We frequently have customers become confused when they're told that they can't "add a board" to their 300 Series satellite receiver package to pick up an SCPC (Single Channel Per Carrier) signal. "After all, they're both audio and they are on the same satellite, aren't they?"

They're on the same satellite, but like AM, FM, and Television broadcast signals that are in the same space on earth, it takes different receiving equipment for each. Each kind of signal uses a different transmission system.

In a Subcarrier system, a bunch of individual audio signals are converted to different low frequency FM signals, called subcarriers. These are all added together and treated as one signal called a baseband. That baseband signal modulates a satellite uplink transmitter which usually takes up an entire satellite transponder.

Since all audio subcarriers must be added together and sent to the satellite from a common dish, all the originators of the audio signals must budget their signals transported to the common uplink site.

Thus, your subcarrier receiver has two jobs to do. First, it has to tune and recover the baseband. This is done with circuitry usually referred to as either a "satellite receiver" or "baseband receiver". Typical Zephyrus boards to do this job are the 391 and 392.

Second, the baseband signal is routed to subcarrier demodulators which tune and demodulate the individual audio signals. Examples are the Zephyrus 321 and 322 Audio Demods.

From a concept standpoint, SCPC is the simplest, most straightforward of the two systems. An individual audio signal drives a modulator that is on a discrete frequency in a band between 52 and 88 MHz. This carrier is upconverted to a frequency compatible with the particular satellite allocation. The upconverted signal goes to a dish (usually located at the origination point of the audio signal) and up to the satellite. Thus, instead of a group of subcarriers, the Carrier contains only a Single Channel of audio. For stereo, two individual carriers, sharing the same dish are used.

A typical SCPC receiver downconverts the received satellite transponder back down to the 52-88 MHz band. At this point , a tuner, much like a mono FM radio receiver tunes the selected carrier and recover the individual audio signal. Two such tuners are required for stereo. The Zephyrus 700 SCPC receiver system operates in this manner.

SCPC was the first method used for broadcast network transmission. Ironically, SCPC receivers are the most expensive. It was that high cost that fostered the later widespread use of subcarrier systems.