Future Digital Norfolk

DAB and DAB+ services use up their data capacity for a variety of reasons.

Transport Layer
Firstly, a limited amount of data is required to provide a framework for the carriage of the various broadcast services transmitted on the multiplex.

Programme Layer
Secondly, a larger amount of data is used to deliver the various station’s programme audio content, which, together with programme related and non-programme related text (as well as still images where broadcast) is encoded for transmission within the DAB multiplex.

Error Protection Layer
Thirdly, each multiplex uses additional data to make the transmission of each broadcast service robust against interference, using what is known as “error protection”. In the UK, three levels of error protection are available for both DAB and DAB+ services. These error protection levels are known as UEP 3, UEP 2 & UEP 1 for DAB services and as EEP 3A, EEP 2A & EEP 1A for DAB+ services. The original international DAB and DAB+ standards also include further (weaker) levels of error protection (UEP 5 & UEP 4 / EEP 5A & EEP 4A respectively), but these are not permitted for use in the United Kingdom. Just as each individual programme service can operate at its own individual bit-rate, each programme service can also operate using its own individual level of error protection.

The trade-off is between programme data rates, error protection levels and the total number of services carried. Higher audio quality services and / or services with higher error protection levels each need a greater total amount of data when compared to lower audio quality services and / or services using a lower level of error protection.

The standard (lowest) level of error protection which must be used is UEP 3 (or EEP 3A for DAB+ services). Instead of using UEP 3 / EEP 3A, individual programme services on a multiplex can make their signals more robust by using either UEP 2 or UEP 1 (EEP 2A or EEP 1A for DAB+ services). However, using increased levels of error protection requires the use of additional amounts of data. In other words, if higher levels of error protection are used less data capacity remains available for other uses. Therefore, as the following table explains, using higher levels of error protection means that, because it has a finite and fixed data capacity, the overall multiplex is then only able to carry a smaller total number of programme services.

NOTES:
The slight differences in the number of capacity units required for some bitrates using DAB or DAB+ can be accounted for by differences in block size. However, the key point is that the bit rate required for a given perceived degree of audio quality is substantially reduced. According to WorldDMB, the DAB+ standard “allows for up to three times as many services per multiplex” such that “[a] 40 kbps sub-channel with HE-AAC v2 provides a similar audio quality (even slightly better in most cases) as MPEG Audio Layer II at 128 kbps” (WorldDMB: 2012: 1).

The enhanced efficiency of the revised standard is reflected in the capacity requirements of receiver design specifications, for example, as set out by the UK government in its ‘Minimum Specifications For DAB & DAB+ In-Vehicle Digital Radio Receivers & Adaptors‘: A receiver must be able to decode a DAB audio service contained in a sub-channel of a size up to and including 280 Capacity Units (e.g. 256 kbps@UEP1)”. DAB audio services are defined in ETSI EN 300 401.A receiver must be able to decode a DAB+ audio service contained in a sub-channel of a size up to and including 144 Capacity Units (e.g. 96 kbps@EEP1A). DAB+ audio services are defined in ETSI TS 102 563 (DCMS: 2013: 13).