This application relates to microphones and, more specifically, to voice activity detection (VAD) approaches used with these microphones.
Microphones are used to obtain a voice signal from a speaker. Once obtained, the signal can be processed in a number of different ways. A wide variety of functions can be provided by today's microphones and they can interface with and utilize a variety of different algorithms.
Voice triggering, for example, as used in mobile systems is an increasingly popular feature that customers wish to use. For example, a user may wish to speak commands into a mobile device and have the device react in response to the commands. In these cases, a programmable digital signal processor (DSP) may first use a voice activity detection algorithm to detect if there is voice in an audio signal captured by a microphone, and then, subsequently, analysis is performed on the signal to predict what the spoken word was in the received audio signal. Various voice activity detection (VAD) approaches have been developed and deployed in various types of devices such as cellular phones and personal computers.
In the use of these approaches, false detections, trigger word detections, part counts and silicon area and current consumption have become concerns, especially since these approaches are deployed in electronic devices such as cellular phones. Previous approaches have proven inadequate to address these concerns. Consequently, some user dissatisfaction has developed with respect to these previous approaches.
For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein:
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein.
The present approaches provide voice activity detection (VAD) methods and devices that determine whether an event or human voice is present. The approaches described herein are efficient, easy to implement, lower part counts, are able to detect voice with very low latency, and reduce false detections.
It will be appreciated that the approaches described herein can be implemented using any combination of hardware or software elements. For example, an application specific integrated circuit (ASIC) or microprocessor can be used to implement the approaches described herein using programmed computer instructions. Additionally, while the VAD approaches may be disposed in the microphone (as described herein), these functionalities may also be disposed in other system elements.
In many of these embodiments and at a processing device, a first signal from a first microphone and a second signal from a second microphone are received. The first signal indicates whether a voice signal has been determined at the first microphone, and the second signal indicates whether a voice signal has been determined at the second microphone. When the first signal indicates potential voice activity or the second signal indicates potential voice activity, the processing device is activated to receive data and the data is examined for a trigger word. When the trigger word is found, a signal is sent to an application processor to further process information from one or more of the first microphone and the second microphone. When no trigger word is found, the processing device is reset to deactivate data input and allow the first microphone and the second microphone to enter or maintain an event detection mode of operation.
In other aspects, the application processor utilizes a voice recognition (VR) module to determine whether other or further commands can be recognized in the information. In other examples, the first microphone and the second microphone transmit pulse density modulation (PDM) data.
In some other aspects, the first microphone includes a first voice activity detection (VAD) module that determines whether voice activity has been detected, and the second microphone includes a second voice activity detection (VAD) module that determines whether voice activity has been detected. In some examples, the first VAD module and the second VAD module perform the steps of: receiving sound energy from a source; filtering the sound energy into a plurality of filter bands; obtaining a power estimate for each of the plurality of filter bands; and based upon each power estimate, determining whether voice activity is detected.
In some examples, the filtering utilizes one or more low pass filters, high pass filters, and frequency dividers. In other examples, the power estimate comprises an upper power estimate and a lower power estimate.
In some aspects, either the first VAD module or the second VAD module performs Trigger Phrase recognition. In other aspects, either the first VAD module or the second VAD module performs Command Recognition.
In some examples, the processing device controls the first microphone and the second microphone by varying a clock frequency of a clock supplied to the first microphone and the second microphone.
In many of these embodiments, the system includes a first microphone with a first voice activity detection (VAD) module and a second microphone with a second voice activity detection (VAD) module, and a processing device. The processing device is communicatively coupled to the first microphone and the second microphone, and configured to receive a first signal from the first microphone and a second signal from the second microphone. The first signal indicates whether a voice signal has been determined at the first microphone by the first VAD module, and the second signal indicates whether a voice signal has been determined at the second microphone by the second VAD module. The processing device is further configured, to when the first signal indicates potential voice activity or the second signal indicates potential voice activity, activate and receive data from the first microphone or the second microphone, and subsequently examine the data for a trigger word. When the trigger word is found, a signal is sent to an application processor to further process information from one or more of the first microphone and the second microphone. The processing device is further configured to, when no trigger word is found, transmit a third signal to the first microphone and the second microphone. The third signal causes the first microphone and second microphone to enter or maintain an event detection mode of operation.
