Note: Descriptions are shown in the official language in which they were submitted.
208~9S9
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Method of Common Transfer of Digital, Audio and Control
Data on a Common Bus Line; Bus System for Implementing
the Method and Interface for Application in the Method
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The invention relates to a method of common transfer of digital
audio and control data on a common bus line in a user system to
which at least one digital audio source and at least one digital
control data signal source belong; a bus system for implementing the
method and an interface for application in the method. :.
In many fields of engineering a local conglomeration of completely
different kinds of electrical and electronic equipment is required :~
to exchange information which thus requires networking in a
sometimes complicated manner. For instance, in m~tor vehicles,
sources of audio signals such as radio receivers, casse~te : ;~
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recorders or CD players require, on the one hand, connecting
together and, on the other, to audio sinks such as amplifier speaker
combinations. It may prove useful to also incorporate audio
equipment which formerly fonmed a separate system, such as car
phones in such a system. In addition, there is a trend towards
increasingly complex control and monitoring of vehicle functions. Of
the innumerable possibilities in this respect, here merely
monitoring the function of external lights on the vehicle or the
inflation pressure or measuring and/or controlling the engine speed -~
or the turbocharging pressure is cited.
For this purpose extensive wiring connections are necessary between
the individual components. Even when, as intended, many items of
equipment and functions are controlled and handled by a vehicle
computer, the problem of the many wires and the length thereof
remains. Also of disadvantage is the fact that many of the known -
components are unable to directly communicate with each other. For
instance, to prompt the audio system to output a recorded text as a
warning via the speakers, in the case of a reduction in tyre
inflation pressure, the expense involved until now was prohibitive.
A great many other examples of networking vehicle components have
already been stated which, however, have only been realized in part,
such as a standardi2ed transfer of audio data, on the one hand, or
control data, on the other, each via a separate network of shielded
wiring. Employing a bus system, although desirable, is hampered by
major difficulties as regards the necessary bus arbitration and the
resulting time losses. Since users of the most different kinds have
to be connected to the common bus line which is required to handle a
data thruput, to transfer two-channel digital audio data in addition
to control and measurement data, for example, conflicting transfer
requirements can hardly be avoided. m~
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The object forming the basis of the invention i91 to achieve a
universal system for the exchange of audio and control data between
many and varied items of electrical and electronic equipment via a
bus system which is reliable, cost-ef~ective ancl exceptionally
powerful.
This object - as far as the method is concerned - is achieved by the
features of claim 1 of the invention. The features of a bus system
suitable for implementing the method according to the invention as
well as a corresponding hardware interface are disclosed by claLms 5 ; ~ ;
and 8. ~
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The achievement according to the invention offers a wealth of
advantages. By all items of equipment to be connected to each other
being circuited in series by a single bus line, wiring can be saved
and the circuiting simplified. Using fiber optic cables for data -
transfer achieves, on the one hand, complete freedom from noise
whilst, on the other, greatly reducing weight and saving material
which in vehicle applications involving the use of very many
electrical and electronic components is particularly significant. ~;
Diqital transfer places only minor requirements on the quality of
the optical transfer medium and the electrooptical conver~ers used
and thus the costs thereof are slight. Since the transfer distances
involved are short, low-cost plastic fiber optic cables can be
employed. Due to the interfaces permitting standardization allowing ~-
the same type of interface module to be used, the electronics can
also be realized at low cost~ This eliminates complicated bus
arbitration since each user has its own transmit channel and control ~ ;
data can be transferred monodirectional on each section of the fiber
optic cable. This channel dedication permits optimum use to be made
of available user channel capacity, since no capacity whatsoever
must be reserved for bus arbitration. Apart from this, channel
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dedication permits exceptionally straight-forward user software
coupling.
In the preferred embodiment of the bus system the series circuit of
the users is a closed ring according to claim lt). Receiving control
data can then be acknowledged according to clai~ 3 by a specific bit
in the signal sequence being modified or removed. since the modified
signal sequence is returned to the transmitting interface via the
closed ring bus system, this interface is directly informed whether
its assigned control data channel is again available.
