Patent Grant
9258641
Achieving higher fidelity sound reproduction has been a goal since advent of sound recordings. Much of the early focus was on the sound reproduction equipment (e.g., power amplifiers, speakers). In the digital age, most of the focus has been on the audio file compression formats that nevertheless can be used for high fidelity playback. However, increases in fidelity are still possible.
For a detailed description of example embodiments, reference will now be made to the accompanying drawings in which:
Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, different companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function.
In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . . ” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection.
“Logarithmic analog-to-digital conversion” shall mean that, for a plurality of analog values converted, a respective plurality of digital values is produced where the digital values are logarithmically related to the respective plurality of analog values. Creating a plurality of digital values linearly related to analog values and later modifying the plurality digital values to have a logarithmic relationship shall not be considered “logarithmic analog-to-digital conversion”.
“Logarithmic analog-to-digital converter” shall mean a device or combination of devices that, for a plurality of analog values applied to the device(s), a respective plurality of digital values are produced where the digital values are logarithmically related to the respective plurality of analog values. A device that creates a digital value linearly related to an analog values in combination with later modifying the digital values to a logarithmic relationship shall not be considered a “logarithmic analog-to-digital converter”. Moreover, incremental changes inherent in binary representation of a continuous function shall not obviate the status as logarithmic analog-to-digital converter. Moreover, localized departure from a logarithmic response (e.g., upper end of a conversion range, lower end a conversion range, temperature dependent changes) shall not obviate status as logarithmic analog-to-digital converter.
“Linear analog-to-digital converter” shall mean a device or combination of devices that produce a digital value that is linearly related an analog value. Incremental changes inherent in binary representation of a continuous function shall not obviate the linearity of a linear analog-to-digital conversion. Moreover, localized non-linearity (e.g., upper end of a conversion range, lower end a conversion range, temperature dependent changes) shall not obviate status as linear analog-to-digital conversion.
“Octave” shall mean a unit of measure corresponding to a range of intensities (e.g., perceived sound intensity), but octave shall not imply any relationship to or number of steps of a set of numbers that may reside within the octave or the number of octaves over the entire range of interest.
“About” in relation to a period of time shall mean that received events occur within 1 milli-second (ms) of each other.
The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment.
The various embodiments are directed to audio capture systems, such as mobile devices (e.g., wireless network-enabled devices), mobile cellular devices, video cameras, and other sound recording equipment. The specification first turns to identifying shortcomings in the related art.
Identifying Shortcomings of the Related-Art
Part of understanding why the example embodiments represent an advance in audio capture technology is an understanding of the shortcomings of related-art systems. In particular, many currently available audio capture devices convert sound pressure level linearly. “Linear” in the context of the identifying the shortcomings of related-art systems indicates that each digital representation of a sound pressure level is related to the analog signal created by a microphone in a straight line sense. That is, each digital value is related to the corresponding analog signal according to the equation:
VALUEDIGITAL=M*(ANALOG VALUE)+OFFSET (1)
where VALUEDIGITAL is the encoded digital value representation, M is gain value, ANALOG VALUE is the instantaneous analog signal created by a microphone, and OFFSET is an offset value.
Thus, digital representations of sound pressure level in many related-art systems are linearly related to the sound pressure level at the time the sample is taken. As an example, consider a system that uses an 8-bit linear analog-to-digital converter, such that digital values for the illustrative system may span the binary range {00000000→11111111} which in decimal is {0→255} (which for convenience will be considered to be {1→256}). Sound is a vibration of air molecules such that pressure at a location fluctuates around ambient pressure. A microphone converts the pressure fluctuations into a time varying electrical signal carried along an electrical conductor, where the electrical signal has both positive portions and negative portions. It is noted that an equivalent description of the function of a microphone can be made in terms of electrical current flow to and from the microphone, but so as not to unduly complicate the discussion the specification from this point forward considers the electrical signal only from a voltage perspective. Ignoring for now negative voltage portions of the electrical signal, in an example captured audio a zero voltage of the electrical signal takes the value 1 and the highest voltage of the electrical signal takes the value 256. Thus, using linear conversion of sound pressure level as converted to an electrical signal by a microphone (and again considering only positive voltage values or sound pressure levels higher than ambient), sound pressure level may be considered to be divided into 256 equally space steps along the range.