In one aspect, either the first VAD module or the second VAD module performs Trigger Phrase recognition. In another aspect, either the first VAD module or the second VAD module performs Command Recognition. In other examples, the processing device controls the first microphone and the second microphone by varying a clock frequency of a clock supplied to the first microphone and the second microphone.
In many of these embodiments, voice activity is detected in a micro-electro-mechanical system (MEMS) microphone. Sound energy is received from a source and the sound energy is filtered into a plurality of filter bands. A power estimate is obtained for each of the plurality of filter bands. Based upon each power estimate, a determination is made as to whether voice activity is detected.
In some aspects, the filtering utilizes one or more low pass filters, high pass filters and frequency dividers. In other examples, the power estimate comprises an upper power estimate and a lower power estimate. In some examples, ratios between the upper power estimate and the lower power estimate within the plurality of filter bands are determined, and selected ones of the ratios are compared to a predetermined threshold. In other examples, ratios between the upper power estimate and the lower power estimate between the plurality of filter bands are determined, and selected ones of the ratios are compared to a predetermined threshold.
Referring now to
The first microphone element 102 and the second microphone element 104 are microelectromechanical system (MEMS) elements that receive sound energy and convert the sound energy into electrical signals that represent the sound energy. In one example, the elements 102 and 104 include a MEMS die, a diaphragm, and a back plate. Other components may also be used.
The right event microphone 106 and the left event microphone 108 receive signals from the microphone elements 102 and 104, and process these signals. For example, the elements 106 and 108 may include buffers, preamplifiers, analog-to-digital (A-to-D) converters, and other processing elements that convert the analog signal received from elements 102 and 104 into digital signals and perform other processing functions. These elements may, for example, include an ASIC that implements these functions. The right event microphone 106 and the left event microphone 108 also include voice activity detection (VAD) modules 103 and 105 respectively and these may be implemented by an ASIC that executes programmed computer instructions. The VAD modules 103 and 105 utilize the approaches described herein to determine whether voice (or some other event) has been detected. This information is transmitted to the digital signal processor (DSP)/codec 110 and the application processor 112 for further processing. Also, the signals (potentially voice information) now in the form of digital information are sent to the digital signal processor (DSP)/codec 110 and the application processor 112.
The digital signal processor (DSP)/codec 110 receives signals from the elements 106 and 108 (including whether the VAD modules have detected voice) and looks for trigger words (e.g., “Hello, My Mobile) using a voice recognition (VR) trigger engine 120. The codec 110 also performs interrupt processing (see
In one example of the operation of the system of
Referring now to
In another approach, the microphone may also track the time of contiguous voice activity. If activity does not persist beyond a certain countdown e.g., 5 seconds, and the microphone also stays in the low power VAD mode of operation, i.e. not put into a standard or high performance mode within that time frame, the implication is that the voice trigger was not detected within that period of detected voice activity, then there is no further activity and the microphone may initiate a change to detection mode from detect and transmit mode. A DSP/Codec on detecting no transmission from the microphone may also go to low power sleep mode.
Referring now to
The power tracker block 304 includes an upper tracker and a lower tracker. For each of these and for each band it obtains a power estimate. The decision block 306 looks at the power estimates and determines if voice or an acoustic event is present.
Optionally, the threshold values can be set by a number of different approaches such as one time parts (OTPs), or various types of wired or wireless interfaces 310. Optionally feedback 308 from the decision block 306 can control the power trackers, this feedback could be the VAD decision. For example the trackers (described below) could be configured to use another set of attack/release constants if voice is present. The functions described herein can be deployed in any number of functional blocks and it will be understood that the three blocks described are examples only.
Referring now to
Referring now to
Referring now to
Referring now to
Referring now to
The sample index number is n, Kaui and Krui are attack and release constants for the upper tracker channel number i. Kali and Krli are attack and release constants for the lower tracker for channel number i. The output of this block is fed to the decision block described below with respect to
Referring now to
Referring now to
The internal operation of the division block 904 is structured and configured so that an actual division need not be made. The lower tracker value loweri(n) is multiplied by Thi(n) (a predetermined threshold which could be constant and independent of n or changed according to a rule). This is subtracted from the upperi(n) tracker value. The sign(x) function is then performed.
