DEVICE FOR PREVENTING POP NOISE IN AN AUDIO OUTPUT APPARATUS AND METHOD THEREFOR

Abstract

An audio output apparatus having an audio codec for generating audio signals delivered to a speaker is provided with a device for preventing an audio pop noise caused by the bias voltage output from the audio codec, which includes a capacitor connected between an output end of the audio codec and the speaker, a capacitor charger for smoothly charging the capacitor to a predetermined voltage level, a switch arranged between the junction of the output end of the audio codec and the capacitor and the capacitor charger, and a control unit for controlling the codec, wherein the control unit turns on the switch and drives the capacitor charger before activating the audio codec, and then turns off the switch and terminates the operation of the capacitor charger if a predetermined charging time has elapsed. A method for preventing the audio pop noise caused by a bias voltage output from the audio codec includes smoothly charging the capacitor to have the same voltage level as the bias voltage of the audio codec before activating the audio codec, and activating the audio codec.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawing in which:

FIG. 1 is a block diagram for illustrating the circuit of the conventional audio output apparatus;

FIG. 2 is a block diagram illustrating a circuit of an audio output apparatus for preventing the pop noise according to the present invention;

FIGS. 3A and 3B are graphs illustrating the charging time of a capacitor varying with the pulse width of the Pulse Width Modulation (PWM) pulse signals according to the present invention;

FIGS. 4A and 4B are graphs illustrating the charging time of a capacitor varying with the pulse density of the Pulse Density Modulation (PDM) pulse signals according to the present invention; and

FIG. 5 is a flowchart illustrating a process of the audio output apparatus preventing the pop noise according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

The inve audio signals delivered through a capacitor to a speaker, which smoothly charges the capacitor to have the same voltage level as the bias voltage of the audio codec before activating the audio codec, so that the voltage difference between the capacitor and the bias voltage of the audio codec occurring when activating the audio codec is substantially eliminated, thereby preventing the audio pop noise.

Referring to FIG. 2, the inventive device for preventing the pop noise in an audio output apparatus includes a capacitor charger 200, a control unit 201, an audio codec 210, a capacitor 220, a speaker 230, and a switch 240. The capacitor charger 200 is driven by the control unit 201 for charging the capacitor 220 through the switch 240. The capacitor charger 200 includes a pulse generator 202 and an integrating circuit 203.

The pulse generator 202 generates a predetermined pulse signal under the control of the control unit 201. The pulse generator 202 generates a pulse signal, which may be one of a Pulse Density Modulation (PDM) pulse signal and a Pulse Width Modulation (PWM) pulse signal. The integrating circuit 203 integrates the pulse signals from the pulse generator 202 for producing a capacitor charge voltage for charging the capacitor 220. The control unit 201 drives the capacitor charger 200 and turns on the switch 240 before activating the audio codec 210 for controlling all of the functions of the audio output apparatus so as to prevent the pop noise.

The audio codec 210 converts the digital audio data into the corresponding analog audio signals while adjusting and amplifying the magnitude of the analog audio signals. The audio signals outputted from the audio codec 210 are delivered to the speaker 230.

The switch 240 is connected between the junction of the output end of the audio codec 210 and the capacitor 220 and the integrating circuit 203, so that it turns on the charging path between the integrating circuit 203 and the capacitor 220, and turns off the charging path to prevent the charged capacitor 220 from being discharged to the integrating circuit 203.

However, it should be noted that where the capacitor 220 is rapidly charged to the equivalent level of the bias voltage of the audio codec 210 before activating the audio codec 210, the same pop noise caused by the bias voltage of the audio codec 210 may occur. In order to eliminate this problem, it is necessary to determine the charging time taken for smoothly charging the capacitor. This charging time may be determined by simulation in the course of designing the inventive audio output apparatus. The charging time also may vary with a pulse signal predetermined for smoothly charging the capacitor 220.

Hereinafter described in connection with FIGS. 3A and 3B is the charging time of the capacitor 220 varying with the pulse width of the Pulse Width Modulation (PWM) signal. In this case, FIG. 3A shows the pulse width of the PWM pulse signal rated 20%, and FIG. 3B shows the pulse width of the PWM pulse signal rated 50%.

Referring to FIG. 3A, if the pulse generator 202 generates the PWM pulse signals with the pulse width rated 20% applied to the integrating circuit 203, the time taken for charging the capacitor 220 to the bias voltage level is t_a as shown by the below graph. However, referring to FIG. 3B, if the pulse width is rated 50%, the charging time t_a becomes much shorter compared to the case of the pulse width rated 20%. Thus, the time t_a taken for charging the capacitor varies with the pulse width of the predetermined PWM pulse. Therefore, it is desired to properly adjust the pulse width to be in a range for preventing the pop noise caused by the abrupt charging of the capacitor 220 when there is a predetermined PWM pulse signal. Understandably, the predetermined pulse signal also serves to predetermine the charging time t_a.

