Compression and expansion technique for digital audio data

An apparatus for compressing a digital audio signal to a compressed signal, storing said compressed signal in a memory, retrieving said compressed signal from said memory and expanding said compressed signal to reconstruct said digital audio signal for real-time playback, comprising:

• input means coupled for receiving a first inputted analog audio signal;
• first conversion means for converting said first inputted analog audio signal into a first inputted digital audio signal;
• predictor means for providing a first predicted signal of said first inputted digital audio signal;
• first adder means coupled to receive both said first inputted digital audio signal and said first predicted signal for providing a first difference signal, said first difference signal representing a difference between said first inputted digital audio signal and said first predicted signal;
• first adjustment means coupled to receive said first difference signal for applying a first gain value to said first difference signal in order to adjust the amplitude of said first difference signal and for normalizing said first difference signal, said first adjustment means including shifter means for providing the appropriate amount of shifting for normalizing said first difference signal, said shifter means operating using a first normalizing value in order to normalize the amplitude of said first difference signal, said first adjustment means output being a first adjusted difference signal;
• quantizer means coupled to said first adjustment means for quantizing said first adjusted difference signal in order to provide a compressed signal which corresponds to a quantized compression of said first digital signal;
• memory means coupled to receive said compressed signal for storing said compressed signal;
• adaptive means coupled to receive said compressed signal for evaluating said compressed signal to determine if the amplitude of said compressed signal is at a predetermined value, said adaptive means including an adaptation table and at least one look-up table, said adaptation table coupled to receive said compressed signal for providing a first pointer to said one look-up table, said one look-up table having gain value entries, each said gain value entry being a step multiple of a previous said gain value entry, said first pointer pointing to one of said gain value entries in said one look-up table for selecting said first gain value, said one look-up table coupled to said first adjustment means for supplying said first gain value;
• retrieval means coupled to said memory means for retrieving said compressed signal;
• inverse quantizer means coupled to receive said compressed signal from said memory means after retrieval by said retrieval means for providing an inverse quantization operation to expand said compressed signal into a second adjusted difference signal, said second adjusted difference signal being substantially equivalent to said first adjusted difference signal, wherein said quantizer means and said inverse quantizer means operate in unison to provide a unitary compresser/expander;
• second adjustment means coupled to receive said second adjusted difference signal for applying a second gain value and for denormalizing said second adjusted difference signal, said second gain value being said one of said gain value entries from said one look-up table indicated by said first pointer, said second adjustment means coupled to said adaptive means to receive said second gain value, wherein said second gain value is a reciprocal of said first gain value, said second adjustment means including shifter means for providing an appropriate amount of shifting for denormalizing said second adjusted difference signal, said shifter means operating using a second normalizing value in order to denormalize the amplitude of said second adjusted difference signal, said second adjustment meas output being a second difference signal substantially equivalent to said first difference signal;
• second adder means coupled to receive both said second difference signal and said first predicted signal for providing a first outputted digital audio signal, said first outputted digital audio signal representing a sum of said second difference signal and said first predicted signal, said first outputted digital audio signal being substantially equivalent to said first inputted digital audio signal;
• said predictor means coupled to receive said first outputted digital audio signal for generating a second predicted signal;
• second conversion means coupled to receive said first outputted digital audio signal for converting said first outputted digital audio signal to a first outputted analog audio signal;
• and
• output means coupled to receive said first outputted analog signal for outputting said first outputted analog signal, wherein said first outputted analog audio signal is substantially similar to said first inputted analog audio signal.

In a computer for processing a digital audio signal, a method for compressing said digital audio signal into a compressed signal, storing said compressed signal in a memory, retrieving said compressed signal from said memory and expanding said compressed signal to reconstructed said digital audio signal for real-time playback, comprising the steps of:

• providing a first predicted signal;
• adding said first predicted signal to a first inputted digital audio signal to provide a first difference signal which is the difference between said first predicted signal and said first inputted digital audio signal;
• adjusting a first gain on said first difference signal to provide a first gain adjusted signal;
• normalizing said first gain adjusted signal to provide a first adjusted difference signal;
• quantizing said first adjusted difference signal in order to provide a compressed signal which corresponds to a quantized compression of said first inputted digital audio signal;
• storing said compressed signal in said memory for later retrieval;
• evaluating said compressed signal to determine if said compressed signal is at a predetermined value in order to automatically control an amount of gain to be applied to a second inputted digital audio signal;
• retrieving said compressed signal from said memory for future real-time playback;
• expanding said compressed signal in an inverse quantizer to provide an expanded signal which is substantially equivalent to said first adjusted difference signal;
• denormalizing said expanded signal to provide a second adjusted difference signal substantially equivalent to said first gain adjusted signal;
• applying a second gain to said second adjusted difference signal to provide a second gain adjusted signal substantially equivalent to said first difference signal;
• adding said second gain adjusted signal to said first predicted signal to provide a first outputted digital audio signal substantially equivalent to said first inputted digital audio signal, wherein said first outputted digital audio signal is outputted available for in time for real-time playback.