Abstract:
A home use sound reproduction system for Hi-fi, with digital signal transfer from a playback unit via a control unit to one or more active loudspeakers, each including or arranged beside an amplifier unit. The control unit is arranged to control sound parameters and send both a digital sound information signal and a power signal for powering the amplifier units, and to superimpose the single ended or differential digital signal together with the power signal on at least one common lead in a cable.

Description:
BACKGROUND OF THE INVENTION 
     The present invention concerns a sound reproduction system for Hi-fi with subsystems for integration into conventional passive loudspeakers, in order to transform these loudspeakers into high performance active loudspeakers. Hi-fi is synonymous with High Fidelity and refers in the following to a sound system for home use that in particular strives to provide a sound that is as lifelike as possible. In other words the sound reproduction system is designed to provide a sound that the listener perceives as very or entirely like the original. Such systems require both high precision and great power. 
     SUMMARY OF THE INVENTION 
     The complete system of the invention comprises active loudspeakers  7 , an amplifier unit  9 , a power supply unit  6 , a control unit  4 , equipment  2  for the reproduction of sound that has been stored or is received in a digital or analogue form, and a cable or cables  10 . By active loudspeakers is meant, loudspeakers with integrated amplifiers. In other words, the amplifiers for the loudspeakers are physically located in the same units as the loudspeaker drivers  11 ,  12  and  13  that transform the electric signals into sound. The amplifying units may preferably be printed circuit boards for integration into the loudspeaker enclosures and further comprise components designed to receive and transform a standardised digital electric sound signal into a digital signal with separate sound data and a clock signal  14  suitable for a digital signal processor (DSP)  15 . The amplifying units further comprise digital signal processing components  15 ,  19 , power components  14 ,  18  for providing a power supply to the amplifying unit and the digital circuits, signal conditioning components  16   a ,  16   b , and  16   c  for transforming the digital signals into analogue signals, and amplifying components  17   a ,  17   b ,  17   c  or  20  for amplifying digital or analogue audio signal. 
     By power supply unit is meant a subsystem that converts the alternating voltage of supply mains to one or several DC-voltage levels suited to drive the amplifying units  9 . The power supply unit  6  comprises at least one transformer for the transforming of the AC-voltage level and at least one rectifier for the conversion from AC to DC. Within the concept of the invention this or these rectifiers may be physically separated from the transformer and instead integrated with the amplifier unit  9 . 
     By control unit is meant a unit that conditions and transmits a digital sound signal together with the power supply current via a cable  10  to the active loudspeaker  7 . The control unit may comprise for instance volume control, bass and treble controls, selection between different sound sources or other controls that allow the user to control the sound and may have receivers for interacting with a remote control playback of sound that has been stored or transmitted 
     The equipment  2 , in the following referred to as a playback unit  2 , is a unit presenting a digital sound signal to the system from a live source, as a microphone or musical instruments, or stored sound from for instance CD™, SACD™, DVD™, DVD-Audio™, HD-DVD™, Blu-Ray™, hard drive, alternative types of data memory, or internet. Also TV-/radio receivers, gaming consoles, or other equipment with digital sound output can constitute a playback unit  2 . The playback unit  2  may be an integrated part of the control unit for instance a CD-player, analog to digital converter (A/D converter) for analog sound received from microphones or musical instruments. 
     A cable  10  includes at least one electric lead that is preferably electrically isolated from the surroundings along its length save at the ends of the cable. If a cable contains two or more electric leads these leads are also electrically isolated from each other. A cable may include an electrically conductive shielding intended to prevent electromagnetic interference from being induced in its leads. 
     The state of the art regarding active Hi-fi loudspeakers have several problems. To integrate the amplifier in the loudspeaker enclosure requires, for instance, that the loudspeaker must be designed in consideration of this from the beginning since the power unit will take up a considerable volume in the loudspeaker enclosure. This makes active loudspeakers for Hi-fi very expensive to develop. It is a large step for a manufacturer to develop such speakers from scratch, as they require an entirely new construction and additional advanced electronics. The suggested invention alternatively enables the modification of existing passive loudspeakers to form active ones, since the amplifier unit  9  is of the same physical size as the passive electronics that is found in passive loudspeakers. 
     With today&#39;s technology, this is only realistic for Hi-fi by separating the power supply unit  6  from the active loudspeaker  7 . To integrate larger electronic units than those of a passive loudspeaker, the enclosure design would influence the sound character negatively since the volume and shape are very important parameters to achieve Hi-fi. The invention makes it possible to achieve the advantages of active loudspeakers from traditional, passive loudspeakers. 
