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ABC guide to... Step-by-step guides to understanding digital TV
march 2011
Digital AV Distribution Want to see more?
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It makes sense to distribute TV signals around your home digitally, but only in the past few years has it become practical and affordable Fotolia.com
PCs, TVs and media streamers can now be seamlessly linked over Ethernet and Powerline networks
Just as TV is broadcast and recorded digitally, it makes sense to distribute TV signals around your home digitally, to enable greater flexibility with no loss of quality. However, it’s only in the past few years that transporting digital TV signals around the home has become practical for anyone but the most serious computer or AV enthusiast. Of course, HDMI carries a digital signal from an AV source to one or more screens but HDMI is just too bandwidth-hungry for use all around the home (at the moment) as it deals with uncompressed video signals. For shunting to and fro about your home, compressed video signals (like those received by satellite, DTT and online, and recorded on your PVR or DVD recorder) are more practical, especially as they can be carried on a home’s computer network. Why use a home network?
01 Often, because it’s already there. Just one PC connected to a broadband router by Ethernet (CAT5) cable is a home network in all but intent. If you have two or more PCs connected, it’s a real network and you can share files between the PCs. The TV data can be sent from one 2 What Satellite & Digital TV March 2011
connection to another just like any other data. You can also connect other devices that deal in TV data (such as TVs, receivers, etc) that can swap their TV data over the network too – and that’s digital TV distribution. The versatility of a digital distribution network is limited only by the reach of the network infrastructure and the equipment you have or are prepared to buy to connect to it. How does a network carry TV?
02 There are two basic forms of TV data that a home network can distribute – media files (a complete programme ‘recording’) and streamed media (a continuous flow of TV data). Media files are simple to transfer from one networked device to another (so a PC can easily playback a media file stored on another PC on the same network). While streamed media can play in real time and requires no intermediate storage, streaming requires special software to transmit the media onto the network and to receive it at the other end. Do I need to use a PC?
03 Having a PC (or Mac) at the heart of your digital distribution system, or at least contributing to it, tends to make life
easier. A PC can perform just about any function of putting data onto the network or taking it off. A PC can provide media storage, it can transmit and receive streamed media (this may require some further software unless you have a Media Centre PC), it can receive broadcast media off-air (with a suitable tuner card), and it can directly connect to a TV (via HDMI with a suitable graphics card) to display the programmes on a TV screen. Media Centre PCs also include a ’10ft user interface’ so they can be operated from the sofa. However, there’s also conventional AV equipment that will connect to the network (just as well, as the PC is often far from the main TV, or even the antenna feed). Satellite and terrestrial receivers and dedicated devices can fulfil most of the distribution functions as well as a PC. Some receivers (mostly PVRs, and third-party-programmable Linux devices) can share storage for recorded media files with a networked PC, and/or handle media streaming to or from other devices on the network. There are even networkconnected receivers that only output streamed media over a network. In addition, conventional receivers may be directly connectable (via USB) to a
abc guide to... NAS (network-attached storage) device to provide PVR storage that’s also accessible to other network devices, so you can have one central repository for all your media files collected of-air and on-line. As well as receivers playing the media files and streams, some TVs today can also be networked and receive streamed media from a PC (or even a receiver). For other TVs, add a media player – a networked device that will handle media files from storage (internal, plug-in, or elsewhere on the connected network) and streamed media, and play the material on a connected TV screen. You should choose a media player that can handle the various compression formats (MPEG2, DivX, and so on) used by your other equipment and has a TV connection suitable for the video formats used (HDMI is the most comprehensive). Wired or wireless?
04 To produce a TV picture of the required resolution (and sound too, of course) without drop-outs, the network must be capable of carrying data at a considerable speed (a single highdefinition TV channel clocks in at a data rate of up to about 20Mbit/s). With top speeds of 1000Mbit/s (i.e. 1Gbit/s) for a
CAT5 cabled network and 54Mbit/s to 300Mbit/s for a Wi-Fi wireless network (using the 802.11n standard), there would seem to be no problem. But these are the maximum theoretical data rates and in practice the actual throughput is hugely reduced. With wireless networks in particular, there are big overheads of error correction and encryption, not to mention the speed reduction due to the walls and floors that weaken the signal in real-life homes, and other usage of the network. Although most wireless networks will probably handle an HD channel, they can struggle – and distributing two or more channels at once will in many cases be impossible. A wired network is definitely preferable for AV distribution. An alternative to the expense of cabling your home with CAT5 is to use the mains wiring already in place. HomePlug AV adapters use the mains wiring to carry the network traffic and appear to connected devices like a CAT5 wired connection, with speeds up to 200Mbit/s. You can mix and match HomePlug AV adapters from different manufacturers to build up your network, and move them from room to room with the connected device for near-wireless flexibility.
Network Networked TV
TV & media player
What are a home network’s
05 limitations?
Your home network can link a variety of media playback and storage devices
TV & media player
Even with the most complex of network, with PCs, receivers, storage devices and TVs all talking to one another and freely exchanging media files and media streams, you will soon run across one unavoidable problem – encrypted TV. Providers such as Sky do not want you to transfer digital recordings or stream digital TV around the home unencrypted (not least because you may also be sending it elsewhere), so there is no way to access the digital data received by, say, a Sky Digibox once it’s been decrypted. Of course, you can feed the analogue output from a receiver into a PC to digitise it and distribute this, or use a device like the Slingbox, but that loses most of the advantages of digital distribution. Although there is some provision for rights protection within the various media compression formats (especially the HD ones), the chances are that the future for digital distribution of pay-TV will lie with equipment such as ‘home hubs’ (combined receivers and media storage and streaming equipment) that are both supplied and controlled by the service provider n Geoff Bains
TV & monitor
Network attached storage
Broadcast antenna
Broadcast antenna
Network streaming receiver PC + tuner card
Network switch
Internet
Broadcast antenna Broadband modem router
Network receiver
Wireless networked PC
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