Why does Bluetooth work for calls but not music?
Bluetooth is a short-range wireless technology standard that allows data and voice transmissions between different devices over a set protocol. It was originally conceived by Dr. Jaap Haartsen in 1994 while working for Ericsson to find a way for mobile devices to connect with each other without cables. The name “Bluetooth” comes from Harald Bluetooth, a 10th century Viking king who united Denmark and Norway – symbolic of the ability of Bluetooth wireless to unite devices. Today, Bluetooth has become a ubiquitous technology that enables connections for devices like wireless headphones, speakers, wearables, and more. It allows data transmission over short distances using radio waves in the 2.4 GHz band. Bluetooth applications include wireless handsfree devices, wireless syncing, and file transfers between mobile devices like smartphones.
Sources:
https://theauris.com/blogs/blog/the-history-of-bluetooth
https://en.wikipedia.org/wiki/Bluetooth
Bluetooth Audio Codecs
Bluetooth uses different audio codecs to encode and transmit audio signals wirelessly between devices. The codec determines how efficiently audio data is compressed and transmitted over Bluetooth, affecting audio bandwidth and quality.
The most basic Bluetooth audio codec is SBC (Low Complexity Subband Coding). SBC is mandatory for all Bluetooth devices and provides acceptable call quality, but poorer music playback compared to more advanced codecs. SBC has a maximum bitrate of 328 kbps and sample rate of 48 kHz [1].
AAC (Advanced Audio Coding) is a more advanced audio codec supported on many Bluetooth devices. Compared to SBC, it provides improved audio quality, especially for music. AAC has a higher maximum bitrate of 512 kbps and sample rate of 48kHz. However, actual bitrate varies based on the specific AAC implementation [2].
aptX is another popular Bluetooth audio codec developed by Qualcomm. Like AAC, it delivers better audio quality than SBC, with lower latency. aptX has a bitrate of 352 kbps and sample rate of 48 kHz. There are also improved aptX versions like aptX HD (576 kbps, 48 kHz) and aptX Adaptive (279-420 kbps) [3].
Bandwidth Limitations
One of the key factors that impacts Bluetooth audio quality is bandwidth limitations. The maximum bandwidth of traditional Bluetooth connections is only around 1 Mbps (Bluetooth to Double Data Rates). This is sufficient for phone calls which only require 64kbps, but not for high quality music streaming which can require 1.4Mbps for CD quality audio.
Bluetooth audio relies on compression codecs like SBC and AAC to reduce the amount of data transmitted. But heavy compression leads to loss of audio fidelity, especially in the high and low frequencies (Bluetooth Audio Compression). Newer versions of Bluetooth aim to increase bandwidth to improve audio quality, but legacy devices still face limits.
Audio Compression
In order to understand why Bluetooth audio quality is lower for music playback compared to calls, it’s important to understand how audio compression works. Audio compression reduces the size of audio files by removing certain data from the original recording (source: The Sample Lab). This allows more audio content to be stored in the same amount of space, but it also degrades the quality. There are two main types of audio compression:
Lossless compression removes redundant data from the original file so it takes up less space, but the audio quality remains exactly the same. Lossy compression actually removes parts of the audio data that are less audible to the human ear. This allows much higher compression rates so the file size is vastly reduced, but it also negatively impacts the audio quality (source: Amplified Studio).
Lossy compression works by analyzing the audio waveform and discarding the data that it determines is not crucial. For example, it removes high and low frequencies outside the normal human hearing range. It also eliminates very quiet sounds that would get masked by louder sounds anyway. This “psychoacoustic masking” is why lossy compression works well for music, since the artifacts from data removal are less noticeable (source: How Audio Compression Works). However, the more aggressive the compression, the more audible the quality loss.
Bluetooth relies on lossy audio compression to transmit music between devices. This allows reasonable audio quality given bandwidth constraints, but the compression degrades the fidelity compared to wired transmission of uncompressed audio.
Call vs Music Quality
There is a significant difference between the Bluetooth audio quality needs for voice calls versus music listening. Voice calls require transfer of human speech, which occupies a relatively narrow frequency range of 300 Hz to 3400 Hz and can be intelligibly encoded at 8-16 kbps. Music encompasses the full spectrum of human hearing, 20 Hz to 20 kHz, and requires higher bitrates around 256 kbps for reasonable fidelity [1].
With older Bluetooth versions, voice calls could operate acceptably within the constrained bandwidth of around 64 kbps offered by early Bluetooth specs. However, music transmission suffered notably from artifacts, lack of stereo separation, flattened dynamics, and loss of high and low frequencies due to heavy compression to fit in the limited bandwidth [2].
