Method and system for wireless real-time collection of multichannel digital audio

[0001] The invention relates to a method according to the preamble of claim 1 for wireless real-time signal collection from several independent sources for mainly audio purposes.

[0002] The invention relates also to a system according to the preamble of claim 6 for wireless signal collection from several independent sources for mainly audio purposes.

[0003] The invention relates to an error control method and system and a synchronization method and system for the said purposes.

[0004] The object of this invention is typically a system with the associated apparatus and method for the isochronous, electromagnetic disturbance resistant, wireless transfer of highest studio-quality multi-channel digital audio signals from several independent but synchronized sources to a central station. This same method can also be used as the basis of the high-speed transmission of other digital information with the same kind of real-time and bandwidth requirements such as synchronized digital measurements from several independent sources.

[0005] With the currently known technique, the studio-quality multi-channel digital audio signal from a set of independent signal sources such as microphones is first transferred to the multi-channel digital mixer with the balanced per-channel electrical cables. The analog-to-digital conversion is performed in the mixer and the channels are finally recorded to a digital storage device after the required balancing and mixing operations have been applied. Also, a transmission method with special purpose radio links is known. The physical analog transmission path injects several degrading effect such as noise, interference, distortion, group delays, amplitude and phase errors to the quality of the original signal. The cabling is often clumsy and can be messy looking especially in concert occasions. With careful design and balancing of cables and their wiring layout, these effects can be limited to some extent but seldom completely eliminated. The number and bulkiness of the cables, the need for careful design and tedious installation work increase the costs as well as required skills and time. Cables and their electromechanical connectors are also prone to mechanical failures, which are hard to find and fix. These problems are especially harmful in public performances when the performers and often even the audience move among the cables. Under these conditions, there can be a real hazard of harm and injury with the cabling. During artistic tours, the audio equipment is installed and uninstalled frequently to and from varying environments, which multiplies these problems, efforts, and costs.

[0006] With the use of modern capacitive microphones, having integrated and optimised preamplifiers within them, the analog signals can be of lower power level and also the more noise and interference resistant differential signalling can be employed. The generation of multi-channel differential signals requires, however, rather expensive high-quality analog electronics plus costly differential cabling and connectors independently of what type of microphones are used.

[0007] The currently available wireless audio microphone systems are non-standard radio or infrared solutions typically using lossy audio compression methods thus resulting compromised performance. They are therefore mainly used for supportive purposes such as public address voice transmission.

[0008] The aim of this invention is to solve problems relating to the isochronous real-time collection of the highest studio-quality streaming digital audio signals associated with the techniques described above by constructing a novel, international standards compliant wireless local area network (WLAN) based data communication system, transmitters, receiver plus the necessary firmware and software for the efficient restricted area collection of digital audio signals and the testing, configuration, management and control of such systems.

[0009] The invention is based on the idea that the digital information is transferred using special speeded-up sequential unicast from the different transmitter stations to the central collecting station in the studio-quality digital format with electro-magnetic radio waves without dedicated signal cables using typically internationally standardized and high-volume produced wireless local area networking (WLAN) components. The analog signal is converted to the digital form directly at the signal source and fed locally to the associated WLAN transmitter. This guarantees the ultimate sound quality at the microphone transmitter. Because of the application of the mass-produced WLAN technique and its commercial components plus the very small number of additional standard integrated circuits, the cost of the development work and the actual system components can be kept very reasonable. This part of the system is typically powered by a rechargeable battery pack, which additionally helps in achieving noise free source signals.

[0010] The method introduced here replaces the wired lines with the standard commercial wireless local area network technology as specified in the IEEE 802.11 series of standards. The special characteristics required for the uncompressed real-time transfer of multi-channel studio-quality audio signals have been implemented by the innovative choice of WLAN system coordination functions, communication modes, and control parameters together with a special upper layer firmware implementing the speeded-up sequential unicast.

[0011] In accordance with a preferred embodiment of the invention the audio data formed by samples is organized in audio frames and sent from the individual microphone stations to the receiver station within consecutive beacon intervals, using coordinated, speeded-up unicast messaging. According to the WLAN standards, two co-existing transmission services are possible. The usual mode, widely used in commercial data communication products, is called the contention-based service. The other mode, used seldom, but accurately specified in the IEEE 802.11 standard, is called the contention-free service, and it is the basis for this invention. Special beacon frames are used to control the switching between these two modes of operation. The length of the beacon interval is a programmable parameter and it is adjusted with this invention so that an optimum amount of isochronous audio signal data can be sent to the receiver, with a minimum of system delay. This optimal amount is in one preferred embodiment of the invention for the required amount of isochronous audio signal data for high quality audio broadcasting and recording.

