Pulse width modulation speaker amplifier

• US 5,398,003 A
• Filed: 03/30/1994
• Issued: 03/14/1995
• Est. Priority Date: 03/30/1994
• Status: Expired due to Term

• Current Assignee: Apple Computer Incorporated (Apple Inc.)
• Original Assignee: Apple Computer Incorporated (Apple Inc.)
• Inventors: Heyl, Lawrence F., Austin, Steven E.
• Primary Examiner(s): Mullins, James B.
• Application Number:
  US08/219,956
• Time in Patent Office: 349 Days
• Field of Search: 330/10, 330/146, 330/207 A, 330/251
• US Class Current: 330/10
• CPC Class Codes: H03F 3/2173 of the bridge type

Abstract

An open-loop speaker amplifier and a dynamic range reduction circuit are disclosed. The speaker amplifier includes a bridge driver and a bridge circuit. The bridge circuit includes a first set of switches and a second set of switches. When the first set of switches are activated, a positive current flows through a speaker load. When the second set of switches are activated, a negative current flows through the speaker load. The bridge driver generates a pulse width modulated signal and a delayed pulse width modulated signal to drive the first and second set of switches of the bridge circuit responsive to a modulation signal. The modulation signal may be, for example, a plurality of pulse code modulated samples representing sound. The dynamic range reduction circuit modifies a selected signal to reduce the dynamic range of the sound reproduced based on the modulation signal. The selected signal is either the modulation signal or a reference signal. In one embodiment, the dynamic range reduction circuit modifies the selected signal by processing the selected signal based on a smooth saturating function. In another embodiment, the dynamic range reduction circuit modifies the amplitude of the reference signal based on an envelope size of the modulation signal.

Claims

1. An open-loop amplifier for driving a speaker in a computer system, the open-loop amplifier comprising:

a plurality of switches including a first set of switches and a second set of switches, the first set of switches being connected to the speaker and to a power source, the first set of switches being configured to provide a positive current flow through the speaker when the first set of switches is activated, the second set of switches being connected to the speaker and the power source, the second set of switches being configured to provide a negative current flow through the speaker when the second set of switches is activated;

a bridge driver operatively connected with the plurality of switches, the bridge driver being configured to receive a plurality of pulse code modulated samples representative of sound sampled at a sampling rate, and to activate and deactivate the plurality of switches in response to the plurality of pulse code modulated samples;

wherein the bridge driver includes;

a pulse width modulation generation unit configured to receive a square wave signal and the plurality of pulse code modulated samples and to generate a pulse width modulated signal based on the square wave signal and the plurality of pulse code modulated samples; and

a break-before-make unit connected with the pulse width modulation generation unit, the break-before-make unit receiving the pulse width modulated signal from the pulse width modulation generation unit and generating a delayed pulse width modulated signal, the delayed pulse width modulated signal being the pulse width modulated signal with a predetermined delay;

the first set of switches being deactivated based upon the pulse width modulated signal to cause the positive current flow through the speaker to cease prior to the second set of switches being activated based upon the delayed pulse width modulated signal to cause the negative current flow through the speaker to begin.

2. The open-loop amplifier of claim 1 wherein the bridge driver further includes a square wave generator connected to the pulse width modulation generation unit, the square wave generator generating the square wave signal and transmitting the square wave signal to the pulse width modulation generation unit.

3. An open-loop amplifier for driving a speaker in a computer system, the open-loop amplifier comprising:

a plurality of switches including a first set of switches and a second set of switches, the first set of switches being connected to the speaker and to a power source, the first set of switches being configured to provide a positive current flow through the speaker when the first set of switches is activated, the second set of switches being connected to the speaker and the power source, the second set of switches being configured to provide a negative current flow through the speaker when the second set of switches is activated;

a bridge driver operatively connected with the plurality of switches, the bridge driver being configured to receive a plurality of pulse code modulated samples representative of sound sampled at a sampling rate, and to activate and deactivate the plurality of switches in response to the plurality of pulse code modulated samples;

wherein the bridge driver includes a pulse width modulation generation unit configured to receive a square wave signal and the plurality of pulse code modulated samples and to generate a pulse width modulated signal based on the square wave signal and the plurality of pulse code modulated samples;

wherein the pulse width modulation generation unit includes;

a programmable counter for receiving the plurality of pulse code modulated samples, for storing a most recently received pulse code modulated sample, and for modifying by a predetermined fixed amount the most recently received pulse code modulated sample in response to pulses of a pulse width modulation clock signal;

a comparator connected to the programmable counter for comparing the most recently received pulse code modulated sample to a predetermined value, the comparator generating an indicator signal indicative of when the most recently received pulse code modulated sample has a predetermined relationship relative to the predetermined value;

a modulator signal generator connected to the comparator, the modulator signal generator receiving the indicator signal and the square wave signal, the modulator signal generator generating a modulator signal responsive to the square wave signal and the indicator signal, the modulator signal being the square wave signal delayed for a duration responsive to the indicator signal;

a latch connected to the modulator signal generator, the latch receiving the modulator signal and the square wave signal and generating the pulse width modulated signal responsive to the modulator signal and the square wave signal.

