GB2160070A - Sound reproduction system - Google Patents
Sound reproduction system Download PDFInfo
- Publication number
- GB2160070A GB2160070A GB08511590A GB8511590A GB2160070A GB 2160070 A GB2160070 A GB 2160070A GB 08511590 A GB08511590 A GB 08511590A GB 8511590 A GB8511590 A GB 8511590A GB 2160070 A GB2160070 A GB 2160070A
- Authority
- GB
- United Kingdom
- Prior art keywords
- amplifier
- mixing
- microphone
- speech
- speech signal
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
Classifications
-
- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
- G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
- G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
- G10K11/175—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound
- G10K11/178—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase
- G10K11/1787—General system configurations
- G10K11/17885—General system configurations additionally using a desired external signal, e.g. pass-through audio such as music or speech
-
- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
- G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
- G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
- G10K11/175—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound
- G10K11/178—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase
- G10K11/1781—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase characterised by the analysis of input or output signals, e.g. frequency range, modes, transfer functions
- G10K11/17821—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase characterised by the analysis of input or output signals, e.g. frequency range, modes, transfer functions characterised by the analysis of the input signals only
- G10K11/17827—Desired external signals, e.g. pass-through audio such as music or speech
-
- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
- G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
- G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
- G10K11/175—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound
- G10K11/178—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase
- G10K11/1783—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase handling or detecting of non-standard events or conditions, e.g. changing operating modes under specific operating conditions
- G10K11/17833—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase handling or detecting of non-standard events or conditions, e.g. changing operating modes under specific operating conditions by using a self-diagnostic function or a malfunction prevention function, e.g. detecting abnormal output levels
-
- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
- G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
- G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
- G10K11/175—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound
- G10K11/178—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase
- G10K11/1787—General system configurations
- G10K11/17875—General system configurations using an error signal without a reference signal, e.g. pure feedback
-
- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
- G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
- G10K2210/10—Applications
- G10K2210/108—Communication systems, e.g. where useful sound is kept and noise is cancelled
- G10K2210/1081—Earphones, e.g. for telephones, ear protectors or headsets
-
- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
- G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
- G10K2210/30—Means
- G10K2210/301—Computational
- G10K2210/3013—Analogue, i.e. using analogue computers or circuits
-
- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
- G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
- G10K2210/30—Means
- G10K2210/301—Computational
- G10K2210/3028—Filtering, e.g. Kalman filters or special analogue or digital filters
-
- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
- G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
- G10K2210/30—Means
- G10K2210/301—Computational
- G10K2210/3045—Multiple acoustic inputs, single acoustic output
-
- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
- G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
- G10K2210/50—Miscellaneous
- G10K2210/503—Diagnostics; Stability; Alarms; Failsafe
Landscapes
- Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Acoustics & Sound (AREA)
- Multimedia (AREA)
- Health & Medical Sciences (AREA)
- Audiology, Speech & Language Pathology (AREA)
- General Health & Medical Sciences (AREA)
- Soundproofing, Sound Blocking, And Sound Damping (AREA)
- Circuit For Audible Band Transducer (AREA)
Abstract
A microphone (10) is acoustically coupled to receive sound from an earphone transducer (14) and sound cancellation feedback is provided by a loop between the microphone (10) and transducer (14). This loop includes a first mixing amplifier (16), a high gain phase-inverting amplifier (12) and a second mixing amplifier (18). Speech signal is introduced via each of the mixing amplifiers (16 & 18) and is such therefore that speech is still discernible in the event of failure of the microphone (10), the first mixing amplifier (16) or the inverting amplifier (12). Each mixing amplifier (16 and 18) may be preceded by an equalisation filter (24, 26) and boost amplifier (20, 22) lying in the speech signal paths. The first mixing amplifier (16) and the inverting amplifier (12) can be arranged for suppressing signal in the event of high level noise transients whilst speech signal can still be introduced via the second mixing amplifier (18). <IMAGE>
Description
SPECIFICATION
Improvements in or relating to sound reproduction systems
The present invention relates to sound reproduction systems and more particularly to the reduction of acoustic noise in such systems.
An active noise reduction (ANR) system is one in which the ambient sound field is detected using a sensing microphone and the phase inverted sound signal (i.e. the same sound field with a H phase change) generated to destructively interfere with the original sound field, which reduces the sound pressure level. This type of noise reduction system is termed "active" as the noise level is reduced by generating sound: this is very different to "passive" noise reduction systems which rely on acoustic absorption to decrease the sound pressure level.
