Bit Rate vs Audio Quality

In general, for a certain audio encoding scheme, the subjective audio quality as measured from listening tests of the encoded sound when compared to the original sound on a CD will increase when the bit rate is increased. There is a law of diminishing returns however because when the subjective audio quality approaches ‘near CD quality’ then using a higher bit rate will not gain a great deal. At the lower bit rates though the subjective audio quality is far lower and improves quickly as the bit rate is increased. The following table shows the subjective audio quality results for a test carried out on the MP2 encoder as is used for DAB:

AAC = Advanced Audio Coding, PAC = Lucent PAC (Sirius Radio in the US uses this), LII = MP2, LIII = MP3

The above figure is from the paper: "Subjective Evaluation of State-of-the-Art 2-Channel Audio Codecs", by Soulodre, Grusec, Lavoie and Libault.

In the table above the ‘subjective diff-grade’ is the measure of how different the sound is in comparison to the original where a diff-grade of zero means that the encoded signal is indistinguishable from the original. A diff-grade of zero is classed as CD-quality and small diff-grades such as those for 192kbps and higher bit rates are classed as ‘near CD-quality’. As can be seen from the table, 128kbps scores far worse than the higher bit rates and this cannot be classed as ‘near CD-quality’. The following table shows the relationship between diff-grades and subjective audio quality:

So from the above table, the 128 kbps bit rate for MP2 has been classified as "annoying". Unfortunately this is the bit rate that is most used by 98% of music stations on DAB in the UK!

The reason why the stations use this bit rate are that there are too many stations crammed into the multiplexes. The Radio Authority (now Ofcom) who govern the licensing of commercial radio stations have a set of guidelines that the commercial stations must abide by. In these guidelines the minimum bit rate for stations that play music is given as 128 kbps. What seems to be happening is a similar situation to digital satellite TV where more and more channels appear but the quality of programming is reduced. Here, it is more the case of the audio quality being sacrificed so as to provide the maximum number of stations that can fit into the multiplexes that are available. 

The BBC are the latest broadcaster to succumb to the ‘content is king’ theory. Before Christmas 2001, Radios 1-4 were all being transmitted at 192 kbps. Because of the plans to start transmitting 4 new stations in 2002 (6 Music, 1xtra, The Asian Network and an Network Z which is a temporary name for a light-entertainment station transmitting repeat comedy programmes and similar material) the bit rates of Radio 1, 2 & 4 have been reduced to 128 kbps and 6 Music that started in March 2002 also started at this bit rate. As 3 of the stations have not yet started transmitting more reductions in bit rates are inevitable on the BBC multiplex. Predictions are that Radios 1 & 2 will be set permanently to 128 kbps, Radio 4 will be reduced from 128 kbps and Radio 3 will be reduced from 192 kbps to 160 kbps. Radio 3 requires a higher bit rate because its signal has a higher dynamic range (ratio of the largest amplitude of the signal to the smallest amplitude) and is more difficult to encode well as a result. 

Therefore, because of these low bit rates and because there is no opportunity for DAB to be allocated any additional spectrum so as to add new multiplexes until 2007, this situation is set to continue until then. Even when new spectrum is allocated to DAB there is no guarantee that the same situation as at the moment where the multiplexes are overcrowded won’t happen again. 

At present, the situation with regards to the audio quality on DAB is very mixed. Listeners to Radio 3, Radio 4, and those that listen to AM stations are usually happy with the audio quality while listeners to other stations such as Radios 1 & 2 and the commercial stations, are generally very unhappy.