Audio Source Coding XHEAAC
Extended HE-AAC audio coding (xHE-AAC) For generic coding of both audio and speech content at all bit rates, a subset of the MPEG xHE-AAC toolbox chosen to best suit the DRM system environment is used. For example a standard configuration for use in one short wave channel could be 16 kbit/s stereo.
Specific features of the xHE-AAC stream within the DRM system are:
• Bit rate: xHE-AAC can be used at any bit rate. The granularity of the xHE-AAC bit rate is 20 bit/s for robustness modes A, B, C and D and 80 bit/s for robustness mode E.
• Sampling rates: Permitted sampling rates for the use of xHE-AAC within DRM are selected such that the interface of the xHE-AAC codec towards the surrounding application can easily accept or provide 48 kHz audio signals, respectively. The actual core sampling rate is selected by the encoder upon initialization to ensure the best possible audio signal quality and is typically not visible to higher layers of processing.
• Audio super framing: To ensure the best possible audio quality particularly at lower bit rates, the xHE-AAC encoder can flexibly assign the available bit rate within certain constraints to each audio frame. Audio super frames - as generated by the xHE-AAC audio encoder and inserted into the DRM logical frames - always have a constant size. However, the number of audio frames per audio super frame is not fixed, and audio frames may span audio super frames. This flexibility is achieved by a slight adjustment to the audio super frame header configuration used for the AAC codec in DRM. One audio super frame is always placed in one DRM logical frame in robustness modes A, B, C and D and in two logical frames in robustness mode E (see clause 6). In this way no additional synchronization is needed for the audio coding. Retrieval of frame boundaries is also taken care of within the audio super frame
Source: ETSI
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