Humans perceive changes in sound pressure levels (i.e., sound intensity) non-linearly, and the relationship is approximately logarithmic. Consider, for example, a digital audio image created using the 8-bit analog-to-digital conversion discussed above. If digital signal values are applied to a digital-to-analog converter and then to a speaker, human perception views the difference of sound intensity from a value of decimal 1 to decimal 2 as a doubling of sound intensity; however, the next doubling of sound intensity is decimal 4 (2*2), not a decimal 3 (2+1). Likewise, the next doubling of sound intensity is decimal 8 (4*2), and so on. On the upper end of the digital signal values in the example, human perception views the difference of sound intensity from a value of decimal 128 to decimal 256 as a doubling of sound intensity.
A bit more precisely then, human perception of change of sound intensity follows the Weber-Fechner law, defined mathematically as:
B=k*ln(L/L0) (2)
where B is the change in apparent intensity to a human listener, k is a constant, L is the just-identifiable change in sound intensity, and L0 is the previous sound intensity. Letting k equal 1, and applying the values described above with respect to the system performing 8-bit analog-to-digital conversion, the change in apparent sound intensity as between digital luminance values of decimal 1 and decimal 2 is B=ln(2/1)=0.693. Again with k equal 1, the change in apparent sound intensity as between digital luminance values of decimal 128 and decimal 256 is B=ln(256/128)=0.693. The precise value 0.693 is of little consequence, but note that the change in apparent sound intensity in the two example situations is exactly the same in spite of the number of steps between the respective analyzed values.
The inventor of the present specification has found that the linear analog-to-digital conversion used in related-art audio capture systems, considered with the human's logarithmic perception of sound intensity, degrades fidelity by storing or recording too much information in the higher sound intensity ranges and too little information in the lower sound intensity ranges. In order to highlight this point,
By performing linear analog-to-digital conversion, the related-art audio capture systems store too much information with respect to the higher sound intensity ranges, and too little information with respect to the lower sound intensity ranges. Again, human perception perceives the change in sound intensity between a decimal 128 value and a decimal 256 value as doubling of sound intensity. Thus, as shown in the illustrative case of
Stated in terms of granularity or quantization between doubling of apparent sound intensity, the difference in sound intensity as perceived by a human having a peak value of decimal 128 and a sound intensity having a peak value of decimal 256 will be a doubling of apparent sound intensity, in this case with 128 gradations between. The difference in sound intensity as perceived by a human having a peak of decimal 64 and a sound intensity having a peak of decimal 128 will again be a doubling of apparent sound intensity, with 64 gradations between. The difference in sound intensity as perceived by a human having a peak of decimal 32 and a sound intensity having a peak of decimal 64 will be a doubling of apparent sound intensity, with 32 gradations between. The difference in sound intensity as perceived by a human having a peak of decimal 16 and a sound intensity having a peak of decimal 32 will be a doubling of apparent sound intensity, with 16 gradations between. The difference in sound intensity as perceived by a human having a peak of decimal 8 and a sound intensity having a peak of decimal 16 will be a doubling of apparent sound intensity, with 8 gradations between. The difference in sound intensity as perceived by a human having a peak of decimal 4 and a sound intensity having a peak of decimal 8 will be a doubling of apparent sound intensity, with 4 gradations between. The difference in sound intensity as perceived by a human having a peak of decimal 2 and a sound intensity having a peak of decimal 4 will be a doubling of apparent sound intensity, with 2 gradations between. Finally, the difference in sound intensity as perceived by a human having a decimal 1 and a sound intensity having a peak value of decimal 2 will be still be a doubling of apparent sound intensity.
The information discussed in the immediately previous paragraph is reproduced in the table form below:
Notice how the granularity within each doubling of perceived sound intensity gets smaller at the lower luminance ranges.
