aptX (until 2010 apt-X, “apt” stand for Audio Processing Technology) is a family of proprietary audio codec compression algorithms owned by Qualcomm, with a heavy emphasis on wireless audio applications.
The aptX audio codec is a proprietary audio codec compression algorithm, which allows the real-time streaming of high quality stereo audio over a Bluetooth A2DP connection between a source device and the accessory device.
aptX technology must be incorporated in both the source and the accessory device for it to have any effect. Full backwards compatibility however allows audio streaming even when one of the devices is not aptX compatible, though in this case, the connection won’t benefit from the technology.
Mode of operation
The example CD-quality 16-bit 44 kHz (up to 22 kHz signal bandwidth) stream is divided by two layers of 64-tap QMF (Quadrature mirror filter) into four 16-bit subbands of 11 kHz (up to 5.5 kHz signal bandwidth each). The first 64-tap QMF divides into two bands (0–11 kHz and 11–22 kHz bands), and then each one is fed into another 64-tap QMF dividing into four bands: 0–5.5 kHz, 5.5–11 kHz, 11–16.5 kHz and 16.5–22 kHz. Reduced variance is generally expected to be found in higher bands compared to lower bands, thus ADPCM is employed to allocate bits optimally.
Each band is coded with ADPCM using bit allocation of 8 bits for band 1 (0-5.5 kHz spectrum), 4 bits for band 2 (5.5–11 kHz), 2 bits each for bands 3 and 4 (11-16.5 kHz and 16.5–22 kHz). A future modification is considered with adaptive bit reallocation based on variance analysis of each subband, for example 9, 2, 3, 2 etc.
As a result, for mono channel, 16 bits @ 44.1 kHz=705.6 kbit/s input is converted into (4×16) 64 firstname.lastname@example.org kHz=705.6 kbit/s and then to (8+4+2+2) 16 bits @ 11.025 kHz=176.4 kbit/s.
Optionally (adds a small delay) a short-term RMSE analyzer is used to reduce dynamic range, and thus allocate bits more effectively during quiet passages (i.e. lossy format).
For a stereo signal, a standard PCM 1.4-Mbit stream is converted into 352 Kbit/sec aptX stream.
Details can be found in the EP0398973B1 patent. The main reasoning is that signal variation is reduced at higher frequencies, which makes it amenable to coding with codecs like ADPCM