[0012] In accordance with another preferred embodiment of the invention, an error control system optimised for isochronous digital audio transfer either minimizing the need or totally eliminating the need for retransmissions is used, where the received signal is correlated with other channels, is used for error correction purposes.

[0013] In accordance with a third preferred embodiment of the invention, the transmitters and their signal sampling are synchronized in a coordinated unicast system with the help of an end-of-frame interrupt, generated by the control frame terminating each beacon interval, at the exactly same instance within each beacon interval. This synchronization is further utilized to trigger the accurate coherent sampling of the audio signals of the independent sources and as the reference instance for the individual timers of the signal source transmitter timers that trigger the coordinated unicast transmission at the proper instance so that each transmitter is active at the right period of time without interfering with others.

[0014] In accordance with a fourth preferred embodiment of the invention, the transmission order and sequential timing of the transmitters are synchronized in a coordinated speeded-up burst unicast system with the help of an end-of-frame interrupt, generated by the control frame terminating each beacon interval, at the exactly same instance within each beacon interval and accurate timers in transmitters triggering the actual frame transmission at the right instance of time. This speeded-up arrangement guarantees the best possible usage of the WLAN bandwidth from a set of independent transmitters to a single receiver.

[0015] More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

[0016] Further, the system according to the invention is characterized by what is stated in the characterizing part of claim 6.

[0017] With the help of the invention significant benefits may be obtained.

[0018] With the coordinated per-signal-source transmission of the studio-quality digital audio, all the error factors associated to the traditional signal path can be avoided. Performing the digital-to-analog conversion immediately at the signal source itself maximizes the sound quality by localizing the propagation path of the analog signal on the fixed and optimized converter circuitry in accordance with one embodiment of the invention.

[0019] The signal cables, their connectors and differential signal transmitter/receivers and related material and installation work can be completely avoided. This eliminates all the cost, failure, and installation problems associated with them. As mass produced standard WLAN technique is the basis of the invention, its production cost can be made very low in accordance with one embodiment of the invention.

[0020] As the coordinated, speeded-up burst unicast transmission mode and frequent multicast synchronization are utilized, the sampling synchronization and the inter-channel phase errors can be effectively eliminated in accordance with one embodiment of the invention.

[0021] As optimized transmission frame sizes are used, the system level delay as well as the buffering requirements can be minimized to an insignificant level in accordance with one embodiment of the invention. The proper varying of the frame size further guarantees the smooth, even flow of the data stream.

[0022] With the help of the error control method in accordance with one embodiment of the invention a simple and fast best-effort audio error correction scheme can be obtained.

[0056] In this document, the following terms will be used in connection with the inventions.

