The aptX codec, owned by Qualcomm, has become widely used in Bluetooth speakers as it delivers a higher-quality Bluetooth experience than either AAC or the default SBC codec. Although careless manufacturers and even journalists often describe it as “CD quality”, it is not a lossless codec, so is more accurately described as “near CD quality”.
There has long been an ‘aptX Lossless’ codec available, but only for use via Wi-Fi connections, not at the lower bit-rates of Bluetooth.
More recently an ‘aptX HD’ codec has appeared, and Qualcomm’s website describes this as “High Definition Bluetooth wireless audio”. It “preserves sound data through the audio transmission, resulting in a ‘better-than-CD’ listening experience...you can enjoy listening to your music at the highest audible quality when using a wireless device.”
We recently investigated Sony’s LDAC codec, which similarly claims high-res audio transmission through Bluetooth but is in fact a lossy codec that aims at “near Hi-Res Audio”, by which they mean indistinguishable from a 24-bit/96kHz input (avhub.com.au/ldac).
Qualcomm makes similar claims for aptX HD — “indistinguishable from high resolution audio”, and “24-bit audio quality over a Bluetooth wireless connection”. It requires the codec to be supported by both the sending device (likely an Android phone or tablet) and the receiving device, though is specific in claiming 24-bit/48kHz, not 96kHz.
Products already supporting aptX HD include LG’s V20 and G5 handsets at the source end, and at the ‘sink’ end Naim’s 'New Uniti' series and two models of Audio-Technica headphones, the ATH-DSR7BT and ATH-DSR9BT (pictured).
We contacted Qualcomm with questions, and our thanks to Jonny McClintock, director of aptX Sales and Marketing at Qualcomm, for providing the following answers.
S+I: Can you confirm whether aptX HD is a lossless or lossy codec?
JM: The aptX HD is technically a Lossy codec. However as aptX HD uses a relatively gentle compression ratio of 4:1 it is non-destructive in nature. As it is based on ADPCM and not psycho-acoustic perceptual based principles, aptX HD uses predictive analysis rather than attempting to replicate the performance of the human hearing via a synthetic ear.
S+I: Does it work within the A2DP profile or use additional data?
JM: The aptX HD algorithm uses A2DP for Bluetooth.
S+I: What maximum data rate does it achieve?
JM: To achieve 24 bit word depth and supporting a sampling frequency of 48kHz requires 576kBit/s.
More info: www.aptx.com/aptx-hd