WO2001035696A2

WO2001035696A2 - Digital loudspeaker

Info

Publication number: WO2001035696A2
Application number: PCT/GB2000/004259
Authority: WO
Inventors: Graham Bank
Original assignee: New Transducers Limited
Priority date: 1999-11-10
Filing date: 2000-11-07
Publication date: 2001-05-17
Also published as: WO2001035696A3; AU1288701A; GB9926489D0

Abstract

A hybrid digital loudspeaker (10, 30) comprising at least two separate loudspeakers systems (12, 14, 32), each loudspeaker system being adapted to cover a different frequency range and each loudspeaker system being fed with a signal of different resolution, such that the resolution is varied with frequency according to sensitivity and discrimination of a user's hearing. The invention takes advantage of studies of the properties of hearing which show that the ear does not have a constant level of sensitivity and discrimination over the audible frequency range. Typically the hybrid digital loudspeaker (10, 30) has a low frequency loudspeaker system (12, 32) and an upper frequency loudspeaker system (14). The low frequency loudspeaker system (12, 32) is fed a signal (22) of lower resolution than the upper frequency loudspeaker system (14).

Description

TITLE: DIGITAL LOUDSPEAKER

DESCRIPTION

TECHNICAL FIELD The invention relates to digital loudspeakers.

BACKGROUND ART

The term "digital speaker" includes both a loudspeaker which converts digitally coded signals directly into sound energy and an analogue speaker with on-board integrated digital to analogue conversion and amplification.

With conventional analogue speaker system design it is customary to split the frequency range into two or more bands and assign specialised loudspeaker drive units to each band e.g. a larger higher excursion drive unit dedicated to low frequencies, and a smaller lower mass drive unit optimised for the higher frequency portion of the working range . In contrast, conventional thinking for a digital loudspeaker comprising an array of identical loudspeaker drive units or electroacoustic transducers is that each individual drive transducer covers the whole of the required audio band. The electroacoustic transducers are fed audio signals encoded digitally on a carrier as binary bit weights at high frequencies, sampling at least double the highest required audio frequency. The chosen sampling rate may be from as low as 44.1kHz to as high as about 200 kHz.

In practice electroacoustic transducer designers, particularly for the conversion of electrical energy into sound, find significant difficulty m satisfactorily covering a wide bandwidth, e.g. a frequency range of several octaves, with a single transducer. For example 100Hz to 7kHz is a range of about 6 octaves. At low frequencies, i.e. around and below 100Hz, the transducer needs to be capable of significant excursion to reproduce the lowest frequency modulation component at significant level. At high frequencies, i.e. around and above 7kHz, the transducer must have diaphragm integrity and very low moving mass .

There are thus major problems with designing a wide bandwidth transducer. Additionally, there are numerous but relatively minor practical difficulties associated with directivity and the potential mechanical noise associated with the most significant bit change for normal PCM (Pulse Code Modulation) digital code, [see HOOLEY WO96/31806 for unary modulation] DISCLOSURE OF INVENTION

According to the present invention there is provided a hybrid digital loudspeaker comprising at least two separate loudspeaker systems, each loudspeaker system being adapted to cover a different frequency range and each loudspeaker system being fed with a signal of different resolution, such that the resolution is varied with frequency according to sensitivity and discrimination of average human hearing.

By altering the resolution, advantage is being taken of studies of the properties of hearing which show that the ear does not have a constant level of sensitivity and discrimination over the audible frequency range. The region of greatest aural sensitivity lies near the middle of the audible spectrum with an average ear having a strongly falling sensitivity above 15kHz. At bass frequencies the aural discrimination is substantially lower and continues to fall with frequency. As a result severe second harmonic distortion for fundamentals of about 8kHz and above and severe third harmonic distortion for fundamentals of about 5kHz and above are unlikely to be noticed. Thus at high frequencies, reproduction mechanisms which due to transducer design or to finite chosen resolution create significant harmonic distortion may still be satisfactory.

Similarly, at low frequencies perhaps 5% or even 10% of distortion may not be audible for a 60Hz fundamental at normal sound pressure levels (spl) , e.g. 80dB spi . In contrast, a mere 1% of distortion may be audible for a 600

Hz pure tone .

Accordingly, the resolution of the signal applied to the loudspeaker system covering the middle of the audible spectrum is preferably of high resolution. For high resolution, there are preferably between four and twenty bits m each signal . Thus the data density m each signal is preferable high. Four bits may be sufficient for speech only communication. For high fidelity applications, the number of bits may be preferably 16 rising to 18 or even 20. By using 16 bits or more, it should be possible to achieve a total harmonic distortion of approximately 1% to 2%, possibly as low as 0.4% or 0.5% and provide scope for controlling dynamic range and volume.