• [0057] 1 WLAN, Wireless Local Area Network
• [0058] 2 mixer or recorder
• [0059] 3 collector receiver
• [0060] 4 Base station
• [0061] 5 Remote controller
• [0062] 6 audio source
• [0063] 7 transmitter
• [0064] 8 signal source subsystem
• [0065] 9 audio data format/sample
• [0066] 10 transmission level audio data format
• [0067] 11 error correction code
• [0068] 12 MAC sublayer, MAC=Medium Access Control
• [0069] 13 Distributed coordination function
• [0070] 14 Point coordination function
• [0071] 15 Contention-free communication services
• [0072] 16 Contention-based communication services
• [0073] 17 General MAC frame structure
• [0074] 18 Frame control
• [0075] 19 Duration/ID
• [0076] 20 Address 1
• [0077] 21 Address 2
• [0078] 22 Address 3
• [0079] 23 Sequence Control
• [0080] 24 Address 4
• [0081] 25 Frame body
• [0082] 26 FCS, Frame Control Sequence
• [0083] 27 MAC Header
• [0084] 28 MAC Frame
• [0085] 29 WLAN frame control field
• [0086] 30 Protocol version
• [0087] 31 type
• [0088] 32 Subtype
• [0089] 33 To DS, DS=Distribution System
• [0090] 34 From DS
• [0091] 35 More Frag
• [0092] 36 Retry
• [0093] 37 More data
• [0094] 38 Pwr Mgt
• [0095] 39 WEP, Wired Equivalent Privacy
• [0096] 40 Order
• [0097] 41 Individual Address
• [0098] 42 Group Address
• [0099] 43 Unicast Address
• [0100] 44 Multicast address
• [0101] 45 Broadcast address
• [0102] 46 Generic Beacon Frame
• [0103] 47 Frame control
• [0104] 48 Duration
• [0105] 49 Destination address
• [0106] 50 Source address
• [0107] 51 BSS ID
• [0108] 52 Sequence control
• [0109] 53 Frame body
• [0110] 54 FCS
• [0111] 55 Time stamp
• [0112] 56 Beacon interval
• [0113] 57 Capability info
• [0114] 58 SSID, Service Set IDentity
• [0115] 59 Optional fields
• [0116] 60 Beacon frame as used in this invention
• [0117] 61 Frame control
• [0118] 62 Duration
• [0119] 63 Destination address
• [0120] 64 Source address
• [0121] 65 BSSID, Basic Service Set IDentity
• [0122] 66 Sequence control
• [0123] 67 Frame body
• [0124] 68 FCS
• [0125] 69 Time stamp
• [0126] 70 Beacon interval
• [0127] 71 Capability info
• [0128] 72 SSID
• [0129] 73 CF parameter set
• [0130] 74 TIM, Traffic Indication Map
• [0131] 75 ERP, Extended Rate PHY
• [0132] 76 Extended rates
• [0133] 77 Element format
• [0134] 78 Element ID
• [0135] 79 Length
• [0136] 80 Information
• [0137] 81 TIM element
• [0138] 82 Element ID
• [0139] 83 Length
• [0140] 84 DTIM Count, DTIM=Delivery Traffic Indication Map
• [0141] 85 DTIM Period
• [0142] 86 Bitmap Control
• [0143] 87 Partial Virtual Bitmap
• [0144] 89 ERP information element
• [0145] 90 Element ID
• [0146] 91 Length
• [0147] 92 Non ERP-present
• [0148] 93 Use protection
• [0149] 94 Barker Preamble mode
• [0150] 95 r3-r7
• [0151] 96 Extended Supported Rates element format
• [0152] 97 Element ID
• [0153] 98 Length
• [0154] 99 Extended Supported rates
• [0155] 100 CF Parameter Set element format, CF=Contention-Free
• [0156] 101 Element ID
• [0157] 102 Length
• [0158] 103 CFP Count
• [0159] 104 CFP, Contention-Free Period
• [0160] 105 CFP Max Duration
• [0161] 106 CFP DurRemaining
• [0162] 107 CF-End frame
• [0163] 108 MAC-header
• [0164] 109 CF-end MAC Frame
• [0165] 110 frame control
• [0166] 111 Duration
• [0167] 112 RA, Receiver Address
• [0168] 113 BSSID
• [0169] 114 FCS
• [0170] 115 ERP-OFDM PHY Frame structure, OFDM=Orthogonal Frequency Division Multiplexing
• [0171] 116 Coded/OFDM
• [0172] 117 PSDU, Protocol Service Data Unit
• [0173] 118 PLCP Preamble
• [0174] 119 SIGNAL
• [0175] 120 Rate
• [0176] 121 Reserved
• [0177] 122 LENGTH
• [0178] 123 Tail
• [0179] 124 Parity
• [0180] 125 Service
• [0181] 127 Frame control
• [0182] 128 Duratrion/ID
• [0183] 129 Address 1
• [0184] 130 Address 2
• [0185] 131 Address 3
• [0186] 132 Sequence Control
• [0187] 133 Address 4
• [0188] 134 Frame body
• [0189] 135 FCS
• [0190] 136 Area of interest
• [0191] 137 WLAN Repetation period/Beacon interval (N*TU)
• [0192] 138 Foreshortened contention-free Period
• [0193] 139 Multiplexer and receiver
• [0194] 140 Serial to parallel converter
• [0195] 141 Buffer
• [0196] 142 USB host controller, USB=Universal Serial Bus
• [0197] 143 USB inputs
• [0198] 144 S/PDIF-inputs, S/PDIF=Sony/Philips Digital InterFace
• [0199] 145 Analog inputs
• [0200] 146 Analog buffers and multiplexers
• [0201] 147 A/D-converters, A/D=Analogue-to-Digital
• [0202] 148 MAC/baseband Processor
• [0203] 149 Microcontroller
• [0204] 150 D/A-converter and filter, D/A=Digital-to-Analogue
• [0205] 151 Select analog input
• [0206] 152 A/D-conversion
• [0207] 153 Select Digital input
• [0208] 154 24-bit reformatting
• [0209] 155 Select audio input
• [0210] 156 Number of channels 8
• [0211] 157 No
• [0212] 158 Yes
• [0213] 159 generate FEC and write to buffer, FEC=Forward Error Correction
• [0214] 160 Calculate missing channels
• [0215] 161 generate FEC and write to buffer
• [0216] 162 sample i−1
• [0217] 163 sample i
• [0218] 164 sample i+1
• [0219] 165 corrected sample i
• [0220] 166 ESS, Extended Service Set
• [0221] 167 IBSS
• [0222] 168 CF Pollable
• [0223] 169 CF Poll Request
• [0224] 170 Privacy
• [0225] 171 Reserved
• [0226] 172 Antenna
• [0227] 173 Most significant bits
• [0228] 174 Audio MAC frame
• [0229] 175 Control MAC frame

