# Patent

## Title
Microphone apparatus and method with catch-up buffer

## Abstract

At a microphone, voice activity is detected in a data stream while simultaneously buffering audio data from the data stream to create buffered data. A signal is sent to a host indicating the positive detection of voice activity in the data stream. When an external clock signal is received from the host, the internal operation of the microphone is synchronized with the external clock signal. Buffered data stream is selectively sent through a first path, the first path including a buffer having a buffer delay time representing the time the first data stream takes to move through the buffer. The data stream is continuously sent through a second path as a real-time data stream, the second path not including the buffer, the real-time data stream beginning with the extended buffer data at a given instant in time. The buffered data stream and the real-time data stream are multiplexed onto a single data line and transmitting the multiplexed data stream to the host.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 14/797,310, filed Jul. 13, 2015, the entire contents of which are incorporated herein by reference.


TECHNICAL FIELD
This application relates to acoustic systems, and, more specifically to processing data in these audio systems.
BACKGROUND
Different types of acoustic devices have been used through the years. One type of device is a microphone and one type of microphone is a microelectromechanical system (MEMS) microphone, including a MEMS die having a diaphragm and a back plate. The MEMS die is supported by a substrate and enclosed by a housing (e.g., a cup or cover with walls). A port may extend through the substrate (for a bottom port device) or through the top of the housing (for a top port device). In any case, sound energy traverses the port, moves the diaphragm and creates a changing potential of the back plate, which creates an electrical signal. Microphones are deployed in various types of devices such as personal computers or cellular phones.
Digital microphones now exist that convert the analog data produced by the sensor into digital data. The digital data is utilized by different processing elements in the microphone to perform different sets of functions such as acoustic activity detection. Acoustic activity detection requires time to be performed in a reliable manner. Unfortunately, this time delay in detection incurs latency, which allows real-time data to pile or back-up thereby reducing the efficiency and performance of the system. The latency further requires use of a buffer to store audio data, while the acoustic activity detection is made.
The problems of previous approaches have resulted in some user dissatisfaction with these previous approaches, specially the latency that is incurred and that stays in the audio path impacting user experience in voice recognition tasks.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein:
FIG. 1 is a block diagram of a microphone;
FIG. 2 is a block diagram of a system of two microphones and a host;
FIG. 3 is a block diagram of a host;
FIGS. 4A and 4B illustrate a timing diagram of the operation of the systems described herein according to various embodiments of the present invention;
FIG. 5 is a flow chart of the operation of the systems described herein;
FIG. 6 is a diagram showing one example of stitching;
FIG. 7 is a flow chart showing a stitching approach;
FIG. 8 is a time line and data diagram showing the stitching approach of FIG. 7;
FIGS. 9A and 9 B are a flowchart of another stitching approach;
FIG. 10 is a time line and data diagram showing the stitching approach of FIGS. 9A and 9B.



Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity. It will be appreciated further 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.
DETAILED DESCRIPTION
The present approaches allow a first microphone to be operated in a mode having a real-time data path and a path that includes buffered data. The present approaches utilize a host processing device that enables the buffered audio data of the first microphone to catch up or recover the latency as compared to the real-time or live audio data capture. Among other things, this allows the use of a second microphone where the second microphone does not have a buffer. Consequently, any latency issues associated with the first microphone are traversed.
In many of these embodiments and at a host processing device, buffered pulse density modulation (PDM) data and real-time PDM data that has not been buffered is received from a first microphone. The buffered PDM data and the real-time PDM data have the same data content but are discontinuous with respect to the other when received at the host processing device. The buffered PDM data is processed over a first time interval and the real-time PDM data is processed over a second time interval. The host processing device is operated so that the second time interval is less than the first time interval. The real-time PDM data is stitched to an end of the buffered PDM data. The stitching is effective to time align the buffered PDM data with respect to the real-time PDM data to create an output data stream that is sequentially ordered in time. This allows the latency that is otherwise always present in this class of acoustic activity detection MEMS microphones to be transferred to the host device, where it can be easily recovered by faster than real-time processing.
In other aspects, second real-time data is received from a second microphone, the second microphone not having a buffer. In some examples, the second real-time data is inserted into the output stream after the conclusion of the latency recovery mechanism described herein.
In other examples, the processing of the buffered PDM data comprises determining an existence of a trigger word or phrase in the buffered PDM data. In yet other examples, the buffered PDM data and the real-time PDM data are decimated. In some examples, the buffered PDM data and the real-time PDM data are received in a multiplexed format.
In others of these embodiments, a host processing device includes an interface and a processor. The interface has an input and output, and is configured to receive buffered pulse density modulation (PDM) data and real-time PDM data that has not been buffered from a first microphone at the input. The buffered PDM data and the real-time PDM data have the same data content but having a latency and being discontinuous with respect to the other when received at the host processing device. The processor is coupled to the interface, and the processor is configured to process the buffered PDM data over a first time interval and process the real-time PDM data over a second time interval. The processor is operated so that the second time interval is less than the first time interval. The processor is configured to stitch the real-time PDM data to an end of the buffered PDM data. The stitching is effective to synchronize the buffered PDM data with respect to the real-time PDM data and to create an output data stream at the output.
Referring now to FIG. 1, a low power acoustic activity detection (AAD) microphone 100 is described. The microphone 100 includes a charge pump 102, a transducer 104 (including a back plate and diaphragm), an input buffer 106 (with adjustable gain), a sigma delta modulator 108, a decimator 110, an Acoustic Activity Detection (AAD) module 112, a circular buffer 114, a first up converter 116, a second up converter 118, a control block (processor) 120, an internal oscillator 122, and a clock detector 124.
The microphone 100 provides Voice Activity Detection (VAD) capabilities at ultra-low power. The AAD module 112 (including a (VAD) gain block) detects voice and voice-like activity. The circular buffer 114 receives data in real-time. In one aspect, the buffer may of sufficient size to hold 256 msec of audio. In another aspect, the buffer size may be trimable to sizes other than 256 msec. The charge pump 102 provides charge or energy to the transducer 104, and the transducer 104 converts an acoustic signal into an analog signal, which is stored in the input buffer 106. The sigma delta modulator 108 converts the analog signal into a pulse density modulation (PDM) signal, and the decimator 110 converts the PDM signal into a pulse code modulation (PCM) signal. PCM data has two paths: a first path through the circular buffer 114 to up-converter 118, and a second path for real-time data that flows directly through up-converter 116.
The first up converter 116 and second up converter 118 convert PCM data into PDM data. The control block (processor) 120 determines when transmissions are made to a host. The internal oscillator 122 supplies a clock signal and the clock detector 124 determines whether an external clock has been received from an external host via pin 134.
The AAD module 112 detects acoustic activity in a low power operating mode of the microphone. The sensitivity of this block is partially controlled through the input gain of this block. The VAD gain portion of the AAD module 112 in one aspect has a trimable gain. The AAD module 112 monitors the incoming acoustic signals looking for voice-like signature, without the need for an external clock on clock pin 134 and this operation occurs in the aforementioned low power sensing mode. Upon detection of acoustic activity that meets the trigger requirements, the microphone 100 asserts a SEL/STAT pin 130 to wake up the rest of the system in the signal chain. Further, the microphone 100 provides real-time PDM data on DATA line 132 when a clock is made available on the CLOCK line provided by the system after it wakes up. The buffer 114 stores a previous amount of data (e.g., the previous 256 msec of data or a pre-set trimmed amount which may be different from 256 msec) generated prior to the activity detection. Once a clock signal has been detected on pin 134, the microphone 100 transmits the buffered data to a host via DATA line 132. Data output may start at the same time as the SEL/STAT line 130 indicates detection of voice. Alternatively, data output may start after receiving an external clock via pin 134.
