DTTV is digital transmission television broadcast entirely over earthbound circuits. It is transmitted on radio frequencies through terrestrial space using multiplex transmitters to allow reception of multiple channels on a single frequency range (subchannel). The DTTV transmitter broadcast the multiplex, which contains the digitalized and compressed audio and video. To broadcast the video, the DTTV transmitter uses the channel coding to protect the bitstream and the modulator.
The modulation method in DTTV is COFDM where closely spaced orthogonal subcarrier signals are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with either 16QAM or 64QAM where the last one allows to transmit greater bit rates but it is more susceptible to interference. Moreover, DTTV use hierarchical modulation, where 16 and 64 QAM constellations can be combined in a single multiplex providing a controllable degradation for more important program streams.
DTTV allows to transmit multiple tv channels on a subcarrier, which it means that multiple streams are transmitted on a single frequency range. In the next figure you can see the organization of the spain channels in 2011. As you can see, every multiplex contains different channels: multiplex 55 (545MHz), for example, contains HDvideo (TveHD), 16:9video (Teledeporte), radio (RNE) and data streams with EPG, Teletext and dual language.
The number of channels included in a multiplex depends on the bitrates of that channels. Every multiplex has 20Mbps, so any combination of different channels will be possible to distribute this 20Mbps. The most common bitrates in Spain for the different types of channels are:
- HD – 8Mbps
- 16:9 – 5Mbps
- radio – 320kbps
- radio – 192kbps
Bandwidth requirements for DTTV over the Internet
DTTV can be delivered with our broadband network, where at least 2Mbps is needed to web-based applications on the computer and 64kbps is required to use landline telephone. For that reason, there’s a limitation on the amount of simultaneously tv/radio channels that we can recieve through our broadband Internet.
The bandwith requirement for IPTV depends on the number of devices inside the household and it increas with each device. In minimal usage, to recieve an IPTV requires 13Mbps to process in a household. Currently compressed HDTV content can be delivered at a data rate between 8 and 10Mbps, but if the home of the consumer equipped with several HDTV outputs, this rate will be multiplied respectivetly. For that reason, there are a lot of possible combinations with HDTV streams, StandardTV streams, Audio streams, etc.