Video formats
All around the world two main formats are in use for High Definition Television broadcasting. These two formats are abbreviated as 720P (where P stands for progressive) and 1080i (where i stands for interlaced).
1080i is a format defined in ITU-R Recommendation BT.709-5. It consists of 1080 active lines per picture and 1920 samples per active line.
The Aspect Ratio is 16:9 and the format uses square pixels.
720P is a progressive format defined by the SMPTE television standard 296M-2001. This format provides 720 active lines per frame and 1280 samples per line. Again the Picture Aspect Ratio is 16:9 and the format uses square pixels.
SES ASTRA and its industry partners have agreed to allow both of these formats to be used for HDTV broadcasting in Europe.
The table below lists the scan formats that European HD displays need to support in order to qualify for the HDTV logo.
Interlaced versus Progressive scanning
Traditional television has used so-called ‘interlaced’ scanning in order to conserve bandwidth. In interlaced scanning each frame is displayed on the screen in two passes. During the first pass all the odd numbered lines are drawn in a 1/50th of a second and during the second pass the even numbered lines are drawn in another 1/50th of a second. A complete picture is drawn
25 times per second.
On the other hand in a progressively scanned system the entire frame of pixels is conveyed in every scan sequence (every 1/50th of a second).
There have been many discussions on the respective merits of one or the other of these formats. There seems to be agreement on the following points:
- A progressive format is easier to compress and leads to lower bitrates
- Motion portrayal is better with 720P50, interlaced scanning can introduce image artefacts during rapid motion when shown on native progressively scanned displays (LCD, Plasma,..)
- 720P50 provides overall fewer artefacts than 1080i
Production in 1080i is currently easier due to more available equipment. Overall 1080i is in wider use worldwide than 720P.
A
Advanced modulation
DVB-S2 is the successor to the very successful DVB-S standard. When combined with Advanced Audio and Video Coding it provides the optimal solution for HDTV broadcasting in the coming years.
DVB-S2 provides a capacity increase of +/- 30 % under the same transmission conditions.
Conversely it can provide a ruggedness increase for the same spectrum efficiency.
The new Variable Coding and Modulation (VCM) scheme can provide different levels of error protection to different service components.
Due to the increased number of HD services that DVB-S2 can convey it increases the gains possible in statistical multiplex environments.
Audio coding
HDTV doesn’t just mean great picture, it also means great sound. Dolby Digital can deliver immersive 5.1 channel surround sound to perfectly complement the improved visual experience offered by HDTV. A Dolby Digital 5.1 broadcast can captivate you in a way which stereo cannot, for films delivering a true cinematic experience, for sport placing you with your team right ensuring you don’t miss the action. In fact for any content Dolby Digital delivers the experience the audience demand.
Dolby Digital is the format of choice for film with over 40,000 cinema screens equipped and standard for DVD worldwide entertaining millions of people every year.
Dolby Digital is the format to deliver surround sound for films, sport and drama but its flexibility also allows for mono and stereo broadcasts with seamless switching between mono, stereo and surround its ideal for all direct to home broadcasting. Increasing the flexibility yet further is the ability of Dolby Digital Decoders to take 5.1 sound tracks and create stereo, a process called downmixing, for situations where a 5.1 system is not available. It’s even possible to downmix right down to mono for a portable TV.
Dolby Digital is also easy to use both for the broadcaster and for the end user in the home. In the home it’s as simple as making one digital connection from the set top box to a surround system. Even surround systems available now are easy to set up usually having intuitive colour coding for connections and simple on screen menus to guide you. Speaker positioning is quite straightforward and usually takes only a few minutes, often smaller satellite speakers are used which are even easier to place.
Set top boxes often also feature a stereo analogue output feature for cases when a surround system is not in use. These outputs can deliver a downmix of the Dolby Digital stream avoiding the need for the broadcaster to transmit an addition stereo track using valuable bandwidth.
Discrete means that the sound information contained in each of the six available channels is distinct and independent from the others. These six channels are described as a "5.1-channel" system, because there are five full-bandwidth channels (left, centre and right at the front together with left and right at the Back) with 3 Hz to 20 kHz frequency range plus one "low frequency effects" (LFE) subwoofer channel devoted to frequencies from 3 to 120 Hz.