At low frequencies (e.g. below 200Hz) , a lower resolution may be used and thus fewer digital bits may be used. Even with such a lower data density the perceived sound reproduction should be equally satisfactory as the higher resolution used m the mid-range because of the reduced aural discrimination at low frequencies. Similarly, at high frequencies a lower resolution may also be used.

For high- fidelity applications, lower resolution may be a signal having eight-bit modulation. Thus, eight-bit modulation may be sufficient at h gh frequencies (i.e. above 8kHz) . Similarly, eight-bit modulation may also be sufficient at low frequencies. Thus, the overall sample rate may be reduced Furthermore well known oversamplm , noise shaping and decimation techniques may be employed to downconvert the incoming data to a resolution within the compass of the speaker system design. The hybrid digital loudspeaker may comprise a low frequency loudspeaker system and an upper (or mid to high) frequency loudspeaker system. The hybrid digital loudspeaker may further comprise a digital signal processor having crossover with a crossover frequency set at around 200 Hz. Thus low frequency may be defined as any frequency below 200 Hz and upper or mid to high frequency as any frequency above 200Hz. The low frequency loudspeaker may be fed a lower resolution signal than the mid to high loudspeaker. The mid to high frequency loudspeaker system may comprise an array of small loudspeaker drive units or electroacoustic transducers . The array may be driven by the mid-high frequency content of an input signal via a digital-processing algorithm. The small loudspeaker drive units may be pistonic loudspeaker drive units.

Alternatively, the small loudspeaker drive units may be resonant panel devices, e.g. distributed mode loudspeakers as described m International patent application WO97/09842. Such resonant panel devices generally have an extended high frequency response which may be helpful m view of the high frequency sample rate.

The low frequency loudspeaker system may comprise either a digital or an analogue loudspeaker. The low frequency loudspeaker system may comprise a pistonic loudspeaker woofer The low frequency content of an input signal may be converted with a digital to analogue converter and the output may be fed via an amplifier into an analogue low frequency loudspeaker

Alternatively, the low frequency loudspeaker system may comprise an array of small loudspeaker drive units or electroacoustic transducers. There may be less loudspeaker drive units m the low frequency loudspeaker system than m the mid/high frequency system. The reduced number may be sufficient to reproduce the low frequency region, even though there will be coarse resolution of levels, since the hearing mechanism is less sensitive to errors m low frequency reproduction. Thus poorer resolution m this woofer section may be inaudible. Of course an increased number may by used to reduce the step size m this low frequency region.

By dividing the digital loudspeaker system into two bands, for example low and mid-high frequency, the problems of producing a wide bandwidth electroacoustic transducer as outlined above are alleviated since the transducers do not have to reproduce the whole of the band. Furthermore, good subjective fidelity may be achieved while greatly reducing the quantity of data needed for satisfactory audio reproduction. Reducing the data and useful m the terms of more economical storage and transmission.

In addition prior art algorithms may be used as i required to assist with issues such as the MSB (Most Significant Bit) changeover which may otherwise cause audible stray noise to become audible from the transducer array. BRIEF DESCRIPTION OF DRAWINGS

The invention is diagrammatically illustrated, by way of example, m the accompanying drawings, m which: -

Figure 1 is a diagram of a first embodiment of a digital loudspeaker having low and high frequency arrays of small electroacoustic transducers;

Figure 2 is a diagram of a second embodiment of a digital loudspeaker having a conventional low frequency pistonic subwoofer and a high frequency array of small electroacoustic transducers, and Figure 3 is a diagram showing a digital hybrid signal flow path. BEST MODES FOR CARRYING OUT THE INVENTION

Figure 1 shows a hybrid digital loudspeaker (10) comprising a low frequency loudspeaker system (12) and a upper frequency loudspeaker system (14) . Each of the low and upper frequency loudspeaker systems (12,14) comprises an array of small electroacoustic transducers (16) . The low frequency loudspeaker system (12) has "n" transducers and the upper frequency loudspeaker system (14) has a greater number of transducers. Figure 1 is a schematic drawing and only shows part of the array for the upper frequency loudspeaker system (14) .

The hybrid digital loudspeaker (10) receives a digital audio input signal (18) The input signal (18) is processed m a digital signal processor (20) . The processing may comprise filtering, decimation, noise shaping, oversamplmg and re-codmg steps. The signal processor (20) divides the input signal (18) into two output signals (22,24) according to frequency.

A first output signal (22) contains the low frequency content of the original input signal (18), i.e. content below 200Hz. The first output signal (22) is fed to the low frequency loudspeaker system (12) . A second output signal (24) contains the mid-high frequency content of the original input signal (18) , i.e. content above 200Hz. The second output signal (24) is fed to the upper frequency loudspeaker system (14) . Both the first and second output signals (22,24) are fed to the arrays of transducers via a digital data link (26) .

