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					                            BLUETOOTH
             and wireless networking
                          a primer for audio engineers
                    ireless networking, us-          tions for wireless data communication.      tances within a so-called personal



W                   ing radio frequency
                    links instead of wires,
                    is becoming increas-
                    ingly popular as a
means of connecting digital devices and
computers. It provides greater flexibility
and mobility to the user and does away
                                                     This article puts Bluetooth into context
                                                     and describes wireless data communi-
                                                     cations in relation to audio systems.
                                                     This field is changing fast like most as-
                                                     pects of computing, making it hard for
                                                     standardization to keep pace with tech-
                                                     nology development. A number of
                                                                                                 operating space of 10 m envelop-
                                                                                                 ing the person. WPANs are intend-
                                                                                                 ed mainly for indoor and fixed out-
                                                                                                 door applications and operate at
                                                                                                 data rates of up to about 1 Mbit/s.
                                                                                                 They are a useful next step from
                                                                                                 infrared communications. Blue-
with those tangled strands of wires. And             wireless networking standards have          tooth is the prime example of a
wireless networks can be integrated                  been or are being standardized by the       WPAN. Home RF is potentially
with wired systems, thereby acting as                IEEE, but alternative solutions also ex-    similar, but it sits somewhere be-
extensions of existing networks. Audio               ist that overlap with or differ from        tween a PAN and a LAN (local area
information is increasingly transferred              IEEE standards. As a rule it seems that     network). Fig. 1 shows an example
over networks instead of dedicated digi-             IEEE has attempted where possible to        of the interrelationship between
tal interfaces or analog cabling, making             adopt commercial technology in re-          PANs, LANs, and WANs. WPANs
it important that audio engineers have               sponse to calls for suitable solutions.     are broadly covered in IEEE stan-
an appreciation of the issues involved.                                                          dards work by the 802.15 groups.
   Bluetooth is a technology for wire-               WIRELESS PERSONAL AREA                      The primary concerns when design-
less personal area networks (WPANs)                  NETWORKS                                    ing successful WPAN technology
that has been widely publicized recent-              WPANs connect desktop digital               are power consumption, simplicity,
ly, but it is only one of a number of op-            devices, usually over short dis-            and product size.                ➥




Fig. 1. Example of WPAN, WLAN, and WWAN interacting.

