It has been a long time since I posted last, but I received an email from a gentleman in New Zealand and his questions prompted me to begin where I left off: "What Good IS BER?"
His letter is reprinted, with permission, below. My response is following his letter.
From Bonner:
Hi
As a retiree I run a small antenna business in Katikati New Zealand and
are very interested in picture quality. I have observed that not all
pictures on our COFDM digital system have the same degree of clarity.
Most technical papers would have you believe that a digital picture
stays the same quality right down to where the picture pixelates. In my
opinion there is picture degrading before you get to the point of
pixelation. I believe it is due to the level of BER. I was pleased to
find this article at
http://www.wowvision.tv/signal_strength_meters_BER.htm which lead me to
you.
I believe the following story will help with my explanation.
Katikati is a rural town and is served by two digital transmitter points
and we are a fringe to deep fringe area. Typical signal level 35 to 42
db microvolts. It you are lucky you may get a job where you have 50db
microvolts. The most common digital TV sold in our area is a Samsung
Series 6 50 inch which I have one. On puchasing one I connected it up to
our UHF antenna and got a good picture, not as sharp as I would like and
with only the occasional pixelization on our HD and SD digital channels.
It always annoyed me that I would go and install new antenna for people
who had just purchased a new Samsung 50 inch series 6 TV and they would
get a sharper, more detailed picture than what we have. Most annoying, I
wondered if I had purchased a lemon. Anyhow the other day a local
street light went faulty and started causing a lot of digital
interference every time the street lights came on. So I wondered if I
replaced my 15 year old UHF antenna with a new Wisi phased array
antenna would improve matters. see;
http://www.wisi.de/cgi-bin/online_katalog.pl?prod_id=36 This antanna I
have found to be the best antenna I have ever come across and is used by
TX Australia as their reference antenna. I replaced the antenna and
mast head amplifier and what a difference. I now had the same sharp
picture that I had got for so many of my customers and I had reduced the
faulty street light interference.
As you will appreciate it has strengthened my belief that poor bit error
rate does degrade a digital TV picture and the only article I could find
on the internet is this one;
http://www.wowvision.tv/signal_strength_meters_BER.htm.
Your comments would be appreciated.
Attached is an antenna test I did of 5 different UHF antennas available
here in New Zealand
Thanks
Bonner Martin
Katikati
New Zealand
Dear Bonner,
I am pleased to receive your correspondence! I have written some other things at DTVUSA
Forum, but had left off at this part: "What good is BER?" (I will be posting
this as an article there, or on my blog, but I would like to use your letter, all or in
part, if it is O.K. with you.)
You will be happy to know that your observations are correct. Digital picture quality
does indeed vary!!! But the most likely culprits are plain ole’ signal strength and noise, or more
accurately, the signal-to-noise ratio (SNR).
We have been led to believe that digital is “All or nothing” but nothing is further from
the truth. Picture quality DOES vary. We are being deceived in hopes that digital
technology (in the form of more efficient FEC algorithms) will AGAIN be able to produce a
non-variable picture. (Although digital SD wasn’t actually “all or nothing”, it appeared
to us as though it was.)
When it comes to the reason for the variation, you are very close! BER is a relative
digital value that is determined by SNR. BER will reflect the SNR level, letting you know
that “X” SNR = “X” BER number of errors/time. BER is the number of bits in error over
time, or the rate of errors. SNR is the reason, BER is the result.
You actually need to look no further than the analog measurements that you are already
familiar with to diagnose and repair any deficiency. The place to look is the signal-to-noise ratio (SNR). SNR is STILL the way to achieving high quality viewing!
Let me ask you this:
If 10-6 BER is the suggested minimum benchmark for good HD quality, and 10-6 BER equates
to one visible artifact per hour, then how long would you have to leave your meter
attached to get an accurate reading? (An accurate reading that is not approximated or interpolated…)
You would have to watch that meter for ONE HOUR. Anything less will give you interpolated
results. Digital processing itself relies and depends on interpolated data. (This is
“WHY” higher BER equals less picture quality. The more corrupted data that has to be
covered, or concealed, or recreated, means that more of the final information will be
approximated from nearby or similar data. The more uncorrectable errors, the further from
the original will be your result.)
I would NEVER use BER in the installation process! In my opinion though, there are two
valid and good uses of BER
First, while you are watching television, you can determine the most crucial benchmark of
BER. If you observe one visible error per hour while watching, then you are at 10-6 BER.
This is the “minimum” and starting point of “acceptable quality” HD. If you observe two
or more visible errors per hour, you are at a higher BER. Signal degradation is
considered noticeable at the 10-6 BER, and further down toward signal loss. Above 10-6
BER, the average individual will notice no picture quality differences. This is the point determined by the powers that be, as the point where the most visible degradation occurs, or where it will be noticed. (For “high quality” HD, the BER benchmark is 10-10!)
The second good use of BER, is while troubleshooting. If a BER meter is installed in your
TV or STB, it will have a long time record of performance that will give you a more
accurate BER reference to determine the level of performance. But to affect BER in a
positive way there are only two things to do -- either increase signal, or decrease
noise.
Attention to detail is very important in HDTV! Every detail neglected, is a detail lost
in the picture. All the stuff about proper connections and terminations, grounding,
excess cable, etc. are all back on the table and NOW they will cause problems if ignored.
Since we are now pushing digital to its limits, we need to maximize these systems to get
the best quality.
It is unfortunate (and intentional) that installers in the states are not taught any of
this. They are taught the hard line of “All or nothing” and are TOLD that “picture
quality doesn’t vary”. They are taught that “as long as you have lock” everything’s O.K.
Installers and industry individuals that have been taught by the industry will constantly
say that signal won’t affect picture quality, but it does (and ALWAYS HAS - we just didn’t
have any other picture quality to compare to, and comparing to analog makes it all look
great!) Nothing has changed in the physics of signal transmission and reception.
Regarding Samsung TVs, I have said numerous times, that Samsung is a television I would
avoid (especially the low end products - difficult to calibrate correctly). The second reason that I would avoid them is that their FEC is not as good as others, and the picture quality readily shows this when
it is in the “low-to-reasonable” signal strength range. (I don’t normally include this in
my recommendations because of the distractions it would cause as some people would
desire to argue the point.)
I see that you have also experienced visible interference from known outside sources! COOL! Lights
with ballasts (fluorescent and compact fluorescent, metal halide, High pressure sodium,
etc) emit a LARGE amount of EMI/Rf radiation that will affect systems. To reference this to what
was written above, as you increased your signal strength, you reduced the appearance of a
portion of the noise. You increase the SNR by increasing signal strength. Signal overcomes noise.
Remember this:
Poor BER does not CAUSE picture degradation, it is a measure of rate of
error. Poor SNR causes picture degradation and SNR is what determines BER.
I am happy to write to you and I will be available to answer any more questions you might
have. God bless you this day!
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