TELSAT® COMMUNICATIONS LTD
A Short DIGITAL Primer
IRD - Integrated Receiver Decoder:
The digital receiver (IRD) is the link between the satellite and the terminal. It translates the digital data stream into the picture with sound, the radio programs and the various data signals i.e. teletext and Electronic Program Guides - EPG
Bit:
Abbreviation of the "binary digit, consisting of two values - a "0" or "1" which is on or off which is "0".
Byte:
A sequence of binary elements forming a logical unit of eight "bits".
Data
Bit sequences in a "fixed" code.
DVB Standard:
Abbreviation for the "Digital Video Broadcasting" the video broadcasting standard for digital TV signals according to the MPEG standard - MPEG stands for "Moving Picture Expert Group".
FEC:
Abbreviation for "Forward Error Correction" - a system used to detect errors in the digital data stream. TV Channels use differant levels of FEC depending on various factors including "how noisy" the path is. FEC is a rate of redundancy information which is added to the digital signals to recognize and correct typical errors which occur during transmission. The FEC rate should be selectable. Rates are typically 1/2, 3/4, 5/8 and sometimes 7/8.
Symbol Rate:
This rate varies from TV channel to TV channel according to the proportion of transponder space (bandwidth) used. Most digital channels use around 28,000Msym/s but some use around 4140Msym/s - so it is very important to purchase a receiver the can select these rates.
DiSEqC:
DiSEqC is a new standard for switching between the different polarization's, frequency bands and LNB's as needed to tune into the channel required. It enables you to control up to four sets of LNB's. Switching is required for selecting polarisation i.e.: Vertical or Horizontal, frequency bands i.e.: upper or lower.
The old method used to different voltages to do the switching between the two polarization's and a 22kHz tone to switch between the two frequency bands - this method does not allow you to switch between two or more LNB's.
The basic principal of DiSEqC is to use the existing 22kHz tone to send digital control signals around the system along the existing cabling.
The 22kHz signal is switched very fast to form a digital like signal - this digital signal can occur in between the standard 22kHz tone switching thus providing complete backward compatibility with existing systems.
SCPC:
Abbreviation for "Single Carrier Per Channel" - used when there are several digital services are uplinked from different locations. Lower bit rates are also utilized between 1.5 and 35Mbits/s.
Msym/s:
Abbreviation for "Mega (10-6) Symbols per Second - some digital channels occupy the whole 36mHz - like the Chinese regional TV services broadcast via AsiaSat-2.
MPCP:
Abbreviation for "Multiple Carrier Per Channel" - used when several channels are uplinked from a single uplink location.
BER:
Abbreviation for "Bit Error Rate" - the threshold level of a digital receiver - the same as the c/n in a analogue receiver - a figure of merit. BUT there is one major difference - with a analogue signal as the c/n degrades so does the picture quality but with a digital service you either have a perfect picture or NO picture at all - there is no deterioration either you have it or not.
QPSK:
Abbreviation for "Quadrature Phase Shift Keying" -
QAM:
Abbreviation for "Quadrature Amplitude Modulation" - QAM is most suited to cable TV distribution systems.
NOTE: To be continued.
Telsat® Communications Ltd
P O Box 1537, 17 Westhaven Grove,
Palmerston North, (Knowledge City)
NEW ZEALAND. Established in 1988.
Tel: +64-6-356-2749, Fax: +64-6-355-2141.
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