[0230] In accordance with FIG. 1, the system comprises one or several audio signal sources 6, which may be either digital or an analog sources. In FIG. 1 these are represented by studio microphones. The sources 6 are digitised, if necessary, and fed to the WLAN adapter and transmitter 7, which includes an antenna arrangement for robust wireless transmission to the collector receiver 3 and from there to the sound consoles, mixers, recorder(s) 2 or to broadcast subsystems. The receiver 3 and the base station 4 are typically controlled by a remote controller 5 or a computer. The signal from the signal station 7 is sent via a WLAN based network 1 using a sequence of isochronous, coordinated unicast messages to the receivers 4 from the signal source subsystem 8, consisting, for example, of several microphones 6. In other words the audio signal from sources 6 is transformed into digital data by elements 7 and transferred to the collector receiver as standard WLAN digital data.

[0231] FIG. 2 shows a simple example version of the collector receiver base station 4 and the audio storage and broadcasting equipment 2. The collector receiver base station 4 is typically a 108 Mbit/s extended IEEE 802.11g WLAN MIMO Access Point station, which receives a specified number of digital audio signals from the source transmitters. 108 Mbit/s is practically the lowest possible standard bit rate for the system of this invention. In the future, higher WLAN transmission speeds are expected and can be used to increase the number of signal sources in proportion to the increased transmission speed. They will also make it possible to improve the error correction methods using selective retransmissions. The received digital analog signals from the source transmitters it is converted to S/PIDIF or AES3 bit streams for processing, recording, and broadcasting.

[0232] Within the collector receiver station 3 there is a 48 KB memory ring buffer 141 or FIFO buffer for the intermediate storing of the incoming data. After initialization the collector receiver station 3 uses the contention mode traffic to initialise the signal sources. Each source is identified based on its unique MAC address and is assigned a sequence number ranging from 1 up to a maximum of 16. This sequence number is used as the basis of the coordinated sequential speeded-up unicast transmission described later. To start the collection the collector station changes its operation to the contention-free mode setting the beacon interval to 6 TUs and sending to the source stations a command to start the signal sampling from the synchronizing end-of-frame interrupt of the next CF-End control frame. From this point the coordination of the transmission is allocated to the cooperating signal source stations as described later. The WLAN part of the collector receiver station (and the source transmitters) conforms to the IEEE 802.11g standard with the range and transmission rate extensions introduced by Atheros Inc. and Airgo Inc. A MIMO antenna arrangement 172 is typically also used. The nominal bit rate is 108 Mbit/s. These implementations of the extended IEEE 802.11g WLANs also contain a powerful transmission error correction mechanism that effectively distributes the eventual transmission path burst errors to single bit reception errors at reception and is capable of correcting all of them on the octet level. This feature is taken advantage of in the specified application layer forward error correction method.

[0233] Contention-based, individually addressed messaging between the base station 4 and the receiver stations is used for the configuration, status monitoring, and control of the signal transmitters as well as the signal source equipment attached to them. There is an infrared handheld remote controller receiver, a USB 2.0 computer communication receiver/transmitter and a USB 2.0 general-purpose receiver/transmitter for Bluetooth and WLAN handheld remote controller adapters in the collector receiver station 3.

