Data processing: generic control systems or specific application – Specific application – apparatus or process – Digital audio data processing system
Reexamination Certificate
1999-06-11
2002-10-29
Mei, Xu (Department: 2644)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Digital audio data processing system
C381S104000, C381S106000, C381S107000
Reexamination Certificate
active
06473662
ABSTRACT:
TECHNICAL FIELD
The invention relates to a signal compressor for an audio apparatus, in particular a motor vehicle audio apparatus, which contains a volume control that is inserted in the audio signal path and is adjustable via a digital setting signal delivered by a digital controller.
BACKGROUND OF THE INVENTION
With mass-produced articles, such as e.g., audio equipment for motor vehicles, which contain at least a radio along with a cassette unit and a CD unit, it is desirable to combine as many circuit parts as possible in an integrated circuit for the reduction of space and production costs. Integrated circuits can be equipped also with features that facilitate handling and improve the sound pattern.
While numerous individual components of an audio apparatus can be combined in an audio processor, there are still certain functions realized as before through discrete components or component groups that could not be designed as an integrated circuit.
The present invention is concerned with signal compression for the dynamic compression of audio signals. Depending on the nature of the memory medium, audio signals, in particular for instance with classic music, have a large dynamic range (e.g., up to about 85 dB). This large dynamic range reflects essentially the situation in a concert hall. In a piece of music, there are very soft passages and extremely loud passages. While the true reproduction of these very soft and very loud passages of a piece of music is desirable as such, the high dynamic range in case of motor vehicle audio equipment and other equipment operated in environments with high background noise level frequently results in undesirable effects. For example, soft passages almost cannot to be heard because of the high background noise level, while loud passages are felt as disturbing, particularly when they distract from another activity, e.g., the driving of a motor vehicle.
For this reason it is already known to provide an audio apparatus with a signal compressor, which reduces the very large dynamic range of the audio signal, e.g., from 85 dB to 60 dB. The effect of such a signal compressor consists in that the loudness is increased in very soft passages of a piece of music, while the loudness in very loud passages is decreased.
The applicant has already developed a signal compressor that is used together with an audio processor operating in digital fashion to the largest possible extent. For example, by means of a signal compressor connected upstream of the audio processor, the non-compressed output signal of a CD apparatus (compact disk apparatus) is fed to the signal compressor, compressed therein, and the compressed output signal is fed to the audio processor.
These previous signal compressors were designed as circuits operating in an analog manner. To obtain the desired signal compression of the analog signal, resistors and capacitors were used. Such components, which were usually employed as external components, in addition to integrated components, are contrary to mass production at low costs.
To convert the non-compressed signal to a compressed signal, typically an analog multiplication of the non-compressed analog signal by an appropriate factor is carried out.
What is disadvantageous with the known multiplier is, however, that the compressed signal obtained displays a strong distortion, in particular with input signals of high level.
Since the known signal compressor designed as a separate component displays a series of active signal processing stages, the signal to noise ratio (S/N ratio) of the signal deteriorates, particularly at low signal levels.
SUMMARY OF THE INVENTION
The invention is directed to providing a signal compressor for an audio apparatus in which the desired signal compression can be realized using a comparatively fewer number of components. In particular, space is saved by saving components of the entire audio apparatus, and the production is less expensive.
The invention achieves a reduction in the harmonic distortion and noise inevitably created with conventional signal compressors.
The invention provides a signal compressor of the type indicated at the outset, which has the following features:
a) the audio signal is branched off from the audio signal path upstream of the volume control and is processed by a signal processing circuit so as to form a tracking signal that is dependent on the intensity of the audio signal;
b) the tracking signal is compared in a comparator with a reference value;
c) depending on the comparison result, a counter designed as up/down counter is increased or decreased; and
d) the contents of the counter are linked with the digital setting signal before the setting signal is fed to the volume control.
A favorable prerequisite for the invention is an apparatus having a volume control that is adjusted by a digital setting signal. Such a volume control has already been developed by the applicant (DE-A-195 27 338). In this volume control the level of the analog audio signal at the input of the volume control is changed on the basis of a digital setting signal, such that when there is a change in volume, a soft or smooth signal transition without any “clicking noise” results. When the digital value of the setting signal changes (which can happen suddenly), the audio signal at the output of the volume control normally would perform an amplitude jump, which expresses itself as a “clicking noise” to a listener. The effect achieved by the measures described in detail in said DE-A-195 27 338 is that this signal transition at the output of the volume control takes place in a soft and smooth manner, so that the undesirable clicking noise is avoided.
The known volume control has a digital input for a digital setting signal. In the invention this digital setting signal is modified depending on the level of the audio input signal. The tracking signal dependent on the level of the audio signal upstream of the volume control is compared with a reference signal, and depending on the comparison result, the count of an up/down counter is changed. The contents of the counter then are linked in suitable manner with the digital setting signal.
The various linking types possible are described in more detail further below. One advantage of the invention is that very weak levels of the audio signal cause the volume control to change the signal towards a higher volume, while at high levels of the audio signal, the volume control performs a change in the audio signal in the sense of a volume reduction.
As will be described in more detail below, the components of the signal compressor can be designed completely as an integrated circuit. Due to the fact that use is made in part of already existing components in the audio processor, considerable space is saved for the circuit on the circuit board as compared to earlier designs, i.e., a signal compression with the aid of an independent signal compressor.
Because no multiplication of analog signals is required, the audio signal changed in intensity has little, if any, detectable distortion. As there are no additional components necessary in the signal path proper, the signal to noise ratio is not affected.
The aforementioned linkage of the digital setting signal with the contents of the counter can be accomplished in different ways. In one embodiment, an adder is provided that receives on its first input the digital setting signal and has a second input coupled with the output of the counter, the counter having its count raised by the output signal of a comparator when the tracking signal corresponds to a reduced level of the audio signal.
To elucidate the operation of this specific embodiment comprising the adder, some numerical examples shall be used: When the digital setting signal for the volume control has 8 bits, there are theoretically 256 volume levels possible. It shall be assumed that the volume control is set to the level “100”, i.e. that the digital setting signal is the binary representation of the decimal value “100”. When the up/down counter has six digits, the contents
Henkel Johann
Kirchlechner Peter
Lübbe Jürgen
Jorgenson Lisa K.
Mei Xu
Seed IP Law Group PLLC
STMicroelectronics GmbH
Tarleton E. Russell
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