Referring now to FIG. 2, another example of a system with a catch-up buffer is described. The system includes a first microphone 202, a second microphone 204, and a host 206.
The first microphone 202 includes a transducer 222 (including, for example, a diaphragm and back plate), a sigma delta converter 224, a decimator 226, a buffer 228, a first up-converter 230, a second up-converter 231, a transmitter 232, a buffer control module 234, a control module 236, an Acoustic Activity Detection (AAD) module 238, and an internal clock 240.
The second microphone 204 includes a transducer, but does not include a buffer. In these regards, the second microphone 204 may be a micro electro mechanical system (MEMS) device that converts sound energy into an electrical signal. The second microphone 204 may include a back plate and a diaphragm. Other examples of microphones are possible.
The host 206 is, in one example, a processing element such as a codec or digital signal processor. The structure of the host 206 is described with respect to FIG. 3. The host 206 receives data streams (that may be multiplexed over a PDM data line 280). The first data stream is from the buffer 228 and the second data stream is un-buffered data. The buffer 228 introduces latency (delay), but is needed because the first microphone 202 needs time for the AAD module 238 to determine whether there is voice (or other acoustic) activity. Additionally, the host processor requires time to wake up from a low power mode and be ready to receive data. The buffer also provides important contextual information to a speech trigger recognition engine to allow it to perform better in noisy conditions. Because of the delay and latency, the two data streams (of the same data content) will be discontinuous and time delayed with respect to each other. The host 206 operates to synchronize the two data streams at its output, and eliminates any discontinuous aspects with respect to each other. In other words, the host guarantees that at some point in time, input data that it is receiving (from one or both of the first or second microphones) is the same data that it is outputting.
The transducer 222 (which may be a micro electro mechanical system (MEMS) device) converts sound energy into an analog electrical signal. The sigma delta converter 224 converts the analog electrical signal into a pulse density modulation (PDM signal). The decimator 226 converts the PDM signal into a pulse code modulation (PCM) signal. The buffer 228 stores the PCM signals. The up-converter 230 converts PCM signals into PDM signals. The transmitter 232 transmits a multiplexed signal (of the first and second data streams) over the data line 280. The transmission is initiated with the receipt of the external clock on line 284. The buffer contents are monitored by the buffer control module 234. When the buffer has transmitted the pre-determined amount of data, for example 256 msec and some additional extension data (by “extension data” it is meant as data beyond the buffer length), the buffer control module 234 sends a buffer empty (bempty) signal 285 to the control module 236, which causes the transmitter 232 to stop multiplexing the contents of the buffer 228. The AAD module 238 detects whether there is voice or other acoustic signals and sends a SEL/STAT signal 282 when acoustic activity is detected. The host 206 responds with a clock signal 284, which is sent to the first and second microphones 202 and 204. The second microphone 204 is also controlled via the GPIO 286 which keeps microphone 204 disabled. The effect of the clock signal 284 is to cause microphone 202 to transmit data. A GPIO 286 is used to control power to the second microphone 204 and to select the second microphone 204. The GPIO 286 is asserted only after stitching is completed at the host. The term “stitching,” means combining the real-time data stream at the end of the buffered data stream in the host, such that a continuous data stream is presented to the application.
In one example of the operation of the system of FIG. 2, the first microphone 202 stores or buffers data in the buffer 228 in order for acoustic activity detection to be performed by AAD module 238 on the buffered data. The host 206 is awaken by the SEL/STAT signal 282 and responsively sends the clock signal 284 to the first microphone 202. Receipt of the clock signal allows the first microphone 202 to clock data out over data line 280.
The first microphone 202 sends multiplexed data (of the first and second streams) to the host 206. This multiplexed data will include real-time and buffered data of length X time units (e.g., 256 ms).
The host 206 processes the X units of buffer data until the processing is complete. X units of real-time data is also waiting for processing by the host 206. The host 206 processes the real-time data over a second time period that is much, much less than the first time period. The host 206 may be operated faster to accomplish this function. The host 206 stitches the real-time data to the end of the buffered data. The goal is that the data being input into the host 206 is being output from the host 206 in real-time.
In order to support low power applications that require or prefer to reduce the signal latency due to the buffer 228, a burst mode is provided in the system of FIG. 2. Burst mode provides the capability for faster than real-time data transfer. Burst mode implements two data paths, one for the real-time data and the other for the buffered data, both of which go through the decimation and interpolation functions needed to run the AAD module 238, for example, at 16 kHz/16 bits PCM. In one aspect, the burst mode utilizes two interpolators to ensure that the signal paths for both signals have the same phase response, excluding any coding and decoding associated with the buffering operation.
