Electrical audio signal processing systems and devices – Noise or distortion suppression – Soft switching – muting – or noise gating
Reexamination Certificate
1998-02-24
2003-02-04
Isen, Forester W. (Department: 2644)
Electrical audio signal processing systems and devices
Noise or distortion suppression
Soft switching, muting, or noise gating
Reexamination Certificate
active
06516067
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to the field of anti-pop circuitry for electronic appliances including at least one loudspeaker.
BACKGROUND OF THE INVENTION
It is known in such appliances that during some operations of the appliance an undesired sound, known as a pop or click sound, may be heard, coming from the loudspeaker. Five operations can trigger said pop sound. They are “power on” of the circuit, or going from “on” or “off” to “standby”, or for instance in a television set “changing channel”, or “muting the sound”, or “switching off” the apparatus. Such a pop sound is caused in each of those five cases by a spurious signal which is created and introduced, when said operation are performed, at the input or output of an amplifier driving the loudspeaker. In each of said operation a power supply of the loudspeaker amplifier applies to said amplifier voltages that are changing quickly causing transient currents to flow in circuitry in an uncontrolled manner. In addition to possible damages to the loudspeaker such a pop sound is not agreeable to hear. It is the reason why, circuitry have been already designed in order to prevent any spurious signal reaching the amplifier input or output while any one of said five operations is performed. The purpose of said circuit is to control the level of a biasing signal of the amplifier and/or the level of the output, in such a way that such levels are set on or off with a steady and slow enough variation that will cause no rapid movement, of a membrane of the loudspeaker, to create an audible sound.
Examples of such attempts to reduce transient voltages at an output of an audio amplifier is shown in UK patent application n° GB 2 096 423 (D
1
). EP patent application n° EP 0562 414 A1 (D
2
) in the name of the applicant also shows a circuit to prevent pop sound coming from an intermediate frequency amplifier. Generally the steady and slow enough variation of the biasing or output voltage is got through an RC integrating network. The time constant of said network is great enough to prevent pop sound to occur. Such a solution is good but does not always prevent a transient arising. For instance, in D
1
a prior art solution is described and discussed in relation with
FIGS. 1 and 2
. It is explained that due to necessary differences in time constant of RC networks connected one to the inverting input, and the other to the non inverting input of an operational amplifier of sound, a sharp transient is appearing while the amplifier is turn on. Said voltage transient is reproduced as undesired sounds by a loudspeaker connected to the amplifier (page 2 column 1 lines 42-45). To cope with this situation, it is proposed in D
1
to add a control transistor
16
(see
FIG. 3
of D
1
). Said control transistor ensures that voltage at the output of a push-pull amplifier is increasing as the charge of a capacitor. Said kind of gate transistor does not always prevent any pop from being heard. For instance in D
2
it is explained that in spite of such a control transistor, a pop was still heard. To avoid said pop an anti-pop circuit has been provided at the level of an intermediate frequency amplifier. It is always difficult when conceiving a circuit to forecast all the transient currents that may appear and have an effect at the level of the sound amplifier. The present invention concerns an improvement of a circuit in which a pop sound was still heard when the apparatus was switched from on to off. As in most recent appliances, in preferred embodiments, the main functions of the apparatus are under control of a microprocessor. The micro-processor needs some time to actually detect the existing of a power interruption and to provide respective power interrupt subroutines to the different circuitry of the set. This point will be commented and explained here below.
The different voltages which are necessary for instance in a television set are produced, out of a main supply source by means of two different circuits. A first power supply circuit known in the art as switched mode power supply (SMPS) delivers two important outputs known as UB and UA supplies. UB supply is generally of 180 volts and is used for the horizontal deflection system of the cathode ray tube. UA supply is generally of about 22 volts and is used among others for the audio amplifier, for some integrated circuits processing video signal, and for producing a 5 volts supply for the microprocessor. More information about such power supply of television receivers is available for instance in two articles of VAN SCHAIK headed “An introduction to switched mode power supplies in television receivers” and “Control circuits for SMPS in TV receivers” published pages 93 to 108 in n° 3 of vol. 34 September 1976 and pages 162 to 180 of n° 4 of said volume of December 1976 of the review “Electronic applications bulletin” from PHILLIPS'. Said articles have also been published in n° 135 and 136 of the British review “Mullard technical communications” respectively of July and October 1977. The main reason of using the supply of the switched mode power supply to feed the microprocessor is because it is a supply which is available during stand-by time of the apparatus. The microprocessor is then still fed in order to be ready to execute orders coming for instance from a remote control of the apparatus. The second power supply is taken out of a fly-back transformer whose primary side receives a return pulse produced by an horizontal deflection coil of the cathode ray tube each time an electron beam of the tube is driven from an end of a line of the screen of the tube, to the beginning of next line. Due to this fact, voltage produced out of this second power supply is available only when the electrons beam of the tube is in scanning mode. The characteristics of all auxiliary power outputs produced out of the fly-back power supply compared to the supply produced out of the switched mode power supply are shorter discharging time and sharper roll off waveform. Said in an other way the switched mode power supply takes longer time to shut down its operation as compared to the fly-back supply. This results from the large capacitors of the switched mode power supply which are necessary to store energy to cope with sudden changes in the instant consumption and to provide the starting energy to start the apparatus when passing from stand-by to on.
An example of a known control circuit to mute the loudspeakers of an apparatus is shown in figures one and two attached to the present application. The control circuit is based on a microprocessor
1
. In
FIG. 1
only the features, and in particular inputs and outputs of microprocessor
1
, useful for the understanding of the generation of the pop sound have been represented. Microprocessor
1
has an input
2
to detect a power interruption signal, and a reset input
3
. The input
2
receives a 5 volts voltage through resistors
4
and
5
. Input
2
is also connected to an interrupt circuit that will pull down voltage at input
2
when the power supply is switched off. The microprocessor detects the change of level at pin
2
and then starts interrupt sub-routines to switch off various circuit of the apparatus. The interrupt circuit comprises substantially two transistors, a first
6
, a second
7
, and a zener diode
8
. The first and second transistors
6
,
7
have each a collector
9
,
10
respectively, an emitter
11
,
12
respectively and a base
13
,
14
respectively.
In normal operation 5 volts is supplied to pin
2
through resistors
4
and
5
, as explained above. The collector
9
of first transistor
6
which is coupled to pin
2
receives also 5 volts. The base
13
of first transistor
6
is at about the same potential as the emitter
11
of said first transistor
6
. Said emitter
11
is connected to earth and said base
13
is short-circuited to earth through second transistor
7
collector
10
emitter
12
path which is conducting. It results from this fact that the base emitter voltage of transistor
6
is not sufficient to
Grier Laura A.
Isen Forester W.
Shedd Robert D.
Thomson Licensing S.A.
Tripoli Joseph S.
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