METHOD FOR ANALOG DECIMATION OF IMAGE SIGNALS

Abstract: In a digital image processing device (10) such as a portable camera, an analog decimation circuit is provided as a front end for the digital video imaging circuit so that the data is reduced without substantial loss of fidelity so that power dissipation is minimized. In a decimation circuit, the data rate is reduced by reducing only higher frequency spectral components of a captured image without substantial loss of fidelity in the remaining spectral components of the captured image. The required amount of digital data processing is so substantially reduced that considerable power savings result. A mixture of decimation modes may be effected using the same circuit.

Description: METHOD FOR ANALOG DECIMATION OF IMAGE SIGNALS BACKGROUND OF THE INVENTION This invention relates to digital image processing for particular application to image capture in a portable digital video imaging de\ ice such as a camera There is a need to be able to capture images at a first sampling rate while prov iding an accurate representation of the sampled image to output elements at a second sampling rate In particular, there is a need to reduce the data rate to output elements in order to decrease system power dissipation and data storage requirements in power and storage-sensitive applications, such as preview modes of images in digital cameras As the conventional analog signal processing systems are replaced by digital systems, many processes can be compromised to take advantage of inherent benefits of digital processing For example, it is sometimes necessary to reduce the amount of data captured so that it can be processed efficiently and economically Whereas sample rate reduction reduces the amount of data, the widespread use of digital processing in a signal processing system typically requires more power than comparable analog systems Subsamphng the input signal after the analog to digital conversion has the drawback of requiring high pow er dissipation in upstream analog to digital conversion subsystems that work at a higher clock rate Subsamphng at any stage reduces information content at the expense of signal fidelity, such as increased noise and coarser granularity in processed images What is needed is a technique for reducing the data rate in a digital image acquisition system w hile saving power and without a correspondingly large reduction in signal fidelity SUMMARY OF THE INVENTION According to the invention, in a digital image processing device such as a portable digital camera, an analog decimation circuit is provided as a front end for the digital video imaging circuit so that the data rate is reduced without substantial loss in fidelity so that pow er dissipation is minimized In a decimation circuit, the data rate is reduced by reducing only higher frequency spectral components of a captured image without substantial loss in fidelity of the remaining spectral components of the captured image The required amount of digital data processing is so substantially reduced that considerable po er sa\ ings result A mixture of decimation modes may be effected using the same circuit The im ention w ill be bettei understood by reierence to the following detailed description in connection w ith the accompanying drawings BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 is a block diagram of an analog decimation subsystem according to the invention Fig 2 is a circuit diagram of one form of analog decimation subsystem functional in both 1/3 and '/- decimation modes Fig 3 is a timing diagram of an analog decimation process for 9 input samples in a 1/3 decimation mode Fig 4 is a timing diagram of an analog decimation process for 8 input samples in a 1/2 decimation mode DESCRIPTION OF THE SPECIFIC EMBODIMENTS Reference is made to Fig 1 , which shows a block diagram of a decimator 10 according to the invention A plurality of video input paths Vιn(l) through Vιn(N) are coupled to a common video signal input Vιn(t) and provide signals to N number of sample and hold amplifiers (S H) 12, all of which aie under control of a synchronous clock 14 having a clock rate of fclk The outputs of the S/H amplifiers 12 are summed at a clocked summing amplifier 16, which itself is under synchronous control of clock 14 However it is clocked at a rate of fclk/N The output is a sequentially clocked analog signal Vout suitable for direct conversion to a sequence of digital samples The output signal at each output clock 1/N is an average over an N sample window Referring to Fig 2, there is shown an example of a programmable analog decimator 100 illustrating the principle of analog decimation and averaging using switched capacitors of value C and 2C The reasoning behind this evident disparity of values will be apparent hereinafter A plurality of sampling modules, as represented by sampling module 101 , each includes a plurality of switches 102, 103, 105 and 107 and an analog value storage capacitor 104 of either value C or value 2C In this embodiment, ping-pong processing is employed such that two separate output signals are multiplexed in time and share a common output amplifier 160 While ping-pong processing is not a necessary element of the invention, it provides certain practical advantages Ping-pong processing, among other advantages, allows extra settling time for the amplifier 160, reduces pow er dissipation, and avoids missing input data samples in the hold/amplify mode while the alternate bank is in the sampling mode With ping-pong piocessing, no input samples are missed, and settling time is adequate for a typical operational amplifier at the sampling rates of interest In proper operation, phases Φ5 and Φ6 are nonoverlapping and phases Φl 1 and Φ12 are nonoverlapping and the other phases of each set (respectively Φ1-Φ4, Φ7-Φ10) must have ended first In operation, the ping bank is controlled by switches with phases Φ1-Φ6, while the pong bank is controlled by switches with phases Φ7-Φ12 A first gating subsystem 1 10 and a second gating subsystem 1 12 alternately route the feedback signals and ultimately establish the stage gain Reference is made to the timing diagrams of Fig 3 and Fig 4 for a more detailed understanding of operation of the circuit According to the invention, 1/3 decimation and V. decimation can be performed by the same sampling system by proper choice of capacitance values and phasing whereby parallel paths are selectively combined Fig 3 illustrates timing for analog decimation in the 1/3 decimation mode The phases of the switches are indicated by the index on the term Φ Duπng a first set of phases, sampling takes place Duπng a second set of phases, signal averaging and decimation occur In the ping or even set of phases, the first analog input sample is stored on capacitor 104 during phase Φl Duπng phase Φ2 (which is the same as phase Φ3 in 1/3 decimation mode), the second analog input sample is stored on capacitor 108 (and on capacitor 1 16 in 1/3 decimation mode) During phase Φ4, the third analog input sample is stored on capacitor 120 Phase Φ5 ov erlaps phases Φl , Φ2, Φ3 and Φ4 Phase Φ5 is used to discharge the feedback capacitor 122 and prepare it for the subsequent averaging phase Phase Φ6 is used for averaging together the signal samples which were captured during phases Φl , Φ2, Φ3 and Φ4 During phase Φ6, all of the sampling capacitors 104, 108, 1 16 and 120 are simultaneously switched into the input port of amplifier 160 to produce an output signal at output port 161 which is the sum of the charge on the capacitors 104, 108, 1 16 and 120 divided by the capacitance value on first feedback capacitor 122 in first gating subsystem 1 10, which has been enabled to provide feedback In this embodiment the values of capacitors 104, and 120 are the same and are also equal to the sum of the \ alues of capacitors 108 and 1 16 As a result, in this embodiment, the three input samples have equal w eight in the output value Moreover, the sum of all of the v allies of the sampling capacitors 104, 108, 1 16 and 120 is equal to the value of the feedback capacitor 122, yielding unity gain In alternative embodiments, the weighting and the gain can be modified to tailor the transfer characteπstics A similar complementary set of phases is employed to process signal samples duπng the odd or pong phase set One-half decimation mode processing operates as follows, as illustrated in Fig 4 During a first set of phases, sampling takes place Duπng a second set of phases, signal av eraging and decimation occur In the ping or even set of phases, the first analog input sample is stored on capacitor 104 during phase Φl (and on capacitor 108 during phase Φ2 which is the same as phase Φl) During phase Φ3 (which is the same as phase Φ4 in 1/2 decimation mode), the second analog input sample is stored on capacitor 116 (and on capacitor 120 in 1/2 decimation mode) Phase Φ5 overlaps phases Φl , Φ2, Φ3 and Φ4 Phase Φ5 is used to discharge the feedback capacitor 122 and prepare it for the subsequent averaging phase Phase Φ6 is used for averaging together the signal samples which were captured during phases Φ1/Φ2, and Φ3/Φ4 During phase Φ6, all of the sampling capacitors 104, 108, 1 16 and 120 are simultaneously switched into the input port of amplifier 160 to produce an output signal at output port 161 which is the sum of the charge on the capacitors 104, 108, 1 16 and 120 divided by the capacitance value on first feedback capacitor 122 in first gating subsystem 1 10, which has been enabled to provide feedback In this embodiment the values of capacitors 104, and 120 are the same and are also equal to the sum of the values of capacitors 108 and 1 16 As a result, in this embodiment, the two input samples have equal weight in the output value Moreover, the sum of all of the values of the sampling capacitors 104, 108, 1 16 and 120 is equal to the value of the feedback capacitor 122, yielding unity gain In alternative embodiments, the weighting and the gain can be modified to tailor the transfer characteπstics Similarly the capacitors in the pong portion of the circuit are activated in the odd set of phases It is noteworthy that the configuration of capacitors having values of 2C, C, C, 2C enables the implementation of both 1/3 decimation and u ι decimation modes with the same hardware Only the timing of the clock 14 needs to be altered, which can be done as often as even, ' other set of phases Selecting different values of capacitance for the sampling modules allows diffeient averaging weights for the output samples, thus changing the transfer characteristics of the filter While only a single ended circuit is shown in Fig 2, it should be understood that the circuit may be fully differential The invention has been explained with reference to specific embodiments Other embodiments will be apparent to those of ordinary skill in the art It is therefore not intended that this invention be limited, except as indicated by the appended claims.