In order to help quantify the difference between the related-art systems and the various example embodiments discussed below, the range of apparent sound intensity in
While each illustrative octave represents a doubling of apparent sound intensity,
Thus, too much information is encoded with respect to the upper sound intensity ranges (the upper octaves), and too little is encoded with respect to the lower sound intensity ranges (the lower octaves).
Before proceeding, note again the discussion has focused only on the positive portions of the electrical signal. However, sound is pressure waves, which the microphone converts to a time varying electrical signal. If one considers a pure sine wave tone at “middle” C (i.e., 440 Hertz (Hz)), the voltage of the electrical signal swings from a positive peak voltage to a negative peak voltage and back to the positive peak voltage 440 times per second. Moreover, for high fidelity audio recording, the original electrical signal may be sampled at the Nyquist rate or above—twice the peak frequency. If the upper limit of human hearing is 20,000 Hz, the Nyquist rate may be 40,000 Hz, meaning there may be about 90 samples of a complete period of the electrical waveform representing the example 440 Hz tone. For a pure tone, half the sampled values will be negative. The same issues with respect to quantization exist for the negative portion of the electrical signal. That is, using linear analog-to-digital conversion results in over-quantization of the greater magnitude values in the negative portions, and under quantization of the values closer to zero volts in the negative portions.
Wider A/D Conversion does not Address the Problem
One approach to obtain higher resolution is merely to use a “wider” linear analog-to-digital conversion. For example, rather than use an 8-bit linear analog-to-digital conversion, one approach is use of 12-bit linear analog-to-digital conversion, or perhaps a 16-bit linear analog-to-digital conversion. However, even a wider linear analog-to-digital conversion does not fully address the issue.
Assume for purposes of explanation that a manufacturer of audio recording equipment wishing to store data with greater resolution modifies a system to have a 16-bit linear analog-to-digital conversion rather than the 8-bit linear analog-to-digital conversion used in the example above. There are a host of problems that would dissuade a manufacturer from making a switch to a 16-bit linear analog-to-digital conversion, not the least of which are increased price, and increased power requirements resulting in shorter battery life for portable devices. Notwithstanding these issues, consider
In particular,
For the same electrical signal range as between
One additional point is made with reference to
A few additional points before proceeding. Firstly, the log of a negative value is not defined in mathematics, but the time varying electrical signal associated with a microphone swings both positive and negative. Through creative circuit design it is possible to create a logarithmic amplifier that operates in a mirror fashion for negative voltages. That is, the logarithmic relationship of positive input voltage to positive output voltage is “mirrored” across the Vin axis to provide output voltage for negative signals. However, the logarithmic amplifiers that function across the zero point do not have sufficient dynamic range for high fidelity audio capture. Expanding the range sufficiently may require piecewise linear systems (i.e., linear in the sense of a continuous function with no step values, as opposed to linear in a straight line sense). That is a plurality of gain sections whose outputs are summed together and, in a piecewise sense, approximate a logarithmic transfer function. Piecewise linear logarithmic conversion, however, introduces errors that are audibly noticeable with respect to high fidelity audio capture.
Example Embodiments
The issues noted above are addressed, at least in part, by use of logarithmic analog-to-digital conversion to create captured audio.
The audio capture system further comprises processor 412 coupled to a program memory 414 and an audio sample memory 416 by way of a communication bus 418. In cases where the audio capture system 400 is a portable device, the audio capture system 400 may further comprise a battery 420. The electrical connections of the battery 420 to the various other components of the audio capture system 400 are omitted so as not to unduly complicate the figure.
