Digital Audio Broadcasting
A Receiver Design
Digital Audio Broadcasting (DAB) on VHF - Key Facts
COFDM: Coded Orthogonal Frequency Division Multiplexing is used.
In the UK, DAB radio uses 174 - 230 MHz on the VHF band.
Within this band, there is space for 14 multiplexes.
There are 1,536 carrier frequencies per multiplex.
Each carrier is both amplitude and phase modulated.
This is called quadrature amplitude modulation (QAM).
Each carrier carries 1.5 kb/s.
This low data rate reduces problems from reflected or delayed signals.
The carriers are spaced 1kHz apart.
The total capacity of one multiplex is 1536 x 1500 = 2.3 Mb/s.
One multiplex can carry about ten radio stations.
Stations are "tuned in" by tuning the radio to the correct multiplex and then decoding the binary data to extract the desired station.
To change channel, the receiver is often NOT re-tuned. Different data is selected from the same multiplex and converted to analogue audio.
Speech stations use less bandwidth than music stations. Bandwidth use can be altered in mid transmission as required.
About 50% of the transmitted information deals with error recovery.
This data adds redundancy. This means it is very fault tolerant.
The receiver still works even if 50% of data is lost.
Transmitters can all use the same frequencies so receiver re-tuning is not necessary as you move around the country.
Bandwidth: Music channels use 192 kilobits/second. Channels carrying mainly speech and phone-ins don't need such high quality and they use as little as 80 kilobits/second. These figures can change at different times of day as the broadcast content changes.
Data Streams: DAB multiplexes can also carry data streams. 32 kilobits per second is a typical data rate. This is used for the station ID and for information about the music, artist, track or other program content.
MUSICAM: is an analogue data compression system. It removes information that the listener would not hear because of the physiology of human hearing. Digital data compression is used too.
Time and Frequency Interleaving make DAB much more likely to work under adverse conditions.
When you drive under a bridge, normal FM radio often blanks out for half a second.
DAB uses a neat trick where not all the information is transmitted at the same time. At any given moment some of the data is sent but more is sent after a time delay and yet more after a further time delay. By now, the car will have emerged from under the bridge and using the incomplete information the receiver can still correctly reconstruct the transmitted signal without a gap.
If the receiver is picking up a signal that travelled by two or more paths, it is a matter of luck whether these signals add or cancel out.
If one frequency is cancelling out, another frequency nearby will be adding because the wavelength is slightly different.
By distributing the data over multiple frequencies, enough will be received successfully for perfect sounding audio.
This also explains how two transmitters avoid interfering. In fact they do interfere but at any moment, most of the carriers are not completely cancelling out so the receiver will work perfectly.
To make this work, the transmitters have to use exactly identical frequencies. This can be achieved by synchronising all the transmitters onto GPS satellite frequencies.
DAB transmitters in different parts of the country use the same transmission frequencies so the receiver does not need to be re-tuned if it moves from one transmitter to another. This would commonly apply to car radios.
This means there are geographical areas where the signal can be received from more than one transmitter.
The receiver picks up transmissions from all the transmitters that are in range.
Some carriers in the multiplex will undergo constructive interference (the signals will add).
Other carriers in the multiplex will undergo destructive interference (the signals will cancel out).
The loss of some carriers doers not prevent successful reception of the DAB signal due to the built-in error correction.
The low data rate used on each carrier makes this technique possible. With higher data rates, different information would be received from each transmitter. This problem is avoided.
Digital Radio Mondiale - MF and HF Digital Radio - Not an exam topic
Analogue and Digital Samples
Here is a downloadable video of a DRM broadcast. There are no nasty effects on the sound. The red, blue and black array of dots is a visual representation of the Quadrature Amplitude Modulation data stream.
Compare this analogue AM sample with the DRM above. The quality is quite nasty. There is a high pitched audible heterodyne whistle and periods of deep signal fading causing serious distortion.
This is very similar to DAB described above but it is designed to work on the MF and HF bands. Each digital transmission occupies the same bandwidth as a traditional AM transmission. The sound quality of the decoded data stream is far superior to AM radio. The transmitters can send audio data and digital data allowing programme schedules and other data to be transmitted alongside the audio. This is a simplified version of the "red button" service available with digital TV.
The screen shot below shows the output from a short wave receiver being decoded by the sound card in a computer. Software called DREAM is used to decode the data stream.
To receive DRM using the Dream software, a modified superhet receiver is needed. The blue RF stages and red IF stages are identical to a normal superhet receiver. Instead of demodulating the IF, a second mixer is needed. Its output is on 12kHz. The bandwidth is 10kHz spanning frequencies from 7 to 17kHz. (All these frequencies are audible). This audio signal is demodulated using the computer sound card and processing power from the computer's CPU.
Frequency division multiplexing is used.
Between 88 and 460 carriers are transmitted depending the the DRM mode.
Carriers are spaced 41 to 107 Hz apart (much closer then VHF DAB).
Each carrier is modulated using QAM.
Audio and/or data streams can be transmitted.
Only one radio station uses each multiplex.
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