Abstract:
Apparatus and corresponding methods, referred to as “stealth recording,” in which long audio segments are recorded into a buffer, then separated into individual phrases for auditioning and application. Stealth recording surreptitiously and continuously records audio processed thereby, then separates, catalogues, and time stamps the audio into phrases using, among other techniques, spectral analysis that compares the recorded audio to a sample of the ambient noise floor. This allows a user to instantly locate any phrase and audition or apply it within its proper context. This has numerous practical applications, ranging from musicians who wish to improvise then apply their most inspired phrases to a particular song, to students reviewing a lecture and replaying audio phrases in context with the visual information present at the time of the audio recording.

Description:
BACKGROUND  
         [0001]    The present invention relates generally to audio recording, and more particularly, to apparatus and methods that surreptitiously record and analyze audio for later auditioning and application.  
           [0002]    Many musicians, when aware that they are being recorded, suffer from “recording anxiety.” Their performances become more constrained, losing some of the emotion and spontaneity that is inherent in the best musical performances. Musicians frequently create their best performances while warming up, experimenting, or improvising. Some musicians attempt to solve the anxiety problem by simply recording everything they play, but this presents its own set of problems, namely, how to audition all the recorded audio and how to find those few inspired performances in a lengthy improvisation.  
           [0003]    Thus, if one wishes to solve the problem of “recording anxiety” by recording every performance, it is desirable to have apparatus and methods that enable one to find, audition, and apply the good performances, while simultaneously deleting the unwanted ones.  
           [0004]    It is therefore an objective of the present invention to provide for apparatus and methods for surreptitiously recording and analyzing audio.  
         SUMMARY OF THE INVENTION  
         [0005]    To meet the above and other objectives, the present invention provides for apparatus and methods that separate long audio recordings into individual phrases, which can be individually auditioned, retained, applied, or discarded later. The present invention is of benefit to a wide range of audio recording applications including musical recordings, audio-for-film, conferencing products, court recording equipment, and classroom recording aids.  
           [0006]    More particularly, the present invention provides for apparatus and a method, referred to as “stealth recording” that implements the following processes.  
           [0007]    (a) The present invention quickly and effortlessly establishes a maximum signal level, which it uses to insure an optimal signal-to-noise ratio.  
           [0008]    (b) The present invention establishes and “fingerprints” an ambient noise floor, which is used as an aid in separating the audio into phrases (as described in step d).  
           [0009]    (c) The present invention surreptitiously records audio signals present at its input into a temporary buffer, whose contents are continuously analyzed (as discussed in step d) until the buffer is either saved or deleted. If the buffer fills without the performer taking action, the oldest buffered recordings will be replaced with newer ones.  
           [0010]    (d) Audio is separated into individual phrases by comparing the spectral content of the recorded audio against the spectral fingerprint of the ambient noise floor. Whenever the spectral signal level rises above the ambient noise floor for a user-specified length of time, a new phrase is created and time stamped.  
           [0011]    (e) A user interface indicates each new phrase in a manner most appropriate for the product. For example, each time a new phrase is detected, a hardware device might light an additional button in a row of buttons that correspond to phrases.  
           [0012]    In the previous product user interface example, any phrase would be auditioned by merely pushing its corresponding button. The phrase, having been time stamped, would play “in synchronization” with any other recording happening at the same time (as in the case of a multi-track recording). Good phrases may be committed to the project at the push of a button. Bad phrases may be deleted just as easily. Entire record buffers may be deleted in a single action.  
