AptX HD, the latest development in Bluetooth aptX wireless audio transmission, was developed by CSR and Qualcomm in order to ensure high-definition sound with compatible Bluetooth headphones and Bluetooth speakers. Yet, what if the only thing that?s HD about Bluetooth HD is its name?
Launched a few years ago, the aptX codec is now handled by a wide variety of devices: computers, Bluetooth headphones, Bluetooth earbuds, TrueWireless earbuds, portable Bluetooth speakers, network amplifiers, connected speakers and multiroom speakers. When aptX HD was introduced, CSR lauded the ?CD-like? quality of its new wireless audio transmission codec.
However, aptX compression does not ensure lossless transmission of the original audio signal. Unlike CD-quality compression, aptX compression is lossy.
Bluetooth HD: lossy compression
To understand the extent to which Bluetooth HD is a lossy compression codec, it is important to consider that a CD-quality audio file (WAV or AIFF, for example) has a bitrate of 1.4Mbps. Meanwhile, FLAC, the well-known lossless compression codec (comparable to ZIP compression) reduces this bitrate to about 800Kbps. AptX compression further reduces it to 352Kbps for wireless transmission. Consequently, audio data is lost, although the difference is not clearly audible to most listeners.
With aptX HD, CSR and Qualcomm claim to offer wireless audio transmission capable of surpassing CD quality. In practice, this is impossible, since aptX HD applies the same lossy compression algorithm as standard aptX.
With aptX HD, the data rate is slightly superior, yet it is no match for the original signal (and inferior to the FLAC format, the reference point for lossless codec).
Nevertheless, there is something noteworthy about aptX HD: it allows for the transmission of 24-bit audio files (versus 16 bits for CD-quality files). However, this is of limited interest as the compression applied is lossy. The promise of an HD audio stream with less bandwidth than is required to ensure CD quality is one which can obviously not be kept.
Are Bluetooth HD compatible devices irrelevant?
It would be a mistake to think so, since the audio quality offered by this technology is more than adequate, even if it is no match for wireless and lossless transmission via a WiFi connection’for which no headphones are yet fitted with a compatible receiver. Bluetooth aptX wireless audio transmission is thus very practical, and it is integrated into a number of excellent headphones, including the Audio-Technica ATH-DSR9BT, Bose QuietComfort 35 and Beoplay H8.
Lossless, CD-quality Bluetooth HD transmission: is this possible?
The advantage offered by aptX and Bluetooth HD (aptX HD) is that they require very little bandwidth. Technically, it would be possible to ensure even higher quality with today?s Bluetooth chips (Bluetooth 4.x), if it weren’t for the additional bandwidth required and the interference which sometimes leads to signal loss when using a Bluetooth device. A bit of patience will be required before Bluetooth 5.0 receivers become widespread. In practice, these receivers offer superior bandwidth and guarantee lossless CD-quality (and even HD) audio transmission.
However, the A2DP Bluetooth audio transmission protocol will need to use a new codec, and this time around, it will need to be a truly lossless one. This could, quite simply, be the FLAC codec.