Although many use Bluetooth headphones nowadays for practicality or because there are less and less cell phones with P2 connection (3.5mm) for headphones, there is still a big doubt about the operation of a wireless system. This article will explain the whole process that happens when we listen to music with Bluetooth devices, starting from the source to the reproduction of sounds through headphones (or Bluetooth speakers).
Source
We consider the source to be the device responsible for reproducing the music files. Tracks can be played locally (files recorded on the device’s storage unit) or via streaming. That is, our source can be, for example, a smartphone, a tablet, a notebook or a desktop (desktop computer).
The source must have a program to play the music files. If the tracks are played locally, we can use, for example, programs such as:
- AIMP
- Foobar2000
- MusicBee
- Neutron Music Player
- poweramp
- USB Audio Player Pro (UAPP)
- winamp
As for the songs reproduced via streaming, we have the application of the music service itself as a file player. Examples:
It is important to note that, in the case of music playback on mobile devices, it is possible to use local playback applications to apply equalization to tracks reproduced by streaming services. An example of this is the USB Audio Player Pro (UAPP) app, which can play music from Tidal with several control options.
Compression and encoding
After the music is played, in order for the audio to be transmitted with lower latency and greater stability, there will be some file degradation, compression and encoding. Here, a file reproduced using codecs such as MP3, OGG, FLAC or ALAC will be transformed into Bluetooth audio codecs such as SBC, AAC, aptX, aptX HD, aptX adaptive, LDAC, among others (understand more in depth about codecs here).
However, the process described above is only part of what happens on a smartphone with an Android operating system, for example. There are more things at play that can substantially affect the latency of Bluetooth signal transmission.
Latency
Latency is known to be important for games, videos, and video conferencing (video calls). But what is latency anyway? Basically it is the time it takes to start the process until its execution (example: you press the button to play a video and it takes a few seconds to start).
In Bluetooth audio, latency is the time it takes from playing the audio file at the source until the sound comes out of your headphones. According to tests carried out by the site soundguys, people cannot perceive any latency below 20ms and many have difficulty identifying that there is a delay if it is something between 40 and 80 milliseconds. However, when moving to a latency of 150ms, the influence of latency during audio playback is noticeable, that is, if we have something below 100ms, there will already be a satisfactory experience on the part of the user.
Over the years, not only Bluetooth and codecs have evolved in their technology, but Google’s mobile operating system as well. Android has undergone several changes in recent years in order to reduce latency between devices and headphones.
When you play any audio file in an Android app, be it a game, a video or any other app that uses audio, a media session is created that feeds the audio to the chosen API (OpenSL ES or AAudio). This API performs the task of managing sample rates and file format conversions between source and destination (here, audio files are also mixed and effects are applied). After this process, the API transmits the audio data to the Hardware Abstraction Layer (HAL), which is nothing more than the chip present in smartphones and Bluetooth headphones responsible for receiving data from the operating system to prepare them for use. send.
All these stages described above, which involve receiving and sending data, have FIFO (first-in-first-out) buffers. Buffers are basically a block of audio files ready to be processed or streamed. When we have very large buffers, there is a greater latency between processing the audio sample and sending it to the next task, that is, the smaller the buffer, the lower the latency, however, not everything is so simple, because by decreasing a buffer , failures can occur if a buffer write is lost or delayed.
Buffer failures can occur when Android is processing data from other programs at the same time on the phone. Here there is a struggle for the audio files to be processed to be inserted into the CPU’s task execution queue. Performance in performing these tasks may vary according to the hardware used on the smartphone and also the way Android is implemented by each manufacturer.
The steps described above about audio processing on Android happen in any audio playback performed by the cell phone. Only then will Bluetooth come into play, requiring even more buffers for codec conversion and packet creation to be sent by the Bluetooth hardware. To give you an idea, the time it takes to perform the encoding for the aptX Bluetooth audio codec is only 2ms, that is, this process is just a part of the whole that influences the total latency for the audio to reach your headphones. by ear.
Currently, in 2021, the operating system of Android smartphones added to the hardware used by them, achieved a good decrease in latency. Below are some popular 2017 cell phone models with their Bluetooth audio streaming latencies released by Google:
See now the latency obtained by smartphones in 2021 in the transmission of audio via Bluetooth released by Google:
Send
After all this processing of audio data and latency for the transmission of the Bluetooth signal to your headphones, we arrive at the stage of sending the files. Here the Bluetooth audio codecs may influence the size of the file transmission rate (bitrate) for the headset. We are on the verge of a significant change in the universal Bluetooth codec that will change from SBC to LC3 (understand more in depth here) with Bluetooth 5.2, however, while this does not happen, it is known that codecs like AAC and SBC do not have a good data transmission capacity, ideally using the aptX variants (aptX HD, aptX Adaptive) and LDAC (present in the vast majority of Android smartphones).
It is important to note that devices with the iOS operating system do not support aptX or LDAC codecs, leaving only support for SBC, AAC and Apple’s proprietary codec for headphones developed by the company.
Recommended articles for reading:
Conversion
As the Bluetooth signal is digital, it is necessary to convert it to analog so that the headphones can oscillate the diaphragm of their drivers and generate the sound. After sending the audio via Bluetooth from, for example, a cell phone, notebook or tablet, the Bluetooth headset will need to contain a DAC, a chip responsible for converting the digital signal to analog (to understand more about DACs, access here). That is, wireless headphones are not like wired headphones that have only wires and drivers to play, there is an internal signal converter, a Bluetooth receiver and a battery for this circuit to work.
Amplification
A DAC alone will not guarantee that the audio signal is reproduced through the headphone. It is necessary that there is an amplification of the signal converted by the DAC so that there is the correct excitation of the diaphragm of the phone and consequently the reproduction of the sound occurs. For this to happen, there is an amplifier integrated into the circuit board of the Bluetooth headset, an amplifier that can be of good quality or not, also influencing the final sound quality of the device.
Driver and acoustic chamber
Of course, amplification is only one part of the whole process and there are still influences from the driver design and the acoustic chamber that the headphone has internally (responsible for controlling the reverberations caused by the speaker’s operation).
Recommended articles for reading:
Conclusion
The performance of a Bluetooth headset depends on numerous factors that go well beyond the concerns taken in the development of a “simple” wired headset. For this reason, it is not surprising that there is a cost charged for this, both on the part of wireless headphones and on the part of the source that needs to have hardware and software in “good tune” for audio reproduction.
This article is made in partnership with Grupo Fones de Ouvido High-End:
Series of articles on Headphones
Selection of some of the main articles in which a new perspective is shown, making everything about headphones uncomplicated:
How it works?
The sound
Technologies
How to choose?
Tips
tutorials
Articles
Reviews
The story
Continue here.
