In recent years, the disappearance of 3.5 mm ports from smartphones is causing wireless audio to gain a lot of strength. And among all wireless technologies, Bluetooth stands out strongly.
With the latest versions of Bluetooth, the connectivity between the devices is very stable. The technology already works. So manufacturers are starting to put more emphasis on streaming audio quality by focusing on improving codecs that compress audio files and stream them wirelessly from source to audio device.
Basic concepts
Bitrate or bit rate (kbps): usually measured in kbps or Mbps. It is the amount of data that is transmitted per second through the Bluetooth connection.
Sampling frequency (kHz): is the number of data per second in an audio file. We have to bear in mind that we need two data to accurately capture a frequency, which is why refresh rates around 40 kHz (44.1 kHz or 48 kHz) are very common, which is twice the frequency range of the human ear.
Bit Depth (-bit): Represents the number of bits saved for each audio sample. Higher bit depth records a signal more accurately. The quality of CDs is 16-bit, but high-resolution files are typically 24-bit.
If we didn’t compress the files, the bitrate could be calculated by multiplying the sample rate by the bit depth.
Best audio codecs for Bluetooth
Codecs are encoding and decoding algorithms that compress audio into manageable data packets for more efficient transmission.
The efficiency of the codec will determine the speed at which the audio data is sent and also the resulting audio quality.
One very important thing to keep in mind is that, to use a certain codec, both the audio source and receiver must be compatible with it.
This means that, even if my headphones are aptX, if my mobile doesn’t support the aptX codec, the connection between the two can never be aptX. The Bluetooth protocol will negotiate the connection and choose the next best codec. If there are none in between (AAC, for example), the final connection will end up using the SBC codec (which is universal).
Next, we are going to see the most used audio codecs in Bluetooth connections:
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SBC (low-complexity SubBand Codec)
sbc
The SBC codec was developed by the SIG (Special Interest Group), the organization responsible for developing Bluetooth technology, for the A2DP (Advanced Audio Distribution Profile) audio profile. This codec is one of the minimum requirements that any Bluetooth audio device must be able to use in order to connect to a wireless source. This means that all Bluetooth audio devices have to be capable of working with this codec as a minimum. ref
The SBC was created in 1993 and requires very little computing power. The downside is that the compression efficiency is not very good, so even at its maximum bitrate of 328 kbps, it does not achieve remarkable sound quality either. Also, the sound is quite delayed.
AAC (Advanced Audio Coding)
aac
The AAC codec was developed by several companies (AT&T, Fraunhofer Institute, Dolby Laboratories, Sony Corporation and Nokia) and was announced internationally by the MPEG group (Moving Pictures Experts Group) in April 1997. Besides being a codec used by the protocol Bluetooth, AAC is also one of the most popular codecs on the internet thanks to being used extensively by Apple and YouTube.
The AAC is characterized by having a much higher audio quality for the same bit rate as the SBC codec, however the latency is usually even worse. Ref
aptX, aptX LL, aptX HD, and aptX Adaptive
aptx
AptX (audio data reduction technology) is a codec designed in the 80s of the last century and used in the cinema and on the radio. The codec was later acquired by the company CSR (Cambridge Silicon Radio) which in turn was bought in August 2015 by Qualcomm.ref
The codec is characterized by offering better sound quality, but it requires more processing power. Its typical compression ratio is 4: 1.
Currently, the codec has three variations (aptX Low Latency, aptX HD, aptX Adaptive) that reduce latency or improve your audio quality.
aptX LL (Low Latency): has latencies close to 30 ms. For comparison, the SBC codec has typical latencies of 170 ms. So it is almost 6 times faster.
