Direct Broadcast Satellite (DBS) -- Forward Error Correction (FEC)

Because the signals from the satellite carry digital information, an increase in signal attenuation (or greater path loss) increases the signal's bit error rate (BER), reducing the accuracy of the recovered digital information. As all DBS satellite television signals use audio and video compression, 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. This type of signal degradation is significantly reduced through the use of 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. Choice of a suitable FEC algorithm ensures that DBS can work reliably with the system's standard small receiving antennas. Without FEC, these small antennas could otherwise only receive a marginal quality signals.

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

However, with DBS, the receiver can't transmit back to the sending location to request a re-send of lost data. Consequently, a technique is used that is able to correct received errors independently, without any intervention from 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 value. 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 error-free.

During the heaviest rains 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 mutes both the sound and video automatically. 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, including the on-screen menu data.

The FEC information data is generated at the transmit location as part of the transmission of the digital compressed television signals to the satellites. The continuously changing details of the television data signal are processed and coded as a separate part of each television signal. A complex mathematical process is used to derive this information known as a syndrome. The incoming digital television signals are 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 are both impaired.

FEC is a pretty 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 sticky gobs of peanut butter on your disks (don't try it!)

For those in the 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.