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Optimally using the Bluetooth subband codec



The Bluetooth Special Interest Group (SIG) has standardized the subband coding (SBC) audio codec to connect headphones via wireless Bluetooth links with the A2DP profile. SBC compresses audio at high fidelity while having an ultra-low algorithm delay and very low complexity. SBC is configurable to a very large extend. Six parameters can be changed including sampling frequency, bit rate and algorithmic delay. In total, more than 60000 different configuration can be selected. In this publication, we objectively measure the audio and speech quality, if SBC runs under those configurations to find the most suitable parameter under given rate and delay constrains. Finally, we present objective tests comparing the audio codecs SBC, CELT, APT-X and ULD.
Optimally Using the Bluetooth Subband Codec
Christian Hoene and Mansoor Hyder
Universität Tübingen, WSI-ICS
72076 Tübingen, Germany
Email: hoene|
Abstract—The Bluetooth Special Interest Group (SIG) has
standardized the subband coding (SBC) audio codec to connect
headphones via wireless Bluetooth links with the A2DP profile.
SBC compresses audio at high fidelity while having an ultra-low
algorithm delay and very low complexity. SBC is configurable to
a very large extend. Six parameters can be changed including
sampling frequency, bit rate and algorithmic delay. In total,
more than 60000 different configuration can be selected. In this
publication, we objectively measure the audio and speech quality,
if SBC runs under those configurations to find the most suitable
parameter under given rate and delay constrains. Finally, we
present objective tests comparing the audio codecs SBC, CELT,
APT-X and ULD.
Index Terms—Bluetooth SBC, audio quality evaluation, VoIP
The Bluetooth SIG, the standardization body for Bluetooth
related technologies, published a specification to support high
quality audio distribution to Bluetooth devices called A2DP [2].
It is intended to connect wireless headsets and headphones via
Bluetooth to an audio source. The A2DP profile defines which
audio codecs should be used over a Bluetooth link. Multiple
codecs are supported the mandatory SubBand Codec (SBC).
The SBC comes with a couple of properties which make it
worthwhile to consider it beyond its original usage scenario of
connecting wireless headphones. More precisely, we use SBC
for phones supporting adaptive Audio over IP .
One of the nice features of SBC is that it is configurable to
a large extend. Most encoding parameters such as the sampling
rate, the number of frequency bands it compresses, the bit rate
and frame size can be freely selected at run-time to cope with
changed requirements. For example, if only speech has to be
transmitted, its bit rate can be reduced or even the bit rate can be
further reduced during silence periods [3]. Rate reductions help
to save energy in case if it is required. Also, on the Internet, if
the bandwidth is too low, then both frame and bit rate can be
Another feature of SBC is the algorithm delay which is in
the order of a few milliseconds. Thus, the SBC codecs can be
used for musician playing over the Internet, which require a
total one-way acoustic transmission delay of about 25 ms [4].
However, to optimally take advantage of SBC, one has to
know which of SBC parameter value sets provide an optimal
quality, bit rate and latency tradeoff. Therefore, we conducted
This work has been funded by the DAAD/HEC and the Universität Tübingen.
This paper is partly based on a technical report [1].
formal subjective listening-only tests following the MUSHRA
method and various objective tests using instrumental methods
standardized such as ITU-R BS.1387-1 [5] and ITU-T P.862 [6].
In light of the results of this work, SBC can now be optimally
used for an audio and wide speech transmissions and for
variable bit and frame rate transmission over the Internet.
Finally, we compared SBC with other codecs having similar
properties, which includes the ultra-low delay (ULD) codec [7],
[8], the APT-X codec [9] and the Constrained-Energy Lapped
Transform (CELT) [10] codec.
This publication continues with a brief description of SBC,
we then describe subjective and objective audio test method-
ology before presenting the results on how well SBC encodes
audio and speech. Before summarizing, we show preliminary
objective results comparing SBC with other similar low-delay
audio codecs.
The Bluetooth’s low Complexity Subband Coding (SBC) is
defined in A2DP specification version 1.0 [2]1and is based on
work of Frans de Bont [11] and Bernard et al. [12]. The SBC
encoders take, as an input, signed 16-bit PCM coded audio
signals having a sampling frequency fsof 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 converts the stereo audio signal into
multiple subbands which are equally spaced. The subband coder
uses 4 or 8 subbands. A polyphase quadrature filters converts
n=subbands audio samples into nsingle subband samples.
These nsamples 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 to the next power of two. Using scale factors, 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 SN R and LO UD NE SS . The SN R mode is simple and
calculates the number of bits needed using (log2scale f act or)
1, where scale f act or is calculated during normalization. The
LO UD NE SS mode calculates the bit needed in a way similar
1The more recent version 1.2 has a couple of editorial errors and thus is
to the SN R mode but it uses a weighting based on subband
position and the sampling rate. More bits are allocated to the
lowest band whereas the higher bands require a lower number
of bits. Also, subbands with a medium loudness get more bits
due to the sacrifice of quiet bands.