Even with modern Bluetooth 5.0 and codecs like aptX and AAC offering higher bitrates, audiophiles still perceive degraded music quality over Bluetooth compared to a direct wired connection. However, for casual listening and voice calls, modern Bluetooth provides an acceptable tradeoff of convenience for moderate loss in pure fidelity.
Bluetooth Profiles
One reason that Bluetooth works better for calls versus music playback is due to the different Bluetooth profiles used. Bluetooth “profiles” are predefined sets of capabilities that allow devices to communicate for different purposes.
For calls, Bluetooth uses the Hands-Free Profile (HFP) or Headset Profile (HSP). These are optimized for the narrow bandwidth requirements of voice calls. They prioritize reducing latency and jitter, which is important for real-time two-way communication like phone calls.
For music playback, Bluetooth uses the Advanced Audio Distribution Profile (A2DP). This profile is designed for unidirectional high quality streaming of audio content. However, A2DP has higher bandwidth requirements and is not optimized for the real-time low latency needs of a phone call.
So in summary, the Headset and Hands-Free profiles work better for phone calls while the A2DP profile is preferable for music but can result in lower quality voice transmission.
Audio Prioritization
Bluetooth prioritizes voice data over music because it uses different Bluetooth profiles which have separate data channels. The Hands-Free Profile (HFP) handles phone calls and voice commands, while the Advanced Audio Distribution Profile (A2DP) handles streaming audio like music.
The HFP channel is given higher priority by the Bluetooth protocol, meaning that if a phone call comes in while music is playing over A2DP, the voice data can interrupt and take precedence. This prevents the audio from a phone call from being disrupted. The music streaming is put on hold until the call ends.
Some older versions of Bluetooth do not handle this handoff between HFP and A2DP very smoothly, sometimes cutting off the beginnings of calls or restarting music abruptly. But newer versions have improved audio prioritization to minimize this effect.
So in summary, Bluetooth gives preference to voice data like calls over music data because of the different underlying profiles. This allows calls to interrupt music while maintaining call quality. But the transition has been smoothed out in newer Bluetooth versions.
Newer Codecs and Protocols
Newer Bluetooth audio codecs like aptX HD, LDAC, and codecs introduced in Bluetooth 5.0 aim to improve audio quality over Bluetooth.
aptX HD has a bitrate of 576kbps and aims to provide high definition 24-bit audio quality over Bluetooth. It can support audio sample rates up to 48kHz which improves audio fidelity compared to SBC and earlier aptX codecs (Source).
LDAC is an audio coding technology developed by Sony that can support a bitrate of up to 990kbps. At its maximum bitrate, it can transmit 24-bit/96kHz hi-res audio. However, connection strength impacts the maximum bitrate so quality may vary (Source).
Bluetooth 5.0 introduced the LESC (Low Energy Secure Connection) codec. While it only supports a bitrate of 345kbps, the lower latency makes it more ideal for transmitting high quality audio vs SBC or AAC (Source).
Improving Bluetooth Audio
New Bluetooth versions and codecs can improve audio quality significantly. Upgrading your headphones or Bluetooth audio device to Bluetooth 5.0 or newer will open up higher fidelity codecs like aptX and LDAC (SoundGuys). Bluetooth 5.0 headphones from quality brands like Sony, Bose, Sennheiser, and others will provide the best experience.
At the OS level, install additional audio codecs like aptX, aptX HD, LDAC, etc. if your device supports them. On Linux distros like Ubuntu, guides are available for installing these codecs and getting the most out of your Bluetooth audio (Improving Bluetooth Audio on Ubuntu).
Adjust audio settings like EQ and sound profiles in your OS and headphone apps. Disable audio enhancements that degrade quality. Try different Bluetooth A2DP hardware offload modes if available. Position your device closer to the audio source and avoid obstructions.
While Bluetooth audio has improved tremendously, wired headphones or speakers will still deliver higher fidelity. But with recent versions and proper setup, Bluetooth audio can provide an excellent listening experience.
Conclusion
Bluetooth audio quality is limited due to bandwidth constraints and the use of lossy audio compression codecs optimized for voice calls rather than music playback. Although newer Bluetooth versions support higher bitrate codecs focused on music, simultaneous voice and audio streaming is still prioritized for calls. While Bluetooth audio quality continues to improve, wired headphones or speakers tend to provide superior fidelity compared to wireless options.
