IRD User Guide 1
General Instrument DSR-4810 Integrated Receiver / Decoder (IRD)
Virtual Channels, Virtual Channel Tables, & Virtual Channel Maps
The GI IRD's do not refer to RF channels in the same sense as tuning to a particular
transponder “channel” with an analog receiver. IRDs use Virtual Channels. A Virtual
Channel (VC) is essentially a pre-configured IRD control setting. When a virtual channel
number is selected; RF tuning, demodulation, and decoding settings, unique to the selected
VC, is automatically entered into the IRD. A Virtual Channel Table (VCT) is a list or
grouping of virtual channels. Table 1 illustrates VCT 241, which is the nominal virtual
channel table for daily operations of the CBS and UPN Television Networks. Table 2
illustrates VCT 242, which is the virtual channel table that would be used in the event that the
Telstar 6 satellite fails and is replaced by Telstar 51. It also includes restoration plan
transponders for Telstar 4 restorations in the event of a transponder failure. Likewise, Table 3
illustrates VCT 243, which is the virtual channel table that would be used in the event that the
Telstar 4 satellite fails and is replaced by Telstar 52. It also includes restoration plan
transponders for Telstar 6 restorations in the event of a transponder failure. Note that
although the VC’s are selected by number, they are operationally referred to by name, as
defined in the VCT and displayed on the IRD itself.
It can be seen from viewing Table 1 that variables such as satellite, transponder, carrier
frequency, polarization, modulation scheme, transport type, and MPEG stream service
number are automatically entered into the IRD when a virtual channel number is selected. A
Virtual Channel Map is a Virtual Channel Table; but more specifically refers to the tuning,
demod, and decode settings that are “mapped” or linked to the particular Virtual Channel. The
words Virtual Channel Table (VCT) and Virtual Channel Map (VCM) are generally used
synonymously.
The satellite and polarization information actually tell the IRD which of its four RF inputs to
use. In a separate IRD initialization process, the IRD is programmed by the user as to which
satellites and polarizations can be found on its four RF inputs. Therefore when a VC is
selected and the VC map contains one of the four possible satellite/polarization combinations,
the IRD will switch to the appropriate RF input.
MPEG Service/Program Number
After carefully viewing Table 1, it should become evident that VCT 241, which is comprised
of 30 virtual channels, is really a listing of ten, 3-virtual channel groups. VC #1 through VC
#3 contain almost identical virtual channel map data. The same can be said for VC #4
through VC #6 as well as VC #7 through VC #9, and so on. The RF tuning and demodulation
settings within each three VC group are identical. Each of the VCs within a three VC group
is, in essence, receiving the exact same satellite digital signal. The only parameter that
changes within each of the three VC groups is the service number and the source name
displayed on the IRD. This service number refers to the MPEG service or program number.
Within each received satellite signal, are three MPEG-2 compressed programs multiplexed
into the demodulated digital data stream. Therefore, for example, although VC #1, VC #2,
and VC #3 are each receiving the exact same satellite signal, each of them extracts or
demultiplexes a different MPEG program from the data stream. This is due to the fact that
VC #1, VC #2, and VC #3 each have a different MPEG service number.
1If Telstar 6 is replaced by Telstar 5, the IRD RF configuration for Antenna Inputs 1 and 2 must have
the Satellite ID number changed from 55 to 19. See “Program and RF Menus (DCII-Manual Mode)”
section.
2If Telstar 4 is replaced by Telstar 5, the IRD RF configuration for Antenna Inputs 3 and 4 must have
the Satellite ID number changed from 12 to 19. See “Program and RF Menus (DCII-Manual Mode)”
section.
