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Jingfan Ke 2024-09-05 13:11:14 +08:00
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<h1><center>21-22年数字信号处理期末考试</center></h1>
## 一、填空题本大题17小题每个空格1分共30分
1. 判断系统$T[x(n)]=2x(n)+3$是否是线性系统(),是否为移不变系统(),是否为因果系统(),是否为稳定系统()。
答案:非线性、移不变、因果、稳定
2. 已知一个因果序列$x(n)$的$z$变换为$X(z)=\frac{0.4+z^{-1}}{1-1.1z^{-1}+0.3z^{-2}}$,求序列$x(n)$的初值$x(0)$为(),终值$x(\infty)$为()。
答案:$0.4$$0$
3. 一个实序列$x(n)$的傅里叶变换($\rm DTFT$)为$X(e^{j\omega})$,则$|X(e^{j\omega})|$为()(偶对称/奇对称),$\arg[X(e^{j\omega})]$为()(偶对称/奇对称)。
答案:偶对称,奇对称
4. 一个线性移不变系统的单位采样响应满足$h(n)=h(N-1-n)$$N$为偶数,采用窗函数设计法,可以设计()(低通、高通、带通、带阻)类型的滤波器。
答案:低通、带通
5. $x(n)=3^nu(n+1)$,则$X(z)=$(),收敛域为()。
答案:$\frac{1}{3}\times \frac{z^2}{z-3}$$|z|>3$
6. 一个$\rm IIR$数字滤波器,如果要灵活地调整该滤波器的某些极点,而不影响到其他极点和零点,可以采用()(典范型/级联型/并联型)形式的结构?
答案:级联型
7. 已知$x(n)=A\sin(\frac{1}{5}\pi n+\frac{\pi}{3})$,判断该序列是否是周期性的(),如果是周期性的,试确定其周期()。
答案:是,$10$
8. 设$x(n)=\{2,1,-1,-3;n=0,1,2,3\}$$x_2(n)=x((n-2))_6R_6(n)=$()。
答案:$\{0,0,2,1,-1,-3;n=0,1,2,3,4,5\}$
9. 采用冲激响应不变法将模拟低通滤波器映射变换为数字低通滤波器,其映射关系为$z=e^{sT}$,其中$T$为抽样周期,则$s$平面的虚轴映射到$z$平面的()。
答案:单位圆
10. 设连续时间信号$x_a(t)$的最高频率为$f_h\rm Hz$,则对信号$x_a(2t)$$x_a(t)*x_a(t)$进行采样能不失真恢复原信号的最小抽样频率分别为()$\rm Hz$和()$\rm Hz$。
答案:$4f_h$$2f_h$
11. 一个无限长序列$x(n)$的傅立叶变换$X(e^{j\omega})$是数字频率$\omega$的()。
A连续的周期函数
B离散的周期函数
C连续的非周期函数
D离散的非周期函数
答案B
12. 设有一谱分析用的信号处理器,抽样点数必须为$2$的整数次幂,要求频率分辨力$\le 5\rm{Hz}$,信号最高频率为$10\rm kHz$,试确定最小记录长度为();抽样周期最大间隔为();在一个记录中的点数为()。
答案:$0.2\rm s$$50\rm \mu s$$4096$
13. 已知一个线性相位$\rm FIR$数字滤波器的一个零点为:$2-2j$,则该系统其他零点为()()()。
答案:$2+2j$$\frac{1}{2-2j}$$\frac{1}{2+2j}$
14. 采用双线性变换法将模拟低通滤波器映射变换为数字低通滤波器,其数字频率$\omega$与模拟角频率$\Omega$是()(线性/非线性)关系。
答案:非线性
15. 一个线性移不变系统的差分方程如下所示,该系统是否具有严格的线性相位性质(),其群延时为()。
$y(n)=x(n)-4x(n-1)+2x(n-2)+5x(n-3)-2x(n-4)+4x(n-5)-x(n-6)$
答案3
16. 一个因果稳定的离散线性移不变系统,其单位抽样响应函数为$h(n)$,假设系统的输入为$x(n)$,则系统的输出$y(n)$为()。