The burst mode operates as follows. The SEL/STAT line 282 is used for signaling the state of the microphone 202 to the host 206. The microphone 202 is normally in sense mode with no activity on the data line 280 and SEL/STAT line 282, when there is no voice and the microphone AAD module 238 has converged to the ambient noise.
When the AAD module 238 detects acoustic activity and asserts the SEL/STAT line 282, the host 206 enters the wake-up mode. This action wakes up the host 206 with some latency. The host 206 in one aspect provides a 768 kHz signal to the clock line 284.
The reception of the clock signal 284 by the first microphone 202 along with acoustic detection puts the first microphone 202 into burst mode. In one example, the first microphone 202 enters burst mode within 10 clock cycles of receiving the external clock at 768 kHz. The burst mode uses a first PDM channel to send the buffer data and a second PDM channel to send real-time data to the host.
In some aspects, the real-time PDM channel may be the default channel, so that the real-time data is valid and may be latched during the rising edge of the clock. Buffered data is valid and may be latched during the falling edge of the clock. The data transfer rate in burst mode is in one example double the normal data rate at 768 kHz. When in the burst mode and in one example, the first microphone 202 will toggle the SEL/STAT pin 282 at 8 kHz, synchronous to the 768 kHz CLOCK edges. When the buffer 228 is emptied via the burst mode, the SEL/STAT pin 282 is held high so the host 206 is signaled that the first microphone 202 is now caught up with real-time data. The host 206 may also use a count of the toggle to verify the amount of data collected to aid in “stitching” the buffered and real-time data. Slower toggle rates will cause lower overhead on host systems. In one aspect, the use of an 8 kHz toggle rate will allow the time between each transition to be the duration of 1 PCM sample.
The signal processing algorithms for decimation may cause pops or clicks at the stitch point of the real-time and buffered audio. By a “pop” or “click,” it is meant that unnatural discontinuities in the audio samples will cause distortions in the output audio signal that resemble a “pop” or “click” sound. Some overlap is expected to be required between the buffered and real-time data to eliminate these pops or clicks. The buffered data will be extended beyond the 256 msec or the specific trimmed size to provide this overlap. During the extended buffer state, the SEL/STAT line 282 is held high. The end of the extended buffer period is signaled by toggling SEL/STAT pin 282 at 16 kHz to allow distinction from the burst mode state.
At the end of the extended buffer period or state, the first microphone 202 enters the Real-Time low power mode. When in Real-Time low power mode, the first microphone 202 only uses one of the PDM channels. Data is valid during the rising edge. This permits the use of the second PDM microphone 204 on the PDM port. The second PDM microphone 204 has to be off during the combined time for burst mode output and extended buffer output durations. The SEL/STAT toggle on line 282 may be used as a signal to determine when the second microphone 204 can be powered on. The SEL/STAT pin 282 will keep toggling until the end of detected voice activity. Thus, the activity of the SEL/STAT pin 282, either high or toggling is an indicator of voice activity. If the host 206 uses internal timers available to it, exact grabbing of the extension buffer may not be necessary, but may be self-regulated by the host 206.
Only after the cessation of voice activity and the external clock 284 from the host 206 will the first microphone 202 re-enter sense mode.
Referring now to FIG. 3, one example of a host 300 (e.g., host 206 from FIG. 2) is described. The host 300 includes a stereo decimator 302 (acting as an interface) and a processor 304. The decimator 302 converts PDM data into PCM data. The processor 304 implements or executes stitching approaches (any combination of hardware and software) that append real-time data to the buffered data. The processor 304 includes a buffer for real-time data.
Data discontinuity exists at the start of a burst when the microphone (e.g., microphone 202) is operated in burst mode. Discontinuity can be represented as x(m)-x(n) and is approximately equal to 256 ms where 256 ms is the buffer length of the first microphone (e.g., microphone 202). A voice trigger algorithm starts recognition on the buffered data, x(m) over a first processing interval, while the real-time data x(n) is saved in a buffer on the host 300 and will be processed by voice trigger algorithm over a second processing interval. Data is stitched by the host 300 (e.g., host 206) after the entire buffer (256 ms) is drained and latency is consequently recovered. Buffer data of the buffer in the first microphone (e.g., buffer 228 in first microphone 202) is extended (e.g., by a length less than 256 ms) to allow the stitch algorithm operated by the processor 304 to synchronize x(m) and x(n) and eliminate signal discontinuity.
After data discontinuity is resolved and synchronization is achieved, real-time data from the first and second microphones (e.g., microphones 202 and 204) can be multiplexed on the incoming data line and output in real-time. This may correspond to a low-power real-time mode.
Referring now to FIGS. 4A and 4B, a timeline showing the operation of the approaches described herein is described. The time line shows the occurrence of voice activity 402. It will be appreciated that this timing diagram illustrates the operation of the systems described with respect to FIGS. 1-3.
Voice is detected causing the SEL/STAT line 404 to go high. SEL/STAT stays high until the clock (e.g., 768 kHz clock) is received from the host. The host sends clock signal 406 back to the first microphone. The first microphone detects the clock signal and sends data out on data line 408 at time 410. SEL/STAT then toggles at a suitably chosen signaling frequency. An example frequency that may be used is 8 kHz. On the rising edge of the clock, real-time PDM data 440 is received over the data line. On the falling edge, buffer PDM data is received over the data line from the first microphone. This is the burst mode.