4. The open-loop amplifier of claim 3 wherein the pulse width modulation generation unit further includes a pulse width modulation clock connected with the programmable counter, the pulse width modulation clock generating the pulse width modulation clock signal and transmitting the pulse width modulation clock signal to the programmable counter.

5. The open-loop amplifier of claim 4 wherein the pulse width modulation clock generates the pulse width modulation clock signal at a rate equal to the sampling rate divided by a maximum value representable by any given pulse code modulated sample of the plurality of pulse code modulated samples.

6. The open-loop amplifier of claim 3 wherein the latch has a clear input, a preset input, and an output, the clear input being connected to the modulator signal generator for receiving the modulator signal, the preset input receiving the square wave signal, the latch generating the pulse width modulated signal at its output responsive to the modulator signal and the square wave signal.

7. The open-loop amplifier of claim 3 wherein the programmable counter decrements the most recently received pulse code modulated sample in response to a pulse width modulation clock signal.

8. The open-loop amplifier of claim 3 wherein the programmable counter increments the most recently received pulse code modulated sample in response to a pulse width modulation clock signal.

9. An open-loop amplifier for driving a speaker in a computer system, the open-loop amplifier comprising:

a plurality of switches including a first set of switches and a second set of switches, the first set of switches being connected to the speaker and to a power source, the first set of switches being configured to provide a positive current flow through the speaker when the first set of switches is activated, the second set of switches being connected to the speaker and the power source, the second set of switches being configured to provide a negative current flow through the speaker when the second set of switches is activated;

a bridge driver operatively connected with the plurality of switches, the bridge driver being configured to receive a plurality of pulse code modulated samples representative of sound sampled at a sampling rate, and to activate and deactivate the plurality of switches in response to the plurality of pulse code modulated samples;

wherein the bridge driver includes;

a digital-to-analog converter, the digital-to-analog converter receiving said plurality of pulse code modulated samples and converting the plurality of pulse code modulated samples to an analog sound signal; and

comparing means for performing a comparison of the analog sound signal to a reference signal, the bridge driver activating and deactivating the plurality of switches in response to the comparison.

10. The open-loop amplifier of claim 9 wherein the reference signal is a triangular wave signal.

11. The open-loop amplifier of claim 9 wherein the bridge driver further includes:

a first voltage splitter for receiving the reference signal and for generating a positively biased reference signal and a negatively biased reference signal;

a second voltage splitter connected to the digital-to-analog converter for receiving the analog sound signal and generating a positively biased sound signal and a negatively biased sound signal; and

an inverter for receiving the reference signal and for generating an inverted reference signal.

12. The open-loop amplifier of claim 11 wherein the comparing means comprises a plurality of comparators, including a first, second, third and fourth comparators, the first comparator receiving the negatively biased sound signal and the inverted reference signal to generate a first control signal, the second comparator receiving the negatively biased reference signal and the analog sound signal to produce a second control signal, the third comparator receiving the positively biased sound signal and the inverted reference signal to produce a third control signal, and the fourth comparator receiving the positively biased reference signal and the analog sound signal to produce a fourth control signal.

13. The open-loop amplifier of claim 12 wherein the first set of switches are activated in response to the first control signal and the fourth control signal, the second set of switches being activated in response to the second control signal and the third control signal.

14. A method for driving a speaker in a computer system, the method comprising the steps of:

providing a positive current flow through a speaker when a first set of switches of a plurality of switches is activated,

providing a negative current flow through the speaker when a second set of switches of said plurality of switches is activated;

causing a bridge driver operatively connected with the plurality of switches to receive a plurality of pulse code modulated samples representative of sound sampled at a sampling rate, and

causing said bridge driver to activate and deactivate the plurality of switches in response to the plurality of pulse code modulated samples

wherein said bridge driver includes a break-before-make unit and a pulse width modulation generation unit,

the step of causing said bridge driver to receive said plurality of pulse code modulated samples including the step of causing the pulse width modulation generation unit to receive the plurality of pulse code modulated samples;

the step of causing said bridge driver to activate and deactivate the plurality of switches in response to the plurality of pulse code modulated samples including the steps of;

causing the pulse width modulation generation unit to receive a square wave signal;

causing said pulse width modulation generation unit to generate a pulse width modulated signal based on the square wave signal and the plurality of pulse code modulated samples;

causing the break-before-make unit to receive the pulse width modulated signal from the pulse width modulation generation unit;

causing said break-before-make unit to generate a delayed pulse width modulated signal, the delayed pulse width modulated signal being the pulse width modulated signal with a predetermined delay;

causing said bridge driver to deactivate the first set of switches based upon the pulse width modulated signal to cause the positive current flow through the speaker to cease prior to activating the second set of switches, and

causing said bridge driver to activate the second set of switches based upon the delayed pulse width modulated signal to begin the negative current flow through the speaker after the first set of switches have been deactivated.