答案:$y(n)=x(n)*h(n)$
17. 一个离散线性移不变系统稳定的充要条件是,该系统的单位抽样响应函数$h(n)$满足()。
答案:绝对可积,或$\sum_{n=-\infty}^{\infty}|h(n)|<\infty$
## 二、判断对错题本大题共10小题每小题1分共10分
1. 有限长单位冲激响应($\rm FIR$)滤波器的结构一定不存在反馈回路。
答案:错
2. 时间抽取($\rm DIT$)基-$2$ $\rm FFT$算法输入一定是倒位序,输出一定是自然顺序。
答案:错
3. 用矩形窗函数法进行$\rm FIR$数字滤波器设计时,一定会造成吉布斯效应。
答案:对
4. 最小相位延迟(滞后)系统的系统函数$H(z)$的极点可以在单位圆内,也可以在单位圆外。
答案:错
5. 序列$x(n)={0.5}^nu(n-1)$是因果序列。
答案:对
6. 采用窗函数设计法设计$\rm FIR$线性相位数字带通滤波器,其窗函数的长度$N$可以为奇数或者偶数。
答案:对
7. 全通系统的零点和极点是关于单位圆镜像对称的。
答案:对
8. 单位冲激响应$h(n)$为有限长序列的数字滤波器一定具有严格的线性相位特性。
答案:错
9. 两个有限长序列的线性卷积和圆周卷积一定是相等的。
答案:错
10. 在时域对连续信号进行抽样得到离散时间信号,其频谱是原连续时间信号的频谱以抽样角频率为周期的周期延拓叠加。
答案:对
## 三、证明题本大题共2小题每小题5分共10分
1. 系统$H(z)=\frac{1+z^{-1}+0.24z^{-2}}{1-1.3z^{-1}+0.4z^{-2}}$是一个最小相位延迟系统。
证明:
最小相位延迟系统的零极点均应该在单位圆内
$$
H(z)=\frac{1+z^{-1}+0.24z^{-2}}{1-1.3z^{-1}+0.4z^{-2}}=\frac{(1+0.6z^{-1})(1+0.4z^{-1})}{(1-0.5z^{-1})(1-0.8z^{-1})}
$$
极点:$z_1=0.5$$z_2=0.8$
零点:$z_1=-0.6$$z_2=-0.4$
均位于单位圆内,故该系统是一个最小相位延迟系统。
2. 若$Z[x(n)]=X(z),R_{x-}<|z|<R_{x+}$$a\ne0$可为实数复数则有
$$
Z[a^nx(n)]=X(\frac{z}{a}),|a|R_{x-}<|z|<|a|R_{x+}
$$
证明:
$$
Z[a^nx(n)]=\sum_{n=-\infty}^{\infty}a^nx(n)z^{-n}=\sum_{n=-\infty}^{\infty}x(n)(\frac{z}{a})^{-n}=X(\frac{z}{a})
$$
收敛域为:$R_{x-}<|\frac{z}{a}|<R_{x+}$$|a|R_{x-}<|z|<|a|R_{x+}$
## 四、计算题本大题共5小题每小题10分共50分
1. 已知确定序列$x(n)=\{2,1,-1,-2;n=0,1,2,3\}$$y(n)=\{1,-2,1,2;n=0,1,2,3\}$,试计算
1线性卷积$x(n)*y(n)$
2$4$点圆周卷积$x(n)④y(n)$
3$8$点圆周卷积$x(n)⑧y(n)$。
解:
14分求线性卷积
$$
x(n)*y(n)=\{2,-3,-1,5,5,-4,-4;n=0,1,2,3,4,5,6\}
$$
24分$4$点循环卷积
利用圆周卷积是线性卷积以$4$点为周期的周期延拓序列的主值序列,即$4$点圆周卷积
$$
x(n)④y(n)=\left\{\sum_{r=-\infty}^{\infty}y_l(n+4r)\right\}R_4(n)
$$
故有:$2+5=7$$-3+(-4)=-7$$-1+(-4)=-5$
$4$点圆周卷积$x(n)④y(n)=\{7,-7,-5,5;n=0,1,2,3\}$。
32分利用圆周卷积是线性卷积以$8$点为周期的周期延拓序列的主值序列,线性卷积长度为$7$,故$8$点圆周卷积的值等于线性卷积的值($7$点),再补一个$0$,即
$$
