History and Fundamentals
Bluetooth was developed in the 1990s as a standard for short-range wireless data transmission to replace infrared connections. Simply put, the technology replaces the use of cables, because the digital data that would traditionally be transferred via USB, audio, or network cables is transmitted wirelessly thanks to Bluetooth – and without line of sight between the devices to be connected. However, the range depends on the respective devices; roughly speaking, it is between ten and one hundred meters.
By the way: The name of the radio technology is derived from the Danish king Harald Blatand (meaning Bluetooth), whose surname translates to "Bluetooth" in English. A reference to him can still be found in the Bluetooth logo: it is composed of the runes ᚼ (h) and ᛒ (b), thus forming the king's initials.
Applications and Pairing
You are probably most familiar with Bluetooth connections from your smartphone, but tablets and laptops also use this wireless technology. This is clear, because after all, the technology is an interface with which different devices can communicate with each other and transmit all kinds of data (including music). This allows headphones, speakers, and other technical devices such as printers or headsets to be easily controlled among themselves. Of course, there are many other devices that can be connected via Bluetooth, but for the HiFi and TV sector, it is sufficient if we limit ourselves to headphones, speakers, televisions, and similar devices.
To connect the corresponding devices, they must be paired with each other. This is called pairing (from the English "to pair"). Up to eight devices can connect simultaneously. The basis for this is a chip with a transmitter and receiver as well as the appropriate software that enables the exchange of data. Then one speaks of the devices being Bluetooth-enabled. This standard is available worldwide and, fortunately, free of charge.
The network technology's data is transmitted via the ISM band (which stands for Industrial, Scientific and Medical Band), which is used by high-frequency devices (transmission of frequencies via audible sound waves). For Bluetooth, the 2.48 GHz and 2.4 GHz bands are required, which are also known from Wi-Fi or microwave ovens. In the worst case, this can even lead to network interference. Unlike Wi-Fi, however, Bluetooth also works without a router.
Bluetooth Versions
Now, the different versions of Bluetooth are important. Between 1999 and 2009, there were a total of seven main and sub-versions of the Bluetooth standard. Like all other functions, Bluetooth consumes battery power, which has gradually been addressed in the versions. Today, at least Bluetooth 4.2, but more likely Bluetooth 5.0 and higher, is installed on devices. Version Bluetooth 4.0 was given the addition "Low Energy" because the focus (as the name suggests) is on low power consumption. The sub-versions Bluetooth 4.1 and Bluetooth 4.2, in turn, received their optimization in terms of security, speed, and power consumption. Since 2016, we have known Bluetooth 5 as the latest standard. This promises a greater range and double the transmission speed. The latest variants, Bluetooth 5.1 and 5.2 (the latter being the newest variant since 2020), excel in automatically finding headphones and providing improved audio transmission, for example. It's nice that the HiFi sector was considered here!
By the way: If you own devices with different versions, that's not a problem. Despite different versions, you can pair speakers, phones, computers, and so on. The only limitations might come from manufacturers who make their products compliant with a specific Bluetooth version. In that case, appropriate adapters can be used.
Bluetooth Profiles
There is a multitude of different profiles that define exactly which data and functionalities are transferred between devices. Needless to say, they only work if both devices support the corresponding profiles. The number of profiles is constantly being expanded. Below is a brief list of profiles that we consider relevant for everyday use and/or HiFi matters.
- "OBEX" (Object Exchange) is used for data exchange.
- You can connect a headset to "HSP" (Headset Profile).
- "HFP" (Hands Free Profile) is used for hands-free systems in cars.
- To transmit audio data, you use "A2DP" (Advanced Audio Distribution Profile). This allows music or your own voice to be played via car radios, headsets, headphones, and wireless speakers. To be precise, it concerns stereo signals. The protocol is slightly different on both sides, as A2DP-SRC works at the source, and A2DP-SNK at the receiver. To ensure good quality during playback, the profile must be supported by a corresponding codec – in this case, it is the SBC codec. MP3, AAC, and aptX are often used, but today almost all Bluetooth devices support A2DP. In connection with this, AVRCP is also often used.
- You use the "AVRCP" (Audio/Video Remote Control Profile) to control a media player, for example. This means you can pause music and video and play the next song. Similar to the previously mentioned profile, this one is also divided: AVRCP-CT for the sender and AVRCP-TG for the device to be controlled.
The A2DP, HSP, HFP, and AVRCP profiles are integrated into most headsets, headphones, and speakers. Therefore, we consider these to be the most relevant. Tech-savvy readers could probably spend an entire evening browsing through all other profiles.
aptX Codecs
Since we just mentioned aptX, and this abbreviation is quite common and widely used, we would like to shed some more light on it. The abbreviation stands for "Audio Processing Technology," and it is an audio codec that aims to reduce the bitrate for broadband stereo audio. There are variants such as aptX, aptX Low Latency, aptX HD, and aptX Adaptive.
- The standard aptX audio codec is used in wireless audio applications via the aforementioned A2DP Bluetooth profile. To take advantage of the acoustic benefits, the technology must be integrated into both the sender and receiver.
- When you use aptX Low Latency, you can enjoy an end-to-end latency of 32 ms. This is smaller than the latency times for audio-video synchronization in television operation (which is between +40 ms and −60 ms). It can be used to control interactive applications for computer games, audio signals from musical instruments, multimedia, and more.
- aptX-HD (also known as aptX Lossless) has a bitrate of 576 kbit/s. This supports high-resolution audio signals with sampling rates of up to 48 kHz and sampling resolutions of up to 24 bits. This is referred to as near-lossless encoding. It offers high-resolution audio quality in the frequency range between 20 Hz and 20 kHz and even a dynamic range of at least 120 dB. Caution, this codec should not be confused with the LDAC codec developed by Sony, which is probably the biggest competitor to aptX Lossless.
- An automatic adaptation of the codec should be achieved with aptX Adaptive. This means: optimal audio quality and optimal latency, depending on the content you are playing. In addition, the codec is backward compatible with aptX and aptX HD.
Security and Conclusion
As you can imagine, Bluetooth is not invincible. After all, it is a technical achievement and, of course, it has its weaknesses. In the past, attempts were made to overcome Bluetooth devices, mainly through PIN queries. However, any attackers would have to be near the devices, as the range of even the best Bluetooth connections does not exceed a short distance. Therefore, the transmission standard is considered very secure. It is also certain that it is future-oriented and will not leave us anytime soon.
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