The aptX audio codec is used for consumer and automotive wireless audio applications, notably the real-time streaming of lossy stereo audio over the Bluetooth A2DPconnection/pairing between a “source” device (such as a smartphone, tablet or laptop) and a “sink” accessory (e.g. a Bluetooth stereo speaker, headset or headphones). The technology must be incorporated in both transmitter and receiver to derive the sonic benefits of aptX audio coding over the default sub-band coding (SBC) mandated by the Bluetooth standard. Products bearing the CSR aptX logo are certified for interoperability with each other.
aptX-HD (also known as aptX Lossless) has bit-rate of 576 kbit/s. It supports high-definition audio up to 48 kHz sampling rates and sample resolutions up to 24 bits. Unlike the name suggests the codec is still considered lossy; however, it permits a “hybrid” coding scheme for applications where average or peak compressed data rates must be capped at a constrained level. This involves the dynamic application of “near lossless” coding for those sections of audio where completely lossless coding is impossible due to bandwidth constraints. “Near lossless” coding maintains a high-definition audio quality, retaining audio frequencies up to 20 kHz and a dynamic range of at least 120 dB. Its main competitor is LDAC codec developed by Sony.
Another scalable parameter within aptX-HD is coding latency. It can be dynamically traded against other parameters such as levels of compression and computational complexity. The latency of the aptX-HD codec can be scaled to as low as 1 ms for 48 kHz sampled audio, depending on the settings of other configurable parameters. aptX-HD performs particularly well against other lossless codecs when the coding latency is constrained to be small, such as 5 ms or less, making it particularly appropriate for delay-sensitive interactive audio applications.
Many lossless codecs possess the benefit of a low computational overhead compared to well-known lossy codecs, such as MP3 and AAC. This is particularly important for deeply embedded audio applications running on low-power mobile devices. aptX-HD promotes low computational overhead by dynamically selecting the simplest coding functions for each short segment of audio whilst complying with other operational constraints, such as levels of compression and coding delay. Depending on the settings of other scalable parameters, aptX-HD can encode a 48 kHz 16-bit stereo audio stream using only 10 MIPS on a modern RISC processor with signal processing extensions. The corresponding decoder represents only 6 MIPS on the same platform.
User metadata and special synchronization data can be incorporated into the compressed format at configurable rates. The latter permits rapid decoder resynchronization in the event of data corruption or loss over communications links where Quality of Service (QoS) can vary rapidly. Depending on the settings of parameters, decoder resynchronization can occur within 1–2 ms.
aptX Low Latency is intended for video and gaming applications requiring comfortable audio-video synchronization whenever the stereo audio is transmitted over short-range radio to the listener(s) using the Bluetooth A2DP audio profile standard. The technology offers an end-to-end latency of 32 ms over Bluetooth. By comparison, the latency of standard Bluetooth stereo varies greatly depending on the system implementation and buffering. Solutions are available that use standard SBC encoding/decoding that achieve end-to-end latency of less than 40 ms. The recommended latency for Audio to video synchronization in broadcast television is within +40 ms and −60 ms (audio before/after video, respectively).
Anyway, aptX, aptX HD, atpX LL are three audio code for wireless audio industry. They are now acquired by CSR/ Qualcomm.