DVB Primer

The purpose of this tutorial is to serve as an introduction to the world of digital broadcasting using DVB.

What is DVB?

DVB stands for Digital Video Broadcasting, and is a technology standard allowing the broadcast of video, audio and indeed any type of data. Broadcasting means that the audience can be unlimited, usually millions of people in one or more countries. Compare this to a telephone conversation whereby the number of communicating parties involved is limited, usually two.

The most popular type of content for DVB is television, which consists of a video stream, and an audio stream, and usually teletext data and EPG (Electronic Programme Guide). Anyone who has used a DVB set-top box (STB) from a digital TV service provider will be familiar with these terms.

The medium along which DVB is transmitted can be via cable, via satellite, or via a terrestrial transmitter (digital terrestrial). The DVB standard is widely adopted in Europe, Australia, South Africa, India, while most Asian, African and many South American countries use it for cable and satellite only. Canada, Mexico and South Korea, while adopting DVB for cable and satellite transmissions, have adopted the ATSC standard for digital terrestrial transmissions. ATSC is another digital broadcasting standard adopted widely in the United States for all types of digital broadcasts.

How does DVB work?

Like any digital storage and communication systems, DVB uses ones and zeros to represent the content being transmitted. With the aid of error detection and correction data stored within the signal itself, a DVB signal is more robust to noise, and hence does not suffer interference in the same way that an analog signal does. Furthermore, its digital nature assures that the quality of the picture being transmitted does not deteriorate due to electromagnetic noise introduced from the transmitting station to the receiving entity (e.g. a set-top box). You receive exactly what is being transmitted. This guarantees a sharp picture that is only limited by the quality of the transmission and the quality of one’s television (or screen).

There is another major advantage of digital transmission: the contents are not just limited to audio and video, but data such as Internet packets, teletext and EPG information can be broadcast together with other services. Furthermore, one single channel (frequency band) can carry a number of services (e.g. TV stations), unlike its predecessor, analog transmission, which could only carry only one TV station per channel. This is why a set-top box (satellite, cable or digital terrestrial) may provide hundreds of services, including radio, while analog transmission (VHF or UHF) could only supply a handful. In digital broadcasting, we do not call a TV station a channel, but rather, we call it a service. A service may consist of a group of data streams, such as video, audio, teletext, EPG and subtitle information.

Satellite, Cable or Digital Terrestrial?

The media by which a DVB signal, called a transport stream (TS), is transmitted can be satellite, cable, or digital terrestrial, in the same way that analog transmissions could have used any of these. Due to the nature of the different media, there are slight variations in the TS when originating from the various sources, which are defined by three standards: DVB-S for satellite, DVB-C for cable and DVB-T for digital terrestrial. When purchasing a set-top box or a PCI/USB card for your computer, you have to decide for which medium you need it. This is because the hardware will still consist of an analog tuner that works quite differently for each of the three types of media. One has to keep in mind that, like its analog counterpart, content has to be modulated, i.e. carried onto an electromagnetic wave. You cannot send ones and zeros right into the air! In DVB, the wave will carry a representation of ones and zeros. In an analog transmission, the wave will carry a signal to represent the picture and audio directly by varying intensities of the components that make up the picture, and by varying some central frequency to represent variations in audio (frequency-modulation). The three DVB standards, DVB-S, DVB-C and DVB-T, specify how the ones and zeros are carried over their carrier frequency band.

A frequency band for cable or digital terrestrial is called a MUX (multiplex), while in case of satellites it is called a transponder. Essentially they are the same. Each MUX or transponder carries a transport stream (TS). Each transport stream carries tens of services (TV, radio, data), multiplexed (split in time) inside the stream. Each satellite provider, cable provider or digital terrestrial transmitter will typically broadcast tens of transponders/MUXes, since frequency bands can be tuned into separately from each other (same concept as an analog radio). This effectively means that there will be hundreds of services available in the air or on cable, thanks to the digital revolution!

Signal Encryption

One cannot have an exposition on DVB without mentioning signal encryption. Producing content, such as running TV stations and building and maintaining the distribution of the signal is a commercial activity. If everybody was able to receive digital television without paying, soon these companies will run out of business and there would be nothing more to watch! That is why not all services provided would be free to all (technically called free-to-air). Most services, i.e. TV or radio stations, would be encrypted/scrambled.

To be able to watch encrypted services, you need what is called a Conditional Access Module (CAM), and a key-card compatible with your CAM. The key-card can be bought from your service provider retail outlet. Most set-top boxes supplied by content service providers (CSP) as part of the paid service will already have pre-installed CAMs in the STB. These CAMs will be compatible with the key-cards used by that service provider. If you bought your STB from some other place, you must make sure that the CAM you have is compatible with the key-cards your CSP supplies. The key-card will contain information about the type of service you purchased, hence allowing only decryption of those services for which you paid.

If, instead of a STB, you use a PCI/USB card, there are two choices. Some PCI cards allow an extension to be attached for holding a CAM which you can purchase separately. So in effect you plug your CAM on this CAM holder which installs into another PCI slot inside your computer. The CAM will now be linked to your DVB card via a cable, and if you have the correct key-card inserted, the CAM will be able to decrypt the service for your viewing. Another choice is using a Software CAM. A Software CAM is a programme that does the work of a hardware-based CAM. However, you still need the key-card and a card-reader (which is much cheaper than a CAM). The card-reader must be able to read your type of card. It usually plugs into your computer either through a serial port or USB. There are different Software CAMs available, so make sure you read their documentation first to see what you will require.

Conclusion

This tutorial just skimmed the surface of the information that is available with respect to DVB technology. For instance, we said nothing about the actual video and audio encodings used to broadcast the various services. This would usually be MPEG-2, however recent high-definition (HD) technologies require the use of different encoding types, such as H.264 and AC3 for digital surround sound. Here are a few pointers if you would like to delve deeper into the subject. You may then follow links from there.