It is an object of the present invention to provide an active noise reduction system which allows the noise level to be reduced and the speech signal to noise ratio to be increased. If any part of most of the
ANR system fails, the ANR system will effectively be turned off and under these circumstances speech would still be heard as in a present day communication system.
According to the present invention there is provided a sound reproduction system comprising:
an earphone transducer;
a microphone acoustically coupled to said transducer, being arranged thus to receive sound therefrom; and,
a feedback loop connected between said microphone and said earphone transducer;
wherein said feedback loop includes:
a first mixing amplifier connected to the microphone;
a high gain inverting amplifier connected to the output of this first mixing amplifier; and,
a second mixing amplifier connected to the output of this inverting amplifier;
first and second mixing amplifiers being connected at respective inputs to a speech signal line.
Since speech signal is introduced via two paths, speech can still be discerned in the event of failure of the part of the system that includes the microphone, first mixing amplifier and inverting amplifier.
It is preferable that the speech signals introduced via the first and second mixing amplifiers sum constructively ie. are additive. To this end the first mixing amplifier can be a differential amplifier and the second mixing amplifier, a summing amplifier. In this case speech signal via the first mixing amplifier is substracted from noise signal and then inverted by the inverting amplifier and added to further speech signal by the summing amplifier. It will be appreciated that these two mixing amplifiers can be exchanged to produce equivalent effect.
It is possible to introduce equalisation filters in the speech signal path so that the speech signal will not be distorted and will have an amplitude response "uncoloured" by the system.
Also it can be arranged that the first mixing amplifier and/or the inverting amplifier suppress inordinately large level noise singal. The speech signal can be still heard without distortion.
Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which:
Figure 1 shows a known active noise reduction (ANR) system.
Figure 2 and 3 show known active noise reduction systems with speech addition.
Figure 4 shows an active noise reduction system with speech addition according to the present invention.
The basic principle of an ANR system is shown in Figure 1. A microphone 10 detects the ambient sound and sends the electrical signal to a phase inverting amplifier 12. The output is fed back to an earphone 14 where the generated sound destructively interferes with the ambient sound to reduce the sound level over a wide frequency bandwidth.
The following symbols are defined:
N, is the noise pressure;
N is the total sound pressure, noise plus generated noise, in the earphone; a is the electro-acoustic transfer function of the microphone; ss is the gain of the phase inverting amplifier; and w is the electro-acoustic transfer function of the sound generator.
The self consistent equation is:
N = N, - any N
N= No +Na[3)
It can clearly be seen that to achieve beneficial amounts of active noise reduction the loop gain (assw product) needs to be large.
Figures 2 and 3 show known ANR systems with speech addition.
Figure 2 is similar to Figure 1 except that the speech signal after being amplified by a boost amplifier 20 is added to a mixing amplifier 16.
The following symbols are defined: S is the total signal pressure level in the earphone 14;
V is the signal voltage introduced; and, o is the gain of boost amplifier 20.
The self consistent equation is: N # S, = N, - 0tss (N+S,) therefore N = N
(1 +αssγ) and So = --------
(1+αssγ) In this case the noise and the speech signal have not been reduced by the same amount so that the signal to noise ratio is AVssy No
As the noise has been reduced, the gain of the boost amplifier 20 could be increased to increase the signal to noise ratio whilst maintaining the reduce noise level, without an excessive sound level being generated in the earphone 14 which would be dangerous to the ear.
Figure 3 shows another ANR system with speech addition. This system has the inverting amplifier 12 first in the loop before a mixing amplifier 18.
The following symbol is defined: is the gain of amplifier 22.
The self consistent equation is: N+S, = N, - cupy (N+S,) + Ew V N N
##V and So =
(1 + αssγ) In this case the noise and speech signal have been reduced by the same amount so that the signal to noise ratio is #γV No
If the system is inoperative the signal to noise ration is also the same. However because both the signal and the noise have been reduced, the gain e may be increased so that the signal to noise ratio is increased without excessive sound being generated by the earphone 14.
Figure 4 is a diagram of a circuit that has been modified in accordance with this invention. Noise in the earphone 14 is sensed by the microphone 10 and the electrical signal connected to a first mixing amplifier, in this example a differential amplifier 16. the output is connected to the phase inverting amplifier 12, and the output of this connected to a second mixing amplifier, in this example, a summing amplifier 16. The output to the earphone transducer 14. The speech signal passes through a boost amplifier 20 to differential amplifier 16 and also through a boost amplifier 22 to summing amplifier 18.