Alternatively, the pulse signal generator 202 may be designed to generate the PDM pulse signal instead of the PWM pulse signal, as shown in FIGS. 4A and 4B. Hereinafter described in connection with FIGS. 4A and 4B is the charging time of the capacitor 220 varying with the pulse density, wherein FIG. 4A shows the case of low pulse density, and FIG. 4B shows the case of high pulse density. As shown in the drawings, the time t_a taken for charging the capacitor 220 to the bias voltage level varies with the pulse density. Therefore, it is also desired, as in the case of the PWM pulse, to properly adjust the pulse density to be in the range for preventing the pop noise caused by the abrupt charging of the capacitor 220 when there is a predetermined PDM pulse signal. Understandably, the pulse width of the PWM signal and the pulse density of the PDM signal may be arbitrarily changed considering the charging time within the range for preventing the pop noise.

FIG. 5 is a flowchart for illustrating the process of controlling the audio output apparatus for preventing the pop noise, wherein the pulse generator 202 is assumed to generate the PWM pulse signal. In step 500, the control unit 201 turns on the switch 240 before activating the audio codec 210, then proceeds to step 510 to activate the pulse generator 202.

The pulse generator 202 being activated in the step 510, it generates predetermined PWM pulse signals that are applied to the integrating circuit 203, which in turn integrates the pulse signals to produce the charging voltage delivered to the capacitor 220. Then the control unit 201 proceeds to step 520 to determine if a predetermined charging time has elapsed. Here, the predetermined charging time is the time taken for charging the capacitor 220 to the bias voltage level of the audio codec 210. More specifically describing in connection with FIGS. 3A and 3B, the predetermined time t_a varies with the predetermined PWM pulse signal.

If the predetermined time has not elapsed, the control unit 201 repeats the step 520. Then the predetermined time having elapsed, the control unit proceeds to step 530 to turn off the switch 240. The switch 240 being turned off, the charging path between the integrating circuit 203 and the capacitor 220 is cut off for preventing the capacitor 220 from being discharged towards the integrating circuit 203. Then the control unit 201 proceeds to step 540 to shut off the pulse generator 202, and then to step 550 to activate the audio codec 210.

As described above, the inventive device prevents the pop noise caused by the abrupt Direct Current (DC) changes upon activating the audio codec 210 by smoothly charging the capacitor 220 to the bias voltage level of the audio codec 210 prior to activating the audio codec. Namely, the pre-charged capacitor prevents the abrupt DC changes of the bias voltage, and thus the pop noise.

In the present invention, the pulse generator generates the pulse signals with the same value until being shut off, but the pulse signals may be arbitrarily changed to have different values provided that the abrupt rise of the charging voltage level does not induce the pop noise.

While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as described in the appended claims.

Claims

1. In an audio output apparatus provided with an audio codec for generating audio signals delivered to a speaker, a device for preventing an audio pop noise caused by a bias voltage output from audio codec, comprising: a capacitor connected between the output end of the audio codec and the speaker;a capacitor charger for smoothly charging the capacitor to a predetermined voltage level;a switch arranged between the junction of the output end of the audio codec and the capacitor and the capacitor charger; anda control unit for controlling the codec, wherein the control unit turns on the switch and drives the capacitor charger before activating the audio codec, and then turns off the switch and terminates the operation of the capacitor charger if a predetermined charging time has elapsed.

2. A device as defined in claim 1, wherein the predetermined charging time is the time to charge the capacitor to a level of the bias voltage of said audio codec.

3. A device as defined in claim 2, wherein the capacitor charger comprises: a pulse generator driven by the control unit for generating Pulse Width Modulation (PWM) pulse signals with a pulse width predetermined according to the predetermined charging time; andan integrating circuit connected between the pulse generator and the switch for integrating the PWM pulse signals so as to charge said capacitor.

4. A device as defined in claim 2, wherein the capacitor charger comprises: a pulse generator driven by the control unit for generating Pulse Density Modulation (PDM) pulse signals with a pulse density predetermined according to the predetermined charging time; andan integrating circuit connected between the pulse generator and said switch for integrating the PDM pulse signals so as to charge the capacitor.

5. In an audio output apparatus provided with an audio codec for generating audio signals delivered through a capacitor to a speaker, a method for preventing an audio pop noise caused by a bias voltage output from the audio codec, comprising the steps of: smoothly charging the capacitor to have the same voltage level as the bias voltage of the audio codec before activating the audio codec; andactivating the audio codec.