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Digital Broadcasting

In Europe the Digital Video Broadcasting (DVB) project was set up in 1993 and came from a market led perception that digital broadcasting to the home needed technical standards of transmission to avoid the anarchy of proprietary boxes which has developed in analogue satellite transmission. However, the DVB is not itself a standards making body; it provides a forum for suppliers to agree specifications which are then passed to existing standards making bodies (such as ETSI, ISO etc.) for ratification.

An early decision was not to “reinvent the wheel”, so MPEG-2 was readily adopted. At about half the data rate of DVD, MPEG-2 video still rivals an analogue TV channel when transmitted over the air by satellite or cable, along with the advantages of a digital bitstream such as error correction to eliminate snow and ghosting. The video carried by DTV isn’t just compressed, it’s transmitted digitally in accordance with one of a number of DVB specified modulation standards:

  • QPSK – for digital satellite broadcasting
  • 64QAM – for digital cable transmission, and
  • COFDM – for digital terrestrial broadcasting.

COFDM (coded orthogonal frequency division multiplexing) was largely developed into a practical proposition in France in the late 1980s and found its first major application in Digital Audio Broadcasting (DAB). A modulation technique designed for reception via roof-top aerials, COFDM splits a channel into 2,000 or 8,000 tiny carrier signals, offset to reduce interference. By using this system, over 30 digital channels – offering the same picture resolution and stereo sound – can be transmitted within the same amount of bandwidth required to send just five analogue channels.

For UK DTV each 30Mbit pipe will carry around six MPEG-2 TV channels. There are six such channels, or multiplexes, operating in the UK delivering around 30 channels via the ONdigital service. Bandwidth is also available for a Super Teletext service which promises image-rich interactive applications via MHEG3 (Multimedia and Hypertext Expert Group). The advantage of using a digital transmission system is that it can scrape together unused spectrum otherwise unsuitable for TV. Some of the spectrum currently used by analogue TV will be given over to DTV eventually, but today the terrestrial service has to compete with Sky’s digital operation and imminent cable offerings that can carry several hundred channels. The government hasn’t said when the “big switch off” of analogue will take place, but in the US analogue services are due to close in 2006.

Since MPEG-2 is scalable, the bandwidth needed for each channel isn’t fixed – neither is the number of channels transmitted. The broadcaster decides how much bandwidth is allocated to each one. In the UK, Sky and the BBC have different approaches. The latter chooses to allot a fixed amount of bandwidth to each channel, arguing that a scheme of dynamic allocation is overly complicated when you take regional programming into account. Even if a particular channel needs more than the allotted bandwidth, the picture quality is unlikely to suffer unduly. Sky, on the other hand, allocates its bandwidth dynamically so that when it deems necessary it can allow more to fast action topics, such as sport.

Under normal conditions, the overall bandwidth allocated to a given broadcaster is enough to ensure that all the required information across every channel gets through clearly. However, at time of maximum data throughput – for example, when several channels are simultaneously showing fast, action sequences – the amount of bandwidth available to each channel is reduced. The effect of this is the screen resolution decreases and the picture is momentarily represented by small, easily seen blocks, rather than the usual high resolution pixel dots. This effect is known as “blocking” and means that the information being received is not enough to complete the whole picture. Generally, though, blocking is rare, having been almost completely eliminated by the continual improvement of the source hardware at the transmission station.

Current PAL analogue provides a resolution of 625 lines. Standard Definition Television (SDTV), the standard presently used by most digital broadcasters, sends the same number of lines although the quality will not be impaired because a digital signal does not degrade. High Definition Television (HDTV) can almost double the resolution, with up to 1080 lines being transmitted. However, the HDTV specification most often quoted is 720 lines as this uses progressive scanning – that is, it uses a non-interlaced signal – and as any monitor user will attest, a non-interlaced signal provides a much better-quality picture.

The Advanced Television Systems Committee (ATSC) has established 18 approved formats for the broadcasting of DTV in the United States. These formats are encoded into a stream of binary bits, which are then modulated using a method known as 8VSB into a 6MHz analogue channel “envelope” in preparation for transmission. The broadcaster then “up-converts” the signal to the frequency that has been allocated by the Federal Communications Commission (FCC) – the part of the US government involved in making sure that communications technologies best serve its citizens – and it is sent to the transmission tower. The twelve SDTV formats are the result of all possible combinations of three resolutions with four frame rates. The resolutions are 704×480 with pixels compressed slightly yielding a 4:3 aspect ratio, 704×480 with pixels expanded slightly to yield a 16:9 aspect ratio and 640×480 with square pixels for an exact 4:3 aspect ratio. The frames are refreshed 60 times per second at a 2:1 interlace (yielding 30 complete frames per second), or refreshed progressively at 60, 30 or 24 frames per second. The other six approved formats are referred to as HDTV formats.

Only the highest resolution formats require the majority of the 19.4 Mbit/s per second that make up a DTV broadcast. In fact, depending on the encoding used, an SDTV program can be made to use as few as 4.5 Mbit/s, sometimes even less. This opens new opportunities for broadcasters, who can take advantage of this situation by transmitting more than one program within a single 19.4Mbit stream, and/or adding various kinds of data to the stream. Some uses for this technique include multiplexing several unrelated programs (typically 4) on a single feed, multicasting a single program such as a sporting event from several different camera angles (allowing the viewer to select the point of view by changing to a different program within the stream) and broadcasting multiple time-shifted copies of a program. Other possibilities are to allow for some limited forms of interactivity by letting a viewer select a different ending to a movie, or select an answer to a question during a distance learning program with the choice resulting in positive feedback or a further explanation of the topic.

 

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