[0234] The system configuration, monitoring and control are done from the handheld remote controller(s) or from a (personal) computer application(s) as described above.

[0235] According to FIG. 4 the receiver 6 typically consists of a MIMO antenna subsystem 172, the IEEE 802g conformant WLAN circuit with the Atheros or Airgo range and transfer rate extensions. There are typically software controlled multi-color LEDs to aid the recognition and status of the individual signal sources 7 for the configuration, status monitoring and control operations. The WLAN is operated at the nominal speed of 108 Mbit/s. The received audio data stream is buffered into a 48 KB input ring or FIFO memory buffer and the source signal transmission from the buffer is started using a hardware timer controlled by the CF-End end-of-frame interrupts and the driver firmware. The data of the different sources is combined by a 32-bit processor 149 and fed to a S/PIDIF and AES3 parallel-to-serial converter 150 followed by optical and coaxial cable driver electronics and corresponding connectors. The output channel mode selection is done by the configuration and control software over the contention communication service of the WLAN.

[0236] The source transmitters 6 of the up to 16 channels each have an internal crystal-derived clock to generate the 192,000 Samples/s clock. These clocks are restarted by the end-of-frame interrupt generated by the CF-End control message of each of the 6,144 μs transmission slot to keep the independent signal sources and their sampling operations accurately mutually synchronized.

[0237] Two methods exist for the control of the system, a battery-powered handheld control terminal 5 and a software application available for several platforms including Linux, MS Windows, Apple, and Symbian operating systems.

[0238] The handheld remote controller 5 contains a keypad, a small display, a processor and a communication link to the base station. The keypad functions allow the selection of the output ports 2, the signal source group 8 and individual signal source 7 configuration and control. Signal source groups 8 as well as individual sources 7 can be smoothly activated and deactivated and their programmable features can be remotely adjusted. The handheld remote controller communicates with the collector receiver station 4 via an infrared, Bluetooth or WLAN link. The receiver station 4 relays the controls to signal sources through the individual signal transmitters using contention mode communication and either group or individual addressing. There is a panic key and function in the remote controller 5 that causes the smooth immediate muting of all signal sources 7.

[0239] The system described above can be fully controlled by a computer running the configuration, monitoring, and control application software. The commands and responses are communicated with the transmitter base station using a Bluetooth, IrDA, LAN, WLAN, or USB link.

[0240] According to FIGS. 5 and 6 the invented apparatus transmits isochronously, in real time, up to 16 fully independent but synchronized, strongly encrypted and uncompressed channels of 24-bit 192 000 Sample/s digital audio streams 11 from the individual signal sources to a common collector receiver station. A group 10 of 688 (or exceptional 689) discrete 24-bit samples 11, totalling 2 064 (or 2 067) sample octets, will be called transmission level source data block format in the rest of this presentation. The sustained application level digital audio data bandwidth requirement is thus 73,728 Mbit/s. Additionally there are the overheads caused by the PHY and MAC framing, encapsulation with the Advanced Encryption Standard based CCMP encryption, and the effects of the IEEE 802.11 contention traffic time allocation. These make even the largest IEEE 802.11g WLAN bit rate of 54 Mbit/s insufficient for this application. With today's standard WLAN techniques, the required performance cannot be achieved. The novel transmission method described below is based on the innovative use of the contention-free speeded-up unicast transmission with the Point Coordination Function (PCF) as specified in the IEEE 802.11 standards. With careful parameter tuning the bandwidth of the WLAN can be optimally divided between the PCF contention-free medium access mode and the usual Decentralized Control Function (DCF) contention access mode so that the isochronous multi-channel digital audio transfer becomes possible. With the 108 Mbit/s extension of the IEEE 802.11g WLAN network and by using the ERP-OFDM PHY layer framing it is possible to transmit the aimed sixteen (16) independent 24-bit, 192 kSample/s digital audio streams isochronously together with normal contention based WLAN data traffic. The same is, of course, also possible with the highest bit rates of the IEEE 802.11n equal or higher than 108 Mbit/s. The high number of channels, the high resolution, and the high sampling rate guarantee the wireless collection of the best sound quality commercially available today.