           [0013]    The present apparatus and methods, while they are specifically designed to benefit musicians as discussed herein, has many applications in various audio recording environments. Filmmakers, videographers and news reports, for example, could search audio phrases to rapidly locate important visual selections, which are synchronized to the time-coded audio. Secretaries taking notes in a classroom, meeting room, or courtroom could instantly locate random sections of a meeting for review or clarification.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:  
         [0015]    [0015]FIG. 1 illustrates exemplary apparatus and “stealth recording” methods in accordance with the principles of the present invention; and  
         [0016]    [0016]FIGS. 2 and 3 are simplified flow charts illustrating how recording levels are automatically optimized in the apparatus and “stealth recording” methods illustrated in FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0017]    Referring to the drawing figures, exemplary apparatus  10  (FIG. 1) and “stealth recording” methods  100  (FIG. 3) in accordance with the principles of the present invention are shown. FIGS. 2 and 3 are simplified flow charts illustrating how recording levels are automatically optimized in the apparatus  10  and stealth recording methods  100 . FIG. 2 shows a flow chart for a noise floor analysis sub-process  200 , and an automatic gain sub-process  300  used in the stealth recording apparatus  10  and methods  100 .  
         [0018]    The exemplary stealth recording apparatus  10  comprises a microphone or instrument input  11  for receiving audio input signals from an instrument or microphone, which is coupled to an input of a preamplifier  12 . An automatic gain sub-process  300  generates a gain control signal that controls the gain of the preamplifier  12 . An output of the preamplifier  12  is coupled to an analog-to-digital (AID) converter  13 . An output of the analog-to-digital converter  13  is coupled to a recording device  14 , comprising a collection of buffering processes  400 ,  400 - 2 , etc., using digital signals processing techniques  420 , to separate and buffer the recordings A, B, C, D, etc., that implements the stealth recording method  100 . A user interface  15  allows a user to operate the apparatus  10 .  
         [0019]    Audio recorders are used in many disciplines and, consequently, come in many forms. Presented below is a detailed description of each step in an exemplary stealth recording method  100  that is implemented in the apparatus  10 , using a single “real world” example of how that step might be implemented in an actual musical recording product (the apparatus  10 ), although other product categories are supported by the present stealth recording apparatus  10  and methods  100 .  
         [0020]    The stealth recording method  100  first automatically establishes a proper gain setting in the automatic gain sub-process  300  for an optimum signal-to-noise ratio of the audio output signals input at the microphone or instrument input  11 . The automatic gain sub-process  300  is illustrated in FIG. 3. The automatic gain sub-process  300  comprises the following steps.  
         [0021]    A user is prompted by way of the user interface  15  whether to automatically adjust the input gain  310  (i.e., to set an optimized gain level  300  of the preamplifier  12 ). If the user does not agree (by selecting a No button (N) on the user interface  15 , for example), a previously-used or default gain level  380  is used. If the user agrees (by selecting a Yes button (Y) on the user interface  15 , for example) to automatically adjust the input gain  310 , the input gain of the preamplifier  12  is digitally reduced  320  to a lower amplification level (−40 dB, for example). At this point, the apparatus  10  samples  330  the microphone or instrument input  11  for a predetermined amount of time (“X” seconds) and the user inputs the loudest sound that is likely to be made into the microphone or instrument input  11 . For instance, a vocalist shouts into the microphone, or a musician plays a loud chord or note.  
         [0022]    If the user is not satisfied  340  (No) with the maximum volume sample, the gain of the preamplifier  12  is again digitally reduced  320  to a lower amplification level. Once the user is satisfied  340  (Yes) with the maximum volume sample, the maximum peak level is measured  350  and the gain of the preamplifier  12  is automatically adjusted upward  360  such that the measured level is equal to 0 dB. The automatic gain setting sub-process  300  insures that recordings always have the best possible signal-to-noise ratio, freeing the performer from “riding” signal levels during a recording session.  
         [0023]    The stealth recording method  100  then performs a noise floor analysis  200  using a noise floor digital signal processor  420 . Details of this process are illustrated in FIG. 2. The noise floor analysis  200  first requests  210  a user-definable length of silence, typically 2-3 seconds. This length of time is input at the user interface  15  such as by using a keypad  16 , for example. If the ambient noise floor is not continuous (city sounds or television audio in background, for example), a longer sample can be requested by inputting a new value using the keypad  16 . During this time period, the user refrains from singing, speaking, or playing. The noise floor digital signal processor  420  in the recording device  14  records  220  the ambient noise in the room, including any wind noise, hum, electrical noise, fans or other ambient sounds that might be present.  