Upper and lower tracker signals are estimated by upper and lower tracker block 902 (this block is identical to block 706). The ratio between the upper tracker and the lower tracker is then calculated by division block 904. This ratio is compared with a threshold Thi(n). The flag R_flagi(n) is set if the ratio is larger than the threshold Thi(n), i.e., if sign(x) in 904 is positive. This operation is performed for each channel i=1 to 5. Thi(n) could be constant over time for each channel or follow a rule where it actually changes for each sample instance n.
In addition to the ratio calculation for each channel i=1 to 5 (or 6 or 7 if more channels are available from the filterbank), the ratios between channels can also be used/calculated. The ratio between channels is defined for the i'th channel: Ratioi,ch(n)=upperi=ch(n)/loweri≠ch(n), i, ch are from 1 to the number of channels which in this case is 5. This means that ratio(n)i,i is identical to the ratio calculated above. A total number of 25 ratios can be calculated (if 5 filter bands exist). Again, each of these ratios is compared with a Threshold Thi,ch(n). A total number of 25 thresholds exist if 5 channels are available. Again, the threshold can be constant over time n, or change for each sample instance n. In one implementation, not all of the ratios between bands will be used, only a subset.
The sample rate for all the flags is identical with the sample rate for the faster tracker of all the trackers. The slow trackers are repeated. A voice power flag V_flag(n) is also estimated as the sum of three channels from 500 Hz to 4000 Hz by summation block 908. This flag is set if the power level is low enough, (i.e., smaller than Vth(n)) and this is determined by comparison block 910 and sign block 912. This flag is only in effect when the microphone is in a quiet environment or/and the persons speaking are far away from the microphone.
The R_flagi(n) and V_flag(n) are used to decide if the current time step “n” is voice, and stored in E_flag(n). The operation that determines if E_flag (n) is voice (1) or not voice (0) can be described by the following:
The final VAD_flag(n) is a smoothed version of the E_flag(n). It simply makes a VAD positive decision true for a minimum time/period of VAD_NUMBER of sample periods. This smoothing can be described by the following approach. This approach can be used to determine if a voice event is detected, but that the voice is present in the background and therefore of no interest. In this respect, a false positive reading is avoided.
Hang-on-count represents a time of app VAD_NUMBER/Sample Rate. Here Sample Rate are the fastest channel, i.e., 250, 125 or 62.5 Hz. It will be appreciated that these approaches examine to see if 4 flags have been set. However, it will be appreciated that any number of threshold values (flags) can be examined.
It will also be appreciated that other rules could be formulated like at least two pair of adjacent channel (or R_flag) are true or maybe three of such pairs or only one pair. These rules are predicated by the fact that human voice tends to be correlated in adjacent frequency channels, due to the acoustic production capabilities/limitations of the human vocal system.
Preferred embodiments are described herein, including the best mode. It should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the appended claims.
This application is a continuation of U.S. application Ser. No. 14/525,413 (now granted as U.S. Pat. No. 9,147,397), entitled “VAD Detection Apparatus and Method of Operating the Same,” filed Oct. 28, 2014, which claims the benefit under 35 U.S.C. §119 (e) to U.S. Provisional Application No. 61/896,723, entitled “VAD Detection Apparatus and method of operating the same,” filed Oct. 29, 2013, both of which are incorporated herein by reference in their entireties.
