Abstract: This paper presents a timbre model of computer musical instruments based on the trapezoid wave，in order to describe the timbre of the computer musical instruments，to construct the software musical instruments，and to make it possible that the instrument timbre can be controlled by modulating the parameters.This model includes two sub models，the vibration model and the amplitude envelope model.This paper makes the parameter values of the amplitude envelope invariable during analyzing the model，defines the timbre parameters，including the time coefficient，the unsymmetrical degree，the trapezoid degree and the triangle degree，and gets the frequency spectrums of the model in different parameters using one dimensional DCT in VC++ environment.This paper also finds the rules of the parameters acting on the instrument timbre，by analyzing how the timbre parameters influence on the frequency spectrums.The analysis shows that the base tone is always the strongest one in the frequency spectrums，and there are no even harmonic waves if it is an isosceles trapezoid.Furthermore，the high frequency components are rich when the unsymmetrical degrees away from 1，the trapezoid degrees，or the triangle degrees away from 0.25，are great；thus the instrument timbre is bright，contrarily the timbre is tedious.So it is important practical value and fundamental significance for the timbre composition of computer musical instruments and algorithmic musical composition.

摘要： 为了描述计算机乐器的音色，构建软件乐器，达到通过调节参数来改变计算机乐器音色的目的，建立了基于梯形波的计算机乐器音色模型。该模型包括振动子模型和振幅包络子模型。保持振幅包络参数取值不变。定义调节音色的参数，分别为时间系数、非对称度、梯形度和三角度。在VC++环境下，利用一维离散余弦变换获取该模型在不同参数下的频谱。通过分析音色参数对频谱的影响，找到这些参数对乐器音色的作用规律。分析表明，该模型的频谱中基音总是最强，等腰梯形时，没有偶次谐波；而且在非对称度偏离1越远，梯形度越大或者三角度偏离0.25越远时，频谱的高频成分就越多，音色越嘹亮；反之音色就越沉闷。对计算机乐器音色的合成和算法作曲具有一定的实用价值和基础意义。