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*Data services share 32kbps channel (all BBC stations are sampled at 48kHz)
The above bit rates are completely adequate for the type of programme material. For example, speech programmes don't need 192kbps because speech can be compressed into far fewer bits than music can be and this is why Radio 5 Live and Sports Extra and the World Service are transmitted at lower bit rates. At the above rates their audio quality will be far superior to that which can be produced on AM. The main Radios 1-4 stations are at the bit rate that DAB was designed to use. At these bit rates the audio quality was excellent. A multiplex is not configured to fit into a maximum bit rate as such but must ensure that there are no more than 864 capacity units (CU) transmitted. There are different error protection levels which allow the total bit rate to be increased or decreased but the 864 CU value must not be exceeded. The DAB standard changes the number of CUs that are required for a given bit rate. The table above is for a protection level (PL) of 3. The BBC could change to a protection level of 4 which would increase the maximum bit rate to increase from 1184 kbps to 1424 kbps but increasing the protection level (which is a confusing term because this actually means using a weaker error correction code rate) requires the signal to noise ratio (SNR) at the receivers to increase. They can't turn the power up at the transmitters because that will cause increased interference to other services so to change to PL4 would require the coverage area to be reduced. They will not do this at present because their main aim will be to increase the coverage until they have achieved say 95% or 99% population coverage. So, at least until the BBC have either rolled-out their network across the whole of the UK or the BBC get another multiplex (2007 is the earliest possible time for this) they will use PL3 which has a maximum bit rate of 1184 kbps, although this is dependent on the CU values of the different streams. The following table gives the number of capacity units for protection levels 3 and 4:
Current BBC DAB Bit RatesJust before Christmas 2001 the BBC reduced the bit rates of the stations on their multiplex. In March 2002 BBC 6 Music began transmitting, in June 2002 1Xtra began, and in October 2002 the Asian Network and BBC 7 began transmitting. The present BBC bit rates (November 2002) are as follows:
*part-time service, which when transmitting requires Radio 3, Radio 4 and Radio 5 Live to transmit at the lower bit rates listed depending on what content is being broadcast on the other stations. Radio 4 LW also requires the bit rates to be reduced when it is transmitting. Here's an excerpt from What HiFi magazine April 2001 concerning the bit rates on the BBC's DAB multiplex:
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Uproar as BBC Cuts Digital Radio Quality Digital radio users are fuming at the BBC's decision to reduce the bit rate of its Radio 1, 2 and 4 digital broadcasts. Only Radio 3 is still transmitted at the highest-quality rate of 192kbps, with Radio 1, 2 and 4 reduced to 128kbps. BBC Director of Radio Jenny Abramsky says: "We're launching five new digital stations this year, and to do this we have to allocate the bit rate for other services. We will have to change the bit rate of some of our national services, but we will not drop as low as some of our independent rivals, and Radio 3 will stay at 192kbps." Bit rates vary widely across the digital network. For example, Classic FM transmits at 160kbps and Ministry of Sound at 128kbps. The first of the new BBC digital services, 5 Live Extra, went on air on 2 February, with 6 Music - playing tunes from '70s punk to contemporary indie - going live later this month. My Comment on what Jenny Abramsky said: She says that "we will not drop as low as some of our independent rivals". If you look at the music stations on the Commercial Bit Rates page then you can see that there is actually one station that is lower than the BBC music stations and that is Prime Time radio transmitted at 96kbps mono. The majority of music stations are at 128kbps which is the same as the BBC stations. Her statement is therefore technically correct, but in reality it is totally false. Since the Asian Network and BBC 7 have started transmitting Jenny Abramsky's comments regarding the bit rate of Radio 3 always being at 192 kbps has proved to be false as Radio 3 has been transmitted both at 128 kbps and at 160 kbps when Radio 5 Sports Extra or Radio 4 LW or both have been transmitting. Also Radio 4 has been reduced for long periods to 64 kbps mono and Radio 5 has also seen its bit rate cut to 64 kbps for long periods. Secondly, the promise that Radio 3 will always remain at 192kbps is too biased towards Radio 3 (and someone at the BBC has confirmed that Radio 3 will not *always* be transmitted at 192 kbps). The following table shows the number of listeners of the 5 main BBC stations on analogue:
Source RAJAR Quarterly Summary
Period Ending September 2004 Ofcom is the regulator of the commercial stations on DAB, and their recommended minimum bit rates are as follows:
(same colour coding as used above, green stands for mixed speech, drama and music, lilac stands for data) Virtually all stations use the 48 kHz sampling rate and the rest use a 24 kHz sampling rate (LSF = Low Sampling Rate). Because of Nyquist's Sampling Theorem the 24kHz sampling rate only allows an audio bandwidth of up to 12kHz to be used, which is only adequate for speech services because music requires a wider audio bandwidth (about 16kHz or higher). As you can see by comparing the 2 tables above that the BBC stations are mainly using their respective minimum bit rates recommended by the Radio Authority with the odd exception such as Radio 3 which uses a higher bit rate because classical music is deemed by the BBC to be more worthy of a higher bit rate than their other music stations..
Proposed BBC DAB Bit RatesThe BBC could re-configure their DAB multiplex without removing any radio stations to provide a fairer allocation of capacity to their radio stations. The main station that sticks out like a soar thumb is Radio 3, which uses 192kbps, which is 50% higher than all of the other BBC music stations on DAB, and yet classical music isn't as difficult to encode as other forms of music. Radio 3 fans object to the bit rate of their station being reduced on DAB (for obvious reasons), but Radio 3 is available at 192kbps on Freeview and digital satellite (and probably digital cable too), so they can still receive Radio 3 at that bit rate. More importantly, though, classical music is actually easier to encode than the vast majority of music played on Radio 1, Radio 2, 6 Music and 1Xtra, mainly as a result of Radio 3 not using dynamic range compression (which makes Radio 3 easier to encode due to the psychoacoustic masking involved in MPEG encoding). At the moment, Radio 3 is basically allocated a higher bit rate solely out of BBC snobbery. The only reason they give for using a bit rate 50% higher than the other music stations is that they say that more people actually listen to the music on Radio 3, whereas they don't really listen to the music on the other music stations. Apart from that being extremely condescending, it's very unlikely to be true. For example, if you just take the number of hours of listening to Radios 1, 2 and 3 from the table above, the total for Radios 1 & 2 is 263,874 hours, which is 22 times the number of hours people listen to Radio 3. The other service on the BBC DAB multiplex that I propose should have its bit rate reduced are the BBC data services, which almost nobody receives, so for the cost of slowing down the downloading speed of very, very few people, this frees-up bandwidth that is, to say the least, at a premium on the BBC DAB multiplex. The proposed bit rates are in the following table:
* part-time stations, only requires bit rate when on-air So, at the expense of reducing the bit rate of Radio 3 from 192kbps to 160kbps and the bit rate of the data services from 32kbps to 4kbps, Radios 1 & 2 -- the 2 BBC stations that most people listen to -- can increase their bit rates from the very poor 128kbps, and BBC7 can have its bit rate increased from 80kbps mono to 96kbps, which will allow the transmission to be in stereo, so long as the sampling rate is changed from 48kHz to 24kHz (which doesn't affect the listeners). None of this gets away from the fact that the BBC should acquire more bandwidth for their DAB services, so that non-ideal proposals such as the one above would not be necessary.
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