Deconvolution of MLS response data

Details Deconvolution of MLS response data Deconvolution of MLS response data

1995-10-20

1998-03-31

Trammell, James P.

Electricity: measuring and testing

Measuring, testing, or sensing electricity, per se

Frequency of cyclic current or voltage

324337, G06K 1100

Patent

active

057348278

DESCRIPTION:

BRIEF SUMMARY
This application is a 371 of PCT/GB94/00862 filed on Apr. 22, 1994.


FIELD OF THE INVENTION

This invention relates to a method of and apparatus for the deconvolution of response data obtained from transmission of maximum length sequences (MLS), similar sequences or variants of similar sequences. References to MLS hereinafter are intended to include such similar or variant sequences.


BACKGROUND OF THE INVENTION

The applicants' prior International patent application No PCT/GB93/00639. relates to the use of a maximum length sequence technique to record otoacoustic emissions. One field of application of the present invention is the deconvolution of response data in the technique proposed in the prior patent application. However, the present invention is not restricted to that field of application.
For signals that have to be extracted from noise, a conventionally applied method, as described in the prior patent application, is to stimulate with an MLS, record the response or incoming signal generated by that MLS and, using sampling techniques, store the received waveform in a memory buffer whose length is equal to the length of the MLS multiplied by the number of ADC samples that are required per MLS entry in order to avoid aliasing errors.
Another MLS is immediately delivered and its received waveform added to that already in the memory buffer. This process is continued until sufficient received waveforms have been added to improve the signal-to-noise ratio to an adequate degree.
The original response, or input signal, then has to be recovered by a deconvolution technique. One example of a suitable technique, proposed in the prior patent application, is as follows.
Let L equal the length of the MLS. The MLS represents stimuli that will be presented at a specified rate. If the minimum time between presentations of stimuli, corresponding to the elements in the MLS, is such that n samples are required to characterise the response or incoming signal then that period is known as a slice of the MLS and n is the number of samples in one slice. Once the responses to a set of MLSs have been recorded, as described above, then the resultant waveform is contained in a raw data buffer whose length is equal to nL. A reconstruction memory buffer, of equal length to the raw data buffer, is zeroed. The recovery sequence is then generated and comprises a 1 for every instance for which the MLS contains a 1 and a -1 for every occurrence of a zero in the original MLS. The data in the raw data buffer are then each multiplied by the first element in the recovery sequence. The product is added to the reconstruction buffer. The data in the raw data buffer are then rotated left by one slice. They are then each multiplied by the second element in the recovery sequence and again added to the reconstruction buffer. This operation is continued until all the elements in the recovery sequence have been used. The reconstruction buffer will then contain the original waveform deconvolved from the MLS. The procedure is illustrated in FIG. 1 of the accompanying drawings.
The disadvantages of the above-described deconvolution technique may be illustrated by an example using evoked otoacoustic emissions recorded with an MLS.
1. As the response is being averaged from noise then it is useful to be able to avoid the noisiest periods (excess noise) during the recording of the signal. Typically this is done by rejecting, that is not adding to the average, any periods of the response where the waveform recorded from the subject or patient exceeds a certain limit. However, as the stimulus is also recorded then, using conventional techniques, the stimulus period has to be excluded from this rejection criterion. With the MLS technique and the overlapping of stimuli and responses, it is not possible sensibly to apply the described rejection methods. This is a problem with using the above-described deconvolution technique for the MLS. The signal-to-noise ratio is poorer than it might otherwise be because periods of high noise activity have to be include

REFERENCES:
patent: 4922362 (1990-05-01), Miller et al.
patent: 4964103 (1990-10-01), Johnson

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