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The 2012 London Olympics really brought home to me just what a massive technological jump in digital media has occurred during the last few years. There has been significant advances in digital compression and transmission. 

 

Digital-Revolution

This year, in addition to high definition broadcast, which made its appearance in the 2008 Beijing games, 3D television was also added to the line up, offering more channels and choices. With analogue television broadcast almost becoming extinct, digital televisions promise of delivering more for less has become a reality. Now, how did we arrive at this point and what does the future hold for digital multimedia? 


Prior to the digital switchover, analogue television was resource hungry in terms of the amount of bandwidth required to carry a single channel. This is typically between 6 - 8 MHz depending on the type of video standard being used. This limited the number of channels which could be transmitted, since there is a finite amount of spectrum that must be shared with other services such as mobile, radio and two way communications.

What the digital standards of ATSC (North America) and DVB (Rest of the World) provided was the ability to reuse the existing analogue spectrum more efficiently. This meant a typical 8 MHz carrier used for analogue broadcast could be converted to DVB-T (Digital Video - Terrestrial) making it possible to carry 9 standard definition channels or 3 HD channels plus one SD channel for the same amount of bandwidth.

It would have required in excess of 70 MHz of frequency spectrum to achieve this with the old analogue standard. In addition to squeezing more channels into less space, digital television is much clearer and doesn't suffer from ghosting or other artifacts which troubled analogue systems. Being digital also allows other features like improved digital sound, electronic program guide and subtitle support to be included.

 

Televisions are sold with the digital decoder integrated and older televisions can use a separate set top box. As technology advances, we will also see improvements in the compression techniques used, which means even more content for digital media, already this has enabled 3D broadcasts for some events such as the Olympics.

The Future

Eventually as fibre to the home is deployed worldwide, the all IP enabled set top box will replace the DVB standard, since the IP set top box has a distinct advantage over digital broadcast technologies, specifically multicast join requests. Unlike DVB-T or DVB-S, IP multicast allows the receiver to send a join message to the network for the desired channel then if the request is successful the broadcast is routed to the receiver, only the bandwidth for the requested channel is used. With the DVB standard, all available channels are being broadcast simultaneously, and the channel count is limited by the finite amount of channel bandwidth regardless of the compression techniques being used.

The IP set top box can support both selective multicast (one to many)  and on demand unicast (one to one) broadcast, this allows for virtually unlimited amount of content. However, unlike DVB, IP set top boxes have to worry about latency and QOS, since there is traffic contention with both residential broadband and IP Telephony. A poorly implemented IPTV deployment can behave like analogue television in an over subscribed service provider network, unless the correct traffic management is in place.
 
High Definition

Today HD is regarded as premium content by most operators and is charged at a higher rate than SD. However, over time this will change as people upgrade their televisions to HD models. Today there are two standards for digital HD broadcast, 720p and 1080i. The ‘p’ means progressive and the ‘i’ means interlaced. In 720p broadcast, the picture is made of 720 horizontal scan lines and a vertical resolution of 1280 pixels, which has the advantage that one frame represents a complete image.

In 1080i broadcast, the picture is made from two 540 horizontal scan images which when combined make 1080 lines. The vertical resolution becomes 1920 pixels. Most modern televisions support playback of 1080p, which is definitely more desirable than 1080i especially in fast moving sequences where motion blur can be experienced. However, on modern televisions the difference is barely discernible.

Initially the public uptake of HD was slow, the receivers were expensive and the available content was limited. HD television has really been an evolution rather than a revolutionary change for most of us and this is also true of digital television in general. As digital switchover continues worldwide and consumers replace their televisions, digital will become the new standard. However, it is unfortunate that technology won’t help to improve the content.

By Angela Sutherland

 

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