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The MPEG-4 Low Delay Audio Coder (a.k.a. AAC Low Delay, or AAC-LD) is audio compression standard designed to combine the advantages of perceptual audio coding with the low delay necessary for two-way communication. It is closely derived from the MPEG-2 Advanced Audio Coding (AAC) standard. It was published in MPEG-4 Audio Version 2 (ISO/IEC 14496-3:1999/Amd 1:2000) and in its later revisions.

The MPEG-4 Low Delay Audio Coder (AAC-LD) combines the advantages of perceptual audio coding with low delay necessary for two way communication. The main requirements are an algorithmic delay of no more than 20 ms and high audio quality for both speech and music. In this way, the AAC LD coding scheme bridges the gap between speech and audio coding schemes. Compared to known speech codecs, the AAC LD codec is capable of coding both music and speech signals with high quality. Unlike speech coders, the achieved coding quality scales up with bitrate.

SPIRIT AAC LD Codec is a highly efficient codec solution targeted to various embedded appliances. The codec is optimized for TI and ARM platforms to achieve the best performance and save more system resources.


  • Fully compliant to the ISO MPEG-4 Low Delay codec standard
  • Profiles supported: LD
  • 512 or 480 samples per frame
  • Low CPU usage and memory footprint
  • Sampling rates from 22.05 to 48 kHz
  • Bit rates from 8 Kbps to 288 Kbps for mono and from 16 Kbps to 576 Kbps for stereo signal
  • TI C6xx version is fully XDAIS-compliant (including parent/child support for paging tables)
  • Code is reentrant, supports multithreading and dynamic memory allocation

To integrate H.239 into your video conferencing application or service please send us a request at sales@spiritdsp.com

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Other components for IP voice and video calling apps provided by SPIRIT: