Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Reexamination Certificate
1998-05-26
2001-01-16
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
C341S131000
Reexamination Certificate
active
06175321
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of sigma-delta modulation with reduced periodic noise.
DESCRIPTION OF RELATED ART
In most audio devices, for example, mobile telephones or CD-players in which an analogue/digital or digital/analogue conversion is carried out, often a sigma-delta modulator is used in the analogue/digital or digital/analogue converter.
A sigma-delta modulator according to prior art comprises a number of integrators, a number of amplifiers and a quantifier. These are arranged in a way characteristic to the sigma-delta modulator. The configuration of a sigma/delta modulator is described below.
In a sigma-delta modulator there arises, when its input signal is low, constant, or changes slowly, a so-called periodic noise, or idle tones. Even if the periodic noise has a relatively low amplitude it is fully audible to the human ear. This noise must therefore be reduced to a non-audible level.
In EP 0 709 959 A2, a sigma-delta modulator reducing periodic noise by means of a Dither signal is disclosed. The Dither signal is a random signal, for example, a pseudo-noise code (PN-code). This Dither signal may be added in one or more places in the sigma-delta modulator. Depending on where the addition of the Dither signal in the sigma-delta modulator is carried out, the signal is filtered by a particular filter before the addition.
A preferred length of the PN-code is that the period of the PN-code is much-longer than the period of the lowest frequency to be treated by the sigma-delta modulator. The PN-code should be at least 21 bits long. The rectified AC-power of the Dither signal is dependent on the order of the sigma-delta modulator.
One disadvantage of this solution is that depending on where it is chosen to add the Dither signal, one or more filters will be needed.
Another common solution for reducing the periodic noise in the sigma-delta modulator is adding the Dither signal in one of the integrators. The Dither signal is added without filtering.
This solution also has the disadvantage that the performance of the sigma-delta modulator is deteriorated.
A deterioration of the signal
oise ratio in the sigma-delta modulator implies that the complexity of the sigma-delta modulator must be increased. This means that a larger number of integrators must be used in the sigma-delta modulation to maintain the desired level of performance.
If the sigma-delta modulator is comprised in a D/A-converter, there is another solution to the deterioration of the signal
oise ratio. It is then possible to increase the over-sampling rate in an interpolation filter in the D/A-converter or increase the complexity of a low-pass filter arranged at the output of the signal-delta modulator.
The three solutions mentioned above, however, lead to an increased power consumption and an increased complexity of the sigma/delta modulator, which is not desirable for radio communication devices, such as mobile telephones.
SUMMARY OF THE INVENTION
The present invention attacks the problem of reducing periodic noise (idle tones) in a sigma-delta modulator.
Another problem attacked by the present invention is to maintain, when reducing the periodic noise, the signal
oise ratio for output signals from the sigma-delta modulator without increasing the complexity of the sigma-delta modulator.
Yet another problem attacked by the present invention is, when reducing the periodic noise when the inventive apparatus and the inventive method for sigma-delta modulation are comprised in a D/A-converter, to maintain the signal
oise ratio for output signals from the D/A-converter without increasing the complexity of the D/A-converter.
One object of the present invention is therefore to provide an apparatus and a method for sigma-delta modulation with reduced periodic noise in the modulated output signal.
Another object is to obtain, when reducing the periodic noise, a good signal
oise ratio for the output signal of the sigma-delta modulator without increasing the complexity of the sigma-delta modulator performing the modulation procedure.
The above-mentioned problems are solved according the present invention by adding two different signals to the sigma-delta modulator. The first signal has a relatively short period. Said first signal is added to one of the most significant bits of the sigma-delta modulator. The second signal has, compared to said first signal, a long period. The second signal is added to the least significant bit of one of the integrators comprised in the sigma-delta modulator.
One advantage of the inventive apparatus and the inventive method is that the reduction of periodic noise is carried out without causing a deterioration of the signal
oise ratio of the output signal from the sigma-delta modulator.
Another advantage of the present invention is that, since the complexity can be kept down when reducing the periodic noise, a sigma-delta modulator consuming relatively little power is obtained.
The invention will now be described in more detail by means of preferred embodiments and with reference to the appended drawings.
REFERENCES:
patent: 3999129 (1976-12-01), Kasson
patent: 4761634 (1988-08-01), Yamaguchi et al.
patent: 4968987 (1990-11-01), Naka et al.
patent: 5010347 (1991-04-01), Yukawa
patent: 5055843 (1991-10-01), Ferguson, Jr. et al.
patent: 5073777 (1991-12-01), Fukuhara et al.
patent: 0709969 A2 (1996-05-01), None
PCT International-Type Search Report, Apr. 7, 1998, Search request No. SE 97/00671, pp. 1-6.
Fremrot Per Henrik
Fr{umlaut over (a)}nnhagen Mats Olof
Jeanglaude Jean B.
Jenkens & Gilchrist P.C.
Telefonakitiebolaget LM
Tokar Michael
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