Satellite installation Pre-BER (and how its one of the most useful satellite alignment tools)
Bit Error Ratio most installation spectrum analysers and various satellite meters provides a scientific notation value to indicate the number of bits in error. This can be somewhat confusing, what do these numbers mean, and how can it help us improve our installations.
For the most part these both analysers and meters utilise a linear scale to represent BER which is much easier to read after all maximising a scale is easier to understand and this is showing us an inverse measurement of the bits in error (the higher the scale goes the lower the number of bits in error) at least for most measurement devices.
So, let us look at these numbers closer and improve our understanding of BER. In this scenario we have already roughly aligned our satellite dish on Astra-2 at 28.2° east and we are now in the process of peaking up (maximising our Signal levels and signal Quality).
There are two types of BER that we are going to focus on they are:
Pre-BER the bits in error before correction (this is our primary area of interest during the alignment of the satellite dish)
Post-BER the bits in error after correction (if you have a high number of errors here then this can indicate a significant problem).
You have probably heard the term FEC and this is Forward Error Correction. Pre-BER is a measurement before FEC and sometimes referred to as Pre-Viterbi (named after Andrew J Viterbi of Qualcomm who invented it) and this is an algorithm for decoding a bitstream that has been encoded using Convolutional or Trellis code. Post-Viterbi where the measurement for Post-BER is obtained (after correction). We will take a closer look at Post-BER in another Whyte technical post.
Let us get back to Pre-BER its notation and meaning, here is an example of what you may see on your analyser/meter:
Pre-BER = 2.0E-3
Which means 2.0x10-3 this tells us that there is 2 incorrect (bits in error) for every 1000 bits. This is quite poor. The table below will help us understand Pre-BER better:
1/10 or 0.1 = 1 x 10-1 or 1.0E-01 1 of 10 bits is in error
1/100 or 0.01 = 1 x 10-2 or 1.0E-02 1 of 100 bits is in error
1/1,000 or 0.001 = 1 x 10-3 or 1.0E-03 1 of 1000 bits is in error
1/10,000 or 0.0001 = 1 x 10-4 or 1.0E-04 1 of 10,000 bits is in error
1/100,000 or 0.00001 = 1 x 10-5 or 1.0E-05 1 of 100,000 bits is in error
1/1,000,000 or 0.000001 = 1 x 10-6 or 1.0E-06 and so on….
To get to a QEF (Quasi Error Free) state where errors are no longer an issue, we need to be under an error threshold of 2.0 E-4 or 2.0x10-4 in mathematical notation (or 2 in 10,000 bits in error) it is at this point that errors would not be perceivable to a casual viewer that there is a problem with the decoded image or sound. However, were looking for less than one uncorrected error for every hour of reception and the lowest possible number of bits in error.
Let us look at another example:
Pre-BER is the number of bits in error divided the total number of bits. Let us say that 1 million bits are transmitted, and three bits out of the 1 million bits received are in error because of a small misalignment (dish pointing or LNB Skew or LNB Focus). The Pre-BER is calculated by dividing the number of error bits received by the total number of bits transmitted: 3/1,000,000 or 0.000003. We can further express 0.000003 in scientific notation format - the way most Pre-BER measurements are shown on spectrum analysers/meters used for satellite dish installation. Scientific notation is nothing more than a shorthand method of expressing very large or very small numbers. Our example of 0.000003 is written in scientific notation as 3x10-6.
So, the best practice to align your satellite dish precisely is to become familiar with the Pre-BER measurement (and its accompanying linear scale) especially when peaking up using micro adjustments of the LNB to ensure that the best skew position (matching the 90 orthogonally offset linear polarisation of the Astra-2 satellite exactly) and focus is set. You should find that once you have checked a carrier from each polarity (in this case Vertical and Horizontal) there will be very little micro adjustment to ensure a balance of performance from each polarity.
A few extra minutes performing this precise alignment using the Pre-BER will pay dividends in system performance, reliability, and less call backs.