"There's always encountered in microphone and PA system is not expected in a wide variety of sounds such as echo, howling, background noise, sound current, of which the greatest harm to the howling sound system.
Howling sound signal i.e. self-oscillation, which produces a schematic diagram shown in Figure 1:
1 a schematic view of FIG generating howling
Sound speaker sound signal reflected by an obstacle then played superimposed on the microphone, the microphone will be reflected back through the speakers to play out, this cycle will produce superposition shrill screams, which is caused by positive feedback sound signal self-oscillation is observed with an oscilloscope, the waveform of the call is "stable frequency, amplitude stable sine wave", shown in Figure 2, which has two characteristics: 1, frequency stability, frequency and PA system environment in which , generally in the [1kHz, 5 kHz] range; 2, amplitude stability, the amplitude of the microphone and speaker will narrow as the distance increases. And the closer the microphone and a speaker, howling occurs more easily, the larger the amplitude of the howling.
Figure 2 waveforms howling
When the self-excited oscillation, the power amplifier will have a great power output may exceed the tolerance range of sound reinforcement equipment, sound generating device and the power amplifier burn.
At present, the mainstream of the howling detection and suppression pathway through the mixer, equalizer and after frequency shifter manually by a professional tuner gradually increase the volume came howling point, and then eliminated by find the equalizer. If the system is a two-channel sound system, it is necessary to close a channel tuner, howling point adjusting another channel; good tune a channel, this channel is closed, in the same manner to adjust the other channel; both sides after a good tune, you need two channels at the same time push it up and check whether there are other howling point, if you still eliminated by the equalizer. The traditional approach is cumbersome, not intelligent, but also very skilful.
Today and share a smart anti-howling circuit, although the howling suppression effect is not good professional approach to tune out the effect of (temporary debugging results from the author's point of view it is the case, but in theory should be able to do better results howling suppression requirements), but sufficient for general use.
Before describing the details of the start of the design, simply give an account of this mapping tool because it is a little special, there may be some small partners have not used. The use of tools is EasyEDA (unknown so small partner can own Baidu EasyEDA), it is a bit like the old hardware engineer PDM system, we can upload the design up, you can view them online design, to achieve a good hardware open source design, easy to see, is useful only to make use of them. But it is much more than upload and view drawings so simple, it also supports online design, see other people's designs have use value can be copied directly to a personal space for secondary design. Also, every design picture circle of friends just like in the above, we can comment on it thumbs up and encounter did not understand this design, you can also direct message consulting designer, it greatly enhanced the effect of open source , not just given design, also allows users to quickly use, a bit like the documentation of open source software.
Took quite a long time, personally I feel EasyEDA support for open source hardware design is very good, and Github somewhat similar, but focuses on open source hardware design, but personally feel a lot more than Github close to the people. Github is focused on open source software, there are very powerful version management functions, while focusing on online EasyEDA hardware design, and also has good project management and version management functions.
When described below, there may be some lack of clarity drawings, there is little interest partners can search for keywords can oscillation_suppresion in EasyEDA official website. Specific projects linked below:
https://EasyEDA.com/Alexis_Sanchez/osillation_suppresion-a45c7ce3e7fc44f58dd3b15258cfa780
The whole project as follows:
3 Engineering Management
Howling detection principle of this system is that: when the occurrence of howling, the sound signal is shaped by the comparator to 50% duty cycle square wave, the MCU within a fixed time its count of frequency of the square wave can be obtained; the same AD637 signal amplitude obtained by the peak detection circuit. To determine whether the frequency and amplitude stabilized and measure the "stability" as the concept of quantization: MCU 20 samples for each frequency, then find the maximum value of the 20 frequencies fh value, minimum value, and rounding fl fMode after the mode, i.e., in measurements of 20 times, the frequency of the sound signal is within [fl, fh] range. If fh - fl
