Digital Audio Broadcasting - Sound Quality

Sound Quality

See also: MP2 Quality

The original objectives of converting to digital transmission were to enable higher fidelity, more stations and more resistance to noise, co-channel interference and multipath than in analogue FM radio. However, the leading countries in implementing DAB on stereo radio stations use compression to such a degree that it produces lower sound quality than that received from non-mobile FM broadcasts. This is because of the bit rate levels being too low for the MPEG Layer 2 audio codec to provide high fidelity audio quality.

The BBC Research & Development department states that at least 192 kbit/s is necessary for a high fidelity stereo broadcast :

“ A value of 256 kbit/s has been judged to provide a high quality stereo broadcast signal. However, a small reduction, to 224 kbit/s is often adequate, and in some cases it may be possible to accept a further reduction to 192 kbit/s, especially if redundancy in the stereo signal is exploited by a process of 'joint stereo' encoding (i.e. some sounds appearing at the centre of the stereo image need not be sent twice). At 192 kbit/s, it is relatively easy to hear imperfections in critical audio material. ”
— BBC R&D White Paper WHP 061 June 2003

When BBC in July 2006 reduced the bit-rate of transmission of Radio 3 from 192 kbit/s to 160 kbit/s, the resulting degradation of audio quality prompted a number of complaints to the Corporation. BBC later announced that following this testing of new equipment, it would resume the previous practice of transmitting Radio 3 at 192 kbit/s whenever there were no other demands on bandwidth.

Despite the above a survey of DAB listeners (including mobile) has shown most find DAB to have equal or better sound quality than FM.

Notwithstanding the above, BBC Radio 4 has extended the periods it broadcasts programmes with a lower bit rate (80kbit/s) and in mono in 2012, rather than 128kbit/s and in stereo. Programmes which had traditionally been broadcast on BBC Radio 4 DAB in stereo (from 1999 to 2011), can now only be heard in the evenings in mono, even though the same programnmes still go out in stereo on Radio 4 FM, Digital TV and On-Line. The BBC have issued a statement stating that stereo is still their default for BBC Radio 4 DAB, however post the Olympics, this does not appear to be the case in the evenings, making FM broadcasts (in good reception areas) superior. As very few car radios are currently fitted with DAB if the BBC switch FM off as indicated later in the decade, some listeners may be forced to receive mono broadcasts in the future, a somewhat backward step.

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