Re: Digital Radio Communication
The book isn't quite right.
ALL 'over the air' signals are analog but the modulation they carry can be digital. For example, take the simplest digital modulation which is keyed AM, suppose a digital '1' bit is sent by turning the RF on and a '0' by turning it off. The electromagnetic wave is still analog, probably a high frequency sine wave but by turning it on and off it is possible to send digital information. Another example would be FSK (Frequency Shift Keying), where the RF is always present but the '1' and '0' are sent as RF at slightly different frequencies.
DAB and DRM use another system which still sends the audio in digital form but a system called quadrature modulation is used. It is still broadcast as analog waves but they are changed in frequency, phase and amplitude to allow more digital bits to be sent. So a digital audio broadcast works like this:
audio --> digitizer (DAC) ---> RF modulator ---> antenna ~~~~~~ antenna ---> RF demodulator ---> digital to analog converter (DAC) ---> audio.
The benefits of doing it are that the data from the DAC can be compressed and have error correction bits added to it. At the receiver the ADC can expand the data again and 'repair' errors using the correction bits to ensure data integrity. Overall, a much higher audio quailty can be achieved than simple analog modulation and it can withstand much higher interference levels because of the correction bits.
DRM broadcasts on short wave are stereo and as good as MP3 quality and at the same time carry program information as HTML, similar to web pages. In contrast, with only slightly narrower bandwidth, conventional signals are mono with poor high frequency response, suffer from interference and have no program information.
There is lots of information about DRM at
www.drm.org.
Brian.