Number | Name | Date | Kind |
---|---|---|---|
4052568 | Jankowski | Oct 1977 | A |
5577164 | Kaneko | Nov 1996 | A |
5598447 | Usui | Jan 1997 | A |
5675808 | Gulick | Oct 1997 | A |
5822598 | Lam | Oct 1998 | A |
5983186 | Miyazawa | Nov 1999 | A |
6049565 | Paradine | Apr 2000 | A |
6057791 | Knapp | May 2000 | A |
6070140 | Tran | May 2000 | A |
6138040 | Nicholls | Oct 2000 | A |
6154721 | Sonnic | Nov 2000 | A |
6249757 | Cason | Jun 2001 | B1 |
6282268 | Hughes | Aug 2001 | B1 |
6324514 | Matulich | Nov 2001 | B2 |
6397186 | Bush | May 2002 | B1 |
6453020 | Hughes | Sep 2002 | B1 |
6564330 | Martinez | May 2003 | B1 |
6591234 | Chandran | Jul 2003 | B1 |
6640208 | Zhang | Oct 2003 | B1 |
6665639 | Mozer et al. | Dec 2003 | B2 |
6756700 | Zeng | Jun 2004 | B2 |
6832194 | Mozer et al. | Dec 2004 | B1 |
6999927 | Mozer et al. | Feb 2006 | B2 |
7092887 | Mozer et al. | Aug 2006 | B2 |
7190038 | Dehe | Mar 2007 | B2 |
7415416 | Rees | Aug 2008 | B2 |
7418392 | Mozer et al. | Aug 2008 | B1 |
7473572 | Dehe | Jan 2009 | B2 |
7487089 | Mozer | Feb 2009 | B2 |
7619551 | Wu | Nov 2009 | B1 |
7630504 | Poulsen | Dec 2009 | B2 |
7720683 | Vermeulen et al. | May 2010 | B1 |
7774202 | Spengler | Aug 2010 | B2 |
7774204 | Mozer et al. | Aug 2010 | B2 |
7781249 | Laming | Aug 2010 | B2 |
7795695 | Weigold | Sep 2010 | B2 |
7801729 | Mozer | Sep 2010 | B2 |
7825484 | Martin | Nov 2010 | B2 |
7829961 | Hsiao | Nov 2010 | B2 |
7856283 | Burk | Dec 2010 | B2 |
7856804 | Laming | Dec 2010 | B2 |
7903831 | Song | Mar 2011 | B2 |
7936293 | Hamashita | May 2011 | B2 |
7941313 | Garudadri | May 2011 | B2 |
7957972 | Huang | Jun 2011 | B2 |
7994947 | Ledzius | Aug 2011 | B1 |
8024195 | Mozer et al. | Sep 2011 | B2 |
8036901 | Mozer | Oct 2011 | B2 |
8099289 | Mozer et al. | Jan 2012 | B2 |
8112280 | Lu | Feb 2012 | B2 |
8171322 | Fiennes | May 2012 | B2 |
8195467 | Mozer et al. | Jun 2012 | B2 |
8208621 | Hsu | Jun 2012 | B1 |
8275148 | Li | Sep 2012 | B2 |
8321219 | Mozer | Nov 2012 | B2 |
8331581 | Pennock | Dec 2012 | B2 |
8645132 | Mozer et al. | Feb 2014 | B2 |
8645143 | Mozer | Feb 2014 | B2 |
8666751 | Murthi | Mar 2014 | B2 |
8687823 | Loeppert | Apr 2014 | B2 |
8700399 | Vermeulen et al. | Apr 2014 | B2 |
8731210 | Cheng | May 2014 | B2 |
8768707 | Mozer | Jul 2014 | B2 |
8781825 | Shaw et al. | Jul 2014 | B2 |
8798289 | Every | Aug 2014 | B1 |
8804974 | Melanson | Aug 2014 | B1 |
8849231 | Murgia | Sep 2014 | B1 |
8972252 | Hung | Mar 2015 | B2 |
8996381 | Mozer et al. | Mar 2015 | B2 |
9020819 | Saitoh | Apr 2015 | B2 |
9043211 | Haiut | May 2015 | B2 |
9059630 | Gueorguiev | Jun 2015 | B2 |
9073747 | Ye | Jul 2015 | B2 |
9076447 | Nandy | Jul 2015 | B2 |
9111548 | Nandy | Aug 2015 | B2 |
9112984 | Sejnoha | Aug 2015 | B2 |
9113263 | Furst | Aug 2015 | B2 |
9119150 | Murgia | Aug 2015 | B1 |
9142215 | Rosner | Sep 2015 | B2 |
9142219 | Mozer | Sep 2015 | B2 |
9147397 | Thomsen | Sep 2015 | B2 |
9161112 | Ye | Oct 2015 | B2 |
9165567 | Visser | Oct 2015 | B2 |
9439005 | Jensen | Sep 2016 | B2 |
