Deployed since the 90s in Europe, DAB underwent a technical evolution in 2006 with DAB+ by integrating the HE-AAC V2 compression codec, offering superior sound quality. However, the sound quality depends on the compression ratio : the lower it is, the more radios can be played. In France, the compression ratio is 80 kbit/s, which is equivalent to that of FM.
DAB/DAB+ : advantages

Compared to FM radio, DAB+ has several advantages :

• Widest choice of stations
  


• Ease of use : Stations are listed alphabetically and only appear when available
  


• No interference between radios
  


• Continuous listening in the car without changing frequency
  


• Better sound quality : the digital signal is louder and therefore picks up less extraneous noise
  


• Display of information relating to the program being listened to (broadcast title, scrolling text, album cover, weather map... depending on the characteristics of the receiver)
  


• Energy saving (60% less than FM)
  

On the other hand, reception is less good inside buildings; it is therefore advi

DVI

The \Digital Visual Interface\ (DVI) or Digital Video Interface was invented by the Digital Display Working Group (DDWG). It is a digital connection used to connect a graphics card to a screen.
It is advantageous (compared to VGA) on the screens where the pixels are physically separated. The DVI connection so significantly improves the quality of the display to the VGA connection with :

sable to keep an FM station in the house.

The DAB standard allows the digital broadcasting of radio programmes, via terrestrial or satellite airwaves. In good reception conditions, the quality is similar to that of digital music players or audio CD

Operation

players. However, depending on the compression ratio, the quality differs. A report by the CSA4 indicates that with the compression ratio and the 80 kbit/s rate expected in France, the quality is only equivalent to that of FM5.

Each programme can be accompanied by information such as its name, the title of the programmes or songs broadcast on the air, and possibly even additional images and data. A suitable receiver must be used : traditional analogue AM and/or FM radio receivers cannot decode DAB5 digital data.

Compared to FM radio, DAB offers a number of advantages to its listeners :

• absence of background noise ("hiss") due to average reception or disturbances
  


• Ability to stream more stations
  


• Fully automatic station list by the receiver
  


• data associated with programs potentially richer than those offered by the RDS : texts, images, various information, websites
  


• robustness to disturbances when used in mobile reception (car, train) including at high speed.
  

• Audio encoding :
  Audio content is typically encoded using codecs such as MPEG-4 HE-AAC v2 (High Efficiency Advanced Audio Coding version 2). This codec offers excellent audio quality at relatively low bitrates, which is ideal for digital streaming.
  


• Multiplexing :
  Multiplexing is the process of combining multiple data streams into a single composite data stream. In the case of DAB+, the audio data and associated metadata (such as station name, song title, etc.) are multiplexed together into a single data stream.
  


• Encapsulation :
  Once the audio data and metadata are multiplexed, they are encapsulated in a DAB+-specific format for broadcasting. This format includes timing information, error correction information, and other data necessary for efficient and reliable signal transmission.
  


• Modulation :
  The encapsulated signal is then modulated to be transmitted over a specific frequency band. DAB+ typically uses OFDM (Orthogonal Frequency Division Multiplexing) modulation, which splits the signal into multiple orthogonal subcarriers. This allows for efficient use of bandwidth and better resistance to interference.
  


• Transmission :
  Once modulated, the signal is transmitted by the broadcasting transmitters via special antennas. These antennas broadcast the signal in a specific coverage area.
  


• Bandwidth Management :
  DAB+ uses techniques such as dynamic bandwidth compression to adapt to transmission channel conditions and maximize spectral efficiency. This makes it possible to optimise the use of the available radio spectrum.
  robustness to disturbances when used in mobile reception (car, train) including at high speed.
  

• Antenna :
  To receive DAB+ signals, a receiver must be equipped with a suitable antenna. This antenna can be integrated into the receiver or external, depending on the device. It is designed to receive radio waves broadcast by DAB+ transmitters.
  


• Signal reception :
  Once the antenna picks up the DAB+ signals, the receiver processes them to extract the digital data. DAB+ receivers can be dedicated stand-alone devices, modules integrated into radios or reception systems in vehicles.
  


• Demodulation :
  Demodulation is the process by which the receiver converts the picked up radio signal into a form that can be used to extract digital data. For DAB+, this usually involves decoding the OFDM (Orthogonal Frequency Division Multiplexing) modulation used for transmission.
  


• Error detection and correction :
  The receiver also performs error detection and correction operations to ensure that data is received accurately. Techniques such as cyclic redundancy coding (CRC) are used to verify data integrity and correct for possible transmission errors.
  


• Data decoding :
  Once the digital data has been demodulated and errors corrected, the receiver can extract the audio data and associated metadata from the DAB+ data stream. This data is then processed to be reproduced as sound or displayed to the user, depending on the type of receiver and its functionality.
  


• Conversion to audio signal :
  Finally, the audio data is converted into an analog audio signal to be played back by the speakers or headphones connected to the receiver. This conversion can involve steps such as audio codec decoding (such as MPEG-4 HE-AAC v2) and digital-to-analog conversion (DAC).
  

Four modes of transmission are defined, numbered from I to IV :

- Mode I, for Band III, terrestrial
- Mode II for L-Band, terrestrial and satellite
- Mode III for frequencies below 3 GHz, terrestrial and satellite
- Mode IV for L-Band, terrestrial and satellite

The modulation used is DQPSK with the OFDM process, which provides good immunity to attenuation and inter-symbol interference caused by multipaths.

In Mode I, the OFDM modulation consists of 1,536 carriers. The useful period of an OFDM symbol is 1 ms, so each OFDM carrier occupies a 1 kHz wide band. A multiplex occupies a total bandwidth of 1.536 MHz, which is one-quarter of the bandwidth of an analogue television transmitter. The guard interval is 246 μs, so the total duration of a symbol is 1.246 ms. The duration of the guard interval determines the maximum distance between transmitters that are part of the same single-frequency network, in this case about 74 km.

The speed available in a multiplex is divided into "services" of several types :

- primary services : the main radio stations;
- Secondary services : e.g., additional sports commentary;
- Data services : program guide, slideshows synchronized with shows, web pages and images, etc.