... While you may be able to go past a day- once a reminder fires- it self destructs (Last time I checked- Since purchasing 4 DTVPal Pus units ALL the APEX DT502's have gone back in their boxes. WAF (Wife Approval Factor) of the DTVPal Plus units being light years ahead of all the other units, due to the GUIDE, and the timers.).
.

I never had much luck when I tried to register my 502 on the Apex site so that I could ask a question; does it work now?

The Centronics ZAT502 HD is a dual bar STB (but not coupon eligible) that has an on-screen display of the signal bars that looks just like the Apex DT502 screen and it does allow you to add a channel after scan if you know the RF channel number. It will also do HD and clear cable QAM.

So, I got my wish when I said, on the Apex thread, that I would pay extra for that feature. I ordered mine Monday and it came today. Preliminary testing shows that its sensitivity equals the Apex.

The thread for the Centronics ZAT502 HD is here:
http://www.avsforum.com/avs-vb/showthread.php?t=1136626

The June 2009 issue of Popular Communications has an article on p 38 by Bruce Conti--- DTV Transition: Take Two. The article mentions RabbitEars.info, run by AVS member Trip in Va, and has a report by two broadcast engineers Louis Dorren, WB6TXD, and Noland Lewis, WB6CKT, that describes the difficulties that many people are having receiving OTA DTV.

Highlights only:
http://www.popular-communications.co...June%2009.html

Good reading for the tech.

There is some interesting information on the signal meter performance in the NAB-MSTV converter box evaluation. Available on the NABfastroad website under the reports heading. The MSTV lab test only has 7 boxes.
APEX DT250
Digital Stream DTX9950
Zenith DTT901
AccessHD DTA1010
Insignia NS-DXA1
Magnavox TB100MW9
Sansonic FT-300A (the only one with dual meters)

The test where meter levels at
signal strengths: -8dBm,-28dBm,-53dBm,-68dBm & -83dBm
SNR : 25dB & 15dB
echo amplitude: -3dB,-6dB & -15dB

Welcome to the forum!

Thanks for the post about the NAB-MSTV report. I didn't know about it.

Since you are not yet able to post a hot link, I'll do it for you. I entered "NAB-MSTV converter box evaluation" in the search box at Google and the first link that came up was:
http://www.nabfastroad.org/NAB-STV%2...y-report1.html

You are given a choice of the summary or the full report, and the manuals.

The full report mentions that the signal quality readings for the Sansonic are inconsistent, which is what I also noticed during my testing:
http://www.avsforum.com/avs-vb/showp...postcount=9117

happy to help.

Thank you. I tried to make it as easy as i could to search and find it for those interested but a link is always better.

I've been trying out a Zenith DTT901 and thought it was strange seeing a signal meter of 40-45% with no channel acquisition. In reading the best case -83dBm signal is 50% of the meter it makes sense. It's odd dedicating almost half the meter to unusable signal strength, but at least you know something is there. The meter is also erratic until it acquires a lock.

It would be interesting if there was a service menu or factory setting that would report more information on the received signal vs. the dumbed down meter. Maybe that's wanting too much.

That's why I like the boxes that have dual signal bars----they give more information.

I was looking for an inexpensive and easy-to-use piece of measurement equipment that would help the average guy who was having trouble with digital reception. It's also why I have included the Kelvin link in my signature that describes the attenuator-to-dropout method.

I'd agree the dual meters do give more information. I was thinking that if the unit could provide any information like signal strength,SNR and echo that it could be used to evaluate the meter display. Without the equipment to produce a known situation the unit itself would be the easiest way to what conditions exist. If you have more information from an other unit it could be used to see if has the same issues as the Sansonic FT-300A or if the other unit better implemented the dual meter display.

The signal strength bar of the dual bar Apex DT502, which is my current favorite because its readings are more consistent than those of the Sansonic, is entirely adequate to make comparisons. If you need to know the absolute value of the signal strength you would have to use a signal level meter (SLM) like I do.

