. The presence of sounds also varies the threshold. A strong signal in one portion of the audio spectrum will mask weaker ones close to it as shown. By detecting the threshold of hearing and not encoding signals below that level the data rate can be significantly reduced, in this system by a factor of between six and 12 when compared to a linearly encoded system. To extract the perceptible part of the audio, the spectrum is split into 32 equally spaced bands. The signal in each of these bands is quantized in such a way that the quantizing noise from the digitizing process matches the masking threshold. The sampling frequency is generally 48 kHz although 24 kHz can be used for lower quality transmissions. Different encoded bit rate options are available. These are 32, 48, 56, 64, 80, 96, 112, 128, 160 and 192 kbits per second for each monophonic channel. There is a stereo mode, or for low 218 Newnes Guide to Radio and Communications Technology bit rate transmissions a mode known as joint stereo mode may be used. This uses the redundancy and interleaving of the two channels of a stereophonic programme to maximize the overall perceived audio quality. The modulation system uses a large number of low data rate carriers packed closely together. This is achieved by using frequency division mutliplexing, and overlapping the signals slightly. However, by altering the phase SO that they are orthogonal to one another the individual carriers can be demodulated without any interference from one another. A total of just over 1500 carriers occupying a bandwidth of 1.5 MHz (1.536 MHz to be exact) are used to carry the data, and they have a spectrum like that shown in Figure 7.9. A guard band of 250 kHz is placed Amplitude Approx. 1.5 MHz wwwwwwwww Frequency Figure 7.9 Spectrum of a DAB signal between adjacent channels to prevent interference between different transmissions. This allows data at a rate of just over 1.5 Mbits per second to be carried giving the capability of up to six high quality stereo transmissions to be carried