J. Audio Eng. Soc., Vol. 50, No. 11, 2002 November                                                                              979
                                                         BLUETOOTH
WIRELESS LOCAL AREA                         data signal look like noise or very         multipoint. In the latter case one de-
NETWORKS                                    short-term impulsive interference to a      vice acts as master, synchronizing,
WLANs are used over larger dis-             narrow-band receiver in the radio fre-      controlling, and sharing the channel
tances, up to 500 m, in areas where         quency (RF) domain, as shown in Fig.        with as many as seven slaves in a so-
people may move around with digital         2. Most current systems operate in a        called piconet of up to eight devices.
devices but usually remain stationary       license-free region of the RF spectrum      Frequency hopping between 23 or 79
while using the devices. They typical-      between 2.4 and 5 GHz, relying prin-        RF channels is employed to increase
ly operate around hot-spot access           cipally on the low-power and interfer-      robustness, and data packets are each
points, for example in offices, homes,      ence-avoiding mechanisms of systems         transmitted on a different hop frequen-
and schools. Data rates are typically       to operate satisfactorily in this free-     cy. The IEEE 802.15.1 WPAN stan-
higher than WPANs, between 2 and            for-all region.                             dard is based on Bluetooth.
about 50 Mbit/s. WLANs are covered             The 2.4-GHz band is becoming in-            The primary modes for data commu-
in IEEE standards activity by the           creasingly crowded owing to Bluetooth       nication are ACL (asynchronous con-
802.11 groups. Typically above this         devices, wireless networks, and tele-       nectionless) links and SCO (syn-
data rate are WMANs (wireless               phones, so there can be significant RF      chronous connection-oriented) links.
metropolitan area networks) that inter-     interference in this band. The 5-GHz        The latter are limited to a data rate of
connect LANs over large regions.            band is somewhat quieter in the United      64 kbit/s each, and up to three concur-
                                            States, but signals have a much shorter     rent SCO links are allowed per Blue-
WIRELESS WIDE AREA                          range at this frequency and are more        tooth channel. They are set up between
NETWORKS                                    easily obstructed by walls, doors, and      a master and a single Bluetooth device
WWANs have more to do with global           other objects. Communication there-         at a time for time-critical purposes
telecommunications systems, for ex-         fore tends to be over line-of-sight paths   such as voice audio. SCO links are es-
ample mobile phone operators using          or short distances. Some chipsets are       tablished as real-time links between
GPRS (General Packet Radio Service)         appearing that will operate in both         two points for the duration of the con-
equipment, and are mainly designed          bands for IEEE 802.11 standards. The        nection. Packets are never retransmit-
for applications in which people move       issue of interference is covered in         ted, owing to their real-time nature. In
around while using devices. They are        greater detail later in this article.       the time remaining within each trans-
not featured in this article as they have                                               mission slot the master can set up an
little to do with potential audio appli-    EXAMPLES OF WPAN                            asynchronous communication with one
cations in consumer and professional        TECHNOLOGY                                  or more slaves. ACL communications
environments.                               Bluetooth operates in the 2.4-GHz           operate rather like conventional pack-
                                            band at data rates up to 1 Mbit/s over      et-switched networks, with the packet
General radio frequency issues              distances of up to 10 m or 100 m de-        header being used to indicate the desti-
Wireless networks typically use low-        pending on the implementation. A rel-       nation on the piconet.
power, spread-spectrum, and frequen-        atively simple binary FM modulation            It is possible to operate either one
cy-hopping techniques to ensure ro-         method is used because Bluetooth has        asynchronous and three synchronous
bust communications in the face of          to be implementable in basic devices.       voice channels (bidirectional) or one
interference and fading. They typically     Communication between devices can           channel that handles both asyn-
use coding techniques to make the           be either point-to-point or point-to-       chronous data and synchronous voice




Fig. 2. Wireless networks use low-power, spread-spectrum, and frequency-hopping techniques for robust communications.

980                                                                                     Audio Eng. Soc., Vol. 50, No. 11, 2002 November
                                                               BLUETOOTH




Fig. 3. Data rates of common wireless technologies.