If you want more signal information, the next step up in measuring equipment would be a laptop computer with a USB ATSC tuner and special software. With this you could measure SNR and bit errors. The signal QUALITY bar of the Apex is related to bit error rate (BER).

I'm resisting the urge to make that upgrade because I recently bought a digital SLM and the Apex is doing what I need for now. I have recently purchased a Centronics ZAT502 HD tuner that gives a similar dual bar readout; it even uses the same tuner as the Apex:
http://www.avsforum.com/avs-vb/showthread.php?t=1136626

The Centronics does HD and you can add a channel after scan, which the Apex can't do.

The FCC and NTIA didn't give any firm guidance on this, so I am not surprised that DTV tuners are all over the map when it comes to how how signal quality is calculated.

There are at least three predictive metrics that an ideal box would use to display signal quality -

1. RF signal level
The NTIA specifications for CECB tuners require them to work over the RF signal range of -83 dBm to -5 dBm. Decibels are logrithimic, so this range corisponds to a huge power level shift of about 63 million to one. With a digital signal, only the first few dB of signal change within this range will cause noticable changes in picture quality. Once the signal is a few dB above the boxes minimum signal threshold we will have a perfect picture, so some CECB manufatures tend to blow off RF signal level monitoring in favor of simple BER (Bit Error Rate) monitoring. However, In fringe reception areas, day/night changes in temprature and humidity, rainfall, and seasonal fluctuations in folage growth can change signal levels several dB or more, so it is important to be able to judge whether you have enough signal margin. We can't use bit error monitoring alone to estimate this margin, because the BER (Bit Error Rate) only shows variations over a very very limited range of signal levels, so some kind or real RF signal level monitoring should also be provided.

2. Raw error rate for low level correctable errors
There are two levels of error correction built into the ATSC signal, TRELLIS ENCODING, and REED-SOLOMON FEC. This error correction combo is so powerful that raw error rates of thousands of low level symbol errors per second can be corrected to the point where the final MPEG video stream is virtually error free (perfect video). Unfortunately the error correction process that does such a good job of cleaning up marginal signals also insures that when the signal does drop out, it lets go very quickly, within a dB or so, like falling off a cliff, so BER monitoring can only give us useful info about signal quality over a limited range.

3. Adaptive Equalizer Margin
All CECB chipsets provide Adaptive Equalization to compensate for multipath. Without this feature, it would be virtually impossible to receive 8VSB signals under real world conditions. These equalizers have a limited range of echo delay and amplitude values that they can deal with. You can have a very strong signal that none the less will be impossible to decode if multipath is present which exceeds the boxes equalizer range. Currently, I am not aware of any CECB which provides a separate 'Multipath' tuning readout, but some may factor the remaining adaptive equalizer margin into the overall signal quality number.

I currently have three CECB's, the CM7000, the Tivax STB-T8, and the DTV Pal Plus. Sadly, all three of these boxes provide only a simple single bar readout for signal quality, but there are some differences that are worth noting.

The CM7000 has a great tuner, but it is the WORST when it comes to trying to tweak your antenna for ideal reception. The CM7000 signal level meter seems to be based only on Bit Error monitoring, so the range between 100 percent, where the picture is perfect, and 30 to 50 percent, where it starts to break up, is very narrow. Also, you can't directly enter an RF channel on the CM7000 (you have to just keep repeating the 'Add Scan' while endlessly re-tweaking your antenna).

The DTV Pal Plus is a little better, because although it has only a single bar 'strength' readout, this readout seems to respond to both RF signal level and to BER (and perhaps even multipath). Signals that are nearly error free, but right at threshold RF power wise, will give 'strength' readings in the 50's to 70's at most, where stronger signals 10 to 20 dB above threshold give readings in the high 90's. This makes it much harder to get a 100 percent readout, but that's exactly what you DO want when you are trying to tweak your antenna for best reception. The Pal Plus also has a special channel add setup mode that lets you directly enter a REAL RF CHANNEL and then will tune directly to that channel without having to scan for it.

- Delphin