Method of distortion-free signal compression
An audio signal in which an audio signal is received as a stream of digital samples, each being a numerical value representing a sampled signal level. A first zero crossing point is identified and the received audio samples are stored until a second zero crossing point is identified, thereby storing a first half-wave of samples. The highest intensity sample is identified from the stored samples and this is compared against a predetermined threshold. All stored samples are scaled by an initial scaling factor so that the intensity of the highest intensity sample is not above this threshold. A second half-wave of samples is stored in which all samples of the second half-wave are below the threshold. All stored samples of the second half-wave are also scaled but by a modified scaling factor derived from a combination of the initial scaling factor and a decay factor.
1. A compression apparatus for compressing the level of an audio signal, comprising an input device, an identification device, a storage device, and a scaling device, wherein:
said input device is configured to receive an audio signal as a stream of digital samples, each sample being a numerical value representing a sampled signal level;
said identification device is configured to identify a first zero crossing point;
said storage device is configured to store the received audio samples until a second zero crossing point is identified by said identification device, thereby storing a first half-wave of samples;
said identification device is configured to identify the highest intensity sample of said stored samples, and to compare said highest intensity sample against a threshold intensity;
said scaling device is configured to scale all stored samples of said first half-wave by an initial scaling factor that is less than unity so that the intensity of said highest intensity sample is not above said threshold intensity;
said storage device is configured to store a second half-wave of samples in which all samples of the second half-wave are below said threshold intensity; and
said scaling device is configured to scale all stored samples of said second half-wave by a first modified scaling factor having a value between that of the initial scaling factor and unity, and which is derived by combining the initial scaling factor with a decay factor.
2. The compression apparatus as claimed in claim 1 , wherein said storage device is configured to store a third half-wave of samples in which all samples of the third half-wave are below said threshold intensity; and
said scaling device is configured to scale all stored samples of said third half-wave by a second modified scaling factor derived from a combination of the first modified scaling factor and said decay factor.
3. The compression apparatus as claimed in claim 1 , wherein said storage device is configured to store a third half-wave of samples in which a sample of said third half-wave is above this threshold intensity; and
said scaling device is configured to scale all stored samples of said third half-wave by a newly calculated initial scaling factor without reference to a previous scaling factor.
4. The compression apparatus as claimed in claim 1 , wherein said identification device identifies the lowest intensity sample of said stored samples and compares said lowest intensity sample against a second threshold intensity and defines an expanding threshold intensity such that all values below said second threshold intensity are increased by an expansion factor.
5. An audio signal processing system responsive to and including a program control that, when executed by said processing system, causes said processing system to perform procedures of:
receiving an audio signal as a stream of digital samples, each sample being a numerical value representing a sampled signal level;
identifying a first zero crossing point;
storing the received audio samples until a second zero crossing point is identified, thereby storing a first half-wave of samples;
identifying the highest intensity sample of said stored samples and comparing said highest intensity against a threshold intensity;
scaling all stored samples of said first half-wave by an initial scaling factor that is less than unity so that the intensity of said highest intensity sample is not above said threshold intensity;
storing a second half-wave of samples in which all samples of the second half-wave are below said threshold intensity; and
scaling all stored samples of said second half-wave by a first modified scaling factor having a value between that of the initial scaling factor and unity, and which is derived by combining the initial scaling factor and a decay factor.
6. The audio signal processing system as claimed in claim 5 , including further program control that, when executed by said processing system, causes said processing system to perform additional procedures of:
storing a third half-wave of samples in which all samples of said third half-wave are below said threshold intensity; and
scaling all stored samples of said third half-wave by a second modified scaling factor derived from a combination of the first modified scaling factor and said decay factor.
7. The audio signal processing system as claimed in claim 5 , including a further program control that, when executed by said processing system, causes said processing system to perform additional procedures of:
storing a third half-wave of samples in which a sample of said third half-wave is above said threshold intensity; and
scaling all stored samples of said third half-wave by a newly calculated initial scaling factor without reference to a previous scaling factor.
8. The audio signal processing system as claimed in claim 5 , wherein said threshold intensity is a first threshold intensity defined as an unexpanding threshold intensity and a second threshold intensity is defined as an expanding threshold intensity, such that all values below said second threshold intensity are increased by an expansion factor.