A list of common questions on DRM Update…
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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.
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.
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.
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.
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).
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.
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.
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.