15. A method for driving a speaker in a computer system, the method comprising the steps of:

providing a positive current flow through a speaker when a first set of switches of a plurality of switches is activated,

providing a negative current flow through the speaker when a second set of switches of said plurality of switches is activated;

causing a bridge driver operatively connected with the plurality of switches to receive a plurality of pulse code modulated samples representative of sound sampled at a sampling rate, and

causing said bridge driver to activate and deactivate the plurality of switches in response to the plurality of pulse code modulated samples;

wherein said bridge driver includes a pulse width modulation generation unit;

the step of causing said bridge driver to activate and deactivate the plurality of switches in response to the plurality of pulse code modulated samples including the steps of;

causing the pulse width modulation generation unit to receive a square wave signal;

causing said pulse width modulation generation unit to generate a pulse width modulated signal based on the square wave signal and the plurality of pulse code modulated samples;

wherein said pulse width modulation generation unit includes a programmable counter, a modulator signal generator, a latch, and a comparator, the step of causing the pulse width modulation generation unit to receive the plurality of pulse code modulated samples including the step of causing said programmable counter to receive the plurality of pulse code modulated samples,

said step of causing said pulse width modulation generation unit to generate a pulse width modulated signal based on the square wave signal and the plurality of pulse code modulated samples including the steps of;

causing said programmable counter to store a most recently received pulse code modulated sample;

causing said programmable counter to modify by a predetermined fixed amount the most recently received pulse code modulated sample in response to pulses of a pulse width modulation clock signal;

causing said comparator to compare the most recently received pulse code modulated sample to a predetermined value;

causing the comparator to generate an indicator signal indicative of when the most recently received pulse code modulated sample has a predetermined relationship relative to the predetermined value;

causing the modulator signal generator to receive the indicator signal and the square wave signal;

causing the modulator signal generator to generate a modulator signal responsive to the square wave signal and the indicator signal, the modulator signal being the square wave signal delayed for a duration responsive to the indicator signal;

causing a set input on the latch to receive the modulator signal;

causing a preset input on the latch to receive the square wave signal; and

causing the latch to generate the pulse width modulated signal at its output responsive to the modulator signal and the square wave signal.

16. The method of claim 15 wherein the pulse width modulation generation unit further includes a pulse width modulation clock connected with the programmable counter, the step of causing said programmable counter to modify by a predetermined fixed amount the most recently received pulse code modulated sample in response to a pulse width modulation clock signal including the steps of:

causing the pulse width modulation clock to generate the pulse width modulation clock signal,

causing the pulse width modulation clock to transmit the pulse width modulation clock signal to the programmable counter.

17. A method for driving a speaker in a computer system, the method comprising the steps of:

providing a positive current flow through a speaker when a first set of switches of a plurality of switches is activated,

providing a negative current flow through the speaker when a second set of switches of said plurality of switches is activated;

causing a bridge driver operatively connected with the plurality of switches to receive a plurality of pulse code modulated samples representative of sound sampled at a sampling rate, and

causing said bridge driver to activate and deactivate the plurality of switches in response to the plurality of pulse code modulated samples;

wherein the bridge driver includes a digital-to-analog converter, the step of causing a bridge driver to receive a plurality of pulse code modulated samples comprising the steps of;

causing the digital-to-analog converter to receive said plurality of pulse code modulated samples;

causing the digital-to-analog converter to convert the plurality of pulse code modulated samples to an analog sound signal;

causing the digital-to-analog converter to transmit said analog sound signal to said bridge driver;

the step of causing said bridge driver to activate and deactivate the plurality of switches including the steps of;

performing a comparison between the analog sound signal to a reference signal;

causing the bridge driver to activate and deactivate the plurality of switches in response to the comparison.

18. The method of claim 17 wherein the step of performing a comparison between the analog sound signal and a reference signal includes the step of performing a comparison between the analog sound signal and a triangular wave signal.

19. The method of claim 17 wherein the step of causing the bridge driver to activate and deactivate the plurality of switches in response to the comparison includes the steps of:

generating a positively biased reference signal and a negatively biased reference signal;

generating a positively biased sound signal and a negatively biased sound signal;

generating an inverted reference signal;

generating a first control signal based on the negatively biased sound signal and the inverted reference signal;

generating a second control signal based on the negatively biased reference signal and the analog sound signal;

generating a third control signal based on the positively biased sound signal and the inverted reference signal; and

generating a fourth control signal based on the positively biased reference signal and the analog sound signal;

causing said bridge driver to activate the first set of switches in response to the first control signal and the fourth control signal; and

causing said bridge driver to activate the second set of switches in response to the second control signal and the third control signal.