Figure 2 is similar to Figure 1 and thus features common to both have the same reference number. Figure 2 shows a hybrid digital loudspeaker (30) comprising a low frequency loudspeaker system (32) and a upper frequency loudspeaker system (14). In contrast to the embodiment of Figure 1, only the upper frequency loudspeaker systems

(14) comprises an array of small electroacoustic transducers or loudspeaker drive units (16) . The low frequency loudspeaker system (32) comprises an analogue pistonic loudspeaker (34).

As m Figure 1 a signal processor (20) divides an input signal (18) into two output signals (22,24) o according to frequency A first output signal (22) contains the low frequency content of the original input signal (18) and a second output signal (24) the mid-high frequency content The first output signal (22) is fed to the low frequency loudspeaker system (12) via a digital data link. The second output signal (24) is fed to the upper frequency loudspeaker system (34) via a digital to analogue converter (36) and an amplifier (38) . As shown m Figure 3 , the digital signal processor (20) (of Figures 1 and 2) receives an input signal (22) and a sampling rate (40) The input signal (18) is processed into two output signals (22,24) each covering a different frequency range and each having a different resolution To take advantage of the fact that a listener's ear does not have a constant level of sensitivity and discrimination over the audible frequency range, the first output signal (22) is of lower resolution than the second output signal (24) . This is because the second output signal (24) covers the middle audible range, i.e. the frequency range for which a user's ear is most sensitive. INDUSTRIAL APPLICABILITY

The invention thus provides an improved digital loudspeaker system.

Claims

1. A hybrid digital loudspeaker (10,30) comprising at least two separate loudspeaker systems (12,14,32), each loudspeaker system being adapted to cover a different frequency range and each loudspeaker system being fed with a signal of different resolution, such that the resolution is varied with frequency according to sensitivity and discrimination of average human hearing.

2. A hybrid digital loudspeaker (10,30) according to claim 1, wherein the resolution of the signal (24) applied to the loudspeaker system (14) covering the middle of the audible spectrum is of high resolution.

3. A hybrid digital loudspeaker (10,30) according to claim 2, wherein for high resolution there are between four and twenty bits m each signal.

4. A hybrid digital loudspeaker (10,30) according to claim 3, wherein the number of bits is greater than 16.

5. A hybrid digital loudspeaker (10,30) according to any one of claims 2 to 4 , wherein a high resolution signal produces an acoustic output having a total harmonic distortion of approximately 1% to 2%.

6. A hybrid digital loudspeaker (10,30) according to any one of the preceding claims, wherein the resolution of the signal (22) applied to the loudspeaker system (12,32) covering low frequencies is of low resolution.

7. A hybrid digital loudspeaker (10,30) according to any one of the preceding claims, wherein the resolution of the signal applied to the loudspeaker system covering high frequencies is of low resolution.

8. A hybrid digital loudspeaker (10,30) according to claim 6 or claim 7, wherein a low resolution signal is a signal having eight -bit modulation.

9. A hybrid digital loudspeaker (10,30) according to any one of the preceding claims, comprising a low frequency loudspeaker system (12,32) and an upper frequency loudspeaker system (14) .

10. A hybrid digital loudspeaker (10,30) according to claim 9, comprising a signal processor (20) having a crossover frequency set at around 200 Hz.

11. A hybrid digital loudspeaker (10,30) according to claim 9 or claim 10, wherein the low frequency loudspeaker system (12,32) is fed a lower resolution signal than the upper loudspeaker system (14) .

12. A hybrid digital loudspeaker (10,30) according to any one of claims 9 to 11, wherein the upper frequency loudspeaker system (14) comprises an array of loudspeaker drive units (16) .

13. A hybrid digital loudspeaker (10,30) according to claim 12, wherein the loudspeaker drive units (16) are resonant panel devices .

14. A hybrid digital loudspeaker (10,30) according to any one of claims 9 to 13, wherein the low frequency loudspeaker system (32) comprises an analogue pistonic loudspeaker woofer.

15. A hybrid digital loudspeaker (10,30) according to any one of claims 9 to 13 , wherein the low frequency loudspeaker system (12) comprises an array of loudspeaker drive units (16) .

16. A hybrid digital loudspeaker (10,30) according to claim 15, when dependent on claim 12, wherein there are less loudspeaker drive units m the low frequency loudspeaker system (12) than m the upper frequency system (14) .

17. A hybrid digital loudspeaker (10,30) according to claim 12 or claim 15, wherein the array of loudspeaker drive units are driven via a digital-data link (26) .