Then at time 412, extension mode is entered. On the rising edge of the clock real-time PDM data is received over the data line and on the falling edge of the clock extension buffer data is received over the data line. This allows the host to stitch the real-time data to the buffer data. The extension period may last a pre-determined time. In one example, this extension period is less than 128 ms and in other examples, this extension period could be 32 msec, 16 msec or 8 msec or another suitable time interval. SEL/STAT toggles at a suitably chosen signaling frequency different from the burst mode signaling frequency until AAD goes inactive. An example frequency could be 16 kHz. At this point, real-time PDM data alone is being received over the data line. Optionally, at time 414, a second microphone (without a buffer) may be powered on the falling edge of the clock after the buffer extension period. On the rising edge of the clock real-time PDM data from first microphone is received over the data line and on the falling edge of the clock real-time PDM data from second microphone is received over the data line.
Referring now to FIG. 5, one example of the state transitions is described. It will be appreciated that this flow chart illustrates the operation of the systems described with respect to FIGS. 1-4.
At step 502, the system is powered ON. At step 504, determine if the SEL/STAT line is VDD or floated.
If at step 504 VDD and acoustic activity detection (AAD) is off, then at step 506 the external clock rate is determined. In one aspect of the invention, if the clock rate is 0-315 kHz, at step 508, the microphone goes to sleep mode. If the clock rate is between 315 and 1400 kHz, at step 510, the microphone is operated in low power mode. If the clock rate is between 1.4 to 4.8 MHz, the microphone goes to normal operating mode at step 512.
If at step 504 the SEL/STAT is floated, then at step 514 it is determined if there is an external clock being received at the microphone. If the external clock is detected to be 1 to 4.8 MHz, execution continues with step 526 where the microphone is operated in normal operating mode. If the external clock is at 768 kHz, execution continues with step 524 at a low power real-time mode. If the answer at step 514 is negative, at step 516 the microphone enters PDM sensing mode. At step 518, wake up is performed. If no external clock is being received at the microphone, execution continues with step 516. If external clock is being received at the microphone, burst mode is entered at step 520. At step 520, burst mode is executed as has been described herein. If at step 524 or step 526, the external clock is stopped, then the execution reverts to block 516 and the microphone enters the PDM sensing mode.
Referring now to FIG. 6, one example of stitching data from the buffer and real-time data is described. It will be appreciated that this example shows how data may be stitched together in the system of FIGS. 1-5.
A buffer (e.g., the buffer 228 in the first microphone of FIG. 2) includes buffered data 602. An audio phrase is received. “He” which (in this example) is the first part of the phrase “Hello VT.” Real-time data 604 also is received and this may be “llo VT” from the last part of the phrase “Hello VT.” The stitching algorithm in the host (e.g., host 206) receives these two data streams and stitches “llo VT” to the end of the buffered data to make stitched data 606 “Hello VT.” The processing of the buffer data must proceed at a real-time rate as it is received at a real-time rate with the latency determined by the buffer size in the microphone. The processing of the real-time data may be made much faster than real-time, because of the accumulated data in the host after the stitching process is completed. Thus, the stitched continuous data stream present at the output of the host recovers the latency and catches up to the live signal with significantly reduced latency. The buffered data 602 and the real-time data 604 are now ordered sequentially with each other and the host can process the data received from one or more microphones in real-time without needing to consider synchronization issues between the first and the second microphone.
Referring now to FIG. 7 and FIG. 8, one example of a stitching approach is described. The discussion with respect to FIG. 7 and FIG. 8 assumes a microphone and host device, for example, as described previously above.
Transients occur whenever PDM data is fed into a decimation filter or when it is stopped. In some aspects, when buffered data is followed by the real-time data, the transients will occur in the middle of the combined audio streams. Using an extended buffer of length greater than the end transient of the buffered audio and the start transient of the real-time audio allows the skipping of these time intervals by calculation of the decimation filter characteristics. One stitching approach provides an extended buffer and skips these transients. Thus, first the buffered and real-time signal must be time aligned at the host. This is possible because both streams start simultaneously only after the host clock is received.
The lengths of the buffer and the extended buffer are pre-determined and may be based upon various factors. 256 ms and 16 ms are examples of lengths for the buffer and extended buffer, respectively.
The output data is taken from the buffered audio until it is past the point where the start transient of the real-time audio has damped out. The output data is then switched to the corresponding real-time stream, so that the transient at the end of the extended buffer data may be skipped. This audio stream does not have any transient in the middle of the stream with this stitching strategy.
At step 702, the host is asleep. At step 704, the microphone wakes up the host, for instance, as has been described above.
