SBC (codec)

This article is about the audio codec. For the video compression technique, see Smart Bitrate Control.

SBC, or low-complexity subband codec, is an audio subband codec specified by the Bluetooth Special Interest Group (SIG) for the Advanced Audio Distribution Profile (A2DP).^[1] SBC is a digital audio encoder and decoder used to transfer data to Bluetooth audio output devices like headphones or loudspeakers. It can also be used on the Internet.^[2] It was designed with Bluetooth bandwidth limitations and processing power in mind to obtain a reasonably good audio quality at medium bit rates with low computational complexity.^[1][3] As of A2DP version 1.3, the Low Complexity Subband Coding remains the default codec and its implementation is mandatory for devices supporting that profile, but vendors are free to add their own codecs to match their needs.^[1]

At CES 2020 the Bluetooth SIG announced LC3 as the successor of SBC. LC3 is used in the LE Audio protocol based on the Bluetooth 5.2 Core Specification.^[4][5][6]

Design

SBC supports mono and stereo streams, and certain sampling frequencies up to 48 kHz. Maximum bitrate required to be supported by decoders is 320 kbit/s for mono and 512 kbit/s for stereo streams. It uses 4 or 8 subbands, an adaptive bit allocation algorithm in combination with an adaptive block PCM quantizer.^[1] Frans de Bont has based the SBC audio codec on his earlier work,^[7] and – in parts – on the MPEG-1 Audio Layer II standard. In addition, the SBC is based on the algorithms described in the EP-0400755B1.^[8] The patent owners wrote that they allow the free usage of SBC in Bluetooth applications with a goal of boosting the use of this technology.

Variants

Overview

		SBC[1][9]	SBC profiles[1]		FastStream[10]		Audio CD
			Middle Quality	High Quality
					main stream	back stream	(for reference)
misc.	Launch	–	May 2003		March 2008[11]		1982
	Related patents	EP 0400755B1[8] (expired)	–		US 9398620B1[12] (expired)		–
	Free implementations	FFmpeg, libsbc	libsbc		libsbc		–
	Proprietary implementations	–	multiple hardware implementations		hardware implementation in Qualcomm chips		–
Audio Encoding	Channels	Mono (1) Joint Stereo (2)	Joint Stereo (2)		Joint Stereo (2)	Mono (1)	Stereo (2)
	Sampling rate	16 kHz 32 kHz 44.1 kHz 48 kHz	44.1 kHz 48 kHz		44.1 kHz 48 kHz	16 kHz	44.1 kHz
	Bit rate	up to 510 kbit/s (@ 44.1 kHz) up to 507 kbit/s (@ 48 kHz)	229 kbit/s (@ 44.1 kHz) 237 kbit/s (@ 48 kHz)	328 kbit/s (@ 44.1 kHz) 345 kbit/s (@ 48 kHz)	212 kbit/s (@ 48 kHz)	72 kbit/s (@ 16 kHz)	1411 kbit/s (@ 44.1 kHz)
	Subbands	4 or 8	8				none
	Bitpool	2...86 (@ 44.1 kHz) 2...78 (@ 48 kHz)	35 (@ 44.1 kHz) 33 (@ 48 kHz)	53 (@ 44.1 kHz) 51 (@ 48 kHz)	29	32	none

Middle and High Quality

A2DP recommends encoders to support Middle Quality and High Quality presets as specified in the above table. As a result, most operating systems are using the High Quality profile as the default or even the only one supported encoding profile.^[10]

Higher quality variants

However, A2DP requires decoders to support higher quality streams, up to 512 kbit/s, and there are some experimental encoders that use this feature: for example, SBC XQ, used by Lineage OS,.^[13] With higher bit rate, audio quality is comparable to aptX HD (529 kbit/s).^[14]

FastStream

While A2DP officially supports only one-way audio streams, CSR has found a way to send a voice-back stream opposite to the main stereo stream, making it possible to use A2DP in headsets with microphones. It was implemented in the FastStream codec, which is the SBC codec with set parameters and the voice-back stream added.^[10][15]

Implementations

The A2DP test specification (V1.0) contains a reference implementation of the encoder and decoder for the SBC codec. A Linux implementation is available at BlueZ - The Linux Bluetooth stack.

See also

• Audio codec
• aptX
• Bluetooth profile
• Adaptive differential pulse-code modulation
• List of codecs

References

1. Bluetooth SIG, Specification of the Bluetooth System, Profiles, Advanced Audio Distribution Profile version 1.3. https://www.bluetooth.org/docman/handlers/DownloadDoc.ashx?doc_id=260859&vId=290074
2. C. Hoene, F. de Bont, "RTP Payload Format for Bluetooth's SBC audio codec", IETF draft, work in progress, Dec. 2010, https://tools.ietf.org/html/draft-ietf-avt-rtp-sbc-01
3. Stephen Wray (26 June 2008). "Bluetooth: Sufficient fidelity even for average listeners?". EDN Network. Retrieved 4 January 2012.
4. "Bluetooth Audio Gets a Big Upgrade at CES 2020". Bluetooth Technology Website. 2020-01-13. Retrieved 2022-01-17.
5. Frumusanu, Andrei. "CES 2020: Bluetooth SIG Announces LE Audio Standard: New Baseline For Next Decade". www.anandtech.com. Retrieved 2022-01-17.
6. "What is LE Audio and LC3, the latest in Bluetooth audio?". TIC. 21 February 2020. Retrieved 2022-01-17.
7. F. de Bont, M. Groenewegen and W. Oomen, "A High Quality Audio-Coding System at 128 kb/s", 98th AES Convention, Febr. 25-28, 1995.
8. J.B. Rault, Y.F. Dehery, J.Y. Roudaut, A.A.M. Bruekers, R.N.J. Veldhuis, "Digital transmission system using subband coding of a digital signal", Publication number: EP0400755 (B1), Priority number(s): EP19900201369 19900530; EP19890201408 19890602
9. ValdikSS. "Bluetooth A2DP SBC Codec Bitrate Calculator". ValdikSS. Retrieved 6 April 2021.
10. ValdikSS (18 June 2019). "Audio over Bluetooth: most detailed information about profiles, codecs, and devices". Habr. Retrieved 6 April 2021.
11. CSR. "CSR presents handset makers with revolutionary audio processing technology". CSR. Archived from the original on 2008-05-09. Retrieved 9 May 2008.
12. J.J. Lazzeroni, M.K. Carevich, J.D. Vertz, P.E.H. Hauser, S.J. Kingston, "Simultaneous voice and audio traffic between two devices on a wireless personal-area network", Publication number: US9398620 (B1)
13. ValdikSS (6 July 2019). "Bluetooth SBC Dual Channel HD audio mode". Lineage OS. Retrieved 6 April 2021.
14. Serge Smirnoff (29 June 2019). "Audio quality of SBC XQ Bluetooth audio codec". SoundExpert. Retrieved 6 April 2021.
15. Pali Rohár (2 June 2019). "[PATCH v11 07/11] bluetooth: Add A2DP FastStream codec support". pulseaudio-discuss mailing list. Retrieved 6 April 2021.