Electrical audio signal processing systems and devices – Including amplitude or volume control
Reexamination Certificate
1997-07-09
2001-06-12
Isen, Forester W. (Department: 2747)
Electrical audio signal processing systems and devices
Including amplitude or volume control
C381S107000
Reexamination Certificate
active
06246774
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is related to the following patent applications filed on the same date herewith, all of which are assigned to the same assignee as the present invention, and all of which are hereby incorporated by referenced thereto and made a part hereof as if fully set forth herein:
Hazard-Free Divider Circuit, application Ser. No. 08/333,410; Monolithic PC Audio Circuit, application Ser. No. 08/333,451; Modular Integrated Circuit Power Control, application Ser. No. 08/333,537; Audio Processing Chip with External Serial Port, application Ser. No. 08/333,387; Wavetable Audio Synthesizer with Delay-Based Effects Processing, application Ser. No. 08/334,462; Wavetable Audio Synthesizer with Low Frequency Oscillators for Tremolo and Vibrato Effects, application Ser. No. 08/333,564; Wavetable Audio Synthesizer with an Interpolation Technique for Improving Audio Quality, application Ser. No. 08/333,398; Monolithic PC Audio Circuit with Enhanced Digital Wavetable Audio Synthesizer, Ser. No. 08/333,536; Wavetable Audio Synthesizer with Waveform Volume Control for Eliminating Zipper Noise, application Ser. No. 08/333,562; Digital Signal Processor Architecture for Wavetable Audio Synthesizer, application Ser. No. 08/334,461; Wavetable Audio Synthesizer with Enhanced Register Array, application Ser. No. 08/334,463; A Digital Decimation and Compensation Filter System, application Ser. No. 08/333,403; Digital Interpolation Circuit for Digital-to-Analog Converter Circuit, application Ser. No. 08/333,399; Analog-to-Digital Converter Circuit, application Ser. No. 08/333,535; Stereo Audio Codec, application Ser. No. 08/333,467; Digital Noise Shaper Circuit, application Ser. No. 08/333,386; and Digital-to-Analog Converter Circuit, application Ser. No. 08/333,460.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a digital wavetable audio synthesizer with multiple volume components and two modes of stereo positioning. More particularly, this invention relates to a digital wavetable audio synthesizer with multiple volume components and two modes of stereo positioning for use in system boards and add-in cards for desktop and portable computers. As an example, the wavetable audio synthesizer of this invention may be used in a PC-based sound card.
2. Brief Description of the Invention
Digital audio has become a viable alternative to analog audio. In general, in digital audio, sound waves are represented as a series of number values which can be stored as data in a variety of media including hard disks, compact disks, digital audio tape, and computer RAM and ROM. Digital audio uses such data to provide unique and beneficial editing and signal processing capabilities.
In digital audio, quantization and sampling processes are used to generate the data representing the amplitude (level) element of sound and the frequency (events over time) element of sound. An analog-to-digital converter (ADC) measures the amplitude of a sound signal—in the form of an analog voltage signal—at particular instances or samples. The rate at which the ADC takes these measurements is referred to as the sampling rate. Quantization is a process in which the ADC generates a series of binary or digital numbers representing the amplitude measurements. A digital-to-analog converter (DAC) transforms digital data representing sound into analog voltage signals. These analog voltage signals may then be applied to an audio amplifier and speakers for playing sound.
Several types of digital “synthesizers,” i.e. devices that generate sound through audio digital-signal-processing, are now available. One modern type of digital synthesizer is a wavetable synthesizer. Wavetable synthesizers generate sounds through digital processing of entire digitized sound waveforms or portions of digitized sound waveforms stored in wavetable memory.
Wavetable synthesizers generate sounds by “playing back” from wavetable memory, to a DAC, a particular digitized waveform. The addressing rate of the wavetable data controls the frequency or pitch of the analog output. The bit width of the wavetable data affects the resolution of the sound being generated. For example, better resolution can be achieved with 16-bit wide data versus 8-bit wide data. 16-bit digital audio is becoming the standard in the industry.
The digitized waveform data may comprise a complete sound, sampled in its entirety, or only a selected portion of the sound. If the waveform is complex, it may be necessary to store the entire digitized waveform. For uniform, repetitive sounds, a fundamental cycle of the waveform may be stored in a smaller block of wavetable memory. Then, the synthesizer can loop through this block of wavetable memory to generate continuous uniform, repetitive sound. Alternatively, a complex segment of waveform may be stored in its entirety in a larger block of the wavetable memory while only a fundamental cycle of a repetitive segment of the waveform is stored in a smaller block of memory. Then, during playback, the synthesizer will first address or scan through the larger block of memory to playback the complex segment of the waveform and then loop through the smaller block of memory to playback the repetitive segment of the sound.
Wavetable synthesizers typically use wavetable data interpolation to reduce the amount of data required to generate quality sound, to reduce distortion, and to increase the signal-to-noise ratio of the generated sounds. In wavetable data interpolation, at the beginning of each sound's or voice's processing, two data samples, S
1
and S
2
, are read from wavetable data. See FIG.
121
. The wavetable address contains an integer and a fractional portion. The integer portion addresses S
1
data and is incremented by 1 to address S
2
data. The fractional portion indicates the distance from S
1
towards S
2
to interpolate and generate an interpolated sample, S. The address for S is designated by the complete (integer and fractional portions) and current wavetable address. The equation for obtaining the interpolated sample S is:
S=S
1
+(
S
2
−
S
1
).
T
[
where T
[
is the distance from S
1
, towards S
2
, to S. Through each interpolation, an additional data sample (S) can be created from two data samples (S
1
and S
2
) stored in wavetable memory. Thus, a particular generated sound can be made up of both wavetable data and interpolated data, and thus, the sound will comprise more data than is stored in wavetable memory for this sound. Wavetable synthesizers generate a certain number of voices or sounds at a particular sample rate. The sample rate affects the audio quality of the generated sounds, with slower sample rates degrading audio quality. Since the highest frequency that can be perceived by normal human hearing is 20 KHz, a sampling rate of 44.1 KHz is adequate. 44.1 KHz is the sample rate used by modern CD players. A prior art wavetable synthesizer in a sound card offered by Ultrasound, which is discussed in more detail below, requires a trade off between the number of voices that can be generated at a particular sample rate and the maximum available sample rate. For example, the prior art Ultrasound synthesizer can only generate up to 14 active voices at a 44.1 KHz sample rate but can generate a maximum of 32 voices at a less desirable 19.4 KHz sample rate.
Notes generated by music instruments have a characteristic “envelope” that generally contains attack, decay, sustain, and release segments.
FIG. 122
illustrates an example of an envelope with these segments. The data representing the envelope of sound to be generated can be stored in digitized format in a wavetable. Thus, wavetable synthesizers can generate the envelope along with the sound waveform. However, since the additional envelope data may put a strain on memory resources, wavetable synthesizers have been developed with separate envelope generation capabilities. A wavetable synthesizer can generate an envelope by multiplying volume components with the generated
Norris David
Suggs David N.
Adams Michael P.
Advanced Micro Devices , Inc.
Campbell III Sam G.
Isen Forester W.
Mei Xu
LandOfFree
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