T-DMB vs DAB+

  

NOTE: T-DMB, as described in this article, is now referred to as DMB-Audio, or just DMB-A for short

 

France is planning on using T-DMB -- and possibly DAB+ -- to carry digital radio, so this page briefly compares T-DMB with DAB+ in terms of their suitability for this purpose.

T-DMB & DAB+

T-DMB (Terrestria-Digital Multimedia Broadcasting) was originally designed to be a mobile TV system, but as a mobile TV system has to be able to carry both audio and video, any mobile TV system can also carry radio stations, because they're just audio streams at the end of the day.

As well as mobile TV, T-DMB can also carry MPEG-4 BIFS (Binary Format for Scenes) streams, which is "MPEG-4's scene description language, designed for representing, delivering and rendering interactive and streamable rich-media services (including audio, video, 2D & 3D graphics)."

DAB+, on the other hand, has been optimised to carry radio stations, so, as I'll explain below, it doesn't need to use some of the things that are needed by T-DMB to carry video as well as audio. DAB+ can also carry graphics and still images through a data channel (for example, some of the commercial radio groups have been trialing sending 'slideshows' over DAB in London), but the difference between carrying graphics via T-DMB and via DAB+ is that on T-DMB the graphics can be displayed synchronously with the audio. 

Both T-DMB and DAB+ are upgrades of the old DAB system, and both have incorporated the following technologies:

  • AAC+ audio codec
  • Reed-Solomon error correction coding

The above two technologies makes DAB+ and T-DMB far more efficient than the old DAB system -- this means that they can carry far more radio stations and provide far higher audio quality than the old DAB system provides.

The technologies that DAB+ and T-DMB use are very similar, but there are important differences, and even WorldDMB, the organisation in charge of T-DMB, is saying that T-DMB should not be used for digital radio!  

WorldDMB recently published an article about DAB+ (ignore the low bit rate levels they're suggesting in that document -- the President of WorldDMB is Quentin Howard, who is also the chief executive of the Digital One multiplex in the UK, so WorldDMB documents dare not suggest that the low audio quality provided by 128 kbps MP2 on DAB in the UK is too low) in which they say the following on the subject of DAB+ vs T-DMB in the section on "Comparison of DAB+ and DMB for radio services":

 

"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."

 

T-DMB wastes 10 - 15% of the capacity on overhead

As the above quote mentions, T-DMB uses additional overhead in comparison to DAB+ due to T-DMB supporting video as well as audio rather than just audio like DAB+ does. This overhead consists of two things:

  • At the time when WorldDAB (as it was called then) wrote the T-DMB specification they were trying to dissuade broadcasters from adopting T-DMB for digital radio, so they stupidly disallowed AAC+ audio to be sent on its own, and instead they required that AAC+ streams must be part of a TV stream. Therefore, on T-DMB, each radio station's audio stream must be accompanied by a video stream! The minimum video bit rate is 4 kbps. This video stream can be used to broadcast the station's logo, but as the station logo doesn't change this is still a very inefficient use of capacity (if a station logo consists of, say, a 4 KB GIF file, the total number of bits is 4 x 1,024 bytes per kilobyte x 8 bits per bute = 32.768 bits, which would only take 8.2 seconds to transmit at 4 kbps, so this is clearly an inefficient way to transmit a station logo)
  • The video and audio streams of a mobile TV channel must be sychronised with each other and multiplexed together. This synchronisation and multiplexing of the streams incurs additional overhead that isn't needed for radio stations transmitted over DAB+, and the 'DAB+ audio super framing' block has a very low overhead. The additional processing needed for a T-DMB radio stations also consumes more power than with DAB+.

 

 

The following figure shows that this additional overhead means that a T-DMB radio station wastes 10 - 15% of its bandwidth on unnecessary overhead that isn't needed on DAB+ ("additional audio coding" in the figure actually means DAB+):

 

T-DMB's affect on DAB+

Although obviously it would have been better if France adopted DAB+, because this would have led to a European-wide digital radio standard, and that using two different digital radio standards will lead to fragmentation in the receiver market, which will lead to higher prices than would otherwise have been, France adopting T-DMB will still be very positive to DAB+'s progress.

 

 

 

 

 

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