     In active loudspeakers for Hi-fi that have been designed as such from the start, the performance of the power supply electronics must be weighed against the space it may take in the loudspeaker enclosure and how large and heavy the loudspeaker may be. If the power supply unit is under dimensioned, the performance of the sound system is considerably impaired. Furthermore the power supply unit develops heat near to the output transistors of the amplifiers and both must be cooled in an efficient way. This leads to the amplifier cooling device becoming unnecessarily bulky. 
     In addition to the problems with the development of active loudspeakers, the existing technology also presents a number of problems for the user. One example is the placement of the cables that is far more extensive for the active Hi-fi Loudspeakers of today than for traditional, passive ones. Each loudspeaker enclosure requires power supply cable and a signal cable. Furthermore several solutions require a third cable for control of the integrated amplifier. The presence of several cables in connection to each loudspeaker makes cleaning more difficult and many users also find it unattractive. Many users are unwilling to route long cables to the wall sockets. Also the furnishing alternatives will be limited, since the loudspeakers must be placed in a region with access to wall sockets. 
     Problems with control of the amplifiers in the active loudspeakers of today are also present since there is no control unit  4 . In conventional systems with passive loudspeakers there is an external amplifier or pre-amplifier that is also used to adjust sound volume, for instance. In digital systems with active loudspeakers for Hi-fi such adjustments must instead be made directly at the loudspeakers, physically or via remote control. The implementation of the invention instead looks exactly like a conventional system with passive loudspeakers, since the control unit for the user looks like and functions as the amplifier or preamplifier in the conventional system with passive loudspeakers. 
     The invention solves the above problem by physically separating the power supply unit  6  from the active loudspeaker  7 . This allows the amplifying unit  9  to be made sufficiently small to enable said integration in passive loudspeakers, without having to compromise the sound quality regarding power or precision. The separation of the power supply unit and the amplifier unit enables a solution to the problem of several cables in connection with each active loudspeaker, since one single cable  10  delivers both a digital sound signal and a power supply signal from the control unit  4  to the active loudspeaker  7 . In order to drive amplifiers for Hi-fi, the cable  10  must be able to deliver a continuous power of at least 160 watts to each active loudspeaker  7 . Amplifiers of, for instance, type class-D can be obtained with a very high efficiency. Such a solution could possibly lower the demand for power transfer and thereby, to a degree, reduce the cross section of the leads in the cable  10 . This is however not suitable for Hi-fi with today&#39;s technology since the distortion is greater for class D and similar solutions than for analogue amplifiers as for instance class-A, class-AB and class-B. This distortion is most obvious for sound with higher frequencies, which means that amplifiers for midrange and treble preferably should be analogue. Class-D and similar amplifier technologies with high efficiency are suitable for sound with low frequency (bass) since the distortion in those ranges is comparatively large from, for instance, loudspeaker drivers. This means that the demand on the precision of the amplifier therein is lowered. Furthermore it is bass reproduction that requires the most power. Irrespective of if amplifiers with high efficiency are used, each active loudspeaker for genuine Hi-fi should have amplifiers with a total power output of at least 150 watts. With minimal losses this means that the cable  10  should be designed to deliver an output of at least 160 watts continuously. 
     The invention also enables the use of a standard cable with so few leads that no special connectors are required in the ends for simple mounting. This means that the user may obtain the cable  10  sold by the meter to be cut in optimal lengths for efficient and practical placing of the cables in all types of rooms. 
     Additionally the system provides other known advantages, since the control unit  4  delivers a digital audio signal all the way to the amplifier  9  in the active loudspeaker  7 . This provides noise free signal transfer entirely without audible disturbances all the way to the loudspeaker enclosure, since the amplifying unit  9  can work with exactly the same sound data that is stored in the storage media  1 . In loudspeakers for Hi-fi it is desirable to separate the electric signal in at least two frequency ranges with a so called crossover filter. This is because there does not exist today, loudspeaker drivers for Hi-fi that can handle the entire audible frequency range. Instead, separate loudspeaker drivers must be used for at least low and high frequency sounds (bass and treble respectively). In order to further increase the accuracy of the sound reproduction, the signal is frequently separated into three or more separate loudspeaker drivers, for instance bass, midrange and treble. Thereby, the loudspeaker drivers only have to work in the frequency ranges where their sound characteristics are at their best. By replacing the analogue separation filter in conventional active speakers with digital filters in the processing device  15 , the invention enables a filter design that in reality is impossible with analogue electronics. Furthermore the processing device  15  can compensate and enhance the sound characteristics digitally, which further increase the accuracy of the sound reproduction so that the sound reproduction system produces nearly ideal Hi-fi sound. 