x(n)⑧y(n)=\{2,-3,-1,5,5,-4,-4,0;n=0,1,2,3,4,5,6,7\}
$$
2. 一个因果线性移不变系统为
$$
y(n)+1.1y(n-1)+0.3y(n-2)=x(n)-0.4x(n-1)
$$
1求该系统的系统函数$H(z)$,求出$H(z)$的零极点;
2指出系统的收敛域并判定系统的稳定性
3求该系统的单位抽样响应函数$h(n)$。
解:
13分对差分方程两端进行$z$变换,可得
$$
Y(z)+1.1z^{-1}Y(z)+0.3z^{-2}Y(z)=X(z)-0.4z^{-1}X(z)
$$
经整理后,得
$$
H(z)=\frac{Y(z)}{X(z)}=\frac{1-0.4z^{-1}}{1+1.1z^{-1}+0.3z^{-2}}=\frac{1-0.4z^{-1}}{(1+0.5z^{-1})(1+0.6z^{-1})}
$$
极点为$z_1=-0.5$$z_2=-0.6$;零点为$z_1=0.4$$z_2=0$
23分由于该系统是因果系统因此收敛域为$|z|>0.6$;所有极点都在单位圆内,所以该系统稳定。
34分将$H(z)$进行部分分式分解,即
$$
H(z)=\frac{-9}{1+0.5z^{-1}}+\frac{10}{1+0.6z^{-1}},|z|>0.6
$$
系统的单位抽样响应函数$h(n)$为:$h(n)=-9\times(0.5)^n u(n)+10\times (-0.6)^n u(n)$
3. 画出$4$点按时间抽取基$2$ $\rm FFT$算法的蝶形流图,并利用该蝶形流图求出序列$x(n)=\{1,2,1,-1;n=0,1,2,3\}$的$\rm DFT$值$X(k)$。
解:
1图5分未标出输入输出符号扣1-2分未标出系数和-1扣1-2分
<img src="2.4.3.1.1.drawio.svg" alt="2.4.3.1.1.drawio" />
2求值5分
$$
\begin{array}{ll}
X_1(0)=x(0)+W_{N}^{0}x(2)=x(0)+x(2)=0\\
X_1(1)=x(0)-W_{N}^{0}x(2)=x(0)-x(2)=2\\
X_2(0)=x(1)+W_{N}^{0}x(3)=x(1)+x(3)=3\\
X_2(1)=x(1)-W_{N}^{0}x(3)=x(1)-x(3)=1\\
X(0)=X_1(0)+W_{N}^{0}X_2(0)=X_1(0)+X_2(0)=0+3=3\\
X(1)=X_1(1)+W_{N}^{1}X_2(1)=X_1(1)-jX_2(1)=2-j\\
X(2)=X_1(0)-W_{N}^{0}X_2(0)=X_1(0)+X_2(0)=0-3=-3\\
X(3)=X_1(1)-W_{N}^{1}X_2(1)=X_1(1)+jX_2(1)=2+j
\end{array}
$$
故,该序列的$\rm DFT$为:$X(k)=\{3,2-j,-3,2+j;k=0,1,2,3\}$
4. 一个因果线性移不变系统为$y(n)=0.5y(n-1)-0.06y(n-2)+x(n)-x(n-1)$,画出该系统的级联结构图和并联结构图(以一阶基本阶表示)。
解:
对差分方程进行$z$变换,可得
$$
Y(z)=0.5z^{-1}Y(z)-0.06z^{-2}Y(z)+X(z)-z^{-1}X(z)
$$
经整理后,得
$$
H(z)=\frac{Y(z)}{X(z)}=\frac{1-z^{-1}}{1-0.5z^{-1}+0.06z^{-2}}=\frac{1-z^{-1}}{(1-0.2z^{-1})(1-0.3z^{-1})}
$$
故级联结构图为4分
<img src="2.4.4.1.drawio.svg" alt="2.4.4.1.drawio" style="zoom:80%;" />
对$H(z)$进行部分分式分解
$$
H(z)=\frac{8}{1-0.2z^{-1}}+\frac{-7}{1-0.3z^{-1}}
$$
故并联结构图为4分
<img src="2.4.4.2.drawio.svg" alt="2.4.4.2.drawio" style="zoom:80%;" />
5. 试用矩形窗函数设计一个线性相位$\rm FIