Abstract:
A camera generally including a sensor and an auto-exposure circuit is disclosed. The sensor may be configured to generate a digital signal in response to an optical signal. The auto-exposure circuit may be configured to control a lightness of a picture within the digital signal by (i) adjusting at least one among an aperture, a shutter and an analog gain and (ii) adjusting a digital gain applied to the digital signal, wherein the digital gain adaptively reduces the lightness of the picture.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and/or architecture for digital photography generally and, more particularly, to an automatic exposure with digital gain optimized for noise reduction. 
     BACKGROUND OF THE INVENTION 
     Automatic exposure (AE) is a process by which a still camera or a video camera controls an overall lightness of a picture. For conventional digital cameras, an aperture, a shutter time and a gain are controlled to adjust the lightness. A gain of six decibels (dB) increases a signal voltage by a factor of two. 
     Many combinations of aperture, shutter time, and gain can achieve a given lightness level. Therefore, besides determining the overall lightness level, conventional AE methods operate with the following considerations: (i) all lenses have a maximum aperture and a minimum aperture, (ii) cameras have a minimum shutter time and a maximum shutter time, (iii) for video cameras, the shutter time cannot exceed a frame time, (iv) electro-optical sensors have a minimum gain and a maximum gain, (v) a wide aperture reduces a depth-of-field, which can be helpful (i.e., focus on the subject and blur the background) or not helpful (i.e., less of the scene can be in focus), (vi) a lens will typically achieve a maximum sharpness and a lowest distortion over a range of aperture values that is smaller than the full range of possible aperture values, (vii) a long shutter time increases motion blurriness, which can be good (i.e., show motion in a still picture, make motion appear smooth in a video sequence) or bad (i.e., the picture is too blurry), (viii) high gain increases noise and is therefore usually avoided, (ix) for low light situations, and with the above described limitations of using shutter time and aperture to increase light, a high gain is sometimes appropriate. 
     With reasonably good light, conventional AE techniques will try to keep the total picture lightness at a certain level, independent of the amount of available light. For low illumination levels, the conventional AE techniques will permit the picture lightness to be reduced. Furthermore, the maximum gain used at the low illumination levels is reduced to keep the pictures from being too noisy. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a camera generally comprising a sensor and an auto-exposure circuit. The sensor may be configured to generate a digital signal in response to an optical signal. The auto-exposure circuit may be configured to control a lightness of a picture within the digital signal by (i) adjusting at least one among an aperture, a shutter and an analog gain and (ii) adjusting a digital gain applied to the digital signal, wherein the digital gain adaptively reduces the lightness of the picture. 
     The objects, features and advantages of the present invention include providing an automatic exposure with digital gain optimized for noise reduction that may (i) utilize an adjustable digital attenuation of a picture lightness, (ii) integrate white balancing with the digital attenuation and/or (iii) reduce amplification noise compared with conventional techniques. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
         FIG. 1  is a block diagram of a system in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a block diagram of an example implementation of a processing circuit; and 
         FIG. 3  is a flow diagram of an example method for an automatic exposure control. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following definitions are generally used in describing the present invention.
         1. Aperture. A light gathering capacity of a lens. The wider the aperture (e.g., lower “f” number) the more that light is collected. Increasing the aperture by one f-stop generally doubles the amount of light gathered.   2. Shutter time. An amount of time a sensor is exposed to the light. The amount of light incident on the sensor may be proportional to the shutter time.   3. Gain. An amount that an electrical signal, induced by the light incident on the sensor, is amplified. Gain may be broken into two types of gain:   3a. Analogue gain. A gain applied to an analog electrical signal before an analog to digital conversion.   3b. Digital gain. A gain applied after the analog to digital conversion. The digital gain may be achieved by multiplying each digitized pixel value by a number. The digital gain may be applied internal or external to the sensor. Moreover, the digital gain may be applied as a discrete step or as part of a white balance step. The white balance step generally multiplies the color component values (e.g., red values, green values and blue values) of each pixel by different amounts to account for variations in color temperatures. The white balancing may make the color “white” appear to have a similar amount of the red component, the green component and the blue component. Applying the digital gain as part of the white balance step generally means that a red multiplier, a green multiplier and a blue multiplier are all scaled by the same amount to change the overall lightness.       

     For the same gain amount, the analog gain generally produces a less noisy result than the digital gain. A sensor may provide different analog gain amounts at discrete points that are widely spaced. For example, a ProCamHD™ 2460/2 sensor (ProCamHD™ is a trademark of AltaSens, Inc., Mountain View, Calif.) offers analog gains of 0 decibels (dB), 3 dB, 6 dB, 9 dB, 12 dB, 15 dB, 18 dB, 21 dB and 24 dB. Since the analog gain levels may be available only in discrete steps, the digital gain may be used to achieve gains in-between the analog gain levels. 
     Referring to  FIG. 1 , a block diagram of a system  100  is shown in accordance with a preferred embodiment of the present invention. The system (or apparatus)  100  may be implemented as a digital still camera and/or a digital video camera. The system  100  generally comprises a lens  102 , a shutter  104 , a circuit (or module)  106 , a circuit (or module)  108  and a circuit (or module)  110 . 
     An input signal (e.g., OPT) may be received by the circuit  106  through the lens assembly  102  and the shutter  104 . A signal (e.g., IN) may be generated by the circuit  106  and presented to the circuit  108 . The circuit  108  may generate an output signal (e.g., OUT) presented to the circuit  110 . A signal (e.g., APR) may be generated by the circuit  108  and presented to the lens assembly  102 . The circuit  108  may also generate a signal (e.g., STR) transferred to the shutter  104  or the circuit  106 . A signal (e.g., AG) may be generated by the circuit  108  for use by the circuit  106 . 
     The lens assembly  102  may be operational to provide a focusing capability and an aperture capability. The focusing capability generally focuses the optical pictures (or images) in the signal OPT onto a surface of the circuit  106 . The aperture capability generally determines an amount of light passing through the lens assembly  102 . The aperture capability may be controlled by the signal APR. 
     The shutter  104  may be implemented as a stand-alone shutter. The shutter  104  may be operational to control an exposure of the circuit  106  to the signal OPT. In some embodiments, the shutter  104  may be absent and the shutter functionality may be implemented by the circuit  106 . 
     The circuit  106  may be implemented as an electro-optical sensor. The circuit  106  is generally operational to covert optical pictures received in the signal OPT into electrical representations of the pictures in the signal IN. An analog gain capability may be included in the circuit  106 . The analog gain may be controlled by the signal AG. Furthermore, an analog to digital conversion (ADC) capability may be included in the circuit  106 . As such, the signal IN may be a digital signal due to the ADC capability. Optionally, a shutter capability may be included in the circuit  106  and the stand-alone shutter  104  eliminated. 
     The circuit  108  may be implemented as a video processing circuit. The circuit  108  is generally operational to perform (i) an automatic exposure (AE) control, (ii) pre-compression processing on the pictures received in the signal IN, (iii) compression of the pictures and (iv) presentation of the compressed pictures in the signal OUT. For still pictures, the signal OUT may be compliant with a Joint Picture Experts Group (JPEG) protocol documented in an International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) Standard 10918-1 and/or an International Telecommunication Union-Telecommunications (ITU-T) Recommendation T.81. For moving pictures, the signal OUT may be compliant with an H.264/AVC protocol documented in an ISO/IEC standard 14496-10 and/or an ITU-T Recommendation H.264. Other standards, recommendations and proprietary protocol may be implemented to meet the criteria of a particular application. 
     The circuit  110  may be implemented as a storage (or memory) circuit. The circuit  110  may be operational to store the compressed pictures received in the signal OUT. The circuit  110  may provide a nonvolatile storage capability, such as a FLASH memory, an optical drive and/or a magnetic hard drive. 
     The system  100  may operate in one or more AE modes. A first AE mode generally sets the aperture, the shutter time and the analog gain so that with no additional digital gain, the picture lightness may be as bright as intended or darker. An additional digital gain, if any, may be used to make the picture lighter as appropriate. As such, the first AE mode generally uses a nonnegative (e.g., a zero dB or a positive dB) digital gain. 
     A second AE mode may set the aperture, the shutter time and the analog gain so that with no additional digital gain, the picture may be lighter than intended. Thereafter, a negative digital gain (e.g., a negative dB or a multiplication by a number &lt;1) generally darkens the picture to the intended level. An advantage of the second AE mode over the first AE mode is that the negative digital gain generally reduces noise in the pictures. 
     The noise reduction may be particularly strong in good lighting situations where the aperture and/or the shutter time are not at maximum limits. As such, opening the aperture and/or increasing the shutter time generally increases the amount of light incident on the circuit  106 . A corresponding negative dB digital gain may directly reduce noise in the pictures. 