     Since the invention provides an electric analogue audio signal only in a minimal part of the sound reproduction system, it is very insensitive to disturbances and the resulting noise level will be minimal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an overview of what is referred to as a sound reproduction system. Numerals in the figure followed by a letter indicate several identical components. In particular L, R and C after a numeral indicates left, right and centre examples of the same type of component. This is an example system for 3-way (bass, midrange and treble) 3-channel sound (L, R and C), but the invention is not limited to 3-channel sound but the number of amplifier units  9  and loudspeakers in the system may be arbitrary. Likewise the number of loudspeaker drivers  11 ,  12  or  13 , and the number of amplifiers  17 , or  20  that are connected to each amplifier unit  9  are arbitrary. 
         FIG. 2  depicting the same sound reproduction system as  FIG. 1  but with an alternative cable arrangement between the control unit  4  and the active loudspeakers  7  that is not possible with passive loudspeakers, is illustrated. 
         FIG. 3  depicts a possible design of the amplifier unit  9  with signal transformation from a digital into an analogue signal before the amplifier for each channel. Only key components are illustrated. Numerals in the figure followed by a letter indicate several components of the same type. 
         FIG. 4  depicts another possible design of the amplifier unit  9  with signal transformation for at least some of the channels from digital PCM-data to digital PWM-data  19  followed by an amplifier  20  intended for this type of input signal, for instance an amplifier of the type class-D. Only key components are illustrated. Numerals in the figure followed by a letter indicate several components of the same type. 
         FIG. 5  depicts an example of superimposing signals that enables the transfer of a stepped down direct or alternating current feed and a single ended digital signal via a cable  10  with two leads. Numerals in the figure followed by a letter indicate several components of the same type. 
         FIG. 6  depicts an example of superimposing signals that enables the transfer of a stepped down direct or alternating current feed and a differential digital signal via a cable  10  with two leads. Numerals in the figure followed by a letter indicate several components of the same type. 
         FIG. 7  depicts an example of superimposing signals that enables the transfer of a stepped down direct current signal, with both positive and negative voltage relative the ground, in combination with a differential digital signal via a cable with three leads. Numerals in the figure followed by a letter indicate several components of the same type. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The description follows the path of the signal from the playback unit to the loudspeaker drivers. 
     The signal from the digital playback unit usually conforms to a standard for the digital transfer of sound, for instance S/PDIF (belong to standard IEC-60958), AES/EBU (belongs to standard AES3) or DSD™ (direct stream digital). The signal may either contain information of uncompressed sound samples (for instance pulse-code modulated—PCM), a compressed data stream that has to be decoded before it can be reproduced (for instance Dolby Digital™ or DTS™), or a data stream that directly corresponds to a sound signal if it is to be sent directly through suitable hardware (DSD™). The control unit  4  or the processing unit  15  in the amplifier unit  9  is capable of decoding a compressed data stream. 
     In a preferred embodiment, the control unit  4  prepares the signal with or without said decoding, by transforming it so that it conforms to the AES/EBU-standard for uncompressed stereo sound. In order to enable the control unit  4  to send sound parameters, as for instance volume and other signal adjustments to the amplifier unit  9 , the control unit places all such information in the part of the outgoing data stream that, in the standard, is named “User Data”. This is the part of the protocol that that can be set arbitrarily without violating the specifications of the standard. In addition to the basic re-encoding of the data, additional optional modification of the data, as for instance correction for the acoustics of the room, take place in the control unit  4 . The control unit  4  then superimposes the AES/EBU-signal  8   a  and  8   b  according to  FIG. 6  or  FIG. 7 . High pass filters  21   a ,  21   b  and  23  and low pass filters  22   a ,  22   b    24   a  and  24   b  are arranged to separate the digital signal and the direct current feed on the sending and receiving sides. The high pass filter  23  also contains an AES/EBU-receiver that transforms the AES/EBU-signal into a suitable format, for instance I 2 S, on the data bus  25  that interfaces with the processing device  15 . Between the control unit  4  and the amplifier unit  9 , a cable  10  with at least two leads is arranged to connect these components. Thick leads enable transport of large direct currents for active loudspeakers  7  intended for Hi-fi. 
     In another preferred embodiment, the control unit  4  prepares the signal with or without said decoding, by transforming it so that it conforms to the S/PDIF-standard for uncompressed stereo sound. The control unit can re-encode the signal as in the preceding embodiment with the only difference being that the coding in this embodiment follows the S/PDIF-standard. The control unit  4  then superimposes the S/PDIF-signal  8  according to  FIG. 5 . The high pass filters  21  and  23  and the low pass filters  22  and  24  are arranged to separate the digital signal and the power signal on the sending and receiving side respectively. The high pass filter  23  also contains an S/PDIF-receiver that transforms the S/PDIF-signal into a suitable format, for instance I 2 S, on the data bus  25  that interfaces with the processing device  15 . Between the control unit  4  and the amplifier unit  9 , a cable  10  with two leads is arranged to connect these units. Thick leads enable transport of large currents for active loudspeakers  7 , intended for Hi-fi. 