(this is essentially made up of single            interoperability of Bluetooth devices,     Bluetooth application range of data
SCO packets with two parts). The                  as peer-to-peer networking interoper-      rates, whereas high rate and UWB lie
maximum asynchronous data rate is                 ability is apparently not mandatory for    above the Bluetooth range, as shown
723.2 kbit/s in one direction, with a re-         device certification. Success may          in Fig. 3.
turn rate of 57.6 kbit/s, or 433.3 kbit/s         therefore come down to a question of
symmetrically.                                    which Bluetooth devices actually           EXAMPLES OF WLAN
   In addition to the Bluetooth Core              work together and at what level of so-     TECHNOLOGY
Specification that describes the princi-          phistication.                              Broadband WLAN technology is de-
ples of communication, the Bluetooth                                                         veloping so fast that the regulatory
SIG (special interest group) publishes            High-rate and ultrawide band               and commercial situation changes al-
a number of profile documents that de-            systems                                    most weekly. However, the following
scribe specific approaches to data                The IEEE 802.15.3 group is studying        is an attempt to summarize the current
transfer for particular applications. The         high-rate applications above 20            situation as clearly as possible.
documents of primary interest to audio            Mbit/s. These could ultimately be up
engineers are the Generic Audio/Video             to 100 times faster than Bluetooth.        IEEE 802.11
Distribution Profile (GAVDP) and the              UWB is also being developed for ap-        IEEE 802.11a and IEEE 802.11b are
Advanced Audio Distribution Profile               plications requiring very high data        wireless LAN standards and the basis
(A2DP). These are discussed later in              rates. UWB uses very wide band             of wireless Ethernet or WiFi. WiFi is a
this article.                                     transmissions at extremely low power,      mark for compatible products awarded
   The long-term future of Bluetooth is           requiring receivers and transmitters to    by the Wireless Ethernet Compatibili-
not certain, but the same can be said of          be tightly synchronized. The U.S. Fed-     ty Alliance (WECA). Apple uses this
virtually any data communications                 eral Communications Commission             technology in its AirPort products, as
technology today. For example, Apple              (FCC) has approved UWB within cer-         do many of the consumer wireless net-
and a number of mobile-telephone                  tain low-power limits, but other coun-     work systems currently marketed in
manufacturers have significant invest-            tries have not yet done the same. IEEE     computer stores.
ment in Bluetooth, and this may help              802.15 Study Group 3a is studying al-         The two standards define different
to ensure its future.                             ternative physical layer options for       physical layers, where 802.11b trans-
   Many commentators appear enthusi-              high-rate WPANs.                           mits at 2.4 GHz and 802.11a transmits
astic about the market potential for the                                                     at 5 GHz. The standards do not inter-
technology. It is a relatively low-rate           Low-rate, low-power                        operate directly with each other, al-
system but is intended for low-cost,              consumption systems                        though bridges between them are pos-
low-power implementation in highly                IEEE 802.15.4 is devising a lower rate     sible and chipsets are appearing that
portable devices. Some commentators               standard for simple devices, such as       operate in either mode. 802.11 deals
cite the relative advantages of broad-            joysticks, that do not need high data      with the physical and MAC (medium
band technology such as UWB (ultra-               rates but need small size and long bat-    access control) layers of networking
wide band) that may ultimately be-                tery life. Data rates of 20, 40, and 250   protocol, but the remaining layers are
come more useful in high-rate                     kbit/s are targeted for use within RF      virtually identical to Ethernet, which
multimedia environments. Some con-                bands at 2.4 GHz, 915, and 868 MHz.        is IEEE 802.3. In network terminolo-
cern has also been expressed about the            This clearly lies below the typical        gy layers are different levels in the ➥

Audio Eng. Soc., Vol. 50, No. 11, 2002 November                                                                                 981
                                                        BLUETOOTH

