Bluetooth A2DP User Manual

09 October 2012

Status

Approved

Revision

V10

Document File Name

Improving A2DP Audio Quality
White Paper

Document Owner

A/V Working Group

E-mail Address

[e-mail]

IMPROVING A2DP
AUDIO QUALITY

ABSTRACT: This white paper gives recommendations on how to

optimize audio quality when employing the Advanced
Audio Distribution Prof ile.

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Section 4 updated to indicate that all recommendations assumed A2DP

Revision History

Revision Date Description

D05r01 23-11-2009 1st draft to show possible outline of document
D05r02 14-12-2009 2nd draft, with detailed added for 1st time review from AVWG.
D05r03 14-12-2009 Include changes suggested by John and Rudiger. Better wording need

D05r04 17-02-2010 Updates in response to comments from Rüdiger, Step hen and Sek i -

D05r05 18-02-2010 Ed made some minor changes.
D05r06 12-03-2010 Added Rec. 10, during last week’s conference call it seemed

D05r07 15-03-2010 Remove HF_RD and AG_MP usage as this implied some relevance to

D05r08 15-02-2011 Update after F2F at UPF38.

D05r09 21-06-2011 From ASG meeting suggest to add statement that a SRC should also

D05r10 29-06-2011 Allan asked to combine/review some of the Recommendations which

D05r11 06-09-2011 Add Rec.11 and include Rec.12 based on Allan’s’ YY update from

D05r12 19-09-2011 Response to comments from Allan and Ash on avv-main in the last 7

D05r13 28-09-2011 Response to minutes of conference call on Sept. 20th.
D05r14 08-10-2011 Updated at F2F meeting in Budapest
D05r15 24-10-2011 Corrected table reference in R3, updated reference section + TOC
D05r16 24-04-2012 Updated to resolve comments from BARB review
D05r17 15-05-2012

D05r18 25-09-2012 Formatting, spell checking
V10r00 09-10-2012 Approved by the Bluetooth SIG Board of Directors

for using AVRCP volume control in preference to changing the
loudness within the SBC data. Recommend the AG_MP includes
AVRCP 1.3 command to turn of Equalization or other DSP is may
perform on the SBC data

san. Made it clear that AVRCP volume control should be supported by
the RD and MP so that MP shall not alter the digital bitstream as a form
of volume control.

contentious about describing a quality vs range setting on the IUT,
hopefully Rec .10 resolves this.

HFP which was not intended for this WP.

Address Seattle F2F comments.

use appropriate bitpools.

have overlapping scope.

message on 07/07 from the avv-main.

days.

and necessary A2DP role support, whilst avoiding instances of “shall”.

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Contributors

Name Company

Rüdiger Mosig BMS
Scott Walsh Plantronics
Morgan Lindqvist Ericsson
John Larkin Qualcomm
Stephen Raxter National Analysis Center
Masahiko Seki Sony Corp
Allan Madsen CSR
Ed McQuillan CSR
David Trainor CSR

DISCLAIMER AND COPYRIGHT NOTICE

THIS DOCUMENT IS PROVIDED “AS IS” WITH NO WARRANTIES W HATSOEVE R, I NCLUDING ANY WARRANTY OF
MERCHANTABILITY, NONINFRINGEMENT , FIT NES S FO R ANY PARTICUL A R PURP OSE, O R ANY WA RRANTY OTHERWISE
ARISING OUT OF ANY PROPOSAL, SPECIFICATION OR SAMPLE. Any liability, including liability for infringement of any proprietary
rights, relating to use of information in this document is disclaimed. No license, express or implied, by estoppel or otherwise, to any
intellectual property rights are granted herein.

This document is for comment only and is subject to change without notice.
Copyright © 2012. Bluetooth® SIG, Inc. All copyrights in the Bluetooth Specifications themselves are owned by Ericsson AB, Lenovo

(Singapore) Pte. Ltd., Intel Corporation, Microsoft Corporation, Motorola Mobilit y, Inc., Nok i a Corporation, and Toshiba Corporation.
*Other third-party brands and names are the property of their respective owners.