     In yet another embodiment the control unit sends the digital sound signal through a separate lead in the same cable as the current supply to the amplifier unit  9  which requires three to four leads depending on if the amplifier unit  9  also requires negative voltage supply in relation to ground. 
     In yet another embodiment of the invention, the system looks as in some of the preceding embodiments, but the digital signal, delivered by the control unit  4  to the active loudspeakers  7 , is encoded differently, or alternatively modulated in a carrier wave, that enables the amplifier unit  9  to interpret the digital content of the signal with an appropriate receiver  14 . 
     In another embodiment of the invention, the system looks as in one of the preceding embodiments, but the control unit does not carry out any re-encoding, but instead the processor  15  in the amplifier unit  9  carries out corresponding re-encoding. This solution requires that information of sound volume is provided in the data stream from the play back unit  2 , that the control unit  4  puts this information into an otherwise unchanged, compressed data stream or that the sound data in itself has been altered to adjust the volume. 
     The amplifying unit  9  extracts data with an appropriate receiver  14  and treats the direct or alternating current in leads  26  and  27  ( FIG. 5 ), or alternatively in leads  26   a ,  26   b  and  27  ( FIG. 6  or  FIG. 7 ), to provide the amplifier unit  9  with suitable voltage levels for the different components comprised therein. That the entire power supply unit or its converter  6  is physically separated from the amplifier&#39;s active components, for instance  17  or  20  does not mean a decline in performance. In order to enable delivery of large currents to the amplifiers  17  or  20 , appropriate capacitors  18  are coupled in parallel to the current feed of the amplifiers. The capacitors  18  can, when they are charged, deliver very large currents for a short time. 
     In a preferred embodiment ( FIG. 3 ), the digital to analogue signal converters (D/A-converters)  16   a ,  16   b  and  16   c  are arranged to provide an analogue electric signal that directly corresponds to the sound waves that the loudspeaker drivers  11 ,  12  and  13  are to emit in the room. The analogue amplifiers  17   a ,  17   b  and  17   c  of the type class-A, class-AB, class-B or the like are arranged to amplify the electric audio signal provided by the D/A-converters  16   a ,  16   b  and  16   c.    
     In another preferred embodiment ( FIG. 4 ), the digital PCM-signal to PWM-signal converters (PWM-processors)  19  are arranged to create, after the processing device  15 , a digital PWM-signal on at least one of the channels of the same amplifier unit  9 , for instance the channel for low frequency signals (bass). In order to amplify this PWM-signal, an amplifier of type class-D or the like  20  is arranged, which gives a high efficiency. 
     In a another embodiment, an amplifier with high efficiency  20 , for instance of the type class-D, is arranged in at least one of the channels in the same amplifier unit  9 , for instance the channel for low frequency signals (bass), to amplify an analogue signal created by a D/A-converter  16 . A class-D amplifier delivers a pulsed signal where the duration or mutual distance is controlled so that, when they are integrated by the mechanical inertia in a bass driver, the desired sound is obtained while the overtones generated by the pulse train are extinguished by the same inertia. Since the amplifier principally works as a switch, the power losses and heat generation in the amplifier will be small. This type of amplifier is nevertheless often provided with an electric reconstruction filter at the output which removes high frequency components before the cable that is arranged to deliver the signal to the loudspeaker driver in question. This is in order to provide an improved sound quality and at the same time minimize the antenna effects that otherwise arise. 
     After amplifying and optional reconstruction filtering in the preamplifiers  17  or  20 , a loudspeaker cable is arranged to connect each one of these amplifiers directly with their loudspeaker driver  11 ,  12  and  13 . 
     Within the concept of the invention one can also mount the amplifier units on the back side of their loudspeaker to provide an analogue sound signal that is also filtered by the integrated filter of the loudspeaker. In this way, existing loudspeakers may be used without dismantling. 
     Within the concept of the invention, the amplifier units in or on the loudspeakers may individually, either by adjustment of each amplifier unit or centrally from the control unit, be adjusted so as to alter the frequency band limits, the slope of the filters, and other parameters for the driving of the loudspeaker drivers in the loudspeakers. 
     The invention allows the user to choose many alternative ways to connect the loudspeakers to the control unit, including in a chain after each other and/or with branching off wherever it is practical or desired in order to reduce the amount of cables. The sound information can be delivered to all the loudspeakers and the amplifier for each loudspeaker can sort out its own individual sound information undisturbed by the sound information to the other loudspeakers. Furthermore the cables may be connected lead by lead instead of by connectors, allowing cables to be easily cut to the right length, just as is done with analogue loudspeakers but with fewer meters of cables on the floor. 
     The invention is not limited to the use of only three loudspeakers and the control unit may receive the sound signals in different ways and from different sources including via the local mains through a wall socket or via a wireless transmission.