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Digital audio broadcasting, also known as digital radio and high-definition radio, is audio broadcasting in which analogy audio is converted into a digital signal and transmitted on an assigned channel in the FM frequency range. DAB is said to offer compact disc (CD) - quality audio on the FM (frequency modulation) broadcast band and to offer FM-quality audio on the AM (amplitude modulation) broadcast band.
Digital radio works by combining two digital technologies to produce an efficient and reliable radio broadcast system:
An audio compression system, called MPEG, reduces the vast amount of digital information required to be broadcast. It does this by discarding sounds that will not be perceived by the listener - for example, very quiet sounds that are masked by other, louder sounds - and hence not required to be broadcast, and efficiently packages together the remaining information
COFDM technology, (Coded Orthogonal Frequency Division Multiplex) ensures that signals are received reliably and robustly, even in environments normally prone to interference. Using a precise mathematical relationship, the digital data signal is split across 1 536 different carrier frequencies, and also across time. This process ensures that even if some of the carrier frequencies are affected by interference, or the signal disturbed for a short period of time, the receiver is still able to recover the original sound.
The interference which disturbs FM reception, caused by radio signals 'bouncing' off buildings and hills (multi-path) is eliminated by COFDM technology. It also means that the same frequency can be used across the entire country, so no re-tuning of sets is necessary when travelling, or taking a portable receiver to a different area. Instead of having a different frequency for each radio station, digital radio combines several services together in what is called a multiplex.
The multiplex is able to carry stereo and mono radio channels as well as services such as text and data. The UK has been allocated seven multiplexes by the Radio Authority - in the spectrum 217.5 - 230.0 MHz. It is possible to carry more services on this one frequency allowing the spectrum to be used more efficiently.
The multiplex has a gross capacity of 2,300,000 bits which are used for carrying audio, data and an in-built protection system against transmission errors. Of these about half the bits are used for the audio and data services. Throughout the day, the data capacity allocated to each service can be varied by the broadcaster.
Each multiplex can carry a mixture of stereo and mono audio services and data services too; the number of each dependent on the quality required. A multiplex is a technical term used for "a number of stations sharing just one frequency to transmit its services". It is a digital transmitter located within a region broadcasting stations operated by a company or group (e.g. BBC, Digital One, Switch Digital etc).
So what are DAB's benefits and the cost
Listeners in most major towns and cities in the world and it can receive between 30 and 50 radio stations with digital radio, in many cases that's more than double what's available on analogue. And it's not just more of the same - the content within that choice of stations is unique and exciting, delivering station formats that just don't exist on analogue. The FM spectrum is so clogged right now that there's no room for new stations that would expand listeners' choice with, for example, soul music, or country music, or big band swing, or any of the other 100+ brands that are available uniquely to DAB.
Digital radio receivers have a screen on which stations can transmit information via Dynamic Label Segments (DLS). Some stations already transmit the latest news, travel, and weather, what's on now and next, Web site addresses and phone numbers. Tomorrow's radios will offer much more sophisticated data. The potential for advertisers to use the DLS facility on DAB for targeted advertising is an exciting prospect, and in the future, advertisers can use DAB to deliver Internet-type commercials. Because digital radio uses the spectrum more efficiently than analogue, it is possible to broadcast more channels using the same frequency, making room for broadcasters to expand their station portfolios.
It also offers less noise. DAB digital radio delivers improved sound quality. The technology allows the receiver to lock on to the strongest signal it can find and ignore everything else. This eliminates the hiss, crackle and fade so familiar on analogue radio.
A BLOCK DIAGRAM OF A DAB RADIO
This diagram above is about what goes on in a DAB radio. In order to receive a station the Low-Noise Amplifiers (LNA) boosts the RF signal from the antenna. The frequency synthesizer generates a Local Oscillator (LO) signal that is mixed with the RF input to form the IF signal. The high-speed ADC converts the IF signal into digital samples. Depending on the speed of the signal the DAC outputs as compared to what the DSP or microcontroller can handle, a Digital Down counter may be required.
The power supply is connected to the 12V or 24V board net and regulates down/up to voltages for DSP, uC, memory and ICs and functions in the infotainment system. In some cases there may be 10 or more different power rails, making the design of the power supply a critical task when trying to design for size, cost and efficiency. Linear regulators with low quiescent current help reduce battery leakage current during standby operating modes (ignition off), are load dump voltage tolerant for directly battery connected devices, and need low drop out and tracking for low battery crank operation.
Beyond providing increased conversion efficiencies, switching power supplies provide EMI improvement with slew rate control of the switching FET, Frequency hopping, spread spectrum or triangulation method for attenuation of peak spectral energy, Low Iq, soft start for power sequencing and in rush current limitation, Phased switching for multiple SMPS's regulators to minimize input ripple current and lower input capacitance, higher switching frequency for smaller components (L and C's), and SVS functions for brown out indications
The Audio input front end and audio output is often combined into a single Codec. On the output side ADCs convert the digital output an analog signal, which is amplified to the levels needed by the speakers or headphones used with the system. By using Class-D amplifiers the system's power efficiency can exceed 90% while maintaining low THD. This improved efficiency leads to significant size, weight and heat reductions. TI's class-D car audio solutions exhibit extremely low EMI levels and are being used in OEM systems with stringent EMC requirements.
The audio DSP performs I/Q demodulation and outputs digital audio and data. This includes functions like volume, treble, bass and sound effects.
THE COMPONENT OF A RADIO
A transmitter can be defined as an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications.
A transmitter can be made by coupling the output of an oscillator directly to an antenna. The primary purpose of the oscillator is to develop an rf voltage which has a constant frequency and is immune to outside factors which may cause its frequency to shift. The output of this simple transmitter is controlled by placing a telegraph key at point K in series with the voltage supply. Since the plate supply is interrupted when the key is open, the circuit oscillates only as long as the key is closed.