At step 706, various parameters or variables are initialized. More specifically, Bufl is the length of the buffer and this is initialized to a value, for example, in milliseconds (e.g., 256 ms). Bufl is shown as element 802 in FIG. 8.
Stpt is the stitch point and is a time value as measured from the end of BUFFERPCM. It is also the same time value when measured from the beginning of the RT_BUF, the real-time buffer on the host. Stpt is represented as element 804 in FIG. 8. Extl is the length of the extension buffer in the microphone and is represented by element 803.
Rt_Buf[BufL+StPt] is an amount of allocation of memory space for real-time data in the host. Real-time data will be stored in a real-time buffer in the host. In one example, the real-time buffer Rt_buf could be set to 256 ms+8 ms if 8 ms is the stitch point. pWR and pRD are write and read pointers and these are initialized to zero.
At step 708, a check is made to determine if line 130 (of FIG. 1) is active. If it is not, return to step 702.
If the line is active at step 710, the host inputs the 2 channels (stereo) of data. The host decimates the data from PDM format to PCM format.
At step 712, store the real-time PCM data in a real-time buffer using the pWR pointer to point to the correct place in the buffer to write the data.
At step 714, a check is made to determine if the pWR pointer has gone past the stitch point. If it has not, at step 716 output the buffered data stream (buffered PCM data) so that it can be further processed. At step 718, the pWR pointer is incremented.
If at step 714, the pWR pointer has gone beyond the stitch point, control continues to step 720. A check is made to see if the pRD flag (used as a position pointer in the real-time data buffer in the host) has reached the stitch point. If it has, output real-time data at step 726. If it has not reached the stitch point, real-time buffer data [pRD+StPt] is output. Then, the pRD pointer is incremented at step 724.
It can be seen in FIG. 8 that the output of this approach will have region 830 (from buffered PCM), region 832 (from extended buffer), region 834 (from extended buffer from RT buffer), region 836 (from RT buffer), and region 838 (not from RT buffer), as the data comes in to the host. It is apparent that the transient regions are avoided because the regions that include the transients are not used (as from a particular buffer).
Referring now to FIGS. 9A and 9B and FIG. 10, another example of a stitching approach is described. The discussion with respect to FIGS. 9A, 9B and FIG. 10 assumes a microphone and host device, for example, as described previously above. At step 902 in FIG. 9A, the host is asleep. At step 904, the microphone wakes up the host.
At step 906, various parameters or variables are initialized. Bufl is the length of the buffer and this is initialized to a value, for example, in milliseconds. Bufl is shown as element 1001 in FIG. 10. Trpt is the length of decimator transients and these are represented by elements 1002 and 1004 in FIG. 10. Rt_Buf[BufL+TrPt] is a memory allocation for real-time data. Real-time data will be stored in a real-time buffer in the host. This could be 256 ms+8 ms if 8 ms is the stitch point. pWR and pRD are write and read pointers and these are initialized to zero. Extl is the length of the extension buffer in the microphone and is represented by element 1003. IS is the interpolated sample.
At step 908, a check is made by the host to see if line 130 of FIG. 1 is active. If it is not, a return to step 902. At step 910, if it is active, then input the 2 channels (stereo) of data is made. The data is decimated from PDM format to PCM format.
At step 914, the approach is dealing with transient period lengths TrPt 1002 and 1004 lengths, which are assumed to be equal. A check is made to see if pWR is in that area of data.
If the pWR pointer is not in the transient area, at step 916 buffered PCM data is output from the host and at step 918 pWR (which is the pointer used in the buffer to store real-time data in the host) is incremented.
If the approach has reached the transient portion, pWR is somewhere in the middle of zone 1006. At step 920, a check is made to see if pWR is out of that zone 1006. If the answer is negative, then at step 922 interpolate the output data based on weighting. PCM data that is interpolated is output from the host at step 924. pWR and pRD are incremented at step 926.
If the determination made is that the pointers are out of the 1006 zone, then control continues with step 928 where a determination is made as to whether pRD is out of zone 1004. If not out of zone 1004, at step 930 output real-time buffer data RT_BUF[pRD+TrPt]. At step 932, the pointer pRD is incremented.
If the process moves out of zone 1004 (by the determination at step 928), real-time (unbuffered) data is output from the host at step 934.
It can be seen that an interpolated region in the output steam avoids the transients. The output is a buffered PCM data region 1030; interpolated region 1032 (that avoids the transients of regions 1002 and 1004); and real-time buffer region 1034 (from the real-time buffer); and region 1036, which is real-time data that is unbuffered.
It will be understood that different interpolation approaches may be used. If infinite input response (IIR) filters are used in decimation, then the transient persists in perpetuity though with decreasing energy to meet design goals. In some situations, the stitch point still shows some broadband noise at the stitch point when basic stitching is used. In interpolated stitching, an allowance is made for the most significant energy of the transients to die down. Then, the intermediate time interval is used to linearly interpolate between the buffered and real-time data. The interpolation may be performed in one example as follows.
Let the time interval be given by discrete variable n. The start of the buffered audio may be considered n=0. An assumption may be made that the time for the most significant energy of the transients to die down is TrPt. The output for each section is given by the following equations respectively.
For the first segment 1030:

