Block diagram of DAB equipmentIn Hong Kong, the broadcast history could be traced back to 1928 when there was only AM (Amplitude Modulation) broadcasting. In 1960, FM (Frequency Modulation) broadcasting systems were introduced, since then, both AM and FM systems are in operation, and there is no any major technological advancements made in the radio broadcasting industry the only two upgrades were in 1976 when mono FM broadcasting systems were changed to stereo ones, and in 1991 when similar AM upgrade was carried out.
Why Digital Broadcast?
Throughout 1990s, digital sound broadcasting technologies have been developed in a very fast pace, there are various standards being adopted or deployed in various countries, say, the DAB and DRM in Europe, IBOC in United States, and ISDB-T in Japan, just name a few popular ones.
Simply speaking, digital sound broadcast systems, as from their name, would broadcast the sound information/signal in a discrete digital format, after some sampling, quantizing, encoding, multiplexing, and modulating processes. They have the following intrinsic advantages when compared with traditional analogue sound broadcast systems:
1. Excellent Quality
As all the information are coded into digital formats, error checking algorithms are added into the digital broadcast system, hence, any interference or noise introduced during the transmission path between the transmitter and the receiver could be well detected (and even corrected) by the receiver end. This means musical program content could be perfectly reproduced, resulting in a very high quality audio broadcasting CD-quality and Surround 5.1 musical programs could be retrieved via air. One could either receive an audio program with very good sound quality, or get nothing from the receiver if the receiving condition is too poor.
2. Environmental Friendlines
Because of its higher anti-interference/noise capability, theoretically digital broadcast system could use less power/energy in its transmitter design. That means, it is much more environmental friendly, in addition to savings in energy cost.
Since they are transmitted in digital way, all various contents are encoded to similar digital format first, thus, digital broadcast system could easily incorporate various information sources together. In fact, it can support voice, text, graph, photo, animation, or even video, and this makes multi-media broadcast possible. When the receiver is equipped with concerned output/display devices, users could easily get real-time mobile multi-media information. This is a big opportunity to the broadcast industry as nowadays people are chasing for information (or entertainment) with mobility, multi-media, and no delay.
4. Efficient Usage of Radio Spectrum
Before the information are transmitted/broadcasted, various sources are multiplexed into one physical medium. Hence, the usage of radio spectrum is multiplied. Under digital broadcast technology, 6 voice channels could be transmitted in a same radio frequency which could only carry 1 voice channel in the analogue one. Under current broadcast environment where all radio spectrum are fully utilized, this is certainly a good news. Broadcasters could create more radio channels and produce more programs with better sound quality and multi-media format.
5. Intelligent Receiver
To the receiver side, intelligent devices and processes could be added to further elaborate those received digital information. Program-associated data could be displayed as text/graphic, e.g. talk shows could be heard while displaying the presenters'/topic's background, musical programs could be listened together with display of the CD cover, traffic congestion news could be shown together with alternative routes/maps, financial programs could be received with trend analysis, etc.
6. User Friendliness
Rather than searching wavebands as present, users can select all available stations or preferred formats from a simple text menu. Besides, individual channels' whole day program schedule could be displayed as Electronic Program Guide (EPG) and users could change the receiver to a personal program recorder by assigning the record on/off time according to the EPG.
The DAB Trial in Hong Kong
In Hong Kong, starting from August 2004, RTHK has been putting on trial one of the most popular digital sound broadcasting systems, the Digital Audio Broadcasting System (or named as DAB), using Eureka 147 standard as developed by an international consortium of broadcasters, network operators, consumer electronic industries and research institutes. In December 2004, nearly 600 DAB different services in 35 countries are in operation, covering more than 300 million people around the world.
This project is initiated by RTHK, approved by OFTA, and supported by various consumer electronic manufacturers. It is being implemented in three phases:
Phase 1 (from 5 August to 10 October 2004)
The trial was commenced on 5 August 2004, with one single transmitter installed at Mount Gough to cover Aberdeen, Central and Kowloon Peninsula. Two audio channels A & B were put in service: Channel A was named as“RTHK ALL IN DAB”which complied programs of Radio 1, Radio 2, Radio 3, Radio 4, Radio 5 and Putonghua Channel according to a pre-scheduled timetable; while Channel B was named a“RTHK DAB ONLY”and carried Radio 4 musical program.
About 30 DAB receivers were borrowed from a local DAB manufacturer, they were on loan to some selected audience for them to experience DAB services.
Phase 2 (from 11 October 2004 to end of February 2005)
The second transmitter, located at Beacon Hill was put into service on 11 October 2004, and a Single Frequency Network was established. The transmission from Beacon Hill extends the coverage to Shatin, Sheung Wan and North Point.
Channel B was re-scheduled to carry our AM services from 18 October 2004. It switched among those 3 AM services monthly. Content of Channel A is remained unchanged.
Receivers were on loaned to different interest groups for them to experience DAB services and their feedbacks/comments are collected. The listeners are chosen to be professional audio engineers, officials of the Hong Kong Philharmonic Orchestra, elderly centers, shopping malls, hotels, etc. Their feedback will be consolidated for further analysis.
Phase 3 (from March 2005 )
In this phase, a DAB studio would be set up in Broadcasting House for production of DAB-only programs. RTHK is planning to set up 6 separate audio programs exclusively for DAB services, to fill up the whole DAB ensemble.
Data broadcasting services would be included as well. Texts and images would be broadcasted along with audio program to show the multi-media functions provided by DAB. Road shows and demonstration booths would be set up in various locations, such as shopping malls and university campus, to promote DAB services to the general public.
System set up
Figure 1 shows the block diagram for this trial. Audio encoders and multiplexers are set up in Broadcasting House. Audio programs are digitised, encoded and multiplexed into a DAB ensemble. Through two E1 digital links, the encoded audio signals are then fed to Mount Gough (MG) and Beacon Hill (BH) where DAB transmitters are located. According to the requirement from OFTA, DAB signal is beamed towards Aberdeen and Central of Hong Kong Island, Kowloon Peninsula and Shatin.
By the time of preparing this article, so far all the DAB features are proved in normal working. Detailed measurement of technical data/results are in progress, and we believe such trial could arouse more awareness from the general public, raise their interest, and help in keeping Hong Kong as one of the world class cities in applying new and advanced digital technologies.