If the requested number of bits is calculated, a limited
number of bits are distributed to the bands. Typically, the
number of bits given the bit pool parameter is constant. These
bits are distributed among 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.
Depending on the SBC coding parameters, the length of an
SBC frame, the coding rate, the frame rate and the algorithmic
delay varies to a large extend. Also, the SBC’s algorithmic
delay is variable. The encoder reads blocks subbands samples
and introduces a delay of blocks subband s 1 samples. The
analysis and synthesis filters add a delay of of 10subbands 1
samples. Thus, the total algorithmic delay is calculated as:
delay =((blocks +10)subband s 2)
ITU Recommendation BS.1534 [13] describes a procedure
on how to judge the impact of intermediate audio degradations.
In listening tests, those degraded audio samples are rated
relative to a reference signal. Typically, a continuous scale
called subjective difference grade (SDG) having as anchors the
values 0 (Imperceptible), -1 (Perceptible but not annoying), -
2 (Slightly annoying), -3 (Annoying), and -4 (Very annoying)
is used. These tests have to be done repeatedly with multiple
listeners and the results are then averaged.
The signal items used for our MUSHRA tests are based
on the audio items given in ITU-R BS.1387 and the „Kiel
Corpus Vol. 1“. We generated anchors consisting of IRS48 filter
for narrow-band, P341 filter for wideband, a super-wideband
filtering at 14 kHz (all made with the ITU-T G.191 software),
and a version sampled at 8000 Hz frequency. Distorted samples
contain SBC encoded samples and samples distorted by packet
loss and packet loss concealment. We conducted listening-tests
and noted responses of questions from 12 participants/subjects.
In total, we got 584 assessment values, each ranging from 0 to
Beside the MUSHRA values obtained from the listening-only
tests, we also applied computational methods for perceptually
assessing the quality of speech and audio transmission. The
ITU developed an improved algorithm that is called perceptual
evaluation of audio quality (PEAQ) [5]. PEAQ is intended to
predict the quality rating of low-bit-rate coded audio signal.
Two different versions of PEAQ are provided: a basic version
with lower computational complexity and an advanced version
with higher computational complexity. In addition to PEAQ,
we used ITU P.862 (PESQ) for evaluating speech quality. We
used PESQ for narrow and wide band assessment of the down-
sampled but not IRS filtered sample items. Throughout this
50 100 150 200 250 300
−3.0 −2.0 −1.0 0.0
mono coding rate [kbps]
PEAQ ODG (combined)
8 sb., mono, 16kHz, loudness, 16 bl.
8 sb., mono, 32kHz, loudness, 16 bl.
8 sb., mono, 32kHz, loudness, 12 bl.
8 sb., mono, 44.1kHz, loudness, 12 bl.
8 sb., mono, 48kHz, loudness, 16 bl.
4 sb., mono, 48kHz, lousness, 4 bl.
other SBC modes
Figure 1. Using SBC for mono audio
publication we will use the raw calculation results of PEAQ
denoted as Objective Difference Grade (ODG).
Because objective audio quality evaluation is not as good
as the human interrogation, we need to compare the results of
subjective and objective assessment to figure out the precision,
weaknesses and strength of the objective assessment algorithms.
The results of this comparison have been published in [1].
If one combines both the results of the basic and advance
versions of PEAQ, the prediction performance compared to the
subjective rating increases. For example, averaging both ODG
values and mapping them to MUSHRA yields a goodness of
fitness of R2=0.9072, which results in a good correlation
between subjective and objective results. Thus, we will use
throughout this publication for objective rating the combined
results of PEAQ-AV and BV.
In case of speech, we applied PESQ, which is known to have
a correlation of about R=0.94 for most distortions it has been
designed for. We denote the raw results of PESQ described as
As described in Section II, SBC can be parametrized in
a wide range. Even though, the A2DP defines some recom-
mended parameters to use, we are interested in verifying which
parameter sets are best at a given bandwidth.
To address these questions, we run extensive simulations
with PEAQ varying both the parameters and the reference
samples. For all the reference sample files we calculated all
coding modes varying the allocation mode (SN R,LO UD NE SS ),
the number of subbands (4 and 8), the number of blocks (4,
8, 12, 16), the coding mode (mono, stereo, joint stereo) and
the bit pool value (10, 12, 14, 18, 19, 25, 29, 31, 40, and 50).
