High Frequency Expander Device

cBACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to audio systems. More particularly it pertains to devices used in such systems to compress the range of the amplitude of the audio signals so as to make relatively quiet musical passages louder or more audible while at the same time avoiding system overload during the louder passages.

2. Description of the Prior Art

A variable gain element has been used for compressing the dynamic range of musical signals in sound systems so as to make the quiet passages more audible while avoiding system overload during the louder passages of music. However, the human ear is more sensitive to changes in the loudness of an audio signal in the higher frequency range of audio signals, e.g. 400 hertz to 4 kilohertz, than at lower frequencies, e.g. near 100 hertz. That is, a 1 decibel "db" change in the level of an audio signal at 1 kilohertz is more apparent to the human listener than a 1 db change in the level of an audio signal at 100 hertz. Furthermore, because a larger amount of audio energy typically exists at the lower frequencies, the amount of compression is controlled by the lower frequency components. As a consequence, when the dynamic range of musical passages is heavily compressed in response to the energy in the entire spectrum of the signal, to the human listener the higher frequency components seem to be attenuated during loud passages.

In a typical dynamic range compression device, the audio signal passes through a variable gain element to a detector circuit. The detector circuit generates a control signal which, in turn, controls the gain of the variable gain element. The control signal from detector circuit is connected to the variable gain element so as to provide a lower gain at higher signal levels. As a consequence the dynamic range of the audio signal is compressed. As pointed out above, however, at the higher levels of compression, the high frequency components appear to have been compressed far more than the lower frequency components.

Various methods have been used in an attempt to mitigate this undesirable effect of compression on the higher audio frequencies. A frequency sensitive filter that reduces the sensitivity of the level detection circuit to the higher frequency components has been used but the result has been unsatisfactory because the lower frequency components typically predominate and control the level of compression. A fixed filter that attenuates the lower frequencies more than the higher frequencies passing through the system has also been used but gives unsatisfactory results because it fails to prevent system overload which is one of the purposes of the compression system. If the range of frequencies that have the most energy (i.e. the lower frequencies) do not control the level of compression, then the system is likely to be often overloaded with consequent distortion.

A compressor 2 that is composed of a combination of bandpass filters with an individual compressor for the output of each bandpass filters can be configured so as to compress the lower frequency components more than the high frequency components. However the result is quite different from that described in this invention because the in the multiple compressor device the compression of each band of frequencies is controlled by the energy in each such band of frequencies while in the present invention the initial compression is responsive to the energy in the entire range of audio frequencies.

A combination of a variable gain element operating to compress the dynamic range and a variable gain element operating to expand the dynamic range of the signal have been used in combination called "compansion" in recording systems to avoid overload of the recording device while at the same time retaining the dynamic range of the system. For instance, Haramoto et al, U.S. Pat. No. 4,072,914, discloses a compression and expansion system that is used to avoid overload of an intervening magnetic tape recorder The higher frequency signals are more apt to saturate magnetic tape recording system. Accordingly, in Haramoto, the entire range of input signals are attenuated when the input signal is loud and the high frequency components are attenuated even more than the lower frequency components. The signal that is output from the tape recording undergoes an inverse expansion in order to restore the original signal balance and dynamic range.

The present invention counteracts the apparent loss of the high frequency signals that otherwise occurs in broadband compression circuits by using a broadband, variable gain element that operates as a compressor followed by another variable gain element that operates as a dynamic range expander of the high frequency components of the audio signal. The compression by the broadband variable gain element and the expansion of the high frequency components by the high frequency variable gain element are both controlled by the same voltage generated by the broadband signal level detection circuitry. In some embodiments of the invention the amount of the control signal that is applied to the high frequency expander may be altered relative to the amount of the control voltage that is applied to the broad band compressor.

FIG. 1 is a block diagram of an embodiment of the invention in which the input to the level detection circuitry is obtained from the audio signal that is input to the invention.

FIG. 2 is a functional block diagram of the invention in which the input to the level detection circuitry is obtained from the output of the broad band variable gain element that acts as a compressor.

Referring now to FIG. 1 which is a functional block diagram of an embodiment of the invention. The audio signal is input to broadband compressor 1, which is a broadband variable gain element whose level of amplification is controlled by the voltage or other signal output by level detector 2. The audio signal output from broad band compressor 1 is then input to high frequency expander 3 which is a variable gain element whose level of amplification is also controlled by the voltage or signal output from level detector 2. Some embodiments of the invention may also include a voltage divider 4 that may be used to alter the relative proportion of the control signal that is applied to compressor 1 and to high frequency expander 3. The relative amount of the control voltage applied to compressor 1 and to high frequency expander 3 may then be adjusted to obtain the most desirable apparent balance for the compression of the high and low frequency components of the audio signal. The level of amplification, of the high frequencies in high frequency expander 3 varies through a greater range than the level of amplification for the mid and lower frequencies in response to the voltage or signal output from level detector 2. It should be understood that although broadband compressor 1 and high frequency expander 3 are described as being variable gain elements having a variable level of amplification, either or both of these elements could be devices having a variable level of attenuation.

Broadband compressor 1, high frequency expander 3 and level detector 2 and voltage divider 4 operate in combination to produce an audio signal that is output from high frequency expander 3 and that has a smaller dynamic range than the range of the audio signal that is input to broadband compressor 1. However at high signal levels, the high frequency components in the output audio signal are not compressed as much as the mid and lower frequency components in the output signal.

Referring now to FIG. 2. The embodiment of the invention depicted in FIG. 2 utilizes the audio signal that is output from broad-band compressor 1 as the input to level detector 2. By using the output from compressor 1 as the input to level detector 2 this embodiment provides compression over a greater input signal dynamic range as compared to the circuit depicted in FIG. 1, because in the circuit of FIG. 2, the input to level detector 2 is compressed by its passage through compressor 1. It should also be understood that even though voltage divider 4 is depicted in the path from detector 2 to expander 3 it could instead be placed in the control signal path from detector 2 to compressor 1. It is intended that the magnitude of the control voltage applied to compressor 1 may be greater or less than that applied to expander 3.

In each of the embodiments depicted in FIGS. 1 and 2, the level of gain of compressor 1 and the gain of high frequency expander 3 are both controlled by the same voltage or signal that is output from level detector 2 although in those embodiments that include voltage divider 4, the size of the control voltage applied to compressor 1 need not be the same as that applied to expander 3.