     A noise reduction due to a negative dB digital gain may also take place in poor lighting where an increased analog gain may be followed by a negative dB digital gain. For example, a target total gain of 14 dB may be achieved using an analog gain of 18 dB and a digital gain of −4 dB. The above approach generally reduces an overall amplification noise because the added noise of increasing analog gain by 6 dB (e.g., from 12 dB to 18 dB) is generally smaller than the noise reduction achieved by reducing the digital gain by 6 dB (e.g., from 2 dB to −4 dB). 
     Referring to  FIG. 2 , a block diagram of an example implementation of the circuit  108  is shown. The circuit  108  generally comprises a module (or function)  120 , a module (or function)  122 , a module (or function)  124 , a module (or function)  126  and a module (or function)  128 . 
     The signal IN may be received by the module  120  and the module  122 . A signal (e.g., STA) may be generated by the module  120  and presented to the module  124 . The module  124  may generate the signal AG, the signal STR and the signal APR. A signal (e.g., DGC) may also be generated by the module  122  for use by the module  124 . The module  124  may generate and transfer a signal (e.g., DIN) to the module  126 . A signal (e.g., PIN) may be generated by the module  126  and presented to the module  128 . The module  128  may generate the signal OUT. 
     The module  120  may be implemented as a statistics gathering module. The module  120  may be operational to collect statistics for some to all of the pixels in the pictures received in the signal IN. The statistics may include, but are not limited to, counting a first number of pixels in each picture having an amplitude greater than a first threshold and counting a second number of pixels in each picture having an amplitude greater than a second threshold. The statistical information may be transferred to the module  122  in the statistics signal STA. 
     The module  122  may be implemented as a control module. The module  122  is generally operational to control the automatic exposure of the system  100  based on the statistical information and user-controllable settings. The module  122  may repetitively adjust the signal AG, the signal STR and the signal APR to effect the AE function using the analog gain, the shutter time and the aperture. Control of the digital gain aspect of the AE function may be achieved through the digital gain control signal DGC. 
     The module  124  may be implemented as a digital gain module. The module  124  may also be implemented as a white balance module. The module  124  is generally operational to amplify/attenuate the pixels received in the signal IN to generate corresponding signals in the signal DIN. Amplification may include multiplying the pixels by values greater than one. Attenuation generally includes multiplying the pixels by values less than one. Furthermore, the module  124  may be operational to adjust a color temperature of the pixels by multiplying the respective color components of each pixel by an appropriate color balance value. 
     The module  126  may be implemented as a pre-compression processing module. The module  126  may be operational to prepare the pictures in the signal DIN for compression. Preparations may include, but are not limited to, scaling, decimating, spatial filtering, temporal filtering, interpolating, interlacing, de-interlacing, image sharpening and/or image smoothing. 
     The module  128  may be implemented as a compression module. The module  128  is generally operational to compress the pictures in the signal PIN to generate the signal OUT. Compression formats may include H.264/AVC and MPEG-2 for video and JPEG for still pictures. Other compression formats may be implemented to meet the criteria of a particular application. 
     Referring to  FIG. 3 , a flow diagram of an example method  140  for an automatic exposure control is shown. The method  140  generally comprises a step (or block)  142 , a step (or block)  144 , a step (or block)  146 , a step (or block)  148 , a step (or block)  150 , a step (or block)  152  and a step (or block)  154 . 
     If the aperture, the shutter time and the analog gain are too high, the pixel values may be clipped within the circuit  106 . Clipping generally results in visual artifacts that may not be fixed by the negative dB digital gain. For example, a circuit  106  generating 12-bit pixels may represent the value of each pixel as an integer in a range [0, 4095]. After a 12 dB analog gain, two of the pixels may have different values (e.g., 2500 and 3000). Increasing the analog gain to 18 dB generally results in the two pixels being clipped at the maximum value (e.g., 4095). The clipping may not be corrected by the negative dB digital gain. After both pixels have been clipped to the same value, no method exists to determine which pixel should be brighter than the other pixel. 
     To achieve a final lightness, the aperture, the shutter time, the analog gain and the digital gain may be controlled based on an estimate of clipping that the aperture, the shutter time and the analog gain may cause. The final target lightness may result in a small amount of clipping in each of the pictures. Limiting the amount of clipping may be accomplished, for example, by looking at the statistics derived from the pixel values. The statistics may be used to estimate the amount of clipping that combinations of the aperture, the shutter time and the analog gain generally cause. 