[0241] According to FIG. 25 the aim of the invention is to transfer enough audio blocks (transmission level audio data format) 10 in order to collect high quality audio sound. Firstly, the beacon interval 137 defined by the software settings has to be chosen correctly in order to achieve the aim. The beacon signal, defining the length of the beacon interval 137, is sent in intervals defined by an integer in the IEEE 802.11g WLAN standard. The value of this integer may have values from 1 to N. In other words, beacon interval 137 is a product of the beacon integer and time unit (TU). The length of one TU in IEEE 802.11g WLAN standard is 1,024 μs and therefore the beacon interval 137 is a multiple of TUs (1,024 μs). However, the standard defines, that in each beacon interval 137 there should be enough time reserved for the contention traffic, more precisely enough time for a maximum size frame, ACK, 2 slot times and 2 SIFS. In accordance with the invention, an optimum value for the number of time units TU for a beacon interval 137 is found to be 7. The optimum amount can be defined also as a sufficient amount in one preferred embodiment of the invention. This gives enough time to send 32 audio MAC frames 174 within one beacon interval 137. Each audio MAC frame 174 includes 688 or 689 transmission level audio data format blocks 10, the number of these blocks is defined in accordance with the table of FIG. 23. In this figure one row represents the content of the audio MAC frames 174 in one contention free period 138 of a beacon interval 137. As can be seen from FIG. 23, a predetermined sequence is repeated after each 125 beacon intervals. With the help of this detailed sequence, the average flow rates of the audio sources and WLAN output are matched, and the jitter can be held at the minimum, as shown in FIG. 24. This also results in a minimum requirement of buffer memory both in the transmitter and in the receivers 6.

[0242] According to FIG. 25, in order to guarantee the timely transport of audio source data, the highest possible repetition rate of contention-free periods 138 must be realized. At the same time, the maximum fraction of the network capacity must be reserved for the audio traffic. The IEEE 802.11 standard requires that there must be enough contention traffic time within each repeating contention-free interval for the transmission of one maximum size data frame together with its acknowledgement frame plus two SIFS periods and two slot times. With the 108 Mbit/s bit rate and with the ERP-OFDM PHY framing this requirement equals to 212+40+2×10+2×9=290 μs. As described in the IEEE 802.11 standard, the contention traffic in the beginning if the contention free period 138 may foreshorten the contention period by a maximum value of the sum of an RTS control frame, a CTS control frame, one maximum size data frame, an ACK control frame plus four SIFS. With the 108 Mbit/s bit rate and with the ERP-OFDM PHY framing this requirement is equal to 40+40+212+40+4×10=372 μs. The contention-free period starts with a Beacon frame 67 (FIG. 13) followed by a SIFS. With the 108 Mbit/s bit rate and with the ERP-OFDM PHY framing this requirement equals 76+10=86 μs. The contention-free period ends with a CF-End frame 109 (FIG. 20). With the 108 Mbit/s bit rate and with the ERP-OFDM PHY framing this requirement equals to 40 μs. The remaining time within the contention-free repetition interval is available for the contention-free data traffic. As the granularity of the contention-free interval is one 1,024 μs time unit (TU), the time available for contention-free traffic when the contention free interval is set to one TU is 1024−290−372−86−40=236 μs. Taking into consideration the maximum data frame size as specified by IEEE 802.11, the MAC, CCMP, and PHY encapsulation overheads and the SIFS between successive data frames, only the maximum of 17.7 Mbit/s effective user data speed can be done with this interval set-up. With contention free interval set to seven (7) TUs, the time available for contention-free data becomes 6 352 μs which allows the transmission of up to 32 of the 688 (or 689) sample blocks of 24 bits each. With 32 blocks per interval each of the 16 data sources will transmit a sample data block twice. This arrangement minimizes the sampling rate and transmission rate alignment cycle and simplifies the alignment algorithm.

[0243] To optimize the smooth flow of data and to minimize the buffering needs, the average rate of samples per TU should be kept as close to 1,024/1,000×192=196.61 as possible by varying the size of the data frames in the proper way in accordance with FIG. 23. At the same time the data flow from each of the 16 data sources should be as smooth as possible. The following frame size algorithm, that is one of the key innovations in this invention, is introduced. The contention-free time is first split into 32 block buffers of varying size. Each buffer corresponds to an individual sequential signal source. During each contention-free period each of the 16 sources transmits twice making the total of 32 buffers. These buffers are presented as columns in FIG. 23. The buffer size varies between 688 and 689 sample records each, according to the following set of size adjustment rules. If no exception rule applies, the default size is 688. The exceptional blocks contain 689 sample records each. The first exceptional block number x_j1 for the j-th data source is calculated by the formula

