Audio source coding

With xHE-AAC, your DRM service sounds better than ever before, even at very low bit rates.

Useful bitrates for DRM30 range from 6.1 kbit/s (Mode D) to 34.8 kbit/s (Mode A) for a 10 kHz bandwidth (±5 kHz around the central frequency). It is possible to achieve bit rates up to 72 kbit/s (Mode A) by using a standard 20 kHz (±10 kHz) wide channel.[(For comparison, pure digital HD Radio can broadcast 20 kbit/s using channels 10 kHz wide and up to 60 kbit/s using 20 kHz channels.) Useful bitrate depends also on other parameters, such as:

• desired robustness to errors (error coding)
• power needed (modulation scheme)
• robustness in regard to propagation conditions (multipath propagation, doppler effect), etc.

DRM system provides ability to switch depending on the strength of reception and perceived audio quality. It provides three kinds of audio codecs namely Advanced Audio Codec (AAC), Code Excited Linear Prediction (CELP) and Harmonic Vector Excitation Coding (HVXC), which vary in quality, bit rate requirement, and its application. CELP and HVXC are designed for speech-only services requiring lower bit rates, while AAC provides highest quality. DRM system provides low data rates at high quality by utilizing MPEG xHE-AAC and AAC with PS (Parametric Stereo) and SBR (Spectral Band Replication). The modulation technique and channel coding used are Quadrature Amplitude Modulation (QAM) and Coded Orthogonal Frequency-Division Multiplexing (COFDM) respectively. The DRM+ system is designed for use in any of the VHF Bands I, II and III, each containing its own channel raster7

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When DRM was originally designed, it was clear that the most robust modes offered insufficient capacity for the then state-of-the-art audio coding format MPEG-4 HE-AAC (High Efficiency Advanced Audio Coding). Therefore, the standard launched with a choice of three different audio coding systems (source coding) depending on the bitrate:

• MPEG-4 HE-AAC (High Efficiency Advanced Audio Coding). AAC is a perceptual coder suited for voice and music and the High Efficiency is an optional extension for reconstruction of high frequencies (SBR: spectral bandwidth replication) and stereo image (PS: Parametric Stereo). 24 kHz or 12 kHz sampling frequencies can be used for core AAC (no SBR) which correspond respectively to 48 kHz and 24 kHz when using SBR oversampling.
• MPEG-4 CELP which is a parametric coder suited for voice only (vocoder) but that is robust to errors and needs a small bit rate.
• MPEG-4 HVXC which is also a parametric coder for speech programs that uses an even smaller bitrate than CELP.

However, with the development of MPEG-4 xHE-AAC, which is an implementation of MPEG Unified Speech and Audio Coding, the DRM standard was updated and the two speech-only coding formats, CELP and HVXC, were replaced.

• xHE-AAC (Extended HE-AAC), primarily developed by Fraunhofer IIS and the latest addition to the MPEG AAC codec family, bridges the gap between speech and audio coding. It provides consistently high-quality audio for all signal types, such as speech, music or mixed content, at all bit rates – starting as low as 6 kbit/s for mono and 12 kbit/s for stereo services, up to 500 kbit/s and above. This makes xHE-AAC the audio codec of choice for digital radio and adaptive streaming applications. It is a mandatory audio codec in Digital Radio Mondiale .

xHE-AAC-streaming services sound better than ever, even at low bit rates.

xHE-AAC for DRM 

DRM (Digital Radio Mondiale) is the first digital radio standard to adopt xHE-AAC.By now, the most recent addition to the AAC codec family is on the air in India: already, the first five major transmitter locations in India, two at Delhi, and one each at Bangalore, Pune, and Chennai have been upgraded with xHE-AAC-enabled DRM equipment by RFmondial with Fraunhofer technology components inside. In total, there are 39 transmitter locations (including 35 transmitters that form the domestic medium-wave network), which all are expected to receive this latest upgrade moving forward.  xHE-AAC makes it possible to provide much better, FM-like audio quality, as well as more programs in a single DRM transmission, up to three instead of one. The upgraded DRM encoder equipment also supports the provision of detailed textual information like current news, information, program schedules, traffic updates, and contact information via DRM’s Journaline advanced text service. Eventually, based on DRM’s emergency warning functionality (EWF)