Satellite TV
It is hard to imagine where television services would be without satellites. Or maybe it’s not! TV would not have been the major news and entertainment carrier, there certainly would not have been the current choice of channels, and images from around the world would not be on our screens daily.
Gerdus van Eeden, chief technology officer in the broadcast technology division of Multichoice, says although satellites are the major technology part of their business, the company also uses terrestrial transmissions for its trial mobile TV service and the internet for its DSTV on-demand PC service.
A large amount of the programming arrives in South Africa by satellites from London via a C-band satellite system. Some channels are packaged at the MNet studios next door to Multichoice’s satellite facility which is directly linked to the transmission control centre by fibre links. Channels such as SABC are received via the Telkom ATM network. ATM is also used to deliver signals to Sentech for terrestrial transmission. Multichoice operates in a converged technology domain to deliver its services.
Some signals are still in analogue format and need to be converted to a digital format. "We also handle SDI digital signals coming from the MNet studios. These need to be compressed. All the digital signals are multiplexed before being sent via fibre links to the uplink facility where they are up-converted to Ku-band, amplified and uplinked".
Multichoice uses 12 transponders on Intelsat 7 which is parked at 68,5° east. Intelsat 7 is due for replacement in 2012 but a replacement satellite will be in place well before that date. Its footprint covers the southern Africa target area. The channels are uplinked from the company’s satellite hub in Randburg.
For the Africa service customers Eutelsat W4 is used. This covers most of the rest of sub-Saharan Africa. Uplinking for this service is handled in South Africa and Spain.
From a technology perspective the network is complex and poses a real operational challenge to meet the high service levels the company has set. The other main challenge is environmental. DStv is transmitted on Ku-band which suffers from rain fade during heavy rain.
"We can counter rain fade up to a point by increasing the uplink power. An automatic control system monitors the signals at the satellite receive point and during periods of heavy rain will automatically increase the uplink power up to a point. If the rain density increases during a storm, signal fading may be beyond the automatic gain control threshold and signals will break-up," Van Eeden explained.
"Customers often don’t understand why, when the sun shines in Cape Town, they experience rain fade. It is most likely due to heavy rain at the uplink site in Johannesburg. Sometimes I wish we could control the physics of radio propagation. Ku-band is a practical choice of frequency because of the convenient size of the dishes. While we use a 9 m dish to uplink to the satellite, customers only require a 60 – 90 cm dish to receive signals."
"Mobile TV is another technology that we are engaged in. It is a form of digital terrestrial television using UHF frequencies. M-Net has a mobile TV trial licence. The trial runs in the Johannesburg, Pretoria, and Cape Town areas from some 14 transmitters. We use the DBV-H standard that is designated for handheld applications."
During the trial Multichoice offers a selection of 14 channels from the DStv programme range, changing the selection every month. Depending on the transmission parameters the system can handle up to 18 channels per allocated frequency. A few handsets are available on the market but the range is expected to increase dramatically during the next cellphone manufacturing cycle.
Delivery of TV via the internet is emerging. "In our market this is still severely constrained because of the lack of broadband and the insistence of ISPs that they must cap their services. DStv on-demand PC is available to all premium subscribers by registering on www.dstvondemand.com. If you have a high quality connection it should be possible to stream and download programmes on demand for later viewing."
Even with the focus on so many other systems, submarine cables and satellites will continue to be a major distribution channel.
EngineerIT
Loading ...