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CONTENTS

1 TERMS AND ABBREVIATIONS ................................................................................... 5

2 DOCUMENT TERMINOLOGY ....................................................................................... 6

3 DOCUMENT SCOPE ..................................................................................................... 7

4 CONFIGURATION AND ROLES ................................................................................... 8

4.1 MEDIA PLAYER (MP) ............................................................................................ 8

4.1.1 RECOMMENDATION ................................................................................ 8

4.1.2 MOTIVATION ............................................................................................. 8

4.2 RENDERING DEVICE (RD) ................................................................................... 8

4.2.1 RECOMMENDATION ................................................................................ 8

4.2.2 MOTIVATION ............................................................................................. 8

5 RECOMMENDATIONS AND MOTIVATIONS ............................................................... 9

6 REFERENCES ............................................................................................................. 12

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1 Terms and Abbreviations

Terms and Abbreviations

Abbreviation Term

A2DP Advanced Audio Distribution Profile
AVDTP Audio Video Distribution Transport Protocol
AVRCP Audio Video Remote Control Profile
GAVDP Generic Audio/Video Distri buti on Prof ile
MP Media Player
NA Not Applicable
RC Remote Controller
RD Rendering Device
SBC Sub-band Coding
SEP Stream End Point (as described in Audio/Video Distribution Transport Protocol)
SNK Sink (as defined in Advanced Audio Distribution Profile)
SRC Source (as defined in Advanced Audio Distribution Profile)
UI User Interface. Some possibility for the user to interact with the system, ranging from

simple button clicks to a more complex UI; e.g., a display with keyboard or touch
screen.

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2 Document Terminology

The Bluetooth SIG has adopted Section 13.1 of the IEEE Standards Style Manual, which dictates use of the
words “shall”’, “should’”, “may’”, and “can”’ in the development of documentatio n, as follows:

The word shall is used to indicate mandatory requirements strictly to be followed in order to conform to the
standard and from which no deviation is permitted (shall equals is required to).

The use of the word must is deprecated and shall not be used when stating mandatory requirements; must is
used only to describe unavoidable situations.

The use of the word will is deprecated and shall not be used when stating mandatory requirements; will is
only used in statements of fact.

The word should is used to indicate that among several possibilities one is recommended as particularly
suitable, without mentioning or excluding others; or that a certain course of action is preferred but not
necessarily required; or that (in the negative form) a certain course of action is deprecated but not prohibited
(should equals is recommended that).

The word may is used to indicate a course of action permissible within the limits of the standard (may equals
is permitted).

The word can is used for statements of possibility and capability, whether material, physical, or causal (can
equals is able to).

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3 Document Scope

This white paper describes how to configure A2DP SRC and SNK devices to produce high quality audio.
The recommendations in this white paper that are related to audio coding are relevant for the SBC algorithm.

However, the recommendations that are not related to audio coding are applicable irrespective of the audio
coding algorithm employed.

This white paper does not make specific recommendations regarding the functionality and performance of
audio system components that are outside the scope of the Bluetooth audio subsystem. Examples of such
components include A/D and D/A converters and transducers within microphones and speakers. However, it
should be noted that these components also contribute to system-level audio quality and their specifications
and parameters; for example, frequency response and resolution must be carefully chosen to prevent
significant degradation of the high-quality digital audio provided by A2DP.

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4 Configuration and Roles

4.1 MEDIA PLAYER (MP)

The media player can, among other devices, be a portable media player (MP3 player, video player or
mobile phone) or a fixed media player (home audio/video system or in-car audio/video system).

4.1.1 RECOMMENDATION

The MP is an example of an A2DP SRC device with the following properties:

> It is assumed to support A2DP as defined in [1] , otherwise the recommendations in this white

paper are not applicable
> It should support AVRCP commands as described later in the document.
> It is assumed to support the SRC role defined in [1] , otherwise the recommendations in this

white paper are not applicable
> It should include the ability to configure the SBC SEP on the SNK to the values defined Table

4.7 in [1].

4.1.2 MOTIVATION

The media player complies with the A2DP SRC role to enable streaming of audio/video to a SNK device.
In addition, it should support suitable codec settings and remote control capabilities to deliver high audio
quality.

4.2 RENDERING DEVICE (RD)

The rendering device can, among other devices, be headphones, loudspeakers, in-car audio systems, or
a video display with optional audio capabilities.

4.2.1 RECOMMENDATION

The RD is an example of an A2DP SNK device with the following properties:

> It is assumed to support A2DP as defined in [1], otherwise the recommendations in this

whitepaper are not applicable
> It should support AVRCP commands as described later in the document.
> It is assumed to support the SNK role defined in [1], otherwise the recommendations in this

whitepaper are not applicable
> It should include the ability to configure the SBC SEP on the SNK to the values defined in Table

4.7 in [1] .

4.2.2 MOTIVATION

The rendering device complies with the A2DP SNK role to be able to receive audio from a media player.
In addition, it should support suitable codec settings and remote control capabilities to deliver high audio
quality.

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5 Recommendations and Motivations

This section summarizes all motivations and recommendations used in the different use cases.