                                                                                        namic frequency selection (DFS) and
                                                                                        transmission power control (TPC) have
                                                                                        been required for conformity with Eu-
                                                                                        ropean requirements, and these are
                        USEFUL WEB SITES                                                now virtually completed for 802.11a
                                                                                        (this project was originally coded
                 Bluetooth specifications:                                              802.11h). However, as discussed in the
          www.bluetooth.com/dev/specifications.asp                                      next section, full ETSI approval of
                                                                                        wireless networking in the 5-GHz band
                              HiperLAN:                                                 does appear to be feasible, and this
                           www.hiperlan2.com                                            band is apparently being cleared in
                                                                                        many countries for broadband data
                                                                                        communications.
                                HomeRF:                                                    The range of 802.11 activities is
                              www.homerf.org                                            wide and fast moving. See
                                                                                        http://grouper.ieee.org/groups/802/11/
                             IEEE 802.11:                                               for the latest information.
                   grouper.ieee.org/groups/802/11/                                      HiperLAN
                                                                                        HiperLAN operates in the 5-GHz band
                             IEEE 802.15:                                               up to 54 Mbit/s and requires approxi-
                   grouper.ieee.org/groups/802/15/                                      mately 330 MHz of bandwidth. These
                                                                                        rates and frequencies are the same as
                              Ultra wide band:                                          802.11a, and at the physical layer the
                               www.uwb.org                                              two are almost identical. HiperLAN
                                                                                        has been developed by ETSI and the
                                                                                        Broadband Radio Access Network
                   Wireless networking links:                                           (BRAN), whose founding members
                 www.wireless-nets.com/links.htm                                        are Tenovis (Bosch), Dell, Ericsson,
                                                                                        Nokia, Telia, and Texas Instruments.
                                                                                        Typically, the operating ranges are 30
                                                                                        m indoors and 150 m outdoors. In Eu-
                                                                                        rope specific bands from 5.15 to 5.35
somewhat arcane hierarchy of data          be covered. For example the 802.11a          GHz and 5.470 to 5.725 GHz seem to
communications, with the application       standard can operate at 54 Mbit/s but        have been approved already for dedi-
at the top and the physical medium at      only within a 50-meter range. Alterna-       cated use by HiperLAN. In the U.S.
the bottom. As with cell phones, users     tively, 802.11b operates at a maximum        the 5.15–5.35-GHz and 5.725–5.825-
can roam between wireless access           speed of 11 Mbit/s but can cover 300         GHz bands are unlicensed and usable.
points (APs), using the beacon of the      meters outdoors (100 meters indoors).           The main difference between Hiper-
AP as a judge of signal strength. Col-     As with most packet-switched RF net-         LAN and 802.11a is at the media ac-
lision avoidance and carrier detection     works, contention, interference, and         cess (MAC) level. Whereas 802.11a is
are used for medium access, rather         overheads can reduce the realistic data      an extension of wireless Ethernet, pri-
like Ethernet, and authentication or       rate closer to half the maximum.             marily based on contention mecha-
full encryption can be used for securi-       Contrary to what may have been an-        nisms and packet switching, HiperLAN
ty purposes.                               ticipated, 802.11b is already in wide        is claimed to offer connection-oriented
   Both FHSS (frequency hopping            use but 802.11a products are only just       communication and is compatible with
spread spectrum) and DSSS (direct se-      beginning to appear. In the UK an in-        other circuit-switched network stan-
quence spread spectrum) approaches         ternal agreement is being formulated to      dards such as ATM. As a result it offers
are specified for 1- and 2-Mbit/s rates,   allow 802.11a to operate in a limited        quality-of-service (QoS) guarantees
but only DSSS is allowed for 11-           fashion in the region from 5.15 to 5.25      and therefore will be useful for real-
Mbit/s communications. 5.5 Mbit/s is       GHz. This allows four access points as       time streaming applications.
also allowed and even higher bit-rate      opposed to the eight possible if the
standards were approved in 1999. In        system occupies the full band from           HomeRF
fact it appears that most developers ac-   5.15 to 5.35 GHz. Full approval from         HomeRF is yet another competing
tually went for DSSS even at the lower     the European Telecommunications              technology in the 2.4-GHz band. It of-
rates, for future compatibility with the   Standards Institute (ETSI) is not yet fi-    fers a data rate of 10 Mbit/s and com-
higher rates. Generally, the higher the    nalized because of military and gov-         bines cordless telephone links (up to
radio frequency (and therefore the         ernment use of the 5-GHz band. But           eight concurrently), wireless network-
greater the potential for high data        other countries are striking individual      ing, and data streaming for home en-
rates), the shorter the range that may     agreements rather like the UK. Dy-           tertainment products.

982                                                                                    J. Audio Eng. Soc., Vol. 50, No. 11, 2002 November
                                                                  BLUETOOTH