The microphone 402 may take any suitable form for converting sound pressure waves 424 into signals for further operation. For example, the microphone may be a moving-coil microphone, a carbon microphone, a piezoelectric microphone, or a fiber optic microphone. For microphones that produce an alternating current (AC) signal “riding” a direct current (DC) bias, additional filtering and amplification circuits may be used to create the time varying (i.e., AC) electrical signal carried on the input signal line 408. In the case of an optical microphone, additional circuitry may be used to create the electrical signal on the input signal line 408 responsive to the light modulation from the microphone. Although only a single microphone 402 is shown in
Logarithmic analog-to-digital converter system 404 couples on an analog side to the input signal line 408, and couples on a digital side to the processor 412 either directly, or through the combining system 410. Various example embodiments of the logarithmic analog-to-digital converter system 404 are discussed in greater detail below, but for now consider that the converter system 404 comprises a circuit or combination of circuits such that, for each positive analog signal sampled, the logarithmic analog-to-digital converter system 404 produces a digital value that is logarithmically related to the positive voltage analog signal. In particular, the logarithmic analog-to-digital converter system 404 may create digital values based on analog input signals according to Equation 3:
VALUEDIGITAL=LogB(ANALOG VALUE)+OFFSET (3)
where VALUEDIGITAL is the encoded digital value representation, ANALOG VALUE is the analog signal from the microphone sampled, B is base of the logarithm, and OFFSET is an offset value.
Similarly, logarithmic analog-to-digital converter system 406 couples on an analog side to the input signal line 408, and couples on a digital side to the processor 412 either directly, or through the combining system 410. Various example embodiments of the logarithmic analog-to-digital converter system 406 are discussed in greater detail below, but for now consider that the converter system 406 comprises a circuit or combination of circuits such that, for each negative analog signal sampled, the logarithmic analog-to-digital converter system 404 produces a digital value that is logarithmically related to the absolute value of the instantaneous negative analog signal. In particular, the logarithmic analog-to-digital converter system 406 may create digital values based on analog input signals according to Equation 4:
VALUEDIGITAL=LogB(|ANALOG VALUE|)+OFFSET (4)
where VALUEDIGITAL is the encoded digital value representation, |ANALOG VALUE| is the absolute value of the analog signal from the microphone sampled, B is base of the logarithm, and OFFSET is an offset value.
Still referring to
The illustrative audio capture system 400 further comprises an audio sample memory 416 coupled to the processor 412. As the name implies, the audio sample memory 416 may be the location to which digital representations of the captured audio are stored. In some cases, the processor 412 may read digital values from the logarithmic analog-to-digital converter systems 404 and 406 and write the values to the image memory 416, but in other cases the digital values may be directly written through a direct memory access (DMA) system. In many cases, the audio sample memory 416 may comprise a removable memory card or stick 422, such that the captured audio may be transferred to other devices. The audio sample memory 416 may thus comprise any suitable removable memory system or device, such as a Secure Digital (SD) card memory or flash memory device. The remaining portions of
Having now described the illustrative audio capture system 400, the specification turns to a description of operation using the logarithmic analog-to-digital converter system 404, and how such operation addresses, at least in part, the issues noted with respect to the related-art systems. In particular,
Thus,
While each illustrative octave represents a doubling of apparent sound intensity,
Thus, the various embodiments more evenly distribute the quantization across the octaves from a granularity standpoint as related to human perception. Table 4 shows, for the example eight octaves and 8-bit analog-to-digital conversion, how the linear and logarithmic conversions compare.
With respect to post-processing, in this example there is insufficient data until one reaches the sixth octave in the linear systems to recreate the granularity in the corresponding octaves in the logarithmic system.
Wider Effective Capture Range
The specification now turns to the concepts of dynamic range of capture. Imagine a situation where a loud instrument, such as trumpet played as loud as possible, plays alongside a quite instrument, such as cello played very softly. The sound that propagates to the listener will be a combination of the sound from the two instruments, and thus in the example the sound will have widely varying dynamic range. The example situation, the trumpet may have a peak dynamic range approaching that of human hearing—around 120 decibels (dB). By contrast, an audio capture system with 10-bit linear analog-to-digital conversion theoretically has only a 60 dB capture range (210=1024, and 20 log (1024)=60 dB), though many currently available microphones capture the entire dynamic range of human hearing—again 120 dB. From the study above, however, it is clear that in spite of the theoretical range of a 10 bit linear analog-to-digital conversion, in actuality the effective range is less as caused by the under quantization in the lower octaves. Table 4 above suggests that roughly half the octaves are under quantized, making the effective dynamic range of linear analog-to-digital closer to about 30 dB. Table 5 shows a relationship between a linear analog-to-digital conversion and logarithmic analog-to-digital conversion for a 10-bit system and 8 octaves to highlight again the effective breadth of the audio capture for wider conversion systems.