Recommendation 1:

When device capability and network capacity permit, the SRC device should configure the SNK SEP to use
the SBC codec parameter settings labeled as High Quality in Table 4.7 of [1]. Use of SBC codec parameter
settings that give lower quality than the settings labeled as Medium Quality in Table 4.7 of [1] is not
recommended.

Motivation 1:

The recommended settings configure the SNK audio decoder to support h igh qu al ity audio .

Recommendation 2:

When device capability and network capacity permit, the SRC device should encode and stream all SBC
frames using the maximum SBC bitpool value previously agreed with the SNK device in the A2DP stream
configuration procedur e.

Motivation 2:

The configuration of the maximum SBC bitpool value sets an upper bound on audio quality. However this
upper bound on quality is only achieved when the bitpool value used for the encoding equals the maximum
bitpool value that was configured.

Recommendation 3:

Despite the motivation for high audio quality, the SRC device should not disconnect from a SNK device that
will not accept the High Quality setting described in Table 4.7 of [1]. The AVDTP Signaling Channel should
remain connected. The SRC may then request SNK SEP settings with a lower quality and bitrate.

Motivation 3:

There are two reasons for this. The first is for backwards compatibility with legacy RD devices. The second is
that there may be reasons that an RD does not have the slot-bandwidth required to support that configuration,
for example, the RD may be in a scatternet.

Recommendation 4:

If the audio input to the SBC encoder of the SRC devic e is not one of the four supported sample rates listed in
Table 4.2 of [1], the SRC should perform sample rate conversion to raise the sample rate to the next-highest
sample rate listed in Table 4.2 of [1] . Care should be taken that the filter characteristics of sample rate
converters, including passband ripple, transition band width and stopband attenuation, are appropriate for the
desired system-level audio quality. If the audio input to the SBC encoder of the SRC device is already
sampled at a rate natively supported by SBC then the rate should not be further converted before SBC
encoding.

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Motivation 4:

Extraneous sample rate conversion is avoided and any conversion that is required involves raising the sample
rate and utilizing sample rate converters with suitable characteristics. This approach minimizes audio quality
degradation due to rate conversion.

Recommendation 5:

If the RD does not possess a suitable UI for volume adjustment then both the RD and MP should implement
volume control using suitable signaling from the Audio/Video Remote Control Profile [2], [3] in preference to
direct manipulation of the audio data by the RD. The MP and RD should respectively support the AVRCP CT
and TG roles. An exception to this recommendation is if environmental or legislative constraints make it
unsafe to allow remote volume adjustment, for example in an automotive environment.

Motivation 5:

The recommended approach to volume control a vo ids audio qua lity degradation caused by the SRC device
manipulating audio data to simulate a volume control.

Recommendation 6:

If the RD does not possess a suitable UI for volume adjustment then both the RD and MP should support
AbsoluteVolumeControl as defined in AVRCP 1.4 [3] unless environmental or legislative constraints make it
unsafe to allow remote volume adjustment, for example in an automotive environment. The MP and RD
should respectively support the AVRCP CT and TG roles.

Use of the AbsoluteVolumeControl procedure described in this Recommendation is strongly preferred to other
AVRCP volume control procedures, except for the purpose of backwards compatibility.

Motivation 6:

The recommended approach to volume control audio quality degradation caused by the SRC device
manipulating audio data to simulate a volume control. In addition, the recommended form of volume control
improves volume control synchronization between MP and RD and prevents volume saturation.

Recommendation 7:

The MP should support the AVRCP setting called “Equalizer ON/OFF Status” in the Player Application
Settings.

If the RD tells the MP to set this value to OFF as an argument included in the
SetPlayerApplicationSettingValue command, the MP should turn off all DSP processing it may be performing
on the audio transmitted over AVDTP, for example equalization or spatial effects.

Motivation 7:

The recommended approach avoids audio quality degradation caused by similar audio signal processing
being carried out on both the RD and MP. If the MP does not apply any audio processing, this
recommendation is not applicab le.

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Recommendation 8:

The MP should not alter the digital bit-stream to implement volume control if the RD implements any
alternative form of volume control; see Recommendations 6 and 7 above.

Motivation 8:

Having two methods for adjusting the volume is confusing, and creates the potential for a scenario in which
one volume setting is set to minimum and the other is set to maximum. This can lead to increased levels of
audio distortion.

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6 References

[1] A2DP Specification Version 1.2, April 2007
[2] AVRCP Specif ic atio n Vers i on 1.0, May 2003
[3] AVRCP Specification Version 1.4, June 2008