Bluetooth audio quality (A2DP)
June 29, 2008 4:29 AM
Bluetooth audio devices become more and more popular today – stereo headsets and headphones, wireless audio transmitters/receivers, cell phones and mp3 players with Bluetooth Advanced Audio Distribution Profile (A2DP). Headphones and headsets seems benefit the most from Bluetooth (BT) audio. To be honest first BT stereo headphones were of poor sound quality – dirty sound with annoying “hiss” on background. Definitely that was not because of BT technology limitations but mainly because of poor implementation of the latter and slipshod design of analog audio circuits. The situation changes slowly but steadily. For example modern BT stereo headphones DR-BT50 from Sony can successfully compete with ordinary wired ones already. Their sound quality will be sufficient for vast majority of portable player owners for sure. In no doubt if manufacturers decide they could produce perfectly sounding BT headphones for demanding listeners as well. In that case the way digital audio transmits over BT protocol is a bottleneck in the whole audio transmission chain.
As the data channel used by A2DP is only 721 kbps wide some data reduction scheme is required. While BT specifications allow using of different audio codecs (mp2, mp3, wma, aac and even atrac) the only mandatory codec for all BT audio devices is subband codec (SBC). In fact many BT headphones (including above mentioned DR-BT50) already support mp3 codec but in real life it remains unused. Obviously it was made for the sake of compatibility but the same time portable devices could benefit from direct streaming of mp3 files through BT interface saving both audio quality and battery life. However in today's practice all high quality audio still transfers over A2DP by means of SBC codec.
SBC codec appeared in SoundExpert ratings more than two years ago (see 320+ kbit/s section). The bitrate used for testing (372 kbps) showed good potential of this compression scheme but it’s clear now that above setting doesn’t reflect real-life scenario of the codec use. Let’s look for example at these two major BT applications:
Toshiba Bluetooth stack for Windows (6.10) has three quality modes for SBC codec
- High Quality – 328 kbps
- Middle Quality – 229kbps
- Low Quality – 201 kbps
IVT Corp. Bluesoleil (5.0.5) has two quality modes: High and Middle. It’s not mentioned what bitrates are used but the application has special “Status Window” available for any BT connection. Among other information it shows amount of bytes sent to and received from connected device. Simple calculations reveal the same bit rates for High and Middle quality modes – 328 kbps and 229 kbps. As Bluesoleil is capable of both sending and receiving A2DP audio data its status window helps to discover actual bitrates used by any connected BT device. For example cell phone Nokia 6500 classic also sends music to BT interface at 229 kbps and the bitrate can’t be altered in phone’s settings (may be some advanced music phones and players have such possibility, though).
Such unanimity in choosing SBC codec settings is not surprising. Bluetooth Special Interest Group (SIG) recommends using of those settings in A2DP specifications (A2DP_SPEC, Revision V12):
Table 4.7: Recommended sets of SBC parameters in the SRC device
|SBC encoder settings*
Sampling frequency (kHz)
Resulting frame length (bytes)
Resulting bit rate (kb/s)
|*Other settings: Block length = 16, Allocation method = Loudness, Subbands = 8
Taking all this into account it was decided to add SBC codec to SE testing engine with these new settings:
It’s worth to add that A2DP limits the available maximum bit rate to 320 kbps for mono and to 512 kbps for stereo modes allowing the use of many other codecs besides SBC. So audio manufacturers (of headphones especially) have wide choice of compression technologies capable of delivering high definition sound through the wireless digital channel at reasonable cost.