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Digital Radio is Coming, Part 3

Last month, we discussed the details of the Advanced Audio Coding (AAC+) used in Digital Radio. This month we discuss how the AAC+ data is transmitted and received. The transmissions combine data with program content and the entire transmission is a multiplexed combination of the many programs.

by Alan Hughes

One of the most interesting features of Digital Radio transmission is that more than one station’s program content is embedded into a single carrier frequency. In fact, up to nine different program digital streams can be combined into the one transmission. At the planned introduction* of digital broadcasting, there will be up to 71 radio stations (ABC, SBS and commercial) on 10 DAB+ transmitters.

Click for larger image
Fig.1: a DAB+ transmitter uses multiplexers to combine the program digital streams from up to nine stations with Program Associated Data and Fast Information Channel data.

Fig.1 depicts how the AAC+ encoded program and multiplexers combine data from a number of stations. Multiplexers can be thought of as rotary switches which continually cycle through the available data sources. So if they are switches, does this mean that bits of individual programs are missing, as the switch selects other program sources? No, that is the beauty of switching digital data packets – no program data is lost.

As well as the program digital streams, two other types of data are multiplexed into the transmission. The first, Program Associated Data, includes date & time, station identification & location, and pictures and text.

By contrast, Fast Information Channel inserts small blocks of data, often regardless of the program sources. Typical uses are emergency and traffic information, as well as paging and conditional access data.

Error detection

The effect of corrupted data becomes more drastic as compression is increased, so some error detection and correction will greatly improve the listening experience, particularly for car and portable reception. This takes several forms:

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