12.3 BROADCASTING SYSTEMS

< Day Day Up >

---

12.3 BROADCASTING SYSTEMS

Radio is the most effective medium for broadcasting audio and video. Broadcasting for large areas is done through satellites, but for local programs within a country or state, broadcasting is done through terrestrial radio. Unfortunately, the broadcasting systems have not evolved as fast as other communication systems. Even now, the audio and video broadcasting systems are analog systems. Though a number of digital broadcasting standards have been developed, they are yet to take off on a large scale. Digital broadcasting systems are now being commercialized.

Broadcasting is one of the most important applications of radio systems. However, the present audio and video broadcasting systems are analog systems.

12.3.1 Audio Broadcasting

In audio broadcasting systems, amplitude modulation (AM) is used. The analog audio signal with 15kHz bandwidth is modulated using AM and then up-converted to the desired frequency band. Many such audio programs of different broadcast stations are frequency division multiplexed (FDM) and sent over the air. The AM radio frequency band is 550–1610 kHz. The receiving stations consist of radio receivers that can be tuned to the desired frequency band. The received signal in that frequency band is down-converted and demodulated, and baseband audio signal is played through the speakers. Presently, frequency modulated (FM) audio broadcasting is becoming predominant. As compared to AM, FM gives a better performance, so the quality of the audio from FM stations is much better. FM stations operate in the frequency band 88–108 MHz.

In AM broadcasting systems, the modulating signal of bandwidth 15kHz is amplitude modulated and up-converted to the frequency band 550–1610 kHz. A number of audio programs are multiplexed using frequency division multiplexing.

Note

FM broadcasting is now becoming popular. In FM radio, the frequency band of operation is 88–108 MHz. FM gives better quality audio because of its noise immunity.

12.3.2 TV Broadcasting

The TV broadcasting system is similar to the audio broadcasting system. The TV signal requires a bandwidth of 5MHz. The video signal is modulated and up-converted to the desired band and transmitted over the radio. At the receiving end, the receiver filters out the desired signal and demodulates the signal, and the video is displayed. VHF and UHF bands are used for TV broadcasting.

To provide very high quality video broadcasting using digital communication, efforts have been going on for the past 20 years. In digital video broadcasting, the moving picture is divided into frames, and each frame is divided into pixels. The number of frames and the pixels in each frame decide the resolution and hence the quality of the video. ETSI has developed two standards: digital TV and HDTV (high-definition TV).

Digital TV standard ITU-R BT.601 uses 25 frames/second, with each frame divided into 720 pixels width and 526 pixels height. For this format, the uncompressed data rate is 166Mbps. Using compression techniques, the data rate can be brought down to 5–10 Mbps.

HDTV standard ITU-R BT.709 uses 25 frames/ second, with each frame divided into 1920 pixels width and 1250 pixels height. For this format, the uncompressed data rate is 960Mbps. Using compression, the data rate can be brought down to 20–40 Mbps.

Present TV broadcasting systems operating in the VHF and UHF bands are analog systems. For digital TV transmission, two standards, Digital TV standard and High Definition TV (HDTV) standard, have been developed.

Note

For digital TV transmission, the moving video is considered a series of frames, and each pixel in the frame is quantized. Using compression techniques, the data rate is reduced to a few Mbps.