In accordance with claim 3 the AES3-1985 standard is used as the
transfer format. This standard was created in coordination with the -
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European Broadcasting Union as an internationally standardized
protocol for digital audio transfer which is already used in ~ `
conjunction with CD players and makes a single data channel
available to the user which can be employed to transfer control
data, for example. The fact that only a single data channel is
available shows that this format is not actually intended for use in
a bus system. The bus system according to the invention suffices
with a signal channel due to the time multiplexing on the bus line.
Apart from this, there is no problem in operating the bus system
according to the invention with its own standard; however, using an
international protocol facilitates communication with users such as
CD equipment, using this pro~ocol.
The remaining subclaims relate to further advantageous embodiments
of the invention.
The invention is not only suitable for application in automobiles
but also in particular in aerospace vehicles due to the low weight ~-
of the data transfer circuitry. In addition, networking stationary
video, audio, telephone and data processing systems is also possible
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by means of the bus system according to the invention to reduce the
volume of the circuit connections formerly neceC~sary.
Further features and advantages of the invention result ~rom the
following description of several embodiments ancl with reference to
the drawing in which~
ig. 1 is a schematic diagram showing a system of users employing
the method of the invention,
ig. 2 serves to explain the data transfer formats used in the
method r
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Fig. 3 illustrates the multiplexing step of the method,
Fig. 4 shows a data sequence in a control data channel,
ig. 5 is a schematic diagram showing how one of the interfaces
is equipped for selecting the various audio sources, and
Fig. 6 serves to explain the configuration and function of an ~ -
interface between fiber optic cable sections and a system
user.
In the embodiment shown in Fig. 1 an audio source 2, such as a radio
receiver, for example, presents its audio signal to an input circuit
4 of an interface 6. The input circuit 4 for instance, can be one of
the known circuits 12S, Sony bus or MSB-LSB-First having a variable
format length. The audio source 2 is further connected to a control
unit 8 which outputs certain control signals, for example, control ~
signals which determine which of several loudspeakers is to be ~ ;
signalled. These control signals are applied to interface 6 via a
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suitable parallel or serial interface 10 of the latter where they ~ ;
are converted together with the audio data into a common serial data
format which is described in more detail below and features a data
channel for the control signals. Should no such control signals be
available at interface 6, due to the radio receiver ~ailing to have
such options, for example, the bits representing the control signals -~
are set to zero. The data clock is determined by the clock of the
audio data stemming from the audio source 2, when it sends the data -
in a suitable data format or by means 12 comprising a clock source
and data buffer belongin~ to the interface. The data clock frequency
is 32.0, 44.1 or 48.0 kHz, for example, corresponding to a baud rate
of 2.0, 2.8 and 3.0 Mbaud respectively.
The seguence of audio and control data in common is converted by an -~
electrooptical converter 14 into a synchronous light signal which is
applied to a fiber optic cable section LWL A. This embodiment is
intended for a relatively compact system so that low-cost plastic
fiber optic cables and converters of the simplest kind can be used.
Fiber optic cables of plastic can also be circuited with very small - -
radii of curvature which is of a great advantage in an autombile,
for example. The fiber optic cable section LWL A ends in an
optoelectrical converter 16 of a second interface 6a which has an ~ ~
identical configuration to that of the interface 6 and which ~ -
connects an audio processor 2a. The latter may be, for e~ample, a
multi-channel bass/treble control. The audio data is made available
to the audio processor 2a via a suitable output circuit 18 where the
data is modified, as required, and returned to the interface 6a. In
this embodiment the audio processor 2a also features a control unit
8a which is able to ~eact to control data contained in the data
sequence and output by a suitable interface lOa or which is able to --
output control data itself. - -
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The same as in the first interface 6, here too ~he data, which may
be modified where necessary, are then applied to a second fiber
optic cable section LWL ~ for monodirectional entry into a third ~ -~
interface 6b, again having the same configuration as that of
interface 6, by means of which the data activates an audio sink 2b
such as, for instance, an amplifier/speaker combination via which
the audio signals are to be reproduced acoustically. The audio sink ~ ~
2b comprises a control unit 8b for receiving data from the control ~ `
data channel.