Capacitors C2 and C3 can be GANGED (mechanically linked together) to simplify tuning. Capacitor C1 is used to tune (resonate) the antenna to the transmitter frequency. CA is the effective capacitance existing between the antenna and ground. This antenna-to-ground capacitance is in parallel with the tuning capacitors, C2 and C3. Since the antenna has capacitance, any change in its length or position, such as that caused by swaying of the antenna, changes the value of CA and causes the oscillator to change frequency. Because these frequency changes are undesirable for reliable communications, the multistage transmitter was developed to increase reliability.
Reception of a DAB signal
The DAB ensemble is selected from the antenna to the analogue tuner, the output is fed to the demodulator and channel decoder to eliminate transmission errors. The information contained in the FIC is passed to the user interface for selection and is used to set up the receiver appropriately.
Receiving of DAB signal
The ratio demodulator uses a double-tuned transformer to convert the instantaneous frequency variations of the fm input signal to instantaneous amplitude variations. These amplitude variations are then rectified to provide a dc output voltage which varies in amplitude and polarity with the input signal frequency. This detector demodulates fm signals and suppresses amplitude noise without the need of limiter stages.
The input tank capacitor (C1) and the primary of transformer T1 (L1) are tuned to the center frequency of the fm signal to be demodulated. The secondary winding of T1 (L2) and capacitor C2 also form a tank circuit tuned to the center frequency. Tertiary (third) winding L3 provides additional inductive coupling which reduces the loading effect of the secondary on the primary circuit. Diodes CR1 and CR2 rectify the signal from the secondary tank. Capacitor C5 and resistors R1 and R2 set the operating level of the detector. Capacitors C3 and C4 determine the amplitude and polarity of the output. Resistor R3 limits the peak diode current and furnishes a dc return path for the rectified signal. The output of the detector is taken from the common connection between C3 and C4. Resistor RL is the load resistor. R5, C6, and C7 form a low-pass filter to the output.
This circuit operates on the same principles of phase shifting as did the Foster-Seeley discriminator. In that discussion, vector diagrams were used to illustrate the voltage amplitudes and polarities for conditions at resonance, above resonance, and below resonance. The same vector diagrams apply to the ratio detector but will not be discussed here. Instead, you will study the resulting current flows and polarities on simplified schematic diagrams of the detector circuit.
What is amplitude modulation?
Amplitude modulation (AM) can be defined as a technique used in electronic communication, also is use as transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent. In order for a radio signal to carry audio or other information for broadcasting, it must be modulated or changed in some way. Although there are a number of ways in which a radio signal may be modulated, one of the easiest, and one of the first methods to be used was to change its amplitude in line with variations of the sound.
The basic concept surrounding what is amplitude modulation, is quite straightforward. The amplitude of the signal is changed in line with the instantaneous intensity of the sound. In this way the radio frequency signal has a representation of the sound wave superimposed in it. In view of the way the basic signal "carries" the sound or modulation, the radio frequency signal is often termed the "carrier".
What is amplitude modulation, AM
Â When a carrier is modulated in any way, further signals are created that carry the actual modulation information. It is found that when a carrier is amplitude modulated, further signals are generated above and below the main carrier. To see how this happens, take the example of a carrier on a frequency of 1 MHz which is modulated by a steady tone of 1 kHz.
The process of modulating a carrier is exactly the same as mixing two signals together, and as a result both sum and difference frequencies are produced. Therefore when a tone of 1 kHz is mixed with a carrier of 1 MHz, a "sum" frequency is produced at 1 MHz + 1 kHz, and a difference frequency is produced at 1 MHz - 1 kHz, i.e. 1 kHz above and below the carrier.
If the steady state tones are replaced with audio like that encountered with speech of music, these comprise many different frequencies and an audio spectrum with frequencies over a band of frequencies is seen. When modulated onto the carrier, these spectra are seen above and below the carrier.
It can be seen that if the top frequency that is modulated onto the carrier is 6 kHz, then the top spectra will extend to 6 kHz above and below the signal. In other words the bandwidth occupied by the AM signal is twice the maximum frequency of the signal that is used to modulate the carrier, i.e. it is twice the bandwidth of the audio signal to be carried.
Amplitude modulation is one of the most straightforward ways of modulating a radio signal or carrier. The process of demodulation, where the audio signal is removed from the radio carrier in the receiver is also quite simple as well. The easiest method of achieving amplitude demodulation is to use a simple diode detector. This consists of just a handful of components:- a diode, resistor and a capacitor.
AM diode detector
AM Diode Detector
In this circuit, the diode rectifies the signal, allowing only half of the alternating waveform through. The capacitor is used to store the charge and provide a smoothed output from the detector, and also to remove any unwanted radio frequency components. The resistor is used to enable the capacitor to discharge. If it were not there and no other load was present, then the charge on the capacitor would not leak away, and the circuit would reach a peak and remain there.
Most of the Dab radio is use by batteries. Also there three parts is a battery which are an anode (-), a cathode (+), and the electrolyte. The cathode and anode (the positive and negative sides at either end of a traditional battery) are hooked up to an electrical circuit.
The chemical reactions in the battery cause a build up of electrons at the anode. This results in an electrical difference between the anode and the cathode.
In a battery, the only place to go is to the cathode. But, the electrolyte keeps the electrons from going straight from the anode to the cathode within the battery. When the circuit is closed (a wire connects the cathode and the anode) the electrons will be able to get to the cathode. In the picture above, the electrons go through the wire, lighting the light bulb along the way. This is one way of describing how electrical potential causes electrons to flow through the circuit.