Based on the speciality of transistor s non-linearity,using maths method,this article analyses the cause of whistle of frequency conversion circuit and provides method to eliminate.

It has solved the problems of howling in the sound field and the combing frequency response distortion caused by multi-point pickup.

The acoustic feedback even howling is a well-known problem in sound reinforcement system.

Aim at the problems of reverberation、 echo and howling etc of public-address system, the paper research these common solutions :parameters equalizer、 feedback canceller and frequency shifter at current public-address processors, and analyze the design of the most advanced products at home and abroad, finally put forward new application-oriented solutions.