         [0024]    The ambient noise is sampled and recorded by the noise floor digital signal processor  420  until the user is satisfied  230  with the ambient sample (that is, no extraneous or spurious noise was recorded during the sampling). The user depresses a “Satisfied” button  18  on the keypad  16  to indicate acceptance of the ambient sample. Then, a spectral analysis of this ambient noise sample is performed  240  and stored  250  in a memory (or buffer) in the noise floor digital signal processor  420 . There are many types of available spectral analysis techniques, but typically, a series of windowed fast Fourier transforms (FFTs) are computed using an overlap-add technique. For example, a 1024-point FFT may be used with a Hanning window and half window overlap. An average of all the windows is computed and stored, although in general, only the power spectrum needs to be retained.  
         [0025]    At this point, the recording device  14  begins to record automatically. All audio signals present at the input  11  are routed through the preamplifier  12 , whose gain was set automatically by the automatic gain process  300 . The signal is digitized by the A/D converter  13  and is temporarily written to a record buffer  410 .  
         [0026]    The noise floor digital signal processor  420  constantly compares the audio in the record buffer  410  with the ambient noise determined by the noise floor analysis  200 , illustrated at the middle-left portion of FIG. 1. Whenever the audio signal level rises above a noise threshold  421  for a user-specified time, the stealth recording method  100  defines this as the beginning of an audio phrase. When the signal level drops below the noise threshold  421  for a user-specified time, the stealth recording method  100  defines this as the end of the audio phrase. The region between the beginning and end of the audio phrase is a calculated phrase  424 . To assure smooth fade-ins and fade-outs, a user-specified length of buffered audio is added to the beginning  422  and end  423  of the phrase. A preferred embodiment of the invention may have a transition time on the order of from 1 to 100 milliseconds, for example. However, it is to be understood that other transition times may be employed at the discretion of the designer or user, and that the present invention is not limited to the above-cited range of transition times. This entire extended phrase  425  is retained and time-stamped. Buffered audio that is not associated with a phrase is discarded  430  and its space is made re-available newly recorded audio.  
         [0027]    In this manner, audio is constantly being recorded into the record buffer  410  and the stealth recording method  100  is continuously analyzing the audio within the record buffer  410 , to identify phrases, time stamp them, and flush the record buffer  410  of “silent” audio, which it reapplies to recording more phrases. The size of each the record buffer  410  is determined by specifying either a maximum number of phrases or a maximum length of “silent” audio.  
         [0028]    In the case where a maximum number of phrases is specified, because the length of each phrase cannot be known in advance, the actual size of the buffer  410  (in megabytes) expands or contracts depending on the length of the phrases it contains. If the buffer  410  fills  440  without the user taking action  460 , the oldest buffered phrase (and any silence that exists before it) is deleted  470  and replaced with the newest buffered phrase, and so on.  
         [0029]    The result of this buffering is that a performer can play for as long as is desired without performance stress or anxiety. The performer is free to experiment, improvise, or practice as long as is desired. The performer does not interact with the recording hardware until something is played that is liked, at which point the stealth recording method  100  is activated such as by using a “Save” button  17  on the user interface  15 , for example, to save the contents of the record buffer  410 . Compare this to “traditional” recording in which the performer operates the recording device to indicate that “I&#39;m going to record now,” then is “forced” to play something good. No wonder so many musicians suffer from “recording anxiety”.  
         [0030]    The present apparatus  10  and stealth recording method  100  uses multiple buffer processes  400 ,  400 - 2 ,  400 - 3 , for example, so, if a performer chooses to save  480  the contents of one record buffer  400 , the performer can continue to play and performances will begin to aggregate in a new buffer  400 - 2 , for example.  
         [0031]    Because the audio has been digitally recorded, any phrase (A, B, C, D, E, etc) can be accessed immediately. This enables the performer to quickly audition the contents of the saved record buffer  400 ,  400 - 2 ,  400 - 3 , for that “perfect take”.  
         [0032]    Thus, apparatus and methods for surreptitiously recording and analyzing audio has been disclosed. It is to be understood that the described embodiment is merely illustrative of some of the many specific embodiments which represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.