   Up to eight simultaneous real-time                less LANs, for which it was not de-         rection depends on which packet type
streaming sessions are possible, either              signed, intended instead as a simple        carries the audio transmission: HV3
two-way, multicast, or receive only.                 and cheap short-distance cable-             (High-quality Voice 3) packets have
Priority is given to real-time voice                 replacement technology.                     no forward error correction (FEC) and
communications over streaming appli-                    The direct sequence spread spec-         contain 3.75 ms of audio at 64 kbit/s,
cations as the default condition. Asyn-              trum (DSSS) approach, as used by            whereas HV1 and 2 packets have
chronous packets take last place in the              802.11b for example, uses more band-        some error correction and contain
queue. The IEEE describes HomeRF                     width than strictly required by the data    shorter durations of audio, 1.25 and
as a sort of trimmed-down 802.11.                    rate. Data are coded onto chips (sim-       2.5 ms respectively.
                                                     ply a pattern of data bits) that have re-
RF INTERFERENCE ISSUES                               dundant portions spread over the fre-       Bluetooth audio streaming
Interference is a particular problem in              quencies in the band. The data are          using ACL packets
the crowded 2.4-GHz band, which is                   exclusively ordered within an 11-bit        New draft profiles have been created
license free in most parts of the world.             Barker code (a pseudorandom se-             for audio streaming using asyn-
Industrial, scientific, and medical ap-              quence), the bits of which make up the      chronous (ACL) packets that can oc-
plications (ISM) use it, and mi-                     chips, and there are usually an integral    cupy the full remaining bit rate of
crowave ovens are the primary culprits               number of chips per bit. These are          Bluetooth (721 kbit/s, after the voice
when it comes to interference.                       modulated onto the carrier using dif-       quality streams are taken into ac-
   In order to limit the effects of inter-           ferential phase-shift keying (DPSK),        count). These use real-time protocol
ference, frequency hopping spread                    so a narrowband receiver perceives          (RTP) for streaming. RTP was origi-
spectrum (FHSS) techniques are used                  this signal as low-level noise. Even if     nally developed for managing stream-
by Bluetooth and HomeRF. Hopping                     some parts are lost during transmis-        ing connections on asynchronous
reduces channel efficiency in favor of               sion owing to interference, error cor-      packet-switched networks such as
robustness and is simpler to imple-                  rection can be used to recover the          parts of the Internet.
ment than the DSSS techniques de-                    data. The so-called spreading ratio is         Either point-to-point or point-to-
scribed below. Such techniques usual-                related to the degree of redundancy         multipoint (such as one transmitter to
ly employ at least 75 frequencies with               employed (number of chips per bit);         multiple Bluetooth loudspeakers) are
a maximum dwell time per frequency                   the spreading ratio of most wireless        allowed. QoS is not guaranteed with
of 400 ms. This ensures that commu-                  LANs is around eight. This ratio            ACL connections although RTP does
nication does not spend too long on                  makes the use of the band reasonably        provide for some real-time require-
each frequency and the signal looks                  efficient at the expense of higher ro-      ments provided that buffering is used
like random impulsive interference to                bustness. Sometimes the band is split       at the receiver. Better QoS provision
a narrow-band receiver. Both transmit-               between more than one network; a            for streaming has been requested by
ter and receiver follow the same pseu-               maximum of three networks is possi-         the A/V working group of Bluetooth
dorandom pattern of hops, and adap-                  ble in typical current implementations.     SIG in the next revision of the Blue-
tive hopping can avoid frequencies                   Typical current DSSS products occu-         tooth data-link specification. Although
that are known to be blocked. Typical                py 20 to 22 MHz of bandwidth no             Bluetooth supports isochronous com-
FHSS products occupy about 1 MHz                     matter what the data rate. The power        munication (the type required for
of bandwidth. They achieve higher                    within any one band is relatively low,      clock-dependent processes) for
power within each frequency band at                  making the RF signal potentially less       streaming applications through the use
any one time and may therefore result                robust than with frequency-hopping          of higher-level L2CAP (logical link
in a better instantaneous RF signal-to-              techniques, but performance in prac-        control and adaptation protocol) con-
noise ratio than other spread-spectrum               tice depends on the coding scheme           nections, this is always only on a best-
techniques.                                          and spreading ratio employed.               effort basis. The only truly syn-
   The Bluetooth hopping rate is quite                                                           chronous reserved slots at the
fast (1,600 times per second) compared               BLUETOOTH AUDIO                             baseband level are for SCO packets,
with some other systems. Per second,                 The basic audio functionality in the        basically speech audio.
in any one band, it is said that one is              Bluetooth Core Specification is really         AVCTP is the Audio/Video Control
more likely to encounter an 802.11b                  only suitable for telecommunications        Transport Protocol that can be used for
transmission than a Bluetooth signal.                applications. The bandwidth is typical-     conveying messages intended for con-
There is some concern over the coloca-               ly limited to 4 kHz for telephony, and      trolling Bluetooth A/V devices. The
tion of Bluetooth and 802.11b devices,               the sampling rate is correspondingly 8      AVDTP (Audio/Video Distribution
as Bluetooth transmitters have been                  kHz. Relevant standards are ITU-T           Transport Protocol) uses L2CAP pack-
shown to reduce the performance of                   P.71, G.711, 712, and 714. The 64-          ets to transfer audio with connections
the other network considerably if                    kbit/s voice channels use µlaw or A-        established between a transmitter and a
placed close to a receiver. Bluetooth                law logarithmic PCM coding or CVS-          receiver. This protocol provides a
has limited error handling and retry                 DM (continuous variable slope delta         mechanism for reporting QoS, opti-
mechanisms beyond the frequency-                     modulation). CVSDM is said to be            mizing the use of bandwidth, minimiz-
hopping physical layer. So it is not par-            preferable with respect to quality and      ing delays, and attaching time-stamp
ticularly sophisticated for robust wire-             robustness to errors. Audio error cor-      data to packets for synchronization ➥

