- DRM audio codecs -
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Audio Compression

All audio compression is based on the psycho-acoustic premise that there is an audio ‘masking effect’ where the ear cannot hear certain frequencies within a critical band when there is louder audio signal close by. This critical band gets wider as the frequency gets higher. If the ear cannot hear these masked frequencies then why transmit this audio information?

DRM uses MPEG 4 HE AAC v2  (aacPlus™ Coding Technologies) audio compression. Essentially aacPlus is two perceptual audio codecs (coding decoding) -
 
- HE (High Efficiency) AAC (Advanced Audio Coding).
- SBR (Spectral Band Replication).
 
AAC gives a more efficient audio compression than previous MPEG layer 2/3 codecs.

SBR is a technique that increases bandwidth of the received audio signal from a combination of the lower bandwidth AAC signal (sampled at 24 kHz) and SBR data that reconstructs the higher audio frequency. SBR uses a 48 kHz sample rate.

When encoding music the fundamental frequencies are usually below 6-8 kHz (depending on the audio source). These lower frequencies are coded using AAC. Frequencies above this usually contain mostly harmonic sound. Although these frequencies are required to reconstruct the sound the ear is less sensitive to these higher frequencies. Hence less SBR data needs to be transmitted in comparison to the AAC codec.

Extended HE-AAC (xHE-AAC)

This codec has been adopted for use in DRM receivers from 2014, it is the next generation codec based on the HE-AAC codec technology.

Parametric Stereo

With Parametric Stereo (PS) the data describing the stereo image is transmitted alongside a mono signal derived from the stereo audio.

Parametric Stereo coding uses two parameters to describe the stereo image - Panorama (Pan) and Stereo Ambiance (SA). The Pan parameter contains information about the frequency selective level differences between left and right audio channels, while the SA parameter contains frequency selective information about the stereo ambiance as this tends to be lost when stereo signals are mixed to mono.

The Pan and SA parameters and some control bits are updated every SBR frame. Control data overhead is only 50 bps (bits per second). For complex stereo music the average PS bitrate will be around 1.2 kbps up to a maximum peak of 2.5 kbps (maximum that can be preset in the encoder), less is needed if the music stereo image is closer to mono. Stereo information requires a small fraction of the available bitrate.

Audio Quality

The actual sound quality is determined by the bit rate allocated to the audio service. This in turn is determined by the DRM mode, QAM level, error protection level (code rate), and spectrum bandwidth used for the broadcast (minimum spectrum bandwidth is 4.5 kHz to maximum of 20 kHz for DRM30, and 96 kHz for VHF DRM+ mode E).

DRM background
COFDM
FAC
SFN
MSC
SDC data 1
Multimedia
Bandplan
DRM future
DRM audio
FAC sync
DRM multiplex
DRM modes
SDC data 2