     In the step  142 , the method  140  may measure the pixel values received from the circuit  106  for a given aperture, given shutter time and a given analog gain. In the step  144 , a count of a first number of pixels values (e.g., P 1 ) that are greater than a first threshold (e.g., T 1 ) may be performed. The number P 1  generally determines if the picture is too bright. A comparison between the number P 1  and a first predetermined count value (e.g., C 1 ) may be made in the step  146 . If the number P 1  of pixels above the threshold T 1  is greater than the count C 1  (e.g., the YES branch of step  146 ), the method  140  may (i) increase the digital gain (e.g., less attenuation) and (ii) decrease the aperture, the shutter time and/or the analogue gain (e.g., darken the analog picture) in the step  148 . Step  148  generally darkens the picture prior to digitization then restores the picture lightness with the digital gain. If the number P 1  is less than or equal to the count C 1  (e.g., the NO branch of step  146 ), the method  140  may continue with the step  150 . 
     In the step  150 , a second count of a second number of pixels (e.g., P 2 ) about a second threshold (e.g., T 2 ) may be conducted. The number P 2  may give an indication if the picture is too dark. Next, the number P 2  may be compared with a second predetermined count value (e.g., C 2 ) in the step  152 . In the number P 2  is less than the count value C 2  (e.g., the YES branch of step  152 ), the method  140  may (i) decrease the digital gain (e.g., more attenuation) and (ii) increase the aperture, the shutter time and/or the analog gain (e.g., brighten the analog picture) in the step  154 . Step  152  generally increases the picture brightness before the analog to digital conversion, then decreases the picture lightness by adjusting the digital gain. If the number P 2  is greater than or equal to the count C 2 , then the present settings for the aperture, the shutter time, the analog gain and the digital gain may remain unchanged. 
     An example set of values is provided as follows for a sensor having a 12-bit analog to digital converter:
 
 T 1=98%×maximum pixel value=4013
 
 T 2=70%×maximum pixel value=2867
 
 C 1=1% of the number of pixels (e.g., 80,000 for an 8 megapixel sensor.)
 
 C 2 =C 1
 
     Typically, the amount of negative dB digital gain that may be used will depend on the contrast in the scene and/or the target final lightness. For example, if the scene is high contrast (e.g., some parts have direct reflections of sunlight and other parts are in very dark shadows), the correct overall lightness may be achieved by (i) adjusting the aperture, the shutter time and the analog gain and (ii) zeroing the digital gain. Therefore, many of the pixel may be close to or at the maximum ADC value. As such, any increase in the aperture, the shutter time and/or the analog gain may produce significant clipping. 
     If the scene is low-contrast for a given average lightness throughout, the brightest parts may not be reasonably light. Therefore, the aperture, the shutter time and/or the analog gain may be increased without significant clipping. Thereafter, the correct final lightness level may be achieved with a negative dB digital gain (e.g., an attenuation). If the scene is very dark such that even after all exposure adjustments (e.g., the aperture, the shutter time, the analog gain and the digital gain) have been employed yet the picture lightness remains low, the aperture, the shutter time and/or the analog gain may be increased (even for a high contrast scene) without unacceptable clipping and the digital gain may be adjusted to attenuate the lightness a corresponding amount. 
     The last case (e.g., picture is dark) may occur when the AE control method (i) permits a low output light level in the picture due to low light in the scene and (ii) attempts to avoid excessive noise. Therefore, the last case generally occurs when the aperture and the shutter times are at the maximum values. In such a case, the present invention generally increases the analog gain and uses a negative dB digital gain. However, a low-contrast scene may occur in good lighting conditions or poor lighting conditions. Therefore, any combination of increased aperture, increased shutter time and/or increased analog gain is possible. 
     The function performed by the diagrams of  FIGS. 1-3  may be implemented using a conventional general purpose digital computer programmed according to the teachings of the present specification, as will be apparent to those skilled in the relevant art(s). Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will also be apparent to those skilled in the relevant art(s). 
     The present invention may also be implemented by the preparation of ASICs, FPGAs, or by interconnecting an appropriate network of conventional component circuits, as is described herein, modifications of which will be readily apparent to those skilled in the art(s). 
     The present invention thus may also include a computer product which may be a storage medium including instructions which can be used to program a computer to perform a process in accordance with the present invention. The storage medium can include, but is not limited to, any type of disk including floppy disk, optical disk, CD-ROM, magneto-optical disks, ROMs, RAMS, EPROMs, EEPROMs, Flash memory, magnetic or optical cards, or any type of media suitable for storing electronic instructions. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.