Claims: WH AT IS CL IMED IS 1 An apparatus tor piocessing sampled analog input signals of an image capture element for subsequent digital processing, said apparatus composing a plurality of analog sample storage elements disposed in parallel paths, signal clocking means, first sw itching means for enabling selected ones of said analog sample storage elements to sample said analog input signals according to a first clocking signal of said signal clocking means suichronized with said sampled analog input signals, second switching means for averaging analog samples stored in said analog storage elements over an averaging period to obtain averaged samples, and amplifiei means for outputting said averaged samples according to a second clock signal in order to provide output samples at a different rate than input samples, said different rate being an integer fraction of said first clocking signal 2 The apparatus according to claim 1 wherein said analog sample storage elements are switched capacitors 3 The apparatus according to claim 1 wherein said amplifier means is an analog output operational amplifier in a gain stage whose amplification is established by a ratio of capacitance values 4 The apparatus according to claim 1 wherein said analog sample storage elements are switchably combined to establish a selectable gain for each input sample 5 The apparatus according to claim 4 wherein said selectable gain is uniform for each different input sample, and wherein said first switching means is alterable between a first decimation rate and a second decimation rate while maintaining said uniform _-aιn 1 6 The apparatus according to claim 4 herein said selectable gain is 2 customized ior each different input sample, thereby to alter transfer characteπstics of said 3 averaging function 1 7 The apparatus according to claim 1 further including a first gating 2 subsystem and a second gating subsystem, wherein said analog storage elements is 3 allocated between a first processing phase and a second processing phase such that said 4 first processing phase is sampling while said second processing phase is averaging and 5 said first processing phase is averaging w hile said second processing phase is sampling 1 8 A method for processing sampled analog input signals of an image 2 capture element for subsequent digital processing, said method compπsing 3 enabling selected ones of a plurality of analog sample storage elements 4 disposed in parallel paths to sample said analog input signals according to a first clocking 5 signal of a signal clocking means synchronized with said sampled analog input signals, 6 averaging analog samples stored in said analog storage elements over an 7 averaging period to obtain averaged samples, and 8 outputting said averaged samples according to a second clock signal in 9 order to provide output samples at a different rate than input samples, said different rate 0 being an integer fraction of said first clocking signal 1 9 The method according to claim 8 w herein said sampling step i includes switchably combining said analog sample storage elements to establish a 3 selectable gain for each input sample 1 10 The method according to claim 9 wherein said sampling step 2 includes switching said first switching means alternately between a first decimation rate 3 and a second decimation rate while maintaining said uniform gain 1 1 1 The method according to claim 8 wherein said sampling step 2 includes customizing said selectable gain for each different input sample, thereby to alter 3 transfer characteristics of said averaging function 1 12 The method according to claim 8 further including a first gating ~) subsystem and a second gating subsystem, further including the steps of allocating said analog storage elements between a first processing phase and a second processing phase such that said first processing phase is sampling while said second processing phase is averaging and said first processing phase is averaging while said second processing phase is sampling.

Inventors: KLEKS JONATHAN A, OPRIS ION E, SHINGO KOKUDO, KLEKS, JONATHAN, A., OPRIS, ION, E., SHINGO, KOKUDO
Applicants: KLEKS JONATHAN A, NUCORE TECHNOLOGY INC, OPRIS ION E, SHINGO KOKUDO, KLEKS, JONATHAN, A., NUCORE TECHNOLOGY, INC., OPRIS, ION, E., SHINGO, KOKUDO
Application year: 2000
Main classification: H03M1
IPC codes: G11C27, H03M1, H04B14, H04N5