Coded data generation or conversion – Sample and hold – Having variable sampling rate
Reexamination Certificate
2002-12-23
2004-10-12
Williams, Howard L. (Department: 2819)
Coded data generation or conversion
Sample and hold
Having variable sampling rate
C341S155000
Reexamination Certificate
active
06803868
ABSTRACT:
This invention relates to a method and apparatus for producing a digital depiction of a signal, in particular for producing an adaptive digital depiction.
Conventional analogue to digital converters (ADCs) are well known. These enable an analogue input signal to be converted into a digital depiction of the input signal.
Various types of ADC are known. Possibly the simplest is the zero crossing discriminator wherein the output changes from a logical naught to a logical one when the output crosses a reference voltage of zero volts. More sophisticated ADCs comprise a number of threshold crossing discriminators each with its own reference voltage, where adjacent reference voltages are spaced apart by a common amount or have some other means for achieving an equivalent effect. For example the outputs of sixteen discriminators expressed as a binary code would indicate that the input voltage lay somewhere between two particular reference voltages. However, for any significant amount of noise at the input, the input signal cannot be considered static during the conversion processes causing the output digital depiction to change rapidly and be almost indeterminate. To overcome this problem the input signal is held constant using a ‘track and hold’ circuit and the resulting unambiguous output digital depiction during the ‘hold phase is latched into a register by sample pulses which occur at equally spaced intervals of time.
Analogue to digital conversion in which the input analogue signal is periodically sampled at a predetermined constant rate and covered into a digital depiction are standard. To convert an analogue signal having high frequency components a higher sampling rate must be used resulting in an increased amount of output digital information. Further the high sampling rate results in an increased amount of unnecessary digital information for sections of the analogue input which have a relatively low frequency. For analogue signals having both high and low frequency components a low sampling rate is not appropriate as the high frequency components cannot be then correctly identified. Conventionally the choice of regular sampling rate is subject to the well known Nyquist sampling criteria i.e. that the rate of sampling should be greater than twice the maximum frequency component of the signal.
An alternative to sampling at a constant rate is described in International Patent Application No. PCT/US98/7592. When the input signal changes in amplitude by an amount that exceeds a predetermined threshold level a digital message is output containing information about the change in amplitude, the polarity of the change and the elapsed time for the change. Therefore samples are only taken when the signal itself changes by a predetermined amount and as such the sampling rate is determined by the signal itself. This is an example of an adaptive sampling technique. As used in this specification the term ‘adaptive’ shall be used to indicate a sampling system wherein the generation of the digital output is based on the evolution of the signal.
Also the system described in PCT/US98/27592 is a system which requires feedback. The crossing of a threshold level causes the levels of the apparatus to be reset. This need to change levels means that the system is inherently slow compared with a flow through system, such as a conventional ADC.
U.S. Pat. No. 4, 680, 797 describes a coder for secure communication of speech which determines certain critical points within a signal and determines the vectors between key points and transmits those vectors. The paper by Balasubramanian K et al. (“A Novel Method for Optimal Sampled Data Selection, Transmission and Reconstruction: A Proposal” 1995 IEEE Instrumentation and Measurement Technology Conference. IMTC/95 Waltham Mass. Apr. 23-26, 1995 Proceedings of the Instrumentation and Measurement Technology Conference) similarly discloses a system which identifies key points in the signal and transmits samples only at those points. Both of these systems require analysis of the signal to determine key or critical points however.
Honary B et al: “Adaptive-Rate Sampling Applied to the Efficient Encoding of Speech Waveforms” National Conference on Telecommunications, York, 2-5 Apr., 1989, London IEE, vol. CONF 2, 2 Apr. 1989, pages 352-357 teaches a system where the signal is analysed and, based on that analysis the sampling rate is varied accordingly to ensure accurate sampling. The resultant signal is complex however and the bandwidth of the signal needs to be quickly assessed.
It is therefore an object of the present invention to provide a method and apparatus for producing a digital depiction of a signal which is adaptive and which mitigates at least some of the aforementioned disadvantages.
According to the present invention therefore there is provided an apparatus for producing a digital depiction of a signal comprising a sampling means for sampling the signal at a constant rate determined by a clock and providing a first digital depiction containing information regarding the signal amplitude at each sampling time, and a transformation means responsive to the first digital depiction for producing a second digital depiction, characterised in that the transformation means is capable of determining that the signal has changed in amplitude by a certain threshold amount and determining the elapsed time for the signal to change by said amount and producing the second digital depiction based on the elapsed times for the signal to change by predetermined amounts, and in that the sampling rate is set such that the signal varies by less than the predetermined amount between successive samples.
This is an example of adaptive sampling wherein the evolution of the signal, as represented by the first digital depiction, is used to determine what adapted depictions are output.
By using the evolution of the signal itself to produce the second digital depiction the output of the second digital signal can be significantly lower than the first digital depiction. For example when the rate of change of the signal is low that fact can be used to produce a more compact digital depiction with less information than the first digital depiction. The first digital depiction is sampled at a constant rate and so will over sample a low frequency component of the signal but the second digital depiction can avoid this.
Also the second digital depiction could present the information about the signal in a form which is more suited for a particular application. For instance in waveform matching applications it may be desired to match similar waveforms evolving in different time scales. The constantly sampled first digital depiction has a temporal dependence on the sampling process but the second digital depiction, using the signal evolution, can avoid this.
Further, by using a sampling means run at a constant rate to generate a first digital depiction high speed components and conventional digital sampling technology can be used and the adaptive part of the sampling can be carried out in the digital regime.
Preferably the second digital depiction is a digital representation. As used throughout this specification the term ‘digital depiction’ shall be taken to mean any digital output which contains information about the signal. The term ‘digital representation’ however shall be taken to mean a representation that digitally describes a signal in such a way that the signal could be reproduced directly by feeding the depiction to a suitable reconstituter, such as a digital to analogue converter, without requiring any preliminary processing. The digital messages produced in PCT/US98/27592 for instance would require prior processing and so do not constitute a digital representation.
In one convenient embodiment the transformation means is capable of determining that the signal has crossed a predetermined threshold level and determining the time interval between crossing a first predetermined threshold level and a second different predetermined threshold level and producing a second digital depiction o
Ballantyne Selina A
Coffey Adrian S
Johnson Martin
Jones Robin
Nixon & Vanderhye P.C.
QinetiQ Limited
Williams Howard L.
LandOfFree
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