2 The A2DP Subband Codec

The Bluetooth's Low Complexity Subband Coding (SBC) is an audio coding system specially designed for Bluetooth (AV) applications to obtain high quality audio at medium bit rates and having a low computational complexity. It is defined in A2DP s pecification version 1.0 [1] 1 and is based on work of Frans de Bont [17] and Rault J. Bernard et al. [9]. The specification of SBC was included to ensure interoperability of devices supporting the A2DP profile. Beside SBC, A2DP devices may support different vendor specific codecs su ch as MP3, AAC and ATRAC. However, only SBC is mandatory.

2.1 Functional Description

The SBC encoders take as input signed 16-bit PCM coded audio signals having a sampling frequency fs of 16, 32, 44.1 or 48 kHz. SBC can run in a one channel mono mode or in the two channel stereo, joint-stereo or dual channel modes.

The SBC encoder consists of a polyphase analysis unit, a quantization unit, an Adaptive PCM coder and the final bitstream packing (Figure 2.1).

Figure 2.1: Diagram of the SBC encoder

First, the SBC encoder converts the stereo audio signal into multiple subbands which are equally spaced. The subband coding has been inspired by the similar process in MP3 encoders described in ISO/IEC 11172-3 [18] but SBC uses 4 or 8 subbands as compared to 32 subbands in MP3 and polyphase quadrature filters [19] having a size of 1 0 subbands (instead of a size of 512). The polyphase quadrature filters convert n = subbands audio sample into n single subband samples. These n samples form one block.

SBC collects 4, 8, 12 or 16 blocks before using these blocks to calculate the maximal loudness of each subband. The loudnesses are then rounded up the next power of two. Using scale factors,

1 The more recent version 1.2 has a couple of editorial errors and thus is incomplete.

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Frame header JOIN RFA Scale factors Audio samples Padding

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

Figure 2.2: SBC frame format (SYNCWORD: Always 8 bit set to 0x9C, SF: 2 bits for sampling frequency, BL: 2 bits for number of blocks, CM: 2 bits for the channel mode, A: allocation mode, S: number of subbands, BITPOOL: 8 bits for number of bits used for the bit allocation mode, CRC CHECK: a CRC8 check over all bits of the frame header expect the SYNCWORD and all scale factors, JOIN: subband-1 bits (only in the joint stereo mode) to indicate, whether to decode the stereo channels jointly, RFA: one bit (available in the joint stereo mode) always equal to zero.

the subband audio signals are normalized to values ranging between [−1; 1]. The normalized subband samples are not transmitted in full resolution but are quantized.

SBC supports two different algorithms for calculating how many bits should be allocated to each subband. The two modes are called SNR and LOUDNESS. The SNR mode is simple and calculates the number of bits needed, using (log2 scale f actor) − 1. The LOUDNESS mode calculates the bit needed similar to the SNR mode but it uses a weighting based on subband positions and the sampling rate. More bits are allocated to the lowest band whereas the highest bands require a lower number of bits. Also, subbands with a medium loudness are getting more bits at the costs of quiet bands.

If the requested number of bits is calculated, a limited number of bits are distributed to the band. Typically, the number of bits given the bit pool parameter is constant. These bits are distributed amount all subbands. The bits from a given bit pool are distributed in proportion to the relative number of demanded bits. Subbands that need more bits, get more bits but not necessarily all the bits they have requested for.

The SNR and LOUDNESS bit allocation run once in the mono mode and twice in the dual channel mode. The dual channel mode uses twice the number of bits given in the bit pool variable. In the stereo mode the bits are jointly distributed between the two channels. In the joint stereo mode, it can be decided for each subband channel whether they will be encoded as two separate channels or they will be converted to mid and side channels calculated by summing up the right and left.

Finally, the parameters describing the coding modes (Table 2.1), the scale factors and the quantized subband audio samples are packed into one frame (Figure 2.2), which is then transmitted.

On the receiving side (Figure 2.3), the decoder first calcula tes the bit allocation based on the received scale factors then the subband samples are reconstructed and fed into the reverse

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polyphase quadrature filter which generates the broadband P CM audio signal.

Figure 2.3: Diagram of SBC decoder [1]

Table 2.1: Description of SBC's coding parameters

The A2DP specification allows the selection of most coding pa rameters freely. It only recommends eight sets of the parameters (Table 2.2).

Depending on the SBC coding parameters (Table 2.1) the length of an SBC frame, the coding rate, the frame rate and the algorithmic delay varies. The A2DP specification contains the following equation, which calculates the frame length and the bit rate. The length of frames (in

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Table 2.2: Recommended sets of SBC encoder settings (with block length=16m, allocation method=LOUDNESS, subbands=8)

bytes) are calculated as

The SBC's algorithmic delay is due to the encoder which reads blocks subbands samples and introduces a delay of blocks subbands − 1 samples. The analysis and synthesis filters add a delay of of 10 subbands − 1 samples. Thus, the total algorithmic delay is calculated as:

2.2 Implementation

The appendix of the A2DP specifications contains the C source code of the SBC. Thus, SBC can be implemented quite easily. In addition, Bluetooth SIG provides a reference implementation with which other implementations such as our own can be crosschecked. However, because SBC is frequently used in mobile, battery powered devices considerable effort has been invested in reducing the complexity of the algorithm. For example, Hermann et al. describe a low-power implementation of SBC in [20]. Also, SBC has been implemented for BlueZ Bluetooth support

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of Linux kernel [21]. This implementation is available as open-source under the GPL license. In the preparation of this technical report, we found that the SBC Linux implementation has a number of bugs, resulting in poor audio quality. We used the performance tests described in [22] to find these errors.

Despite the fact that SBC is available as open source and compiled for most Linux systems, its intellectual properties rights are protected by at least one patent [9]. Because of a contract between the patent owners and Bluetooth SIG, the usage of SBC is license free for all Bluetooth devices. However, it is not publicly known which devices are covered by this contract. The patent owners were reluctant to provide precise information in which kind of applications SBC can be used without the requirement of paying license. Luckily, the patent is going to timeout in 2010.

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