op(n)=BUFPCM(n) for 0<n≦(BufL+TrPt)

This equation describes that the output of the host is determined solely based on buffered data.
For the intermediate segment 1032:



  
    
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      )
    
  




  
    
      where
      ⁢
      
        
      
      ⁢
      
        α
        ⁡
        
          (
          n
          )
        
      
    
    =
    
      n
      /
      
        (
        
          ExtL
          -
          
            2
            ×
            TrPt
          
        
        )
      
    
  



This equation describes that data in the intermediate segment is linearly interpolated in both data streams.
For the segment 1034:

op(n)=RTBUF(n) for(BufL+ExtL−TrPt)<n 

This equation describes that the output of the host is determined solely based on real-time buffered data. The above approach results in significantly lower transient broadband energy in the segment where the output is in transition from the buffered data stream to the real-time data stream.
In the equation above, op(n) is output at processing cycle n, n is counter of processing cycles, BUFPCM(n) is buffer PCM sample of processing cycle n, RTBUF(n) is real-time PCM sample of processing cycle n, ExtL(n) is extension buffer PCM sample of processing cycle n, and α(n) is time varying weight factor of processing cycle n. In one aspect, α(n) is defined to increase linearly from 0 to 1 with increasing n.
The first and last equations determine when the output is determined solely by the Buffered data and the Real-Time data and the intermediate equation determine how the data in the intermediate segment is linearly interpolated from both data streams.
This results in significantly lower transient broadband energy in the segment where the output transitions from the buffered data stream to the real-time data stream. In other words, buffered data is used more at the beginning of the interpolation, while real-time data is used less. Real-time data is used less at the beginning and more at the end. The degree of use for each may be described as a linear function.
Preferred embodiments are described herein, including the best mode known to the inventors. It should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the appended claims.

## Claims

1. A method in a microphone apparatus, the method comprising:
clocking the microphone apparatus using a local oscillator of the microphone;
generating a digital data stream from an electrical signal representative of acoustic energy;
determining whether voice activity is present in the electrical signal while buffering data representative of the electrical signal in a buffer of the microphone apparatus, the buffered data having a delay time with respect to real-time data representative of the electrical signal;
providing a multiplexed data stream at an interface of the microphone apparatus after determining the presence of voice activity, the multiplexed data stream including a buffered data stream and a real-time data stream, the buffered data stream and the real-time data stream based on the electrical signal, the buffered data stream including non-real-time buffered data and real-time buffered data, an end portion of the buffered data stream temporally overlapping a beginning portion of the real-time data stream; and
providing the real-time data stream at the interface after discontinuing providing the buffered data stream at the interface.

2. The method of claim 1, further comprising discontinuing providing the multiplexed data stream at the interface of the microphone apparatus after a time period corresponding to a duration of the buffered data stream.

3. The method of claim 2, further comprising:
providing a signal at the interface in response to determining the voice activity is present;
receiving an external clock signal at the interface of the microphone apparatus in response to providing the signal at the interface,
providing the multiplexed data stream at the interface after receiving the external clock signal.

4. The method of claim 3, further comprising synchronizing the multiplexed data stream with the external clock signal by synchronizing the buffered data stream with a first portion of the external clock signal and synchronizing the real-time data stream with a second portion of the external clock signal.

5. The method of claim 1, wherein the generating the digital data stream includes generating a first digital data stream from a first electrical signal produced by a first acoustic sensor and generating a second digital data stream from a second electrical signal produced by a second acoustic sensor, and wherein the providing the real-time data stream at the interface includes providing a first real-time data stream based on the first electrical signal and a second real-time data stream based on the second electrical signal.

