DMB is inefficient compared to DAB+

19th July 2007

Some of the French broadcasters want to use DMB when they launch digital radio next year rather than DAB+, but some figures published in an EBU Technical Review article about DAB+ and DMB show that DMB wastes a large amount of the capacity on overhead compared to DAB+, which would leave a far lower bit rate available for the audio, or the broadcasters would have to spend the extra money on transmission costs.

The figures from the EBU Technical Review document are summarised in Figure 25 on page 15, some of which are copied into the following table along with the bit rate that will be left over after accounting for the overhead :

Sub-channel bit rate kbps	DMB percentage overhead	DMB bit rate available for audio¹ kbps	DAB+ bit rate available for audio² kbps	Percentage increase in audio bit rate for DAB+ relative to DMB
32	42.24	18.5	27.8	50.3
40	35.76	25.7	35.1	36.6
48	31.43	32.9	42.5	29.2
56	28.34	40.1	49.8	24.2
64	26.02	47.3	57.1	20.7
72	24.22	54.6	64.5	18.1

1 - calculated using the formula: bit rate = sub-channel bit rate x (1 - (percentage overhead / 100))

2 - figures copied from DAB+ specification Table E1 (ETSI TS 102 563)

It would seem that the French broadcasters have overlooked just how inefficient DMB is -- and therefore how expensive it is to transmit -- for carrying digital radio stations in comparison to DAB+.

Basically, this is yet another reason why it is such an unbelievable decision that the French want to use DMB for digital radio when DAB+ has just been designed specifically to solve the efficiency problems with DAB -- which is why they were against using DAB in the first place. The only reason I've been able to ascertain as to why they want to use DMB is that it allows BIFS interactive graphics streams to be transmitted alongside the radio stations, but again, I think they seem to have overlooked the fact that BIFS graphics streams can also be transmitted on DAB and DAB+.

Ultimately, DMB wasn't designed to carry digital radio whereas DAB+ is specifically designed to carry radio stations, and it is a botch to carry radio on DMB, which is why there's all this overhead (the overhead is there primarily to allow video to be synchronised with the audio, because DMB is a mobile TV system, not a radio system), and it is also missing some features that DAB+ provides, such as the following that has been copied from a WorldDMB document:

"DMB is based on MPEG audio/video standards and is adapted to DAB. In addition to audio and video, it is also possible to provide additional multimedia information. DMB is designed and optimised for mobile television, but it is not recommend for radio services.

One consideration is that DMB is missing some features which are expected for radio services e.g. PAD, DLS, service following etc.

Also, since DMB is based on MPEG audio/video standards it inherits some of the overhead needed to manage and synchronize audio and video (and possibly also multimedia) streams. This overhead is relatively low for the high bit rates that would be used for a mobile television channel, but the overhead becomes significant for the low bit rates that would be used for a radio service.

In contrast, DAB+ is optimised for radio services (including PAD information) and thus has a much lower overhead (see figure).

At the moment (beginning of 2007), most DMB receivers also expect a video component before they start decoding the audio. However in the future, devices may no longer have this restriction. Therefore, in addition to the audio, at the moment it would be necessary to provide a video component (picture radio) at a high enough frame rate when using DMB. A low frame rate causes a high delay when tuning to the service.

All the functionality available for DAB services is also available for DAB+, but it is not available for DMB (it is not needed for a mobile television service): services following (e.g., to FM, AM, DRM or another DAB radio service), traffic announcements, PAD multimedia (dynamic labels such as title artist information or news headlines; still images such as weather charts or CD covers, etc.), service language and programme type information etc."

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