DAB+ broadcast is nothing more than the digital transmission of bits between the transmitter and a receiver.
The transmitter has a bit rate of 2400 kbps.
Of these 2400 kbps, 96 kbps or (4%) is used for fast information (Fast Information). It can best be compared to RDS on FM, although it is slightly more complex.
The receiver also needs this information to know the composition of the MUX. Without this information, the receiver cannot de-MUX the MUX!
It is therefore important that the receiver receives this information quickly. That is why no time interleaving is applied to the data here. This ensures that the information is quickly available to the receiver.
This 96 kbps is therefore referred to as the FIC (Fast Information Channel).
The remaining 2304 kbps are used for the audio (and the associated error correction).
How are the bits modulated ?
The modulation used is phase modulation, with a period of 1 ms during which the phase state changes.
This length was chosen to be long enough to prevent phase distortion in the transmitted radio waves (a major problem in FM transmissions).
Due to the fact that symbols are modulated at a rate of 1 ms, only 1,000 symbols per second can be modulated. To achieve 2,400 kbps, a solution must therefore be found -> multiple carrier waves -> MODE I -> 1,536 carriers!
How is it transmitted ?
The transmitter transmits successive frames using the 1536 carrier waves. In DAB, we refer to the complete frame as the transmission frame. This transmitted frame lasts exactly 96 ms and is sent into the air synchronously, one after the other.
As modulated symbols, we use 4 different phase jumps across all 1536 carriers in phase modulation. Four phase shifts correspond to two bits (00, 01, 10, 11), resulting in 2×1536 = 3072 bits per 1 ms (or a gross capacity of 3072 kbps).
This does not correspond to the actual capacity of 2400 kbps. The reason is that the frame also has certain waiting times and is therefore not used 100% for the transmission of data bits.
The transmission frame, which spans 1536 carriers in DAB mode I, is divided into 77 fractions.
- The first fraction is the NULL symbol, which is a short pause during which nothing is transmitted and which the receiver uses to synchronise with the received frames.
- The phase synchronisation signal is transmitted on each carrier to indicate the phase of the carrier to which the phase decoder must align itself.
- The remaining 75 modulation cycles across the 1536 carrier.
These remaining 75 fractions contain the 2400 bits/s of information.
Time-slot 3 until 6 is reserved fort he FIC (Fast Information Channel) (96 kbps).
It is easy to calculate that these 3 time slots actually contain 3 times 3072 bits (3072 bits over the 1536 carriers, which represent 2 bits per phase).
3 x 3072 = 9216 bits. The transmission frame lasts 96 ms and 9216 bits/0.096 = 96000 or 96 kbps. So the calculation is correct for the FIC!
Then there are another 72 of these time slots on which modulation takes place.
To get straight to the point, we may need to divide these into 4 times 18 time slots (72 in total). The reason for this is that audio frames in DAB are sent every 24 ms. Given that the frame lasts 96 ms, four containers with 24 ms audio frames can be sent. These containers are called CIFs (Common Interleaved Frames). Unlike the FIC, time interleaving is used here!
The entire 72 slots are called the MSC (Main Service Channel). It is divided into 4 CIFs of 24 ms.
The capacity of a CIF is therefore 18 time slots over the 1536 carrier waves with 2 bits. Calculated
1536 x 2 bits = 3072 bits x 18 slots = 55296 bits/24ms or 55296/0.024 = 2304 kbps, so the calculation is correct here too!
2304 kps for the MSC and 96 kbps for the FIC gives a total of 2400 kbps.
The above figure of 55,296 bits in a CIF later leads to the calculation of the number of CUs (64-bit pieces that are assigned to the radios on the MUX).
Namely, 55,296/64 = 864 available CUs on the MUX (note that the FEC must also be included in this calculation). If you have a stream of 96 kbps with FEC ½, you need a total of 192 kbps for your stream on the MUX. Converting 192 kbps to the time unit of 24 ms for one FIC gives 192 kbps x 0.024 = 4608 bits per CIF, or 4608/64, which is 72 CUs.