Channel 4 proposes to use DAB+ on new national commercial multiplex

The applications for the new national DAB multiplex are available on the Ofcom website, and Channel 4 is proposing to broadcast a podcast service using "the DAB+ format" -- i.e. encoded using AAC+ -- from the outset. The following quotes are taken from the 4 Digital bid:

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DAB radio owners have no grounds for complaint about the introduction of podcasts using the DAB+ format, because there are no DAB receivers that can receive podcasts at present, so which audio codec is used to encode them is entirely irrelevant. 

However, as new receivers would need to be purchased in order to receive podcasts then it makes perfect sense to encode them using AAC+, both because far more podcasts can be made available (if MP2 were used, the podcast service Channel 4 is proposing wouldn't even be viable, because there would be too few available), and it would provide an incentive for people to buy DAB+ receivers, which would speed up the migration to DAB+ so that listeners and broadcasters can take advantage of the benefits that using DAB+ will offer sooner.

The quote about all kitchen DAB radios in the shops supporting DAB+ within the next 24 months clearly vindicates what this website has been saying about the availability of DAB+ receivers. It should also be said that if portable radios support DAB+ then more expensive receivers should also support DAB+, such as micro systems, hi-fi separate tuners and car stereos. However, even if Channel 4 isn't awarded the licence, the vast majority of DAB receivers will support DAB+ within 24 months anyway.

The downside of Channel 4's bid: 112 kbps MP2

The downside of C4's bid is that they're proposing to use 112 kbps MP2 for the stereo stations. This is a disgrace, and I will return to this issue in a few days' time. One saving grace, however, is that as Channel 4 clearly prefers a rapid migration to DAB+ then the very low audio quality 112 kbps provides should be relatively short-lived. 

How will DAB+ be better than DAB?

The following is a short summary of what DAB+ should provide in comparison to DAB:

1.  Far more stations

DAB+ is 3 - 4 times as efficient as DAB, which means that it will be able to carry far more radio stations than on DAB -- up to 3 - 4 times as many. 

2.  Far higher audio quality overall

Some sceptics will tell you that DAB+'s vastly superior efficiency will just result in lots and lots more stations being carried and there being no improvement in the audio quality. But these people are ignoring the BBC's DAB multiplex, because the BBC is not expected to launch any new stations. 

Furthermore, the BBC has said on a number of occasions that it would like to improve the audio quality of its stations, so the following table shows what the BBC DAB multiplex could look like once they've switched to DAB+:

1 - the bit rate includes the bit rate for the MPEG Surround stream data, which is around 3 - 4 kbps

2 - AAC (LC-AAC) is used for higher bit rates and AAC+ (HE-AAC -- High-Efficiency AAC) is used at lower bit rates, but is ideal for stereo speech

I've assumed  they'll use error protection level PL4A

To summarise:

• All of the BBC's music stations plus Radio 4 could be available at audio quality virtually indistinguishable from CD, and therefore the audio quality would be vastly superior to what it is today;
• All of the BBC's music stations plus Radio 4 and BBC7 could have surround-sound streams added using the new ultra-efficient and backwardly-compatible MPEG Surround format;
• All of the BBC's speech stations could be available in stereo and at high quality;
• There would be approximately 1/3rd of the BBC's DAB multiplex left over for other data services to use.

I would also expect that most of the commercial radio stations will choose to provide higher audio quality than they currently provide simply because it is difficult to imagine how any compnay could be so amazingly tight-fisted as to not improve the audio quality considering how little it will cost to transmit at higher quality. 

3.  Better reception quality

DAB+ adds Reed-Solomon error correction coding, which will provide far more robust reception than at present.

4.  MPEG Surround

As well as DAB adopting the AAC+ audio codec, the MPEG Surround format has also been adopted, which, as the name suggests, allows surround sound to be transmitted.

MPEG Surround uses a very low bit rate stream (about 3 - 4 kbps) to carry the surround sound information, and tests have apparently shown that it performs well — it apparently performs in between the high quality discrete multi-channel and the lower quality matrix-based surround sound formats, which is pretty amazing if true given that the bit rate is only 3 - 4 kbps.