A standard DAB MUX (multiplex) provides a maximum of 864 Capacity Units (CU). Each audio programme service uses some CUs to deliver audio and additional CUs for error-correction (information to help the receiver decode the audio information provided). Tests have shown that some DAB receivers (in particular some early designs) do not always function properly when receiving DAB programmes using more than a total of 140 CU. FDN DAB services are therefore limited to bit rates which do not require additional CU above this 140 limit.
In the UK there are three levels of error correction available. The standard protection level is UEP3 and this is used by almost all DAB audio services in the UK. UEP2 and UEP1 provide additional error correction, but consume more CUs to achieve this. (Lower levels of protection specified in the design of DAB (UEP4 and UEP5) are not permitted for use in the UK).
In theory the use of UEP2 should be equivalent to an increase in radiated transmitter power of 1.5dB and the use of UEP1 should be the equivalent of an increase in radiated transmitter power of 3dB (for example a doubling of e.m.r.p. from, say, 100 Watts to 200 Watts). When the FDN trial MUX for Norwich first launched, we tested the use of the maximum error-correction level (UEP1) but found that, in practice, it provided little benefit in terms of improved reception, certainly nowhere near the improvement we would expect from a 3dB increase in transmitter power.
Minimum Audio Standards
FDN believes that the quality of broadcast audio on DAB is important. We have therefore decided to define some minimum bit-rates for use on our MUX and these are set out below:
- Permanent Music-Based Stereo Audio Services = 112kbps
- Permanent Music-Based Mono Audio Services = 80kbps
- Permanent Speech-Based Mono Audio Services = 64kbps
Assuming the use of UEP3 error-correction, a DAB MUX can accommodate the following numbers of audio services or a combination of various such services at different bit-rates:
Stereo Music-Based Audio Services (DAB / MP2):
- 192 kbps needs 140 capacity units per service (840 CU for 6 x services + 24 CU over)
- 160 kbps needs 116 capacity units per service (812 CU for 7 x services + 52 CU over)
- 128 kbps needs 96 capacity units per service (864 CU for 10 x services + 0 CU over)
- 112 kbps needs 84 capacity units per service (840 CU for 10 x services + 24 CU over)
Mono Music-Based Audio Services (DAB / MP2):
- 96 kbps needs 70 capacity units per service (840 CU for 12 x services + 24 CU over)
- 80 kbps needs 58 capacity units per service (812 CU for 14 x services + 52 CU over)
Mono Speech-Based Audio Services (DAB / MP2):
- 64 kbps needs 48 capacity units per service (864 CU for 18 x services + 0 CU over)
Mono Information-Based (e.g. travel or weather) / Temporary Audio Services (DAB / MP2)
– these low bit-rate levels are not normally available for audio services:
- 56 kbps needs 42 capacity units per service (840 for 20 x services + 24CU over)
- 48 kbps needs 35 capacity units per service (840 for 24 x services + 24CU over)
- 32 kbps needs 24 capacity units per service (864 for 30 x services + 0 CU over)
- Permanent music-based services (stereo) must be at least 112kbps
- Permanent music-based services (mono) must be at least 80kbps
- Permanent speech-based services (mono) must be at least 64kbps
- Data rates of under 112 kbps are NOT available for stereo services
- Data rates of under 80 kbps are NOT available for music services
- Data rates of <64kbps are available for data (IP etc.) use only.
- “Half-rate” coding which limits audio frequency response to just under 12kHz, provides no carrying capacity advantage and is not considered necessary with the implementation of MP2 coding used on our MUX.
For more detailed technical information, please see the relevant ETSI specification (Final Draft ETSI EN 300 401 V1.4.1 (2006-01) (Page 50)). (Search at: www.etsi.org/standards).
For information about DAB+, (not covered above) please see the relevant ETSI specification (ETSI TS 102 563 V1.2.1 (2010-05)) also available via the ETSI web-site.
[First Published: Friday 30th October 2015].