If you consider that somewhere between 64 and 128 quantization steps within each octave is the subjective point where degradation becomes noticeable to an ordinary listener. Table 5 thus highlights again that the effective dynamic range for the linear conversion is about half (arguably less than half given the 64 quantization steps in illustrative zone 5) that of the logarithmic conversion in the positive quadrant.
Gain Control
The specification now turns to gain control in accordance with at least some embodiments. As an aid in discussing the concepts of gain control, attention is directed to
However, in accordance with at least some embodiments additional controls may be implemented, the additional controls in the form of gain control within the logarithmic analog-to-digital converter systems 404 and 406. For example, by controlling the gain applied within each of the logarithmic analog-to-digital converter systems, control of the relationship of the digital signal values to the peak sound intensity may be adjusted to further increase the fidelity of the recording. That is, when the peak sound intensity is below dynamic range of the logarithmic analog-to-digital converter systems 404 and 406, the gain of each converter systems may be adjusted to better match, which creates greater quantization within each octave. Example logarithmic analog-to-digital converter systems are discussed more below. For example, by increase the gain with the logarithmic analog-to-digital converter systems, the relationship of the digital luminance values to the sound intensity may be shifted counter clockwise about the origin, as shown by solid line 604. Likewise, if a previous recording used high gain based on a low dynamic range of the sound intensity, by lowering the gain the relationship of the digital signal values to the sound intensity may be shifted about the origin back to that of dashed line 600, or anywhere between.
The gain control in relation to
In some cases, the gain control is implemented based on commands received from the user of the audio capture system 400. For example, as a precursor to capturing audio for storage to the image memory, the system 400 may be configured to initially capture audio and provide to the user an indication of dynamic range of the audio. As part of capturing the initial audio, the system 400 may enable the user to make gain control adjustments. Once the user has adjusted the gain as desired, the final audio capture may begin. It is noted that the gain (or more mathematically, the base of the logarithm) if adjustable may be stored to the audio sample memory 416 such that in the playback process, discussed more below, the correct anti-log may be taken.
In yet still other embodiments, adjustments to gain may be made by a program executing on the processor 412 without user input. In particular, by performing logarithmic analog-to-digital conversion an automatic system for gain may be implemented. In the “automatic” adjustment example, the system 400 may initially capture audio with each of the gains at a predefined “center” or midrange setting. The processor 412 may analyze the initially capture audio to determine the dynamic range, and may adjust the gain based on the dynamic range, again without user input. For example, the processor 412 may locate within the initially capture audio the mid-point digital signal value, and make a gain adjustment such that the mid-point of the range of the digital signal values substantially matches the mid-point of the initially capture audio (if the initially audio could be re-recorded with the new gain settings). Note that as discussed with respect to
The positive logarithmic analog-to-digital converter system 404 in
The amplifier 708 in accordance with these embodiments has a gain response that is logarithmic (e.g., Part No. ADL5310, available from Analog Devices, Inc. of Norwood, Mass.), and the analog-to-digital converter 710 has a linear response (e.g., Part No. LTC2480 16-bit A/D converter available from Linear Technologies of Milpitas, Calif.). That is, the output signal of the amplifier 708 is logarithmically related to the input signal. Logarithmic amplifier 708 may further have a control input, such as a gain control 712. In the illustrative audio capture system 400 the gain control is an analog value and is thus coupled to a digital-to-analog output portion 714 of the processor 412 (such as when the processor 408 is a microcontroller or ASIC). In other cases, an analog value may be created by a digital-to-analog converter distinct from the processor 412, yet communicatively coupled to both the processor 412 and the amplifier 708. In other cases, the gain of the logarithmic amplifier 708 may controlled by way of a digital control signal or signals, and thus the processor 412 may couple to the amplifier by way of a digital communication bus. Regardless of the precise mechanism by which the processor, executing instructions, controls the gain of the amplifier, such control enables the gain control features discussed above.