The self consistent equation is: N+S, = N, - oipy (N+S,) + 8ssw V+V eX N= N, So = Vγ (6ss + #)
(1 + αssγ) When the system is operating ss is large. bss is made large compared to E and then the speech signal introduced into the differential amplifier 16 will become dominant as compared to that introduced into the mixing amplifier 18 for producing speech in the earphone 14. Under these conditions a filter 24 is inserted before boost amplifier 20 which allows the speech signal in the earphone 14 to have the required amplitude response (uncoloured).
If the system is switched off, or any part of items 10, 16 or 12 fail, then speech will still be heard in the earphone via route 22, 18. Under these conditions, setting ss = 0 it follows that S, = VrE. In order to allow the speech signal to have the required amplitude response (uncoloured) a filter 26 is inserted before amplifier 22.
The whole circuit is shown in Figure 4 with both filters present, the important speech path being 20, 16 if the gain is high and 22, 18 if the gain is zero.
Additionally, if the voltage rails of inverting amplifier 12 or mixing amplifier 16 are arranged to be less than those of mixing amplifier 18, then if a large signal is sent by the microphone 10 the signal will "clip" on amplifiers 16, 12 resulting in reduced - active noise reduction but the speech will still be heard in the earphone via amplifiers 22, 18 all-be-it at a lower level.
The advantages of this invention are:
1. In the event of failure of any of the items 10, 16 or 12 the speech signal will still be heard clearly due to its introduction through amplifiers 22 and 18.
2. If the circuit is arranged so that the rail voltages of amplifiers 12 or 16 are less than amplifier 18, the speech signal will still be heard even if an inordinately large signal is sent from the microphone 10.
This invention may be used in any closed loop feedback ANR system to introduce speech and to achieve a large signal to noise ratio, at a reduced noise level irrespective of the design or type of the sound generator or microphone.
Claims (8)
1. A sound reproduction system comprising:
an earphone transducer;
a microphone acoustically coupled to said transducer, being arranged thus to receive sound therefrom; and,
a feedback loop connected between said microphone and said earphone transducer;
wherein said feedback loop includes:
a first mixing amplifier connected to the microphone;
a high gain inverting amplifier connected to the output of this first mixing amplifier; and,
a second mixing amplifier connected to the output of this inverting amplifier,;
first and second mixing amplifiers being connected at respective inputs to a speech signal line.
2. A system, as claimed in claim 1, wherein speech signals introduced via first and second mixing amplifiers are additive.
3. A system, as claimed in claim 2, wherein one of the mixing amplifiers is a differential amplifier, and the other mixing amplifier is a summing amplifer.
4. A system, as claimed in claim 3, wherein said first mixing amplifier is a differential amplifier.
5. A system, as claimed in any one of the preceding claims, wherein one or both mixing amplifiers are each proceded by an equalisation filter interposed in the speech signal line.
6. A system, as claimed in claim 5, wherein each mixing amplifier is preceded by a respective equalisation filter and boost-amplifier.
7. A system, as claimed in any one of the preceding claims, wherein the first mixing amplifier, or the inverting amplifier, or both, are arranged for the suppression of inordinately large signal.
8. A sound reproduction system, constructed, adapted, and arranged to perform, substantially as described hereinbefore, with reference to, and as shown in Figure 4 of the accompanying drawings.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB848412195A GB8412195D0 (en) | 1984-05-12 | 1984-05-12 | Sound reproduction systems |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8511590D0 GB8511590D0 (en) | 1985-06-12 |
| GB2160070A true GB2160070A (en) | 1985-12-11 |
| GB2160070B GB2160070B (en) | 1987-07-29 |
Family
ID=10560880
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB848412195A Pending GB8412195D0 (en) | 1984-05-12 | 1984-05-12 | Sound reproduction systems |
| GB08511590A Expired GB2160070B (en) | 1984-05-12 | 1985-05-08 | Sound reproduction system |
Family Applications Before (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB848412195A Pending GB8412195D0 (en) | 1984-05-12 | 1984-05-12 | Sound reproduction systems |
