Signal processors

Details Signal processors Signal processors

1998-10-23

2003-07-15

Williams, Howard L. (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Differential encoder and/or decoder

C341S076000, C341S155000

Reexamination Certificate

active

06593866

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a 1-bit signal processor.
2. Description of the Prior Art
It has been proposed to process 1-bit signals with Delta Sigma Modulators (DSM). The 1-bit signals may be audio signals and the invention is described herein by way of example with reference to audio signals.
Background to the present invention will now be described by way of example with reference to
FIGS. 1
,
2
and
3
of the accompanying drawings of which
FIG. 1
is a block diagram of a known Delta-Sigma Modulator,
FIG. 2
is a block diagram of a previously proposed Delta-Sigma Modulator configured as an nth order filter section and
FIG. 3
shows a noise shaping characteristic.
It is known to convert an analogue signal to a digital form by sampling the analogue signal at at least the Nyquist rate and encoding the amplitudes of the samples by an m bit number. Thus if m=8, the sample is said to be quantized to an accuracy of 8 bits. In general m can be any number of bits equal to or greater than 1.
For the purpose of quantizing to only 1 bit, it is known to provide an analogue to digital converter (ADC) known either as a “Sigma-Delta ADC” or as a “Delta-Sigma ADC”. Herein the term “Delta-Sigma” is used. Such an ADC is described in for example “A Simple Approach to Digital Signal Processing” by Craig Marven and Gillian Ewers ISBN 0-904.047-00-8 published 1993 by Texas Instruments.
Referring to
FIG. 1
in an example of such an ADC, the difference
1
(Delta) between an analogue input signal and the integral
2
(Sigma) of the 1-bit output signal is fed to a 1-bit quantizer
3
. The output signal comprises bits of logical value 0 and 1 but representing actual values of −1 and +1 respectively. The integrator
3
accumulates the 1-bit outputs so that value stored in it tends to follow the value of the analog signal. The quantizer
3
increases (+1) or reduces (−1) the accumulated value by 1-bit as each bit is produced. The ADC requires a very high sampling rate to allow the production of an output bit stream the accumulated value of which follows the analogue signal.
The term “1-bit” signal as used in the following description and in the claims means a signal quantized to an accuracy of 1 digital bit such as is produced by a Delta-Sigma ADC.
A Delta-Sigma Modulator (DSM) configured as nth order filter section for directly processing a 1-bit signal was proposed by N. M. Casey and James A. S. Angus in a paper presented at 95th AES Convention Oct. 7-10, 1993 New York, USA entitled “One Bit Digital Processing of Audio Signals”—Signal Processing: Audio Research Group, The Electronics Department, The University of York, Heslington, York YO1 5DD England.
FIG. 2
shows a 3rd order (n=3) version of such a DSM filter section.
Referring to
FIG. 2
, the DSM has an input
4
for a 1-bit audio signal and an output
5
at which a processed a 1-bit signal is produced. The bits of the 1-bit signal are clocked through the DSM by known clocking arrangements which are not shown. The output 1-bit signal is produced by a 1-bit quantizer Q which is for example a comparator having a threshold level of zero. The DSM has three stages each comprising a first 1-bit multiplier a
1
, a
2
, a
3
connected to the input
4
, a second 1-bit multiplier c
1
, c
2
, C
3
connected to the output
5
, an adder
6
1
,
6
2
,
6
3
and an integrator
7
1
,
7
2
,
7
3
.
The 1-bit multipliers multiply the received 1-bit signal by p bit coefficients A
1
, A
2
, A
3
, C
1
C
2
, C
3
producing p bit products which are added by the adders
6
1
,
6
2
,
6
3
and the sums passed to the integrators
7
. In the intermediate stages the adders
6
2
,
6
3
also sum the output of the integrator of the preceding stage. A final stage comprises another 1-bit multiplier A
4
connected to the input which multiplies the input signal by a p bit coefficient A
4
and an adder
6
4
which adds the product to the output of the integrator
7
3
of the preceding stage. The sum is passed to the quantizer Q.
Within the DSM, two's complement arithmetic is used to represent the positive and negative p bit numbers. The input to the quantizer Q may be positive, quantized at the output as +1 (logical 1) or negative quantized at the output as −1 (logical 0).
As observed by Casey and Angus “a one bit processor . . . will produce a one bit output that contains an audio signal that is obscured by noise to an unacceptable level and it is imperative the quantization noise is suitably shaped”. The noise which obscures the audio signal is the quantization noise produced by the quantizer Q.
The quantizer Q may be modelled as an adder which has a first input receiving an audio signal and a second input receiving a random bit stream (the quantization noise) substantially uncorrelated with the audio signal. Modelled on that basis, the audio signal received at the input
4
is fed forward by multipliers a
1
, a
2
, a
3
, a
4
to the output
5
and fed back by multipliers c
1
, c
2
, c
3
from the output
5
. Thus coefficients A
1
to A
4
in the feed forward path define zeros of the Z-transform transfer function of the audio signal and coefficients C
1
-C
3
in the feed back path define poles of the transfer function of the audio signal.
The noise signal is fed-back from the quantizer by the multipliers C
1
-C
3
so that coefficients C
1
-C
3
define poles of the transfer function of the noise signal. The transfer function of the noise signal is not the same as the transfer function of the input signal.
The coefficients A
1
to A
4
and C
1
to C
3
are chosen to provide circuit stability amongst other desired properties.
The coefficients C
1
-C
3
are chosen to provide noise shaping so as to minimise quantization noise in the audio band, as shown for example in
FIG. 3
by the full line
31
.
The coefficients A
1
-A
4
and C
1
-C
3
are also chosen for a desired audio signal processing characteristic.
The coefficients A
1
-A
4
and C
1
-C
3
may be chosen by:
a) finding the Z-transform H(z) of the desired filter characteristic—e.g noise shaping function; and
b) transforming H(z) to coefficients.
This may be done by the methods described in the paper
“Theory and Practical Implementation of a Fifth Order Sigma-Delta A/D Converter, Journal of Audio Engineering Society, Volume 39, no. 7/8, 1991 July/August by R. W Adams et al.”
and in
the paper by Casey and Angus mentioned herein above using the knowledge of these skilled in the art. One way of calculating the coefficients is outlined in the accompanying Annex A.
The various papers mentioned herein above consider only nth order filter sections.
For high quality audio recording it is conventional to use a microphone having a differential or double-ended analogue output. Proposed 1-bit signal processors including DSMs require non-differential or single-ended signals. One proposal for converting differential signals to non-differential signals is to add using an analogue adder the analogue differential signals output by the microphone and then converting the resultant non-differential signal to 1-bit digital form.
SUMMARY OF THE INVENTION
According to the present invention there is provided a signal processor having a pair of inputs for receiving first and second analogue signals which are a differential pair of signals,
a pair of 1-bit analogue to digital converters coupled to the respective inputs to convert the first and second signals to 1-bit digital form, and
a Delta Sigma Modulator having a pair of inputs coupled to receive the respective first and second 1-bit signals for combining the differential pair to form a non-differential 1-bit signal.
In a preferred embodiment of the invention the pair of input signals are produced by an audio signal source such as a differential microphone. By converting the two differential analogue outputs of the microphone to corresponding 1-bit signals and then combining the two 1-bit signals in a DSM, the signal to noise ratio is improved (compared to the prior proposal discus

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