Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
2002-11-18
2004-07-20
Tran, Khai (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S346000
Reexamination Certificate
active
06765970
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a method and an apparatus for preventing interference in systems processing discrete-amplitude signals, particularly in telecommunications systems operating in burst mode.
Particularly in the audio paths in such systems, interference can arise when the signal has a signal level below a threshold level value and/or the signal has a constant amplitude over time. The reason for this is because one or more processing units in the system react to the aforementioned characteristics of the signal being processed by generating an interference signal and/or by amplifying a signal that is present to an unwanted degree. The signal that is present is, by way of example, also an interference signal, for example, microphone noise, which is fed into the reception path in a telephone terminal.
A first example of such a processing unit is a digital filter that is arranged in the digital section of an audio path, i.e. upstream of a digital-analog converter (DAC) in the processing direction. Often, such filters are fed back to the input of the digital processing section, so that the problem of “limit cycles” arises: the filter reacts to a signal having such a low level by representing the discrete-amplitude signal as a zero signal by generating a cyclic signal. This interference signal is transmitted to the downstream processing units. The system is thus subject to unwanted interference.
A similar situation arises when a signal having an amplitude that is constant over time is present on the digital filter. In this case, the filter generates an interference signal that is superimposed on the signal that is present.
Another example of such a processing unit is a “noise shaper”, which is normally connected upstream of a DAC in an audio path, as seen in the signal processing direction. The noise shaper is used to cut off or suppress noise signals having low level values. Noise shapers exist which react in the same way as the digital filter described above. There are also noise shapers that feed or inject interference signals into the processing path to an unwanted degree when the signal being processed that is present on the noise shaper has a level below a threshold level value. The interference signal is thus generated only when a user signal having a low level is present. With such interference in the system, signal-to-noise ratios are obtained that are much worse than the ratios demanded by the respective standard.
To solve this problem, it is known practice to use additional filter structures that filter out the interference signals. A drawback in this case is that such solutions are normally complex and hence cost-intensive.
It is also known practice to use “dither techniques”. These involve injecting a dither signal in the signal processing direction upstream of the processing unit, which can cause the interference. The dither signal is a noise signal that is added, by way of example, in the digital processing path as a succession of random numbers. This technique is described, for example, in the book “Digitale Audiosignal-Verarbeitung” [Digital Audio Signal Processing], 2
nd
edition, 1997, by Udo Zölzer, published by Teubner Verlag, Stuttgart, Germany (called Zölzer '97 for short below). Section 2.2.1 of this book explains dither techniques. The dither signal causes linearization of the quantization levels, i.e. of the levels of the signals' amplitude quantization. The aforementioned limit cycles can therefore be prevented even for signals that have a much lower signal level than the level that corresponds to the lowest quantization level.
However, a drawback of the dither techniques is that signal processing needs to be performed more accurately than is required for signal processing without a dither signal. This is manifested particularly by providing additional bits in a digital word in order to represent the dither random numbers. Accordingly, computation power needs to be provided and the processing units need to be of a complex design. This solution therefore also generates costs as a result of preventing the interference that is possible because of limit cycles or other effects.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an apparatus and a method for preventing interference which overcome the above-mentioned disadvantages of the prior art apparatus and methods of this general type.
In particular, it is an object of the present invention to provide a method and an apparatus of the type mentioned in the introduction that allow the interference to be prevented and that are inexpensive to implement.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for preventing interference. The method includes steps of: providing a system for processing a discrete-amplitude signal in which interference occurs when the discrete-amplitude signal has a signal level below a threshold level value and/or when the discrete-amplitude signal has an amplitude that is constant over time, the system having a processing path; and adding an artificial interference signal having at least one interference signal frequency into the processing path of the system such that the signal level of the discrete-amplitude signal exceeds the threshold level value or such that at least an amplitude that is constant over time is prevented.
With the foregoing and other objects in view there is also provided, in accordance with the invention, an apparatus for preventing interference in a system for processing at least one discrete-amplitude signal in which the interference occurs when the discrete-amplitude signal has a signal level below a threshold level value and/or when the discrete-amplitude signal has an amplitude that is constant over time. The apparatus includes: a signal processing path having a section, the signal processing path configured such that the at least one discrete-amplitude signal can be processed and transmitted along the signal processing path; an interference signal generator for generating an artificial interference signal having at least one interference signal frequency; and a device for injecting and adding the interference signal into the section of the signal processing path.
The inventive method involves an artificial interference signal, having at least one interference signal frequency, which is added into the processing path in the system. For the situation in which the interference occurs when the signal has a signal level below a threshold level value, the signal level of the interference signal is chosen such that the signal level exceeds the threshold level value overall. Preferably, the signal level of the interference signal is defined at a fixed value. However, the addition of the interference signal having at least one interference signal frequency ensures in each case that the signal amplitude is not constant over time. The previously described interference, in particular, can therefore be prevented.
An artificial interference signal having at least one interference signal frequency can be generated in a simple and hence inexpensive manner. In particular, the interference signal having at least one interference signal frequency can be generated in a particularly simple manner in the digital signal processing range, for example, by inserting individual bit values based on the at least one interference signal frequency into the signal being processed. This can be achieved by hardware and/or software solutions.
The invention is not limited to the addition of a digital interference signal, however. By way of example, the interference signal can be added in analog form, for example before or after the transmission of a user signal via a radio link. An analog interference signal generator for generating an interference signal having at least one interference signal frequency can also be implemented in a simple and hence inexpensive manner.
The invention is based on the insight that the previously describ
Greenberg Laurence A.
Infienon Technologies AG
Mayback Gregory L.
Stemer Werner H.
Tran Khai
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