SBC Sub-band codec

The SBC (Sub-band Codec) is an audio coding format that is commonly used for Bluetooth audio transmission. It is designed to compress and decompress audio signals for efficient transmission over limited bandwidth channels while maintaining acceptable audio quality.

Here's how SBC works at a high level:

1. Sub-band Analysis: The input audio signal is divided into multiple frequency sub-bands using a filter bank. Each sub-band represents a specific range of frequencies.
2. Quantization: In this step, the samples in each sub-band are quantized. Quantization involves representing the continuous amplitude values of the samples with a limited number of discrete levels. This reduces the bit depth and hence the amount of data required to represent the audio signal.
3. Bit Allocation: The available bits are allocated to each sub-band based on their importance. Sub-bands with more critical audio information are assigned more bits, while less important sub-bands receive fewer bits.
4. Huffman Coding: Huffman coding is used to further compress the quantized sub-band samples. Huffman coding assigns shorter codes to frequently occurring values and longer codes to less frequent values, reducing the overall number of bits required for transmission.
5. Frame Structure: The quantized and Huffman coded sub-band samples are organized into frames for transmission. Each frame typically contains several sub-band samples along with control information such as bit allocation and coding parameters.
6. Transmission: The compressed frames are transmitted over the Bluetooth channel to the receiving device.

On the receiving side, the reverse process takes place:

1. Frame Reception: The compressed frames are received over the Bluetooth channel.
2. Huffman Decoding: The Huffman codes are decoded to reconstruct the quantized sub-band samples.
3. Sub-band Synthesis: The sub-band samples are synthesized by combining the samples from each sub-band using a synthesis filter bank.
4. Reconstruction: The quantized samples are converted back to their original amplitudes through inverse quantization.
5. Sub-band Combination: The sub-band samples are combined to reconstruct the original audio signal.

The overall audio quality of SBC depends on various factors such as the bit rate, bit allocation strategy, and the performance of the encoder and decoder. Higher bit rates result in better audio quality but require more bandwidth for transmission. SBC supports different quality modes and can adapt the bit rate dynamically based on the available bandwidth and the capabilities of the Bluetooth devices involved in the communication.

It's worth noting that while SBC has been widely used for Bluetooth audio streaming, newer audio codecs like aptX, LDAC, and AAC have gained popularity due to their ability to provide better audio quality at lower bit rates.