x_j,1=8 mod (13−j)+1,

resulting values 5, 4, 3, 2, 1, 8, 7, 6, 5, 4, 3, 2, 1, 8, 7, and 6 for the signal sources from 1 to 16, respectively. After this an exceptional data blocks repeats after each seven default size blocks until the limit of 250 source blocks is reached. Yet another exception rule is applied. For sources 1, 2, 3, 6, 7, 8, 9, 10, 11, 14, 15, and 16 the blocks 200, 221, 242, 11, 32, 53, 74, 95, 116, 137, 158, and 179 each will contain 689 sample records. After 250 blocks the full cycle is repeated. The full cycle thus contains 125 intervals of 7 TUs each resulting a full cycle time of 125×7×1 024 μs=896 ms. Each independent signal source transmitter implements its own sequencing. This algorithm guarantees, in accordance with FIG. 24, that the buffering jitter remains below +/−1.5 sample within all the buffer sets and becomes zero at the end of each 125^th sample buffer set. With this adjustment algorithm there is a worst-case margin of 80 μs within the contention-free data transfer time. This arrangement also makes it possible to support the effective user data contention traffic of up to 5 Mbit/s along with the real-time audio transmission. The contention traffic is available for system configuration and control as well as for other independent data exchange.

[0244] As shown above, the choice of at least seven TUs for the duration of the Beacon Repetition interval is required to reserve enough bandwidth for the contention-free isochronous audio traffic and to keep the rates alignment algorithm manageable. Selecting the minimum value of seven TUs further minimizes the system delay and buffering requirements. Also, selecting the value of seven TUs instead of any bigger ones, creates a maximum bandwidth for the contention-based traffic, in addition to the contention-free isochronous audio traffic.

[0245] According to FIGS. 6 and 7, the error control method is optimised for simplicity and speed under the assumptions of human listening of multi-channel studio-quality voice and music audio sound. This means a simple and fast best-effort error correction scheme that reduces the audible effect of the errors to a non-observable level. The method takes advantage on the long 24-bit audio data samples and the high 192 kSample/s sampling rate as well as the inherent property of the extended IEEE 802.11g implementation to transform transmission path originated burst errors to single-bit errors in reception. However, this error correction scheme is not appropriate for applications where no errors can be tolerated.

[0246] Thanks to the WLAN transmission error correction method, almost all the residual reception errors are single-bit errors. It is therefore sufficient to correct the effects of single bit errors. The error detection is done by comparing a sample to the average of the immediately preceding and following samples. If the difference is larger than a predefined maximum inter sample difference limit then all the 24 one bit variants of the sample prepared by bitwise exclusive or function of all the bit locations are compared to the calculated average and the one with the smallest absolute difference is chosen to replace the erroneously received sample. This process is illustrated in FIG. 7. Because of the high sampling rate, the residual errors are not audible by the human ear.

[0247] According to FIG. 20, the synchronization within the system is based on the repetitive appearance of the end-of-frame interrupt generated by the CF-End frame 109 at exactly 6 802 μs after the beginning of each repeating 7 168 μs contention-free repetition interval. The end-of-frame interrupt of this control message 109 synchronizes all the signal transmitters 6 in regard of signal sampling, transmission block size calculation, and transmission timing within the inaccuracy of the interrupt latency time difference of the receivers. Because all the receivers are programmed to wait for this particular interrupt, the system level synchronization jitter caused by the interrupt latency is of the order of one instruction execution cycle (added with the very small processor-to-processor crystal oscillator phase jitter). In practise, this total jitter is of the order of 100 ns and cannot possibly be noticed by human listener. For comparison, the 192 kSample/s audio sampling cycle is 5.21 μs.