20020054588 | Mehta | May 2002 | A1 |
20020116186 | Strauss | Aug 2002 | A1 |
20020123893 | Woodward | Sep 2002 | A1 |
20020184015 | Li | Dec 2002 | A1 |
20030004720 | Garudadri | Jan 2003 | A1 |
20030061036 | Garudadri | Mar 2003 | A1 |
20030144844 | Colmenarez | Jul 2003 | A1 |
20040022379 | Klos | Feb 2004 | A1 |
20040234069 | Mikesell | Nov 2004 | A1 |
20050207605 | Dehe | Sep 2005 | A1 |
20050240399 | Makinen | Oct 2005 | A1 |
20060074658 | Chadha | Apr 2006 | A1 |
20060233389 | Mao | Oct 2006 | A1 |
20060247923 | Chandran | Nov 2006 | A1 |
20070168908 | Paolucci | Jul 2007 | A1 |
20070278501 | MacPherson | Dec 2007 | A1 |
20080089536 | Josefsson | Apr 2008 | A1 |
20080175425 | Roberts | Jul 2008 | A1 |
20080201138 | Visser | Aug 2008 | A1 |
20080267431 | Leidl | Oct 2008 | A1 |
20080279407 | Pahl | Nov 2008 | A1 |
20080283942 | Huang | Nov 2008 | A1 |
20090001553 | Pahl | Jan 2009 | A1 |
20090180655 | Tien | Jul 2009 | A1 |
20100046780 | Song | Feb 2010 | A1 |
20100052082 | Lee | Mar 2010 | A1 |
20100057474 | Kong | Mar 2010 | A1 |
20100128894 | Petit | May 2010 | A1 |
20100128914 | Khenkin | May 2010 | A1 |
20100131783 | Weng | May 2010 | A1 |
20100183181 | Wang | Jul 2010 | A1 |
20100246877 | Wang | Sep 2010 | A1 |
20100290644 | Wu | Nov 2010 | A1 |
20100292987 | Kawaguchi | Nov 2010 | A1 |
20100322443 | Wu | Dec 2010 | A1 |
20100322451 | Wu | Dec 2010 | A1 |
20110007907 | Park | Jan 2011 | A1 |
20110013787 | Chang | Jan 2011 | A1 |
20110029109 | Thomsen | Feb 2011 | A1 |
20110075875 | Wu | Mar 2011 | A1 |
20110106533 | Yu | May 2011 | A1 |
20110150210 | Allen | Jun 2011 | A1 |
20110208520 | Lee | Aug 2011 | A1 |
20110264447 | Visser | Oct 2011 | A1 |
20110280109 | Raymond | Nov 2011 | A1 |
20120010890 | Koverzin | Jan 2012 | A1 |
20120052907 | Gilbreath et al. | Mar 2012 | A1 |
20120232896 | Taleb | Sep 2012 | A1 |
20120250881 | Mulligan | Oct 2012 | A1 |
20120310641 | Niemisto | Dec 2012 | A1 |
20130044898 | Schultz | Feb 2013 | A1 |
20130058506 | Boor | Mar 2013 | A1 |
20130183944 | Mozer et al. | Jul 2013 | A1 |
20130223635 | Singer | Aug 2013 | A1 |
20130226324 | Hannuksela | Aug 2013 | A1 |
20130246071 | Lee | Sep 2013 | A1 |
20130322461 | Poulsen | Dec 2013 | A1 |
20130343584 | Bennett | Dec 2013 | A1 |
20140064523 | Kropfitsch | Mar 2014 | A1 |
20140122078 | Joshi | May 2014 | A1 |
20140143545 | McKeeman | May 2014 | A1 |
20140163978 | Basye | Jun 2014 | A1 |
20140177113 | Gueorguiev | Jun 2014 | A1 |
20140180691 | Vermeulen et al. | Jun 2014 | A1 |
20140188467 | Jing | Jul 2014 | A1 |
20140188470 | Chang | Jul 2014 | A1 |
20140197887 | Hovesten | Jul 2014 | A1 |
20140244269 | Tokutake | Aug 2014 | A1 |
20140244273 | Laroche | Aug 2014 | A1 |
20140249820 | Hsu | Sep 2014 | A1 |
20140257813 | Mortensen | Sep 2014 | A1 |
20140257821 | Adams | Sep 2014 | A1 |
20140274203 | Ganong | Sep 2014 | A1 |
20140278435 | Ganong, III | Sep 2014 | A1 |
20140281628 | Nigam | Sep 2014 | A1 |
20140324431 | Teasley | Oct 2014 | A1 |
20140343949 | Huang | Nov 2014 | A1 |
