Apt-X

• Website: http://www.aptx.com/Technology-Portfolio.aspx
• Decoder: https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/aptxdec.c

apt-X is an established audio coding technology for high-performance applications in broadcast, pro audio, consumer electronics, and gaming. The technology is developed and licensed out by APTX, a privately-owned company which previously traded as Audio Processing Technology or APT.

There are several variants of apt-X, each optimised for key applications in audio-over-networks (eg, AoIP, Massively Multi-player On-line Gaming) and wireless audio (for microphones, speakers, Bluetooth headphones, cordless intercoms, etc): standard apt-X, Enhanced apt-X, apt-X Live, and apt-X Lossless.

For more up-to-date information, visit the apt-X entry on Wikipedia.

Apt-X and apt-X HD work by splitting audio signal into four sub-bands using QMF filter and applying ADPCM on each band and packing quantised result into 16-(for apt-X) or 24-bit word (for apt-X HD). Words for different samples are interleaved.

Packed word for apt-X HD (in MSB):

 2 bits - bits 1-2 of high band quantised sample
 1 bit  - parity bit
 2 bits - band 2 quantised sample
 4 bits - band 1 quantised sample
 7 bits - low band quantised sample

Packed word for apt-X HD (in MSB):

 4 bits - bits 1-4 of high band quantised sample
 1 bit  - parity bit
 4 bits - band 2 quantised sample
 6 bits - band 1 quantised sample
 9 bits - low band quantised sample

Each band has the same order-2 ADPCM applied to it. There are no differences between apt-X and apt-X HD beside the latter operating on 24-bit audio and outputting 24-bit words.