J. Audio Eng. Soc., Vol. 50, No. 11, 2002 November                                                                                   983
                                                           BLUETOOTH

purposes. The AVDTP document pro-             at least one of 44.1- or 48-kHz sam-         can be ameliorated using adequate
vides some advice in an appendix re-          pling frequencies, and other lower           buffering and resynchronization.
lating to the synchronization of devices      rates can be specified. The encoder          Broadcasting gets around this problem,
either to each other or to a separate         should be able to handle at least mono       but packet loss can be encountered and
network clock. It also briefly mentions       and one stereo mode (such as dual            not recovered because there is no dy-
an approach to measuring timing jitter.       channel, stereo, joint stereo) and the       namic retransmission method for
Broadly, the approach relies on the           receiver should be able to decode all of     broadcast mode. However, there is a
timing mechanisms inherent in RTP             these. Similar requirements exist for        fixed retransmission option for broad-
and RTCP (real-time control protocol).        MPEG 1, Layer I, II or III, for MPEG         cast mode, which appears to act rather
   The A2DP (Advanced Audio Distri-           2 and 4-AAC, and for ATRAC codecs.           like a form of permanent redundancy,
bution Profile) describes a configura-        MPEG codecs are also allowed vari-           whereby packets are always retransmit-
tion of layers in the Bluetooth stack         able bit-rate (VBR) encoding in the          ted at the expense of a reduction in
and higher application layers that can        profile.                                     available bit rate on the channel. Floros
be used for the conveyance of audio. It                                                    et al.2 found broadcast mode with no
was authored by the audio/video work-         Quality and Robustness of                    retransmissions only just acceptable for
ing group consisting of members from          Bluetooth Audio Streaming                    audio streaming owing to packet loss
Sony, Toshiba, Nokia, Philips, Erics-         For adequate audio quality the A2DP          on the wireless link. One retransmis-
son, and Matsushita.                          profile requires that the audio data rate    sion reduced audio data loss to 2 to 3%
   Considering the bit rate, there is re-     be sufficiently lower than the available     compared with 7 to 8%, but reduced
ally no way that uncompressed high-           link data rate to allow for the retrans-     the effective bit rate from 551 to 325
quality audio can be carried, requiring       missions that will avoid packet loss.        kbit/s. Because the losses were in com-
that some form of low bit-rate coding         The margin allowed for this obviously        pressed audio data, the resulting un-
be employed. Specified in A2DP is a           depends on the robustness expected of        compressed audio was badly affected.
mandatory audio codec, which is a             the application in the face of interfer-     They claim that retrieval mechanisms
low-complexity, subband codec                 ence or long distances. The profile          in the Bluetooth lower levels and appli-
(SBC), whereas other codec types              limits the SBC bit rate to a maximum         cation layer could be adapted to mini-
(MPEG and ATRAC in the current                of 320 kbit/s for mono and 512 kbit/s        mize the effects of packet loss on audio
version) are listed as optional. The          for stereo. The maximum available bit        quality, but have some reservations
SBC codec was developed for Blue-             rate is 721 kbit/s. The data overhead        about the use of the approach for syn-
tooth but based upon an earlier Philips       when one transmitter is communicat-          chronous compressed multichannel au-