The negative logarithmic analog-to-digital converter system 406 in
The negative logarithmic analog-to-digital converter 406 has an additional component in the form of inverting amplifier 724. That is, the inverting amplifier reverses the sign of the electrical signal created by the microphone.
The example system 400 of
However, in other cases the user may interact with the programs executing on the processor by way of physical buttons accessible on the outside cover 724 of the audio capture system, such as illustrative externally accessible switches 704. In particular, the illustrative externally accessible switches 704 couple to the processor 412 by way of digital inputs 736 of the processor 412 (such as when the processor 412 is a microcontroller or ASIC). In other cases, the digital values may be read by a digital input device distinct from the processor 412, yet communicatively coupled to both the processor 412 and the externally accessible switches 704. While illustrative externally accessible switches 704 are shown as two normally open pushbutton devices, other types and number of externally accessible switches may be used.
Other Example A/D Converter Systems
The specification to this point has expressly illustrated a logarithmic analog-to-digital conversion, and shown possible implementations of logarithmic analog-to-digital converter systems in
The logarithmic analog-to-digital converter 802 in accordance with these embodiments has a response that is logarithmic. That is, the digital output values created by the logarithmic analog-to-digital converter 802 are logarithmically related to the input signal. Thus, the combination of the linear amplifier 800 and logarithmic analog-to-digital converter 802 may be used to implement some or all the various embodiments discussed above.
Still referring to
Combining the Digital Signal Values
Returning to
Now consider the negative half cycle of the sine wave 726 of
Thus, in the example system two digital signal values are created that correspond to the same point in time relative to the sine wave 726. In accordance with some example systems, the dual digital signal values are combined by the combining system 410 to create a single digital signal value for the particular point in time. The combining system may take many forms, but logically the combining system may be viewed as a summation function 752 which adds the digital signal value from the positive logarithmic analog-to-digital converter system 404 with a negative version (as shown by minus sign 754) of the digital signal value created by the negative logarithmic analog-to-digital converter system 406. Stated differently, the summation logic subtracts the digital signal values created by the negative logarithmic analog-to-digital converter system 406 from the digital signal value created by the positive logarithmic analog-to-digital converter system 404. The resultant digital signal value may then be stored by the processor 412.
During the positive half cycle of sine wave 726, the digital signal value produced by the negative logarithmic analog-to-digital converter system 406 will theoretically be zero, and thus will not degrade the representation of the digital signal value created by the positive logarithmic analog-to-digital converter system 404. In practice, a small non-zero value will likely be present in the digital signal value produced by the negative logarithmic analog-to-digital converter system 406, but such will be small in relation to the other digital signal value and thus will not noticeably degrade the quality, or may be considered to cancel similar noise in the positive logarithmic analog-to-digital converter system 404 channel.
During the negative half cycle of sine wave 726, the digital signal value produced by the positive logarithmic analog-to-digital converter system 404 will theoretically be zero, and thus will not degrade the representation of the digital signal value created by the negative logarithmic analog-to-digital converter system 406. In practice, a small non-zero value will likely be present in the digital signal value produced by the positive logarithmic analog-to-digital converter system 404, but such will be small in relation to the other digital signal value and thus will not noticeably degrade the quality, or may be considered to cancel similar noise in the negative logarithmic analog-to-digital converter system 406 channel.
The functionality of the combing system 410 may be implemented in hardware or in software. For example, the functionality of the combining system 410 may be performed by physical logic circuits that read the digital signal value from each system 404 and 406, perform the summation, and then make available the resultant for reading and storage by the processor 412. In other cases, the processor 412 (executed program instructions) may directly read the digital signal values from each system 404 and 406, and perform the summation in software (i.e., the processor performs the summation operation).