All interfaces 6, 6a and 6b have an identical configuration to
rationalize their production; however, it is not always the case
that they are used in their entirety.
Another feasible arrangement would be that the control unit 8b
belonging to the audio sink 2b has a control data output to prompt
corrections of the bass/treble control, should, for instance a
speaker failure occur. It is good practice not only in this case to
connect the third interface 6b via a third fiber optic cable section
LWL C to the first interface 6 as is indicated by the dashed line in
Fig. 1. Such a ring arrangement of users is, in fact, a preferred
embodiment of the invention and is described in more detail below.
Fig. 2 shows the smallest unit 20 of the data format preferably used
in view of the intended maximum-possible compatibility in accordance
with the AES-EBU protocol for audio transfer in the consumer range
and professional studio range AES3-1985 ( ANSI S4.40-1985, Standards
and Information Documents, Audio Engineering Society, Inc., New York
1985). This unit 20 is termed a subframe and contains 32 bits. A
first group 22 of 4 bits in the unit 20 serves to synchronize and
distinguish between the left-hand and right-hand stereo channel. A
second group 24 of 8 bits following in time sequence is available
for audio data which is, however, still hardly made use of -~-
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currently. This is followed by a third group 26 of 16 bits for audio
data which is already used. The termination o~ a subframe 20 is
formed by a sequence of discrete bits 28, 30, 32 and 34 of which bit
30 is termed the user bit and can be utilized by the user
arbitrarily without affecting audio transfer. A~ regards the
significance of the bits 28, 32 and 34 reference is made to the
cited AES-EBU protocol. Each stereo channel is assigned a subframe
20, two of which follow in sequence in each case to form a frame,
192 frames then forming what is termed a block.
The user bit 30 causes a serial data channel to be opened which is
subdivided into any number of subchannels by a time multiplex
procedure. Fig. 3 shows, for example, the divisioning into 10
subchannels. Each 1st, 11th, 21st etc bit of the data sequence
contained in the data channel opened by the user bit 30 is assigned
as a subchannel to a channel 1, each 2nd, 22nd, 32nd etc bit to a
channel 2, and so on. Each user transmits on a channel. For a total
baud rate of 2 Mbaud a control data baud rate of 64 kbaud results
for a 32 bit long subframe 20. In a preferred embodiment 8
subchannels are provided for each of which a transfer capacity of 8
kbaud thus remains, such a capacity being sufficient to transfer
even complex infonmation.
Fig. 4 shows a special data sequence in one of the 8 user channels
of four blocks n, n+1, n+2 and n+3, each 48 bits long ~384 -
subframes/8 subchannels~. Each block bgsins with a bit 40 set to
logical 1 to mark the start of the data sequence. This is followed
by a start bit 42, this being set to logical 1 on commencement of
the data message to be transferred and to logical 0 when the data
message is continued with the next block. The start bit is followed
by a 7 bit long address 44, each user being assigned a specific f
address. The address 44 is followed by a bit 46 which the -~
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2~859~9
receiver of a data sequence can change to acknowledge receipt or
inform the sender whether a channel is again vacant or whether the
message was rejected. Bit 46 is followed by a bit sequence 48
containing coded information as to the length oi~ the subsequent
actual data sequence. This data is transferred ln groups 50,
identified in Fig. 4 by Datal, Data2, etc.
Fig. 5 is a schematic diagram showing how two further audio sources
Auxl and Aux2 are coupled to the bus system via an interface 6. The
configuration of interface 6 corresponds to that of the interfaces
6, 6a and 6b shown in Fig. 1. The selectors 52 and 54 which may also
be implemented electronically permit the choice of signal feed from
the audio sources Auxl and Aux2. In the special case in which the
selectors 52 and 54 are both in the UP position in Fig. 5 the audio
signals of the bus system pass the interface 6 unchanged.
The salient parts of one of the interfaces 6, 6a or 6b shown in Fig.
