Fullscreen HTML5Med Res (??MB)HTML4

Digital Radio Part 3: Transmitters & Receivers 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 O NE OF THE MOST interesting features of Digital Radio trans­ mission 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. 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 traf- 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. siliconchip.com.au fic 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: Cyclic Redundancy Check: this adds parity bits which are related to the data. This will enable the decoder to ignore corrupted data. Reed-Solomon Error Correction: for every 110 bits, 10 bits are added which are mathematically related to the data. This will allow up to five bits in error to be corrected in the decoder. DRM Error Correction: this uses the Cyclic Redundancy Check and ReedSolomon error correction. Audio frames are divided into two. The first half, representing the loudest frequencies, has more error correction than the second half which is less significant. Huffman Codeword reordering is used for AAC. Camouflage: the data is shuffled in a April 2009  11 powered linear amplifier increases the power to be applied to the transmitting antenna. In the DAB+ case, the antenna is vertically polarised. Map characteristics Fig.2: block diagram of a COFDM modulator. This results in quadrature amplitude modulation (QAM) of the transmitter signal. predetermined order, prior to transmission, so that the effect of impulse noise is distributed, making the like­ lihood of complete correction greater. This makes reception more reliable until the noise is continuous. COFDM multiple carriers In the process of multiplexing, the single serial data stream is subjected to Coded Orthogonal Frequency Division Modulation (COFDM). This is similar to taking a serial signal and converting it to parallel. So for DAB+, each serial pulse becomes 1536 times longer in duration. This allows reflected signals to be ignored. It also enables Single Frequency Networks (multiple transmitters in the same coverage area on the same frequency) to be used. Fig.2 shows the COFDM process while Fig.3 shows the resulting map of the modulation. The data from the nine program sources is used to address a block of RAM (random access memory), one bit at a time. A ‘one’ is written into that location. For transmission, the memory is read so that the column value is obtained. This is fed into an analog-to-digital converter (DAC). It is used to vary the carrier level in a double sideband suppressed (DSB) carrier modulator. If the value is greater than 3.5, the carrier will be inverted to make the left side of the modulation graph in Fig.3. This is called the in-phase or “I” axis. A second DAC is fed with the row data. It is identical to the “I” axis but its carrier is delayed by 90 °. This gives the Quadrature (Q) axis. Once the outputs of the two DSBSC modulators are added together, the result is quadrature amplitude modulation (QAM). This signal consists of a suppressed carrier and a set of sidebands. The frequency of the sidebands depends on the data being modulated. A Fast Fourier Transform is applied to this signal to generate the many carriers which are characteristic of COFDM. The suppressed carrier is usually a standard intermediate frequency (IF) which is the same in every transmitter. To get to the allocated transmission frequency, a second modulator is used to increase the frequency. A highFig.3. this diagram shows the quadrature amplitude modulation (percentage modulation) vs phase modulation for a COFDM. 12  Silicon Chip The modulation map of Fig.3 shows percentage modulation versus phase modulation. In 4-QAM, there are only four phases and so the amplitude is always at a maximum from the modulator, as shown by the pink spots on the graph. This mode gives the best immunity from noise. 64-QAM can transmit 16 times the data rate as 4-QAM. 64-QAM has nine amplitude levels over a range of 17dB. To get the same reliability of the reception, the radiated power needs to be increased by a factor of 50 times. As a compromise, the data which has the most audible effect can be transmitted using 4-QAM and the more subtle information transmitted at 64-QAM. This is called “hierarchical” coding. Digital radio receivers Fig.4 shows the block diagram of a digital radio receiver. The blue section of the diagram is devoted to the RF and IF sections which more or less constitute a conventional radio tuner. The mauve or pale blue section is devoted to the COFDM demodulator which essential reverses the processes applied in the COFDM in the transmitter. In operation, the microprocessor displays a list of available stations. The listener uses the station select switch to select the wanted program and the micro takes over from there. The antenna signal is filtered to only allow the channel you have selected to be amplified. The microprocessor will tell the tuning section which frequency band and what frequency is to be selected. The output of the tuning and IF amplifier section is digitised. The signal leaves the hardware and with the exception of the audio amplifiers and loudspeakers, the rest is done by the microprocessor and some memory. Audio directional control is performed in the Parametric Stereo section, shown in the pale green section of Fig.4. The central pair of delays and attenuators is used to steer the sound from left to right. There is a fixed delay when the sound source is central. The outer pair of delays is used to produce siliconchip.com.au reverberation by feeding the steered signal back around the section. The duration of the delays and the amount of feedback will control the reverberation time. This adds to the “realism” of the sound. The demodulation can be done using mathematics performed by a processor, however the control microprocessor will have to tell the demodulator which mode to operate in. When the demodulator is operating in the COFDM mode, its output will be decoded by the AAC+ decoder back into AES digital audio. This is then converted to stereo analog sound and fed into speakers. An HDMI output could be used, particularly for 5.1 channel sound to be fed into a hometheatre amplifier. The microprocessor can also decode and send text and images to the display screen. In addition, the processor will use the station list in the DAB+ or DRM signal to check the availability of other signals containing identical program and will switch to it if the DAB+ or DRM signal contains too many errors. Fig.4 shows the DRM/FM/AM tuners in a separate block, for simplifica- Fig.4. block diagram of a DAB+ receiver. The microprocessor controls all functions. tion of the diagram. The combined DAB+/DRM/FM/AM radio only needs the following modifications over a DAB+ only radio: additional firmware, extra tuning coils, plus varicaps and band-switching diodes. A ferrite rod or loop antenna is also required for the MF band (DRM & AM). Radioscape® has a DAB+/DRM/ FM/AM module available to radio manufacturers. Next month, we will conclude with a discussion of the signal format, a comparison of DAB+ and DRM and SC suggested antennas. STOP PRESS* As we went to press on this issue, it was announced that industry-wide switch-on to DAB+ will be postponed to August 1st rather than May 1st 2009. May 1st will begin the widespread testing of new signals but the ABC will not begin test transmissions until June, with all its stations on air from July 1st. Av-Comm DAB+ Digital Radio Receiver ONL Y $149 PLU .00 S P& P Av-Comm’s Q4000 DAB+ receiver is the result of over 12 months product development and market research. Rather than releasing a noncompatible DAB receiver which could have been used during early on-air testing, the company chose to wait until the DAB+ standard was formalised. Originally intended to combine the features of DAB+, FM with RDS and Internet radio, the results of Av-Comm’s market research indicated that different demographics exist for DAB+ and Internet radio. The result is the Q4000 which is a basic VHF only DAB+ receiver. The receiver also has a clock and alarm functions with snooze allowing it to be used as a bedside clock radio. Priced at $149 (plus P&P), the receiver represents an affordable entry point into the world of Digital Radio. The unit is capable of running from 6 AA internally-housed batteries but is supplied with a 9V regulated DC power supply. For those technically minded, the important specs are: (1) RF Input Frequency Range: 174.928-239.2MHz (2) Sensitivity: -100dBm (3) Power supply 9V DC, 800mA (4) DAB+ channels: 5A/B/C/D, 6A,B,C,D, 7A,B,C,D, 8A,B,D,C, 9A,B,C,D, 10A,N,B,C,D, 11A,B,C,D, 12A,B,C,D, 13A,B,C,D,E,F. Av-Comm Pty Ltd, 24/9 Powells Rd, Brookvale 2100, NSW, Australia (PO Box 225 Brookvale 2100, NSW, Australia) Phone: (02) 9939 4377    Fax: (02) 9939 4376   Website: www.avcomm.com.au siliconchip.com.au April 2009  13