Audio transient suppression circuits and methods

Details Audio transient suppression circuits and methods Audio transient suppression circuits and methods

2002-05-16

2003-07-29

Choe, Henry (Department: 2817)

Amplifiers

Combined with automatic amplifier disabling switch means

C330S149000

Reexamination Certificate

active

06600365

ABSTRACT:

FIELD OF THE INVENTION
This invention is generally concerned with audio transient suppression circuits and methods and is particularly applicable to the suppression of an audio transient when an amplifier is switched between an active and a standby mode.
BACKGROUND TO THE INVENTION
When a piece of audio equipment, such as a power amplifier, is switched on a transient signal often appears at the output. This can cause switch-on “thump”, which projects an unsophisticated impression and, in a power amplifier, can damage loudspeakers. A conventional way of reducing such a power-on thump is to temporarily disconnect the amplifier output during switch-on, for example using a relay. However this basic solution is often unsatisfactory, for example where the output load is ground referenced and ac coupled. Moreover it is commonplace in modern audio equipment design for amplifiers and the like to be switched in and out of a standby condition in which internal components are only partially powered, and improved techniques are needed for such designs.
Current audio systems tend to be constructed using integrated silicon technology rather than discrete components. Typically a single-supply, as opposed to a split-supply, is used to reduce power supply cost and to reduce the supply voltage range. This facilitates use of the latest silicon technology in audio integrated circuits of an audio system. In this context single-supply refers to a power supply with a single output (and another, say ground, connection) rather than to a power supply with, for example, positive and negative supply voltages (referenced to ground). In a single-supply arrangement a load is typically ground-referenced whereas an audio signal will typically deviate from a quiescent value midway between ground and the single supply and thus, to avoid a quiescent dc output from the audio system ac coupling (i.e. a capacitor) is generally employed. However, as will be seen in more detail below, this introduces difficulties associated with charging and discharging of the ac coupling capacitor.
The above-described systems are often intelligent enough to have multiple modes, and typically a standby mode is provided for power management. A general problem with such systems, however, is the presence of audible clicks when switching between modes, caused by transient signals on the audio output. Although these clicks may only have a relatively low volume they are nevertheless aesthetically undesirable and distracting to a user. Likewise audio systems usually have at least two channels, to provide a stereo output, and often have several channels. In such systems it is not uncommon for crosstalk to result in switching in one channel causing clicking in another.
It is therefore desirable to be able to minimise audio output transients, particularly when switching between standby and active modes of an audio system such as an audio amplifier. It is further desirable to reduce the risk of crosstalk between different channels of a multiple-channel system.
WO 98/45938 describes an audio transient suppression device in which an FET switch with a variable resistance is used to couple a biassing voltage equal to a “common mode voltage” to an output node. However a difficulty with this arrangement is the need for a common mode bias voltage source with a relatively low output impedance, as will be described in more detail below.
U.S. Pat. No. 5,515,431 (EP 0 642 247 A) describes a speakerphone for a telephone circuit which employs control circuitry monitoring an output node to detect when charging of an ac coupling capacitor is “complete”. However the additional control circuitry described in this document is relatively complex and its implementation would be expensive. Background prior art can be found in U.S. Pat. Nos. 5,805,020, 6,346,854, 4,410,855, 4,054,845 and KR 9,403,349.
Consider now the circuit of
FIG. 1
, which shows a digital audio system
100
with active and standby modes. Two switches S
1
126
and S
2
134
are provided for switching between the active and standby modes. During normal operation (“active” mode) S
1
126
is closed and S
2
134
is open; in standby mode S
1
126
is open and S
2
134
is closed.
The audio system
100
has two supplies, Vdd
102
, a “positive” supply, and Vss
104
, a “negative” supply, in a practical implementation these typically being provided from a single-ended supply comprising, for example, a positive voltage and ground. A potential divider is formed by resistors R
4
106
and R
5
108
which are coupled between Vdd
102
and Vss
104
to provide, at node
112
, a voltage which, where R
4
is equal to R
5
, is a mid-rail voltage Vmid=(Vdd+Vss)/2. Voltage Vmid
112
is decoupled by capacitor C
2
110
to remove Vdd supply ripple and noise.
Voltages Vmid
112
and Vss
104
provide voltage references to a digital-to-analogue converter (DAC)
114
, which has a digital input
116
and which provides an analogue output Vdac
118
. The analogue output is adjustable under control of digital input
116
between Vmid
112
and Vss
104
and, with a digital audio signal, the output swings between maxima of Vmid and Vss and has a quiescent voltage halfway between these two voltages. It will be appreciated that for an audio signal the quiescent voltage with respect to Vss is determined by the reference (or supply) voltages to DAC
114
.
In active mode, switch S
1
126
is closed and operational amplifier A
1
120
is then configured as a non-inverting amplifier (in this example) with resistors R
1
122
and R
2
124
providing feedback from output node X
128
. It will be noted in the circuit of
FIG. 1
that the feedback around operational amplifier
120
is referenced to Vss
104
rather than to Vmid
112
and thus the quiescent dc level output from DAC
114
with respect to Vss
104
is multiplied by (1+R
1
/R
2
), as well as the audio signal. In the case of equal-valued R
1
and R
2
the non-inverting amplifier has a gain of two. When input voltage Vdac at
118
is equal to Vss, output voltage at X
128
is also Vss. When input voltage Vdac at
118
is equal to Vmid, output voltage at X
128
is Vss+2*(Vmid−Vss)=Vdd. When input voltage Vdac at
118
is at the quiescent value (Vss+Vmid)/2, output voltage at X
128
is Vss+2*({Vmid+Vss}/2−Vss)=Vmid. Thus the output signal at X
128
swings between Vss and Vdd, with a quiescent voltage of Vmid.
Where all or part of the audio system
100
is fabricated on an integrated circuit, R
1
and R
2
, and R
4
and R
5
, may be closely spaced and physically similar to provide closely matching component values. In this way the quiescent and standby voltages at output node
128
can be controlled to be close to Vmid despite any manufacturing tolerances in the absolute value of resistors R
1
, R
2
, R
4
and R
5
.
The signal at output node X
128
is ac coupled by a capacitor C
1
130
to drive the ac output signal into an external load R
3
132
. Load
132
has its non-driven end connected to an external ground
106
, typically at Vss potential.
In the above-described audio DAC system
100
as previously mentioned, typically Vss is ground and Vdd is provided from a single positive supply. The circuit then provides a maximum audio output swing of ground to the supply voltage from a well-decoupled reference Vmid. It will be appreciated for the purposes of the following discussion of the manner in which audio clicks arise and may be suppressed, that the principles may be applied to any audio system and are not restricted to the particular illustrated example in which an audio signal happens to be generated by a digital-to-analogue converter.
As shown in
FIG. 1
the audio system has a standby mode in which much of the system may be powered down, for example during temporary absence of a signal. Moreover an audio system employing a DAC subsystem as shown in
FIG. 1
may often have multiple channels, not all of which need to be active all of the time. Switches S
1
126
and S
2
134
implement such a “standby” mod

Affiliated with

Also associated with

Rating

Audio transient suppression circuits and methods does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Audio transient suppression circuits and methods, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Audio transient suppression circuits and methods will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3049948