How is a DAB+ transmitter composed?
At the input of the DAB+ transmitter, a continuous digital stream of bits is offered from the (Ensemble) MUX. The bit rate (bit/s) depends on how many radios the MUX contains. The more radios, the more audio bits and the higher the bit rate at the transmitter input. This can range from 114 kbit/s for one radio station (of 64kbit/s) to almost 1217 kbit/s for 12 radiostatios (of 96kbps) or 18 stations (of 64kbit/s). So the bitstream speed at the input of a DAB+ station does not have a fixed bitrate at its input, it depends on the number of radio stations and their audio bitrate. This transmission between (Ensemble)-MUX and transmitter can be done through different connections and will be discussed in detail later.
The first thing the transmitter does with this bitstream is encode the bitstream. This encoding has nothing to do with the audio encoding (HE-AAC). In fact, the audio is already encoded in HE-AAC v2 for the input of the MUX.
The whole encryption method aims to encrypt the stream coming from the MUX in various ways and add a number of bits that also allow the receiver to fix errors. This encryption is actually standard in digital transmissions and aims to make the digital transmission more reliable.
The DAB+ transmitter will handle the encrypted stream in separate groups of 3072 consecutive bits. Remember that due to the added bits for convolutional error correction, there will now actually be more bits than the number of bits received from the MUX. The groups of 3072 consecutive bits, will be offered per two consecutive bits to 1536 carriers (2 bits per carrier). The transmitter will transmit these 3072 bits for 1 ms.
The OFDM modulation method was invented as early as the mid-1960s. Until the 1980s, it was quasi-impossible to realise this method practically with the electronics that existed at the time. The precision of each modulator and its carrier was difficult to achieve in practice and so the system was rather limited to about 30 carriers and mostly in lab environment. It was also economically unthinkable to fabricate such electronic circuits for mass production. The solution came only with the emergence of faster signal processors. The emergence of these processors, at market-based prices, made OFDM modulation possible on a large scale.
In groups of 3072 bits, a DSP (processor) is going to compose the spectrum: 1536 carrier waves modulated by 2 bits each. The speed at which this happens is 1 thousandth of a second (1 ms). This is necessary because closely spaced carrier waves (at 1kHz distance) would not interfere with each other. The rule is simple: the distance between the carrier waves must be exactly the modulation frequency, 1kHz distance corresponds to a period of 1/1000 second. The carrier waves are said to be Orthogonal (the “O” in OFDM). The result of the calculations results in two time signals (the I-signal and Q-signal). This time signal is calculated by applying the “Inverse fast (Fast) Fourier Transform (IFFT)” algorithm.
The question then arises: why two signals I and Q (or Q/I signal)? I/Q stands for the I signal (In faze) and Q signal (In Quadrature = 90° shifted) on the one hand. Reason is that the baseband signal has to be shifted to the final DAB+ frequency. If you apply the same signal to the I-input and Q-input of an I/Q modulator, you get a double sideband signal (twice the baseband signal in mirror image with suppressed carrier wave). However, if you put different signals at the input of the UP-CONVERTER, such as the I-output and Q-output of the OFDM-MODULATOR, then you are only going to generate one sideband. In other words, you are shifting the baseband signal to a higher frequency, and that is precisely the intention of the up-converter. The I/Q modulator plays a major role in digital modulations and thus certainly also in DAB+. We will try, as soon as possible, to explain the operation of this modulator in detail on our webpage.
After a DAC (digital Analogue Converter), the analogue baseband I/Q signals go back to an I/Q modulator to convert them to the transmission frequency (shift in the frequency spectrum).
Now this signal only needs to be amplified and fed through a filter to the transmitting antenna.