Electrical audio signal processing systems and devices – Including frequency control
Patent
1998-02-10
1999-10-26
Isen, Forester W.
Electrical audio signal processing systems and devices
Including frequency control
381103, 381106, 333 28T, H03G 500, H03G 700
Patent
active
059741562
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a digital circuit arrangement for influencing the frequency response of a digital audio signal as a function of frequency and/or amplitude.
2. Description of the Related Art
Such circuit arrangements are particularly necessary in noise suppression systems which are to suppress the noise arising after a sound carrier has been recorded and subsequently reproduced. Such known systems are, for example, the DBX, the HIGHCOM system and the various Dolby systems. In the latter noise suppression systems, a predefined effect on the audio signals, is set as a function of the frequency and the amplitude of a signal. Such systems are usually referred to as sliding-band systems.
An encoder is provided for these noise suppression systems which increases the upper frequency range of the audio signal at the recording end, for example, as a function of the frequency and/or amplitude of the signal, and a decoder which reduces the same frequency range in a mirror-inverted manner.
For most noise suppression systems of the type described above, it holds that only circuit arrangements operating in the analog mode are known for them. The cause of this is that the circuit arrangements are generally used only for analog audio signals. Of late, however, digitally operating devices for reproducing audio signals have increasingly been marketed, which are also to be suitable for the reproduction of, for example, audio signals.
SUMMARY OF THE INVENTION
In this connection, it is an object of the invention to provide a digitally operating circuit arrangement which, similarly to a noise suppression system, can influence the frequency response of the digital audio signal as a function of frequency and/or amplitude.
According to the invention, this object is achieved in that the input signal of the circuit arrangement is applied to a signal filter having variable filter coefficients, whose output signal, after addition to the input signal of the circuit arrangement, represents the frequency response influenced output signal of the circuit arrangement, and in that the coefficients for the signal filter are selected in dependence on the output signal of a control circuit to which the output signal of the signal filter is applied and which includes a control filter, a rectifier, a pulse shaper stage, a logarithmic means and a circuit for determining the coefficients that produces the filter coefficients for the signal filter.
Contrary to the known analog arrangements, the circuit arrangement according to the invention does not operate recursively as this causes problems in digital signals. Yet, the circuit arrangement is capable of copying the properties of analog systems not only with respect to the amplitude behavior, but also with respect to the pulse behavior. A basic problem is in this context, that not the desired frequency and/or amplitude-dependent frequency-response curves can be indicated as a standard, but that certain main features of analog circuit arrangements are prescribed. The behavior of these circuit arrangements is not laid down, but substantially automatically results from its structure. If the behavior of these analog circuit arrangements is to be almost copied in the digital area, a corresponding digital circuit arrangement is to be arranged very flexibly i.e., it must be possible to obtain a substantially arbitrary amplitude and/or pulse behavior. If such a characteristic of a certain noise suppression system is to be copied, the respective frequency-response curves and the dynamic behavior of the circuit can only be determined by experiments.
In the digital circuit arrangement according to the invention, these requirements are satisfied in that a signal filter is included which works with variable filter coefficients. These filter coefficients can be set as desired, depending on the frequency and/or amplitude of the input signal of the circuit arrangement. This holds both for the static case where there is an input signal present having
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Goodman Edward W.
Isen Forester W.
Mei Xu
U.S. Philips Corporation
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