Country Status (1)
| Country | Link |
|---|---|
| GB (2) | GB8412195D0 (en) |
Cited By (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1987005430A1 (en) * | 1986-03-07 | 1987-09-11 | Centre National De La Recherche Scientifique (Cnrs | Methods and devices to improve the intelligibility of electroacoustic communications |
| GB2188210A (en) * | 1986-03-21 | 1987-09-23 | Univ Southampton | Acoustic noise reduction in ear defenders |
| DE3719963A1 (en) * | 1986-09-26 | 1988-03-31 | Deutsch Franz Forsch Inst | Protective device against the effects of noise |
| WO1988006779A1 (en) * | 1987-02-24 | 1988-09-07 | British Airways Plc | Noise attenuation |
| EP0390386A2 (en) * | 1989-03-25 | 1990-10-03 | Sony Corporation | Noise reducing device |
| GB2234881A (en) * | 1989-08-03 | 1991-02-13 | Plessey Co Plc | Noise reduction system |
| WO1998024088A1 (en) * | 1996-11-27 | 1998-06-04 | Panphonics | Method and apparatus for processing sound |
| US6118878A (en) * | 1993-06-23 | 2000-09-12 | Noise Cancellation Technologies, Inc. | Variable gain active noise canceling system with improved residual noise sensing |
| US7103188B1 (en) | 1993-06-23 | 2006-09-05 | Owen Jones | Variable gain active noise cancelling system with improved residual noise sensing |
-
1984
- 1984-05-12 GB GB848412195A patent/GB8412195D0/en active Pending
-
1985
- 1985-05-08 GB GB08511590A patent/GB2160070B/en not_active Expired
Cited By (19)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2595498A1 (en) * | 1986-03-07 | 1987-09-11 | Centre Nat Rech Scient | METHODS AND DEVICES FOR MITIGATING EXTERNAL NOISE FROM TYMPAN AND ENHANCING THE INTELLIGIBILITY OF ELECTRO-ACOUSTIC COMMUNICATIONS |
| EP0237454A1 (en) * | 1986-03-07 | 1987-09-16 | Etablissement Public dit: CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE (CNRS) | Processes and devices for attenuating noise from an external origin arriving at the ear drum, and for improving the intelligibility of electro-acoustic communications |
| US4833719A (en) * | 1986-03-07 | 1989-05-23 | Centre National De La Recherche Scientifique | Method and apparatus for attentuating external origin noise reaching the eardrum, and for improving intelligibility of electro-acoustic communications |
| WO1987005430A1 (en) * | 1986-03-07 | 1987-09-11 | Centre National De La Recherche Scientifique (Cnrs | Methods and devices to improve the intelligibility of electroacoustic communications |
| GB2188210A (en) * | 1986-03-21 | 1987-09-23 | Univ Southampton | Acoustic noise reduction in ear defenders |
| GB2188210B (en) * | 1986-03-21 | 1989-11-15 | Univ Southampton | Improvements in or relating to acoustic noise reduction |
| DE3719963A1 (en) * | 1986-09-26 | 1988-03-31 | Deutsch Franz Forsch Inst | Protective device against the effects of noise |
| DE3719963C2 (en) * | 1986-09-26 | 1998-01-15 | Deutsch Franz Forsch Inst | Protection device against noise |
| US5018203A (en) * | 1987-02-24 | 1991-05-21 | Scientific Generics Limited | Noise attenuation |
| WO1988006779A1 (en) * | 1987-02-24 | 1988-09-07 | British Airways Plc | Noise attenuation |
| EP0390386A2 (en) * | 1989-03-25 | 1990-10-03 | Sony Corporation | Noise reducing device |
| EP0390386A3 (en) * | 1989-03-25 | 1991-10-23 | Sony Corporation | Noise reducing device |
| US5138664A (en) * | 1989-03-25 | 1992-08-11 | Sony Corporation | Noise reducing device |
| GB2234881B (en) * | 1989-08-03 | 1993-09-15 | Plessey Co Plc | Noise reduction system |
| GB2234881A (en) * | 1989-08-03 | 1991-02-13 | Plessey Co Plc | Noise reduction system |
| US6118878A (en) * | 1993-06-23 | 2000-09-12 | Noise Cancellation Technologies, Inc. | Variable gain active noise canceling system with improved residual noise sensing |
| US7103188B1 (en) | 1993-06-23 | 2006-09-05 | Owen Jones | Variable gain active noise cancelling system with improved residual noise sensing |
| WO1998024088A1 (en) * | 1996-11-27 | 1998-06-04 | Panphonics | Method and apparatus for processing sound |
| US6711267B1 (en) | 1996-11-27 | 2004-03-23 | Panphonics Oy | Method and apparatus for processing sound |
Also Published As
| Publication number | Publication date |
|---|---|
| GB8412195D0 (en) | 1984-06-20 |
| GB8511590D0 (en) | 1985-06-12 |
| GB2160070B (en) | 1987-07-29 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 732 | Registration of transactions, instruments or events in the register (sect. 32/1977) | ||
| PE20 | Patent expired after termination of 20 years |
Effective date: 20050507 |