A list of common questions on DRM Update…

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1. What is an audio codec?

An audio codec (encoder-decoder) dramatically reduces the capacity required to store, download or transmit audio signals. Well-known examples for audio codecs standardized by MPEG are mp3 (MPEG Audio Layer 3) and AAC (Advanced Audio Coding) / HE-AAC (High Efficiency AAC).

General-purpose or music codecs such as mp3 and AAC attempt to stay as truthful to the original audio content as possible, while speech codecs such as HVXC, CELP or AMR-WB+ are tailored to the coding of speech signals only.

2.What is DRM’s enhanced audio codec?

DRM has enhanced the existing audio codec (MPEG HE-AAC v2) to the latest MPEG standard xHE-AAC (Extended HE-AAC). xHE-AAC is a superset of HE-AAC v2. It provides improved audio quality at lowest bitrates – from as low as 6 kbps – and works equally well for both music and speech content.

3. Why improve the DRM codecs? What triggered the enhancement?

When DRM was originally developed, it included MPEG HE-AAC v2, the best music and general-purpose codec available at the time. To support very low bit-rate transmissions, DRM in addition selected two speech codecs (HVXC and CELP). However, broadcasters quickly discovered that there is hardly any pure-speech programme available – basically all programmes at some point contain non-speech content such as stations jingles, advertisements, life reports with real-world sounds, etc. Therefore DRM broadcasters exclusively relied on HE-AAC v2, which however was not designed and never intended for operation at the very low bit rates as for example available for very robust DRM transmissions in the AM bands.

Therefore adopting the latest member of the MPEG AAC family – Extended HE-AAC or xHE-AAC – was a natural choice as it provides a single solution for all bit rate ranges and content types with a single codec. This single solution simplifies the selection of the optimal codec for broadcasters and facilitates the implementation for receiver manufacturers’ side. At the same time xHE-AAC is a superset of the MPEG HE-AAC v2 codec, keeping all DRM broadcasts valid and unaffected. 

4. What is happening with the existing codecs?

DRM initially offered a choice of three audio codecs – MPEG HE-AAC v2 for general-purpose audio content, and the two speech-codecs HVXC and CELP operating on speech-content only.

With the introduction of MPEG xHE-AAC, support for the redundant speech-only codecs HVXC and CELP is dropped to reduce complexity; this however does not affect any DRM broadcasters or programmes on-air, given that all DRM transmissions used MPEG HE-AAC v2.

MPEG HE-AAC v2 remains a valid DRM codec, keeping all DRM broadcasts valid and unaffected.

5. What are the biggest benefits?

DRM30 has been adopted by the ITU as a suitable system for digital sound broadcasting under Recommendation ITU-R BS.1514 “System for Digital Sound Broadcasting in the broadcasting bands below 30MHz”. DRM+ is currently seeking adoption by the ITU into Recommendation ITU-R BS. 1114 “Systems for terrestrial digital sound broadcasting to vehicular, portable and fixed receivers in the frequency range 30-3 000 MHz”. DRM30 transmissions are thus permitted by Rec. BS.1514 and DRM+ transmissions can be authorised by individual administrations on the basis that they will not cause interference to exiting licensed services. Both DRM30 and DRM+ are standardised at ETSI as ES 201 980 “Digital Radio Mondiale (DRM); System specification”.

The basis of most regulation of the use of spectrum is the limitation of interference. The re-use of a particular channel is authorised on the assumption that the transmitter power and antenna directivity characteristics of one service do not cause undue interference to another service using the same channel within its intended service area; and, of course, vice versa. The relevant ITU-R recommendations define the acceptable levels of interference. The idea can be extended to interference caused by transmissions in adjacent channels if it is known how much energy from the transmission falls outside the specified channel bandwidth. Whilst this is likely to be much smaller than the in-band energy, it still has the potential to interfere with other transmissions, particularly if the service areas are geographically close. To be able to manage the situation and calculate the interference potential, the ITU-R Recommendations put limits on the amount of energy that is permitted outside the specified band for different modulation schemes.

