Abstract: Variable frequency sawtooth waveforms are often utilized as an excitation signal in a digital musical synthesizer. A problem exists at lower sampling rates in such systems due to an aliasing problem which occurs at frequencies near the Nyquist rate. Low pass filtering may be utilized to mask this problem; however, low pass filtering is very time consuming to implement in a ditigal signal processor. The method and apparatus of the present invention reduces the upper harmonic content of a sawtooth waveform by proportionally converting the sawtooth waveform to a triangle waveform in response to variations in the frequency of the sawtooth waveform. This is accomplished by adding a selectable offset to the sawtooth waveform and then taking the absolute value of the resultant waveform. By restoring this waveform to a zero offset, the sawtooth waveform excitation signal will be converted to a triangle waveform having a substantially reduced upper harmonic content.

Abstract: A method and apparatus are disclosed for simultaneously outputting digital audio and MIDI synthesized music utilizing a single digital signal processor. The Musical Instrument Digital Interface (MIDI) permits music to be recorded and/or synthesized utilizing a data file containing multiple serially listed program status messages and matching note on and note off messages. In contrast, digital audio is generally merely compressed, utilizing a suitable data compression technique, and recorded. The audio content of such a digital recording may then be restored by decompressing the recorded data and converting that data utilizing a digital-to-analog convertor. The method and apparatus of the present invention selectively and alternatively couples portions of a compressed digital audio file and a MIDI file to a single digital signal processor which alternately decompresses the digital audio file and implements a MIDI synthesizer.