system described by de Bont et al. 1          ing multiple ACL streams to different        dio. They concluded that one could eas-
NonA2DP codec types can be accom-             receivers can lower the overall band-        ily transmit stereo audio at 256 kbit/s
modated, although the rubric is rather        width available for audio, hence the         per channel plus control information,
confusing in relation to this eventuali-      number of channels is tightly limited        within the bandwidth available, using
ty. This is supposed to be achieved ei-       even with data compression. An alter-        two separate ACL links. The best data
ther by upgrading the codec to optional       native is to use broadcast mode in           rate was obtained with DH5 packets,
status within A2DP (this requires the         which the full data stream for all audio     getting close to the upper limit of 721
manufacturer to submit clear defini-          channels is broadcast to all receivers,      kbit/s, but all DH packets have no for-
tions of certain required characteris-        requiring them to separate the chan-         ward error correction and so are more
tics) or by transcoding the audio data        nels themselves (they therefore need         prone to data loss on the link. DM
to SBC if the receiver does not support       to have a means of channel identifica-       packets have forward error correction
the decoding of the data type; this way       tion). The maximum number of audio           (FEC) and a correspondingly lower
interoperability is maintained as much        channels in this mode is seven, that is      overall data rate.
as possible.                                  the maximum number of connections               Whether Bluetooth stands the test of
   The profile document does not de-          to a master device.                          time remains to be seen, but wireless
fine anything in relation to nonA2DP             There are some problems with point-       technology for audio transmission will
codecs except that the vendor is sup-         to-multipoint connection for audio us-       undoubtedly continue to expand. See
posed to use a Bluetooth Assigned             ing ACL and RTP because different re-        the box on p. 982 for useful web sites
Number to identify itself, and its pa-        transmission rates will apply on each        with more information.
rameters must be signalled within the         connection and possibly affect inter-
standard packet headers. Audio may be         channel synchronization. The result of       FURTHER READING
encrypted for content protection or not,      this is timing differences between the       Specification of the Bluetooth System,
but this is application dependent. The        audio channels or phase distortion. This     version 1.1 (2001).
mandatory subband codec should use
                                                                                           Bluetooth Audio/Video Working
                                              2
                                               A. Floros, M. Koutroubas, N. Alexander-     Group (2002), Advanced Audio
1
 F. de Bont, M. Groenewegen, and W.           Tatlas, and J. Mourjopoulos, “A Study of
Oomen, “A High Quality Audio-Coding           Wireless Compressed Digital Audio Trans-
                                                                                           Distribution Profile 0.95b.
System at 128 kb/s,” presented at the 98th    mission,” presented at the 112th Conven-     Bluetooth Audio/Video Working
Convention of the Audio Engineering So-       tion of the Audio Engineering Society, J.
ciety, J. Audio Eng. Soc. (Abstracts), vol.   Audio Eng. Soc. (Abstracts), vol. 50, p.     Group (2002), Generic Audio/Video
43, p. 388 (1995 May), preprint 3937.         498 (2002 June), preprint 5516.              Distribution Profile 0.95b.

984                                                                                       J. Audio Eng. Soc., Vol. 50, No. 11, 2002 November