Overall, 4800 PEAQ ODG-BV and ODG-AV values have been
calculated. In the one channel mode, we have compared the
degraded files to the mono version of the references file. In the
stereo modes, the degraded samples were compared with the
original stereo reference file. In addition, we approximated the
quality for remaining bitpool parameters between 11 and 49
with a natural spline function in order to save time.
In Figure 1, we plot the averaged ODG values versus the
coding rate. The ODG results of parameter sets which differ
only in the bitpool values are interconnected by lines. We also
highlighted the best parameter sets with coloured lines. In the
mono mode up to a rate of about 96 kbps the 16 kHz, 16 blocks,
LO UD NE SS coding mode is the best. Then between 96 and
72 kbps, the 32 kHz sampling rate should be chosen. Further
up the axis multiple best coding alternates at fast pace.
In the stereo mode, choosing the right mode is simpler. Up
to 106 kbps, the 16 kHz, 16 block, LOUD NE SS mode is best.
Both the stereo and joint-stereo mode seem to encode equally
good. Then up to 237 kbps, the 32 kHz sampling rate is the
best. At higher quality, the 44.1 kHz stereo encoding mode can
be chosen.
Some of coloured lines match those recommended value
in the A2DP standard. However, if a lower audio quality is
required, the results indicate that it is better to use the 32 kHz
coding mode instead of 44.1 and 48 kHz. Also, the joint
stereo mode does not increase significantly the audio quality
as compared to the stereo mode.
In packetized networks, speech frames are also transmitted
in packets, which have packet headers. In the Internet, the
size of packet headers can vary depending on the kind of
protocol used and whether header compression is applied. In a
typical scenario, one frame is transmitted with the RTP, UDP,
IPv4 and IEEE 802.3 protocols and thus each packet contains
packet headers having 12 bytes, 8 bytes, 20 bytes and 18 bytes
respectively. In the end, the gross rate, as measured on the
physical layer is much larger than the actual coding rate. Thus,
we also consider this gross rate in addition to the coding rate.
The gross rate calculates as
rgross =rcoding +packetoverhead f ramer ate (2)
where coding rate gives the coding rate of the SBC codec,
packetoverhead is the number of bits for protocol headers in
each packet (typically 58*8=464), and the framerate is the
number of packets/frames per second.
Considering the gross rate, the best coding mode for band-
width constraint link is shown in Figure 2. As compared to the
Figure 1, the best coding mode hardly changed.
The Bluetooth SIG standardization group currently con-
sidered to use SBC for wideband headsets to transmit the
microphone signal in the upcoming Hands-Free Profile (HFP)
version 1.5. We measured the mean ITU P.862 wideband MOS
results for speech samples including the Kiel corpus samples
and the ITU BS.1387 speech samples (English male, English
female, German male, Suzanne Vega singing). For all SBC
coding modes, the mode with 8 subbands, 16 kHz sampling
rate, kHz sampling rate, LO UD NE SS allocation mode, 16 blocks
and mono provides the best speech quality. It performs slightly
better than ITU G.722 and 48, 56, and 64 kbps.
In addition, we present the results of the SBC 16 kHz coding
mode with samples that were shifted by one octave up. We refer
to this mode as SBC 8 kHz sampling mode, which however is
not standardized. The measured narrowband PESQ values for
this mode are even better than of the wideband PESQ values
at 16 kHz sampling mode.
The SBC might not perform equally well for all kind of
acoustic contents. To avoid a content specific judgement in the
previous tests, we have taken the objective ratings averaged
over multiple sample files. This time, we take the average
of all the sampling modes but keep the sample file to be
fixed. The 16 kHz sampled speech and noisy instrument such
as the snare drum can be compressed rather well. On the
other side, single instruments having high tonal sounds such
as the glockenspiel, the tambourine, the flute, the triangle and
the clarinet are encoded relatively badly. If looking on the
measured speech qualities, it is interesting to note that high
female voices are encoded worse than low male ones. Music
such as the opera, the piano and full band speech show an
average compression efficiency. Expectingly these are also the
most common contents which are transmitted via a hifi-phone.
Several encoding schemes can compress an audio signal
with very low algorithmic delay. One of the simplest encoding
techniques is to use a PCM coding at different sampling rates.
Also, a logarithmic quantization of the samples [14] can be
considered. The classic logarithmic quantization called µ-Law
and A-Law has been standardized in ITU_T G.711 for 8 bits
per sample at a sampling rate of 8000 Hz but the IETF RTP
and SDP standards allow the usage of µ- and A-Law even at
other sampling rates, for example at 48000 Hz. Thus, the use
of logarithmic quantization (or PCM) allows the transmission
of audio signals even with existing standards. The APT-X
stereo codec has an algorithmic delay of 1.9 ms and a rate
between 128 and 384 kbps but it is not available for free.