20140348345 | Furst | Nov 2014 | A1 |
20140358552 | Xu | Dec 2014 | A1 |
20150039303 | Lesso | Feb 2015 | A1 |
20150043755 | Furst | Feb 2015 | A1 |
20150046157 | Wolff | Feb 2015 | A1 |
20150046162 | Aley-Raz | Feb 2015 | A1 |
20150049884 | Ye | Feb 2015 | A1 |
20150055803 | Qutub | Feb 2015 | A1 |
20150058001 | Dai | Feb 2015 | A1 |
20150063594 | Nielsen | Mar 2015 | A1 |
20150073780 | Sharma | Mar 2015 | A1 |
20150073785 | Sharma | Mar 2015 | A1 |
20150088500 | Conliffe | Mar 2015 | A1 |
20150106085 | Lindahl | Apr 2015 | A1 |
20150110290 | Furst | Apr 2015 | A1 |
20150112690 | Guha | Apr 2015 | A1 |
20150134331 | Millet | May 2015 | A1 |
20150154981 | Barreda | Jun 2015 | A1 |
20150161989 | Hsu | Jun 2015 | A1 |
20150195656 | Ye | Jul 2015 | A1 |
20150206527 | Connolly | Jul 2015 | A1 |
20150256660 | Kaller | Sep 2015 | A1 |
20150256916 | Volk | Sep 2015 | A1 |
20150287401 | Lee | Oct 2015 | A1 |
20150302865 | Pilli | Oct 2015 | A1 |
20150304502 | Pilli | Oct 2015 | A1 |
20150350760 | Nandy | Dec 2015 | A1 |
20150350774 | Furst | Dec 2015 | A1 |
20160012007 | Popper | Jan 2016 | A1 |
20160057549 | Marquis | Feb 2016 | A1 |
20160087596 | Yurrtas | Mar 2016 | A1 |
20160133271 | Kuntzman | May 2016 | A1 |
20160134975 | Kuntzman | May 2016 | A1 |
Number | Date | Country |
---|---|---|
2001-236095 | Aug 2001 | JP |
2004219728 | Aug 2004 | JP |
2009130591 | Jan 2009 | WO |
2011106065 | Jan 2011 | WO |
2011140096 | Feb 2011 | WO |
2013049358 | Jan 2013 | WO |
2013085499 | Jan 2013 | WO |
Entry |
---|
U.S. Appl. No. 14/285,585, filed May 22, 2014, Santos. |
U.S. Appl. No. 14/495,482, filed Sep. 24, 2014, Murgia. |
U.S. Appl. No. 14/522,264, filed Oct. 23, 2014, Murgia. |
U.S. Appl. No. 14/698,652, filed Apr. 28, 2015, Yapanel. |
U.S. Appl. No. 14/749,425, filed Jun. 24, 2015, Verma. |
U.S. Appl. No. 14/853,947, filed Sep. 14, 2015, Yen. |
U.S. Appl. No. 62/100,758, filed Jan. 7, 2015, Rossum. |
International Search Report and Written Opinion for PCT/US2016/013859 dated Apr. 29, 2016 (12 pages). |
Search Report of Taiwan Patent Application No. 103135811, dated Apr. 18, 2016 (1 page). |
“MEMS technologies: Microphone” EE Herald Jun. 20, 2013. |
Delta-sigma modulation, Wikipedia (Jul. 4, 2013). |
International Search Report and Written Opinion for PCT/EP2014/064324, dated Feb. 12, 2015 (13 pages). |
International Search Report and Written Opinion for PCT/US2014/038790, dated Sep. 24, 2014 (9 pages). |
International Search Report and Written Opinion for PCT/US2014/060567 dated Jan. 16, 2015 (12 pages). |
International Search Report and Written Opinion for PCT/US2014/062861 dated Jan. 23, 2015 (12 pages). |
Kite, Understanding PDM Digital Audio, Audio Precision, Beaverton, OR, 2012. |
Pulse-density modulation, Wikipedia (May 3, 2013). |
Number | Date | Country | |
---|---|---|---|
20160064001 A1 | Mar 2016 | US |
Number | Date | Country | |
---|---|---|---|
61896723 | Oct 2013 | US |
Number | Date | Country | |
---|---|---|---|
Parent | 14525413 | Oct 2014 | US |
Child | 14861113 | US |