[0248] According to FIG. 25 in the idle state, when no audio signal is present, the collector receiver is programmed to run the beacon interval of one time unit (1 TU). When the audio stream needs to be started, a contention-free mode command is sent to all transmitters using their group address and the beacon interval is reprogrammed to 7 TUs of 1 024 μs each totalling 7 168 μs. The CF-End end-of-frame interrupt of this frame triggers the beginning of synchronous source signal sampling in all transmitters. The transmitters also program their hardware transmitter timers to be started by the same interrupt. The transmission start time for each signal source is determined by the timer value generated by a special virtual token passing method as follows. The point coordination function (PCF) is implemented in the receiver collector of the WLAN access point station. The beacon repetition interval, and hence the contention-free repetition interval, are set to seven time units and every such period contains a contention-free and a contention part. The length of the allocated contention-free period is set to 6 748 μs using the CFPMaxDuration parameter in the Beacon frame 67 and this set-up leaves a guaranteed 290 μs for the decentralized control function (DCF) contention traffic. This time is large enough for the transmission one maximum length data frame during the contention period together with its acknowledgement and the associated inter-frame elements as required by the IEEE 802.11 standard. It also means that a minimum of 2.58 Mbit/s of bandwidth (when maximum size data frames are used) is always available for contention traffic. Under heavy traffic of large frames, the allocated contention-free period becomes foreshortened from the beginning when a frame is being transmitted during the expected start of the contention-free period. Because this contention exchange can include the CTS and ACK control frames with their associated inter-frame elements in addition to a maximum size data frame, up to a maximum of 324 μs may be consumed by the busy medium from the beginning of the contention-free period.

[0249] The worst-case transmission-timing scenario for the audio data is as follows. The expected beginning of the contention period occurs but a maximum length contention transfer sequence was just started. It will cause a 324 μs contention-free period foreshortening. Only after this foreshortening delay, the 40 μs Beacon message that sets the NAV condition, can be transmitted. The first audio data block transmission starts after an additional 10 μs SIFS time has elapsed. This is a total of 374 μs after the expected beginning of the contention-free period. In the case of a smaller or none foreshortening, a quiet filler period is inserted by the transmitter software to reach the 374 μs tick. This arrangement guarantees that the first audio bit is always sent on the same relative tick within the 6 748 μs contention-free repetition interval. The available transfer time for the contention-free audio data is therefore 7 168−374−290−40−10=6454 μs. In the worst-case scenario, all audio buffers contain either 688 or 688 24+8-bit sample records. The time needed to physically send either buffer together with their MAC headers and trailers as well as an AES based CCMP encapsulation overheads is the same 186 μs. Each frame is followed by a 10 μs SIFS period. The time needed to send two full sets of sixteen blocks from the sixteen independent signal sources together with their SIFS periods is thus 2×16 (186 +10)=6 272 μs. The transmission sequence is finally followed by a 80 μs programmed idle delay after which a 40 μs CF-End broadcast frame 109 terminates the contention-free period, also resetting the NAV condition initially set by the beginning of the Beacon frame. This happens exactly at the same time as the contention-free period would have ended based on the timers set by the CFPMaxDuration parameter of the Beacon frame. The time margin within the contention-free period of 80 μs out of the minimum available time of 6 352 μs represents just a 1.26 percent contention-free time margin. At this point, the contention period starts allowing the transmission of a single maximum size frame with an ACK response plus the associated two inter-frame SIFS times and two slot times as specified in IEEE 802.11 standard.

[0250] In accordance with FIGS. 1 -3 based on the commands from the remote controllers 5 the system selects a recording or broadcasting subset out of the possible n AES (S/PDIF) digital outputs. The roles of the signal sources 6 are also programmed at this point with the controllers using the individual addresses of the signal sources 6 and their LED indicators. Also the group address of the signal sources is set now.

[0251] In this application the speeded-up multicast means a procedure, where all transmitters 7 transmit their data packages back-to-back using the same group address and the end of frame interrupts triggered hardware timers for their transmission timing. Thus no polling and no acknowledgements are used. The first transmitter 7 is programmed to transmit 10 μs after the end of the end of frame interrupt of the Beacon frame. Other transmitters 7 are programmed to transmit 10 μs after the end of the end of frame interrupt of their predecessor's frame. Transmitter number 16 is considered the predecessor of transmitter 1. The sequence ends when each source transmitter has transmitted twice. The transmission times are listed in FIG. 25a and illustrated in FIG. 25b. This protocol is called the simplified Virtual Token Passing (sVTP).

[0252] This invention is applicable for various isochronous data transmission systems, but as described here, it is particularly suitable for multi channel audio signal collection purposes.

[0253] Some video applications are also suitable for some embodiments of the present invention.

[0254] In addition to the WLAN transmission medium, this invention is also applicable for UltraWideband radio transmission technology, or HomePlug AV type transmission technology, where the mains power cable is used also for data transmission. In the latter case, the transmission system is not literally wire free, but since active loudspeakers always require external power feeding through a cable, no additional cabling is required for data transmission.