1 are depicted in the block diagram of Fig. 6. Parts which have
already been explained in conjunction with Fig. 1 have received the
same reference numerals. All parts with the exception of the
electrooptical convertors 14 and 16 are implemented in a single ;
integrated circuit to save costs. The converters 14 and 16 are here
considered as elements of the interface, but ln actual
implementation can be connected right from the start with the fiber
optic cables and are not connected electrically to the remaining -~
parts of the interface until the system is installed. A circuit 60
contains a phase locked loop PLL which tunes itself to the
electrical signals coming from the one converter 16. Circuit 60 is
also capable of determining when an address contained in the signals ;
agrees with an address assigned to the corresponding interface. In
addition, the data sequence is applied to the receiver buffer 62.
The audio data is converted via an output circuit 18 into various
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formats of which the one matching a respective connected audio unitis transferred. Control data is applied to a microprocessor of the
connected user and back again via an interface 10 also designed to
handle several formats. The control data to be t:ransmitted is
applied via a transmit buffer 64 tO a multiplexing means 66, the `~
mentioned format being configured, on the one hcmd, and, on the
other, the control data being read into the multiple~ channel
correct in time. Audio data arri~ing from the connected audio unit
via an input circuit 4, or, audio data from the receiver buffer 62
are also incorporated in the format and the complete data sequence
is applied to a converter 14 which converts the electrical signals
back into optical signals. Data management is the job of a control
and status register 68 connected to the receiver buffer 62, the
transmit buffer 64 and the interface 10. When an address contained
in the data fails to agree with the user address the data is passed
on unchanged. Due to buffering a certain shift in time results, this
being arranged, however, so that the data is resorted into its
correct channel. The clock information is received by the transmit
buffer 64 from a clock circuit 70 which receives the clock either ~ -~
from the PLL circuit 60 or from a separate clock generator. This
clock generator need not be a component of the interface, however,
since commercially available audio sources such as CD players
generally contain such a clock generator, this being the reason why
it is not indicated.
As evident from Fig. 1 a basic distinction is made among three
different modes of opPration as regards audio transfer. The mode of
the interface 6 shown on the left in the Fig. is an active mode in ~;-
which the mentioned data format is originally generated, i.e. based ~-
either on the clock of the data stemminq from the audio source 2 or -~ -
on the clock of a separate clock generator. In this so-called master
mode only one interface of a system is ~ ~
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208~959
operative at any one time. The two other interfaces 6a and 6b
generate no clock of their own, but adapt by the!ir PLL circuit to
the clock as specified. This mode of operation is termed slave mode.
Interface 6a shown in the middle of the Fig. ha a special feature,
it being able to receive and also modify audio data when required.
In this case the mode i5 termed a slave/processor mode. The
interface 6b on the right of the Fig. behaves purely passive as
regards the audio data by it merely passing the data on.
An essential feature of the invention is that data transfer via the
fiber optic cables is in one direction only. Only one user operates
in the master mode and thus dictates the cl w k. Together with the
fact that each user employs its own transmit channel and a bus
arbitration is not necessary it is possible to connect a large
number of users to a single network without transfer conflict -~
despite a high data thruput. Transmitting is possible on all control
data channels at the same time, this being the reason why each
control data channel has a mean data transfer rate which is
available at all times. Compared to conventional bus systems of
comparable performance the costs of realizing the system are
exceptionally low. Nhen the first and last user are reconnected
together a closed ring results in which a message can circulate
until its receiver accepts it. The sender of a message is able to ;~
determine whether its message has arrived at the receiver. To this
end, bit 46 is used which as already mentioned in conjun~tion with
Fig. 4 is altered or removed by the receiver of a message to -~
acknowledge the receipt. -~
In one particular embodiment of the invention the audio data is
rerouted already within the interface and directly retransmitted,
namely when the connected user features no audio unit at all or
merely executes control or monitoring functions.For instance,
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2085~59
the system can be used with any of the many types of sensors
employed in a vehicle. In one embodiment of the invention these
sensors can be interrogated within the scope of a vehicle diagnosis
for which a separate diagnosis subchannel is prc~vided by means of a :~
transparent bit which is not manipulated and passes through the ~-
chain of users without being changed.
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