What are Capacity Units?
Necessary prior knowledge: Transmission Frame, MSC, and CIF
Capacity Units indicate the capacity used by a radio station on the DAB(+) ensemble MUX.
Let us start with the CIF frame with 55296 bits every 24 ms.
The CU corresponds to a quantity of 64 bits.
It is then easy to calculate the number of CU’s in one CIF frame.
55296 (bits in one CIF) / 64 (bits in one CU) = 864 CU’s (in one CIF of 24 ms)
So 864 CU containers are repeated every 24ms. 4 of these are transmitted per 96 ms (transmission frame)
Consecutive CU’s will contain the audio bits and the error correction of one radio station.
The error code is a convolution code that entangles audio bits and error bits with each other. The final number of bits is the audio bit rate divided by the error correction factor.
One can now easily calculate the number of CUs for an audio sub-channel in the MUX based on the audio bit rate and the error code.
- Take the bit rate of the audio signal and divide it by the error correction factor. Consider the station bit rate at 96kbps and EEP-3A (factor ½), then give 96 kbps divided by ½ or 192 kbps total bits in the audio (sub) channel.
- We transform this bitrate from 1 second to 0.024s (24ms).
192 kbps x 0,024 = 4608 kbit / 24 ms
- We divide the number of kbits per 24ms obtained by 0.064 kb (64-bit) to get the number of CUs.
4608 kbit / 24ms divided by 0.064kb (64 bit) gives 72 CUs for a 96kbps AAC audio bitrate and error correction EEP-3A
It is, therefore, important to memorize that one CU is 64 bits in size and calculated per 24 ms.
We want to provide the following conversion table for CUs. Remember that the maximum number of CUs in a CIF is 864 (this corresponds also to the total MUX capacity). Also, remember that the transmission area will decrease slightly from EEP-1A to EEP-4A while the number of stations that fit in the MUX will increase.
|48 kbits/s||72 CU’s||48 CU’s||36 CU’s||24 CU’s|
|56 kbits/s||84 CU’s||56 CU’s||42 CU’s||28 CU’s|
|64 kbits/s||96 CU’s||64 CU’s||48 CU’s||32 CU’s|
|72 kbits/s||108 CU’s||72 CU’s||54 CU’s||36 CU’s|
|80 kbits/s||120 CU’s||80 CU’s||60 CU’s||40 CU’s|
|88 kbits/s||132 CU’s||88 CU’s||66 CU’s||44 CU’s|
|96 kbits/s||144 CU’s||96 CU’s||72 CU’s||48 CU’s|
For “Equal Error Protection (EEP),” two options, A and B, are available. Each option consists of 4 different protection levels ranging from 1 to 4.
Option A assumes a division of the audio sub-channels in multiples of 8kbps. These formats give the maximum flexibility for audio channel layout used by DAB(+) providers.
Option B assumes a division of the sub-channels in multiples of 32kbps and is less flexible for audio services. This option is to use with DMB video services.
For example, an audio channel of 80kbps is impossible with option B because 80 is not a multiple of 32!
So option B will only allow 32kbps, 64kbps, and 96kbps as audio bit rates!
Assume that option B is not useful for DAB+ and that mixing options A and B in the same MUX is not an option when rolling out DAB+.