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发表于 2008-1-12 11:16:00
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INTRODUCTION
When powering up DACs and other audio ICs, it is common for settling DC levels within the device to
produce audible clicks or pops at the outputs. In more complex devices, these can often be
eliminated through the use of power up sequences or register writes to the device during power-up.
Sometimes though, the best solution is to use an external mute circuit at the output of the product.
IDENTIFYING POPS USING AN OSCILLOSCOPE
Often, the first step in diagnosing the cause of an audible pop is to try to capture the event using an
oscilloscope. It is useful here to have some idea of the voltage swing expected on the scope trace.
Otherwise, it is easy to accidentally measure events other than the actual pop.
For example, on power up, many audio chips will produce voltages at their output relating to two
events. The first will be a high frequency burst (outside the range of human hearing) and the second
will be a sudden DC step as the internal DC levels settle. It is the second of these events which is
usually audible. However, an oscilloscope will often trigger on the first.
To help asses whether the pop being heard is also the pop being displayed on the scope, the
following guidelines can be followed:
1. Does the event measured on the scope appear as a wave that swings around GND? If so,
this is likely to be oscillation as the device powers up. In general, if there is equal energy
above and below GND, the oscillations can be treated as a periodic waveform. If the
frequency is above 20KHz, it will not be audible. If a pop can still be heard, then it is likely
that it occurs after this initial event.
2. If the event sounds more like a click then is likely to be composed mainly of high frequency
energy and is probably a very short transient event (like a voltage spike or very short
pulse). This can occur when output transistors within audio devices are biased at power
on.
3. If the event sounds more like a pop, it is likely to include greater low frequency energy.
Typically, the largest pops are produced by a sudden change in DC level at the output of a
device (these can occur as VMID is ramped up at power on). After passing through an ACcoupling
capacitor these pops appear as a relatively large pulse with an exponential decay.
Their amplitude and longer duration mean that they will produce significant cone
movement in the output loudspeaker or headphone.
Once the pop is captured on the scope trace, it is easier to identify the cause. In many cases this will
allow the prevention of the pop by changing the power up sequence of the device. In those cases
where this is not possible though, another solution is to use a Mute circuit on the output of the
product. The next section will describe a tested solution that uses the minimum number of easily
available parts.
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SIMPLE LOGIC DRIVEN MUTE CIRCUIT
In more complex ICs, clicks or pops can be produced at times other than initial power up (for
example, applying clocks to the device or bring the device out of a sleep mode). For this reason, the
external mute circuit below is designed to be activated via a digital control line or GPIO from the
application’s DSP or microcontroller. Such control allows flexible software operation of the mute
circuit whenever required.
The circuit is designed to provide good levels of signal attenuation using commonly available
transistors. The circuit can be triggered using a range of input logic levels (from 1.8V to 5V) without
modification and can run on dual rail supplies from ±8V to ±12V. Figure 2 and Figure 3 show how
minor changes to resistor values will allow operation from dual rail supplies as low as ±3.3V.
Figure 1 12V Dual Rail Mute Circuit
The circuit shown in Figure 1 will allow a logic level Mute signal from a DSP or microcontroller to
significantly attenuate the output of a line level consumer audio product. The level of attenuation is
determined by the output transistors (Q3 and Q4). Standard transistors like the 2N3904 will offer
around 35dB of attenuation. Using higher performance devices, with a high Vce Saturation
specification, will improve this further. For example, tests with Zetex FMM617 devices have shown
65dB of attenuation.
This attenuation figure can be further improved by increasing the value of R7 and R8. For example,
increasing these to 200Ω will give a further 6dB of attenuation.
The following two circuits in Figure 2 and Figure 3 show slight modifications to allow similar
performance with different supply rails. As a guideline, the circuit in Figure 1 can be used with dual
rail supplies from ±12V to ±8V, the circuit in Figure 2 can be used with dual rail supplies from ±8V to
±5V and the circuit in Figure 3 can be used with dual rail supplies from ±5V to ±3.3V.
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Figure 2 8V Dual Rail Mute Circuit
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Figure 3 5V Dual Rail Mute Circuit
EXTENDING CIRCUIT FOR MULTI-CHANNEL USE
The circuits above can be easily extended for use in multi-channel applications (6 or 8 channel audio
for home cinema applications for example). The only section of the circuit that needs to be repeated
are the output transistors (Q3 and Q4). Additional transistors should be similarly connected between
the audio signal and ground with the base of the transistor connected to the same point between R5
and R6 as Q3 and Q4. Driving an additional 6 output transistors from this point will represent no
problems for the circuit.
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APPLICATION SUPPORT
If you require more information or require technical support please contact Wolfson Microelectronics
Applications group through the following channels:
Email: apps@wolfsonmicro.com
Telephone: +44 (0)131 272 7070
Fax: +44 (0)131 272 7001
Mail: Applications at the address on last page.
or contact your local Wolfson representative.
Additional information may be made available from time to time on our web site at
http://www.wolfsonmicro.com
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