In the example systems discussed to this point the final digital signal value associated with each sample of the analog input signal is produced by a summation. The summation is fast and requires no decision-making on the part of the hardware or software. However, it is to be understood that other systems could be implemented as part of the combining system 410. For example, in either hardware or software the two digital signal values may be analyzed, the large non-zero value selected as the “true” digital signal value, and the effectively zero digital signal value discarded. However, analog input signals representing sound pass through the zero often (e.g., for a 20 kHz signal, 40,000 zero-crossings a second), and thus selection of a “true” digital signal value and a discard value during periods when the analog input signal is near zero add complexity.
File Format
Assume for this portion of the discussion that the digital signal values created by the converter systems 404 and 406 have been combined and/or selected in some fashion, hereafter the “sampled digital signal value.” In accordance with at least some embodiments, each sampled digital value created may be stored to the image memory 412, such as a series of digital values, each digital signal value representing the instantaneous voltage of the analog input signal at a particular point in time. For example, for a 10-bit logarithmic analog-to-digital conversion system, each digital value may be 10 bits, with an associated sign bit, for a total of 11 bits for each sample. Moreover, as mentioned above, an indication of the base of the logarithmic analog-to-digital conversion may likewise be stored. Assuming the same base across the entire recording, an indication of the base need only be stored one time.
However, in other embodiments the file format may take advantage of the use of octaves. In particular, in some embodiments the digital values in the image memory 412 are stored as a value indicative of octave (which may also be referred to as a radix), a value indicative of graduation or quantization within the octave (the graduation may also be referred to as a mantissa), and a value indicative of sign. Consider, for example, a 10-bit logarithmic analog-to-digital conversion having eight octaves. As discussed with respect to Table 5, there are 128 graduations or quantization steps within each of the illustrative octaves. Storing the digital values in this example thus involves, for each digital signal value, storing a 3-bit indication of octave (23=8), a 7-bit indication of graduation or quantization within the octave (27=128), and a 1-bit indication of color.
Many times in sampling periodic or near periodic waveforms many consecutive sample values may reside within the same octave. By storing digital values based on octave and gradation within the octave, for groups of samples within the same octave the bits related to octave may be omitted. For example, consider a file storage format where, as a default, each digital value comprises a value indicative sign, followed by a value indicative of octave, and then followed by a value indicative of gradation within the octave. In the example system, when a series digital signal values all reside within the same octave, a designator may be inserted into the file along with an indication of the octave, and an indication of the number of subsequent samples to which the octave applies. For the next number of designated samples, only the mantissa (i.e., the gradation within the octave) may be written to the file, omitting the octave.
Note, however, that the example storage systems result in a “lossless” storage of audio data. No compression or loss of data is used in the storage. Thus, the audio reproduction need not suffer based on loss of data for the compression. Finally, while the example system either combines the digital signal values, or selects one of the digital signal values to be the sampled digital signal value, it is possible to omit the combing system 410, and store both the digital signal values corresponding to the same time. During reproduction, the digital signal values corresponding to the same point in time may be combined as discussed, or the reproduction system may implement the selection of the digital sample to be provided to a digital-to-analog converter. While storing both values increases the size of the storage file, there may be advantages (e.g., advantages in post-processing of the data) in such systems.
Single Logarithmic A/D Converter Systems
The various example systems discussed to this point have been based on having a separate positive logarithmic analog-to-digital converter system and negative logarithmic analog-to-digital converter system. However, in yet still other example systems a single logarithmic analog-to-digital converter system may be used, in combination with a circuit to indicate sign.
Turning first to the logarithmic analog-to-digital converter system 900. The logarithmic analog-to-digital converter system 900 is similar to the logarithmic analog-to-digital converter systems previously discussed, in that the logarithmic analog-to-digital converter system comprises a logarithmic amplifier 908 and a linear analog-to-digital converter 910. The operation of the logarithmic amplifier 908 and linear analog-to-digital converter 910 are the same as discussed above, and thus the operation of the devices will not be repeated again here. The logarithmic analog-to-digital converter system 900 also comprises an active rectifier circuit 912. Unlike the previous active rectifier circuits which perform half-wave rectification, active rectifier circuit 912 performs full-wave rectification. Thus, as shown by waveform 914, negative portions of the analog input signal are provided to the logarithmic amplifier 908 as positive portions. Thus, the logarithmic amplifier 908 and linear analog-to-digital converter 910 produce all the digital signal values.