6. The method of claim 5, further comprising synchronizing the first real-time data stream with a first portion of a clock signal and synchronizing the second real-time data stream with a second portion of the clock signal.

7. The method of claim 1, further comprising providing the multiplexed data stream at a single connection of the interface of the microphone apparatus.

8. The method of claim 1, further comprising synchronizing the buffered data stream of the multiplexed data stream with a first portion of a clock signal and synchronizing the real-time data stream of the multiplexed data stream with a second portion of the clock signal.

9. The method of claim 1, further comprising:
operating the microphone apparatus in a first mode before providing the multiplexed data stream at the interface of the microphone apparatus; and
operating the microphone apparatus in a second mode while providing the multiplexed data stream at the interface of the microphone apparatus,
wherein a power consumption of the microphone apparatus operating in the first mode is less than a power consumption of the microphone apparatus operating in the second mode.

10. A microphone apparatus comprising:
a converter having an analog input and a digital output, wherein a digital data stream on the output of the converter is produced from an electrical signal representative of acoustic energy on the analog input of the converter;
a buffer coupled to the digital output of the converter, wherein the buffer is configured to buffer data representative of the electrical signal, the buffered data having a delay time with respect to real-time data representative of the electrical signal;
a voice activity detector (VAD) coupled to the digital output of the converter, wherein the VAD is configured to detect whether voice activity is present in the electrical signal while data is buffered in the buffer; and
a multiplexer coupled to the buffer and to the converter, the multiplexer coupled to an interface of the microphone apparatus,
wherein a multiplexed data stream is provided, by the microphone apparatus, at the interface after voice activity is detected, the multiplexed data stream including a buffered data stream and a real-time data stream, the buffered data stream and the real-time data stream based on the electrical signal, the buffered data stream including non-real-time buffered data and real-time buffered data, a portion of the real-time buffered data of the buffered data stream temporally overlapping a portion of the real-time data stream,
and wherein the real-time data stream is provided, by the microphone apparatus, at the interface after the buffered data stream is no longer provided at the interface.

11. The apparatus of claim 10, further comprising a local oscillator, wherein the microphone apparatus is clocked by the local oscillator at least before providing the multiplexed data stream at the interface.

12. The apparatus of claim 11, wherein the apparatus is an integrated circuit.

13. The apparatus of claim 11, further comprising an acoustic sensor packaged with the integrated circuit, the acoustic sensor coupled to the analog input of the converter, the integrated circuit comprising at least the local oscillator, the converter, the buffer, the VAD, and the multiplexer.

14. The apparatus of claim 10, wherein the buffered data stream of the multiplexed data stream is synchronized with a first portion of a clock signal and the real-time data stream of the multiplexed data stream is synchronized with a second portion of the clock signal.

15. The apparatus of claim 10, further comprising a controller coupled to the VAD and to the interface, the interface including a clock connection, a data connection, and a signal connection, wherein:
a signal of the controller is provided at the signal connection of the interface in response to the VAD detecting voice activity,
the microphone apparatus is clocked by an external clock signal provided at the clock connection of the interface in response to the signal of the controller,
the multiplexed data stream of the multiplexer is provided at the data connection of the interface in response to the external clock signal, and
the real-time data stream is provided at the data connection of the interface after the buffered data stream is no longer provided.

16. The apparatus of claim 15, wherein the buffered data stream is synchronized with a first edge of the external clock signal and the real-time data stream is synchronized with a second edge of the external clock signal.

17. The apparatus of claim 10, wherein the digital data stream includes a first digital data stream generated from a first electrical signal produced by a first acoustic sensor and a second digital data stream generated from a second electrical signal produced by a second acoustic sensor, and wherein the real-time data stream includes a first real-time data stream based on the first electrical signal and a second real-time data stream based on the second electrical signal.

18. The apparatus of claim 17, wherein the first real-time data stream is synchronized with a first portion of a clock signal and the second real-time data stream is synchronized with a second portion of the clock signal.

19. The apparatus of claim 10, wherein the interface has a single data pin, and wherein the multiplexed data stream is provided at the single data pin of the interface after voice activity is detected.

20. The apparatus of claim 10, wherein the microphone apparatus has a first mode of operation before the multiplexed data stream is provided at the interface, wherein the microphone apparatus has a second mode of operation while the multiplexed data stream is provided at the interface, and wherein a power consumption of the first mode of operation is less than a power consumption of the second mode of operation.