In most of the frequency bands used for broadcasting it can usually be assumed that all of the broadcast signals within that band have the same technical characteristics. This simplifies the process of channel allocation because it can reasonably be assumed that the interference potential of any one service is mirrored by the other. Where different modulation schemes, such as DRM and analogue, are used in the same band, the differential interference effects of DRM into analogue and analogue into DRM must be calculated and taken into account in the planning process. As well as the planning parameters necessary to ensure that DRM30 signals do not interfere with each other, in the bands below 30MHz, ITU-R Recommendation BS.1615 includes the differential parameters needed to ensure compatibility between analogue AM services and digital DRM services. Equivalent planning parameters for DRM+ are currently being prepared for submission to the ITU seeking incorporation into the relevant ITU-R Recommendation.

Both DRM30 and DRM+ have been specifically designed to be compatible with the planning rasters in the relevant bands. DRM30 has modes which include all channel bandwidths used for AM broadcasting (e.g. 9 kHz for MF in Region 1, 10 kHz for HF globally, etc.). Similarly, DRM+ occupies 100 kHz such that two DRM+ signals will fit into a conventional 200 kHz FM channel. This means that it is relatively easy to introduce new DRM transmissions alongside analogue transmissions in the existing bands and similarly to replace existing analogue transmissions while staying within the interference limits specified by the ITU.

A DRM signal carrying the same programme content as an analogue signal (a so called “simulcast”) may be placed in any available spectrum, it does not need to be co-located or in an adjacent channel, although these options are also possible. This is because DRM provides an Alternative Frequency Signalling feature. Broadcasters may provide information that keeps receivers constantly updated with the frequency information for alternatives to the currently tuned service and the receiver, by channel sounding, may determine which of these provides the best quality signal. The user therefore does not need to worry about retuning the radio as he moves around.

DRM also offers the option of single frequency networks (SFNs). This allows a multitude of transmitters to all broadcast on the same frequency which can make planning and coverage expansion a simple task.

6. Can I listen to a demo?

 Yes. Please visit www.drm.org to enjoy some sound samples comparing various content types (speech, music, reports) encoded in xHE-AAC and HVXC/CELP.

7. What about licensing?

It is expected that IP licensing for xHE-AAC, the latest member of the MPEG AAC family, will be handled by ViaLicensing – just like all other AAC flavours before. For more information please contact ViaLicensing (www.vialicensing.com). This company also maintains the DRM license pool for DRM equipment manufacturers.

Please note that the DRM Consortium is in no way linked with ViaLicensing or IP licensing in general, and does not benefit from IP royalties.

For implementations of the DRM audio codecs on various platforms, please contact Fraunhofer IIS (www.iis.fraunhofer.de/amm).

8. Who else is using this codec? / Is any other standard using this codec?

MPEG Extended HE-AAC is designed with the latest Internet streaming and mobile download services in mind. It is expected to be supported in future by all those mobile sets, media devices, automotive radios, etc. that already support AAC and HE-AAC today.

Among the broadcast standards, DRM takes a leading role by enabling the most innovative audio technology for the benefit of listeners, broadcasters and device manufactures.

9. Will it improve DRM in the AM and VHF bands?

Yes, thanks to MPEG xHE-AAC the audio quality for the most robust DRM transmission modes can dramatically be improved. Regular MW or SW transmissions benefit from the opportunity for secondary services on the same frequency, and more capacity for added-value services such as Journaline advanced text services. In the VHF bands, the use of xHE-AAC allows to increase the coverage area and/or reduce the transmission power while maintaining the same audio quality as before.

10. Will broadcasters need new transmitters, antennas, DRM Modulators, DRM ContentServers, etc?

No, broadcasters will not need to invest into new equipment. Only if and when they choose to re-configure from HE-AAC to xHE-AAC broadcasts will the ContentServer software need to be updated. No other component of the transmission chain is affected.

Manufacturers of ContentServers may make this upgrade available at no cost to their existing customers – broadcasters may therefore want to check with their equipment providers.

And as long as the current and still valid MPEG HE-AAC v2 transmissions shall remain on-air and unchanged, no immediate action by broadcasters is required at all. 

11. Does this update have an impact on the number of programmes I can carry on the same frequency?

Yes, according to the individual signal configuration of a MW or SW broadcast, xHE-AAC may allow transmitting secondary programmes in FM-like quality and in any case free capacity for added-value services such as Journaline advanced text services.

The same applies to VHF transmissions, where xHE-AAC enables typically three programmes in better-than-FM quality plus added-value services even with the most robust signal configurations for largest coverage are at lowest transmission powers.