Negative portions of the analog input signal are indicated to the combining circuit 904 by way of the sign circuit 902. In particular, during periods of time when the analog input signal is positive, the sign circuit 902 provides an indication to the combining circuit 904 that the digital signal values created are for positive values. During periods of time when the analog input signal is negative, the sign circuit 902 provides an indication to the combining circuit 904 that the digital signal values created are for negative values. For example, the sign circuit 902 may provide a single Boolean value to the combining circuit 904 indicative of the sign of analog input signal corresponding to the each sample. There are a variety of physical circuits which may be used to implement the functionality of the sign circuit 902.
The combining circuit 904 is configured to combine each digital signal value and sign value in some way. In one example system, the combining circuit merely concatenates the sign bit with the digital signal value for reading by the processor (not shown in
Playback Considerations
Playback of captured audio follows directly from the file storage format. Any portion of the logarithmically converted data may be played back directly. Alternatively, because the stored values directly indicate the stored value of the instantaneous voltage of the electrical signal, each stored value may be raised to the appropriate base power based on the base used in the recording stage, apply the digital value to a digital-to-analog converter to create an analog signal, and apply the analog signal to an amplifier and speaker(s). For example, if the recording stage performed logarithmic analog-to-digital conversion as a base 2 log (i.e., each datum proportional to log2(instantaneous voltage), then in playback each datum is raised to the base (e.g., 2[datum] in this example) before being applied to the digital-to-analog converter. In cases where the stored values are encoded as an octave and gradation within the octave, the playback system may read the octave and gradation, produce an appropriate digital value (again taking into account the base power used during storage), apply the digital value to a digital-to-analog converter to create an analog signal, and apply the analog signal to an amplifier and speaker(s).
Returning to
The precise anti-log function may depend on the philosophy implemented by the recording system. In cases where a single digital value is stored for each sampled time, the playback may involve reading the single digital value, performing the anti-log function, and applying the anti-log value to the digital-to-analog converter 450. On the other hand, if the processor 412 stores two values for each sampled time (one for the positive logarithmic analog-to-digital conversion and one from the negative logarithmic analog-to-digital conversion), the processor may (at the time of playback) read both values and combine them to create the digital value applied to the digital-to-analog converter 450.
In some cases, the amplifier 452 may implement gain control at the direction of the processor 412, as shown by the connection 456 between the processor 412 and the amplifier 452. The connection 456 may be an analog connection, where the gain is proportional to the value of the analog signal, or the connection may be a digital communication channel, where the gain is encoded in a digital value exchanged between the processor 412 and the amplifier 452.
It is noted that the various aspects of the playback features need not necessarily play back only audio recorded within the same system. Moreover, while
From the description provided herein, those skilled in the art are readily able to combine software created as described with appropriate general-purpose or special-purpose computer hardware to create a computer system and/or computer sub-components in accordance with the various embodiments, to create a computer system and/or computer sub-components for carrying out the methods of the various embodiments, and/or to create a non-transitory computer-readable storage medium (i.e., other than an signal traveling along a conductor or carrier wave) for storing a software program to implement the method aspects of the various embodiments.
References to “one embodiment,” “an embodiment,” “some embodiments,” “various embodiments”, “example embodiments”, “example systems” or the like indicate that a particular element or characteristic is included in at least one embodiment of the invention. Although the phrases may appear in various places, the phrases do not necessarily refer to the same embodiment.
The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.
This application is a continuation-in-part of U.S. application Ser. No. 13/490,555 filed Jun. 7, 2012 titled “Method and system of image capture based on logarithmic conversion”, which application is incorporated by reference herein as if reproduced in full below.
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| Number | Date | Country | |
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| 20130329914 A1 | Dec 2013 | US |
| Number | Date | Country | |
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| Parent | 13490555 | Jun 2012 | US |
| Child | 13625513 | US |