Also Fraunhofer’s Ultra Low Delay Encoding [8] compresses
stereo audio to 96 kbps with a frame size of 2.7 ms and an
algorithmic delay of 5.4 ms. Again this codec is not available
as open source. Recently, the CELT codec has been developed
by J.-M. Valin et al. [10]. It is open source and has a trade-off
between very good quality and rate and also posseses very low
algorithmic delays. We tested it at various sampling rates and
frame sizes. We used it with an algorithmic delay of 150% of
the reciprocal of the frame rate.
We tried to compare the performance of those coding
schemes. However, we were not able to get a working im-
plementation of ULD and APT-X. Thus we asked fellow
researchers and a company to encode and decode a large sample
file containing multiple samples files. The large sample file
contained the shorter samples used throughout this work but
kept them separated by one second of silence. After getting
back the encoded and decoded large sample files, we removed
aligned the file to the original and splitted it again into small
files. Next we compared the original small samples with the
degraded samples using the combined PEAQ metric. This step
was done multiple times for different codecs for coding modes
both in mono and in stereo conditions.
It is a general consensus that PEAQ is not capable of
comparing different codecs because the kind of distortions
100 300 500 700
−3.0 −2.0 −1.0 0.0
mono gross rate [kbps]
PEAQ ODG (combined)
8 sb., mono, 16kHz, 16 bl.
8 sb., mono, 32kHz, 16 bl.
8 sb., mono, 32kHz, 12 bl.
8 sb., mono, 48kHz, 16 bl.
8 sb., mono, 44.1kHz, SNR, 12 bl.
4 sb., mono, 44.1kHz, SNR, 16 bl.
4 sb., mono, 48kHz, SNR, 16 bl.
other SBC modes
100 300 500 700
−3.0 −2.0 −1.0 0.0
stereo gross rate [kbps]
PEAQ ODG (combined)
8 sb., (joint)stereo, 16kHz, loudness, 16 bl.
8 sb., (joint)stereo, 32kHz, loudness, 16 bl.
8 sb., stereo, 44.1kHz, loudness, 16 bl.
other SBC modes
40 60 80 120 160
2.5 3.0 3.5 4.0 4.5
gross rate [kbps]
PESQ MOS (raw,wideband)
8 sb., 8kHz, loudness, 16 bl.
8 sb., 16kHz, loudness, 16 bl.
ITU G.722 with 8ms
Figure 2. Using SBC for mono audio, stereo audio, and speech measuring the gross rate (including RTP, UDP, IPv4, and Ethernet packet headers)
−3.5 −2.5 −1.5 −0.5
PEAQ ODG (combined)
50 100 150 200 250 300
0 2 4 6 8 10 12
bit rate [kbps]
algorithmic delay [ms]
Figure 3. Objectively audio quality measured with combined PEAQ of samples
encoded with different codecs and different coding modes. We display the ODG
value versus algorithmic delay and bit/gross rate.
might vary to a large extend. PEAQ evaluates different kinds
of distortions on different scales and therefore a comparison of
different codecs without proper subjective verification has to be
avoided. Being aware of these facts, we still included PEAQ
comparison results into this document knowing that they are
not means of suitable performance codec comparison but only
provide an indication of quality. The PEAQ comparison results
are shown in Figure 3.
The PEAQ-ODG ratings in the mono mode are clear. CELT
v0.6 outperforms all other codecs at the tested rate vs. delay
trade-offs. ULD is better as SBC if one considers just the bit
rate and is equally good if looking at the more realistic gross
The main contribution of publication is on the optimal usage
of SBC on the Internet transmission and on wireless (Bluetooth)
connections. Now, we know, which, transmission mode of SBC
to be chosen for given bandwidth and delay constraints and a
Bluetooth A2DP device can operate more efficiently by using
an optimal codec parameter set. Also, the results show the
strength and weakness of SBC. Audio which are difficult to
be coded with SBC are in general audio signals containing
pure tones and stable harmonic series such as the harpsichord
and the pitch pipe. On the other hand SBC is relatively good in
coding audio signals with a high time resolution, e.g. castanets
and applause.
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The Ultra Low Delay (ULD) codec developed at the Fraunhofer IDMT is based on a versatile perceptual audio coding method that achieves very low encoding/decoding delay and is nevertheless capable of high compression ratios. Utilizing a perceptual model for irrelevance reduction, the ULD codec is in principle a variable bit rate codec. To achieve coding with constant bit rate, the use of bit reservoir techniques would result in additional coding delay. This paper presents a rate loop which ensures constant bit rate coding without increasing coding delay. It is shown that this technique does not decrease the decoded audio quality significantly.
Org FoundationThe CELT ultra-low delay audio codec
  • Xiph
Digital transmission system using subband coding of a digital signal
  • R J Bernard
  • D Y Francois
  • R J Yves