Acleris
—
Consultants, Software and Training

3. Forward Error Correction (FEC):

Because the signals from the satellite carry digital information, an increase in signal attenuation (or greater path loss) between the satellite and the receive antenna increases the signal's Bit Error Rate (BER), reducing the accuracy of the recovered digital information. DBS television signals use both audio and video compression, meaning that each data bit received represents what was originally a number of data bits in the audio and video signals prior to their compression. (This type of compression is sometimes known as entropy coding.) If special steps weren't taken, the signal received, even during light rainfall, would be unwatchable — the television program signals would appear as a patchwork of colored blocks or a frozen image, accompanied by audio clicks, pops and squeaks. Such signal degradation is significantly reduced using a technique called Forward Error Correction or FEC.

Of the 40 Mbps (megabits per second) available on each DBS satellite transponder, just 23 Mbps is used for the actual audio and video signals. The remaining 17 Mbps is used for forward error correction information, ensuring that DBS can work reliably with the standard small receive antennas. Without FEC, these small antennas could otherwise only at best provide marginal sound and picture quality during fine weather.

With conventional data transmission, similar to that between your home or office and the Internet, your modem and the one at the distant location carry out a constant two-way exchange of information called "handshaking" to ensure error-free data transmission. If something happens to the data during transmission, the modem receiving the data will request that the sending modem re-transmit the block of data in error, a useful error control technique when two-way communications is possible and just two sites are involved.

However, with DBS, the receiver can't initiate a request to re-send of lost data from the sending location. Consequently, a technique is needed that is able to correct most received errors independently, without any intervention by the transmit site. This technique is known as forward error correction.

When a signal from the satellite passes through heavy rain on its way to the receiver, the signal is considerably attenuated or weakened — perhaps to one-one thousandth of its fair-weather strength. This loss of receive signal level in turn results in significant errors being introduced into the received digital signal, analogous to a snowy analog TV picture. The presence of the FEC information, transmitted together with the compressed video, usually provides the means to automatically keep the recovered signals looking and sounding error-free.

But during the heaviest rain or when the receiver's dish is covered with melting snow, the receive signal level can drop to a very low value, resulting in an extremely high bit-error rate. When this happens, the forward error correction can run out of range, exposing the viewer to artifacts like incorrectly placed small picture blocks, blocks of colored snow, rainbows, or other visual anomalies. Static-like clicking sounds may also be heard. When the signal finally fades to a point beyond the FEC's capabilities, the receiver automatically mutes (turns off) the sound and video. Occasionally, muted video may temporarily appear as a frozen image.

As the rainfall rate decreases, the signal level gradually increases. The receiver eventually reaches a threshold where it can lock on to the incoming satellite signals, a process known as synchronization, and once again becomes able to properly decode the audio, video and data signals which include the on-screen menu.

The FEC information data is generated at the transmit location as part of the digital compression of the television signals before transmission to a satellite. The continuously changing details of each digital television signal are processed and coded as a separate part of each compressed television signal. A complex mathematical process derives this information known as a syndrome. Television signals received from the satellites and processed together with the syndrome. When there are digital signal bits in error, the syndrome provides information for the receiver to faithfully re-create the original signals, at least most of the time.

The syndrome also includes secondary FEC information, to enable it to correct and fully recover its own original information, because under error conditions, both the FEC syndrome and the digitally coded television signals both become impaired.

FEC is a complex technique but it is used for many forms of mass-market digital communications. For example, audio CDs, which are really stored one-way digital communications, use a form of FEC called Reed-Solomon coding, allowing reliable playback even with scratches and fingerprints on your disks.

For those in frostier climes, a winter blizzard won't cause a loss of the DBS signals — only liquid water attenuates the satellite's microwave signals. However, melting snow on a dish will result in significant signal attenuation, similar to that from a heavy rain.

C O N T E N T S

Disclaimer and Copyright © 2006 - 2008 Acleris