Interpreting Bit Error Rate (BER)
The bit error rate of a digital system is the number of errors of bits in relation to time. The bit error rate (BER) is the digital expression of the signal quality or signal-to-noise ratio. BER is digital signal quality and it depends on SNR. (You may want to read “Signal Quality and the RF Front End” if you haven’t.) http://www.dtvusaforum.com/dtv-hdtv-reception-antenna-discussion/15390-signal-quality-rf-front-end.html
How does BER relate to a signal meter?
To understand the BER, let’s put it in the context of a signal quality meter. There are several benchmarks in BER that are identified with performance.
A system can get lock on a signal as low as 10 -1 bit error rate (BER). It would be the lowest signal quality meter reading that you can still have reception. You can in most cases watch a program “as long as you have lock”. With my digital converter box (which has both strength and quality readings) this is around 58-60 in signal strength and 17-19 in signal quality. Everyday for a couple of years I’ve watched programming at this level of signal. I choose to view at this level because this is where I get to see the widest variety, and largest number of compression artifacts.
As we will see, digital isn’t “all-or-nothing” and lock is enough for a picture, but not enough for great picture quality. If you think just having lock gives you great picture, then here is some information that you might be interested in knowing.
MPEG compression (Forward Error Correction) doesn’t even start working until the BER reaches 10-4! Or conversely, FEC stops working when you drop below 10-4 BER. Lock can and does occur at signal level that has insufficient information to even get the FEC to work. If you are watching a program that is very close to lock, the error correction isn’t even working. (As a matter of fact, the misnamed “digital cliff” (more later) is caused by MPEG FEC. The “fast drop” comes with the sudden loss of the correction benefits of MPEG as signal fluctuations cause the BER to cross the 10-4 threshold where MPEG stops working.
It is obvious that not everyone will see a problem at low signal levels. Some will have compromised picture quality but won’t notice. Cool! They don’t have to worry about picture quality concerns at all (save money on calibration, too). Others who have eyes to see, may notice a blurry or grainy picture, and other compression artifacts. Still others (with eyes to see) may not experience deficiency in performance that is visible. The degraded performance may show up in loss of signal, pixilation, timer issues, lip sync, DVR function, and an array of compression artifacts. The bottom line is that errors increase as SNR decreases, and signal quality is the determinant factor in all areas of performance from stability and function to quality.
Another notable bit rate benchmark is 10-6. A bit error rate of 10-6 is the point where, below 10-6, the average person perceives a degraded picture. 10-6 BER is also the Quasi error free (QEF) point described as one visible error per hour. If there HAS to be a minimum to be achieved (versus maximizing your signal at the highest level obtainable which is what should be the goal), 10-6 is that minimum for “good quality” viewing, the Quasi Error Free point. It is at this point that FEC has enough good information from the signal to give its best guesses about the stuff that is corrupted or missing.
With Dish Network, I believe the QEF point of 10-6 to be about 66, including some headroom to stay above 10-6. When watching Dish programming received at 55 on the signal meter, it is clear that this is short of the goal of 10-6 as evidenced by the far greater number of errors seen. Remember that 10-6 is characterized as ONE visible artifact per hour.
MPEG error correction is in full swing at 10-6 BER input and with all of its tools and tricks, the output “resembles” a much lower but higher quality BER. At BER of 10-6 there are still a lot of errors but the forward error correction (FEC) compensates, covers, and hides the errors quite well for a BER of 10-6 and fewer errors.
It is not until we’ve moved further up the signal quality performance scale, to a BER of 10 -10, do we reach the benchmark labeled “High quality video”. Here is where “WOW!” is actually found! For the most discriminating eye and for the highest quality picture with trouble free performance, this is really where we want to be! Beyond a BER of 10-10, the top end of the scale is between 10-12 and 10-13. In this area there becomes too much signal for the digital signal processor and it becomes overloaded resulting in pixilation and loss of signal similar to the bottom of the scale.
Here’s the thing…digital performance is not actually “all-or-nothing”. If the BER performance were actually “all-or-nothing”, a graph of it would be a straight line. A straight horizontal line on a graph represents “no change”, or a constant value; which is exactly what we would expect in an “All-or-nothing” scenario.
When we actually take a look at the digital performance graphs, the first thing we see is that we are not dealing with a straight line of constant performance, but a curved line denoting variable performance. While the “digital cliff” idea only hints at the inaccuracy of the “All-or-nothing” fable, it has been known for quite some time that there isn’t even a digital “cliff”, but rather a digital waterfall. It seems to me that no one wants to tell us about it.
Here’s a quote from an article written in 2002, How Forward Error Correction Works
“Represented graphically, the general error-performance characteristics of most digital communication systems have a waterfall-shaped appearance. System performance improves (i.e., bit-error rate decreases) as the signal-to-noise ratio increases.”
Here is the link: How Forward Error-Correcting Codes Work
I found the graph in this article to be in a strange orientation, versus what I would call a typical graph. I am accustomed to viewing a graph that increases in performance when read from left to right. This graph presents the digital waterfall, but the way it is oriented, you might think, as I did at first, that the “water’ is flowing “down”, from left to right, but that is not correct. For the “waterfall”, to be accurately reflected as flowing “down” (as water does) requires a different orientation.
To view the graph in a more sensible fashion (reading left to right, low performance to high performance, I have included the graph with its new orientation.
This graph gives clear evidence of the BER to SNR relationship and now, with different orientation, represents a performance graph from “nothing” (bottom left), to higher performance as we read to the right.
This graph represents only the portion that used to be called the “digital cliff” but is more accurately called the digital waterfall.
I’ll leave you with a couple more graphs.
Note: You might notice that the graph in the article only represents BER in the range of 10-1 to 10-6, or so. It is quite common in the BER/performance graphs that I have found, for them to only include portions of the total graph. Common among them are graphs that stop at 10-4 or 10-6. These stopping points are common because 10-4 is where you should reach for FEC to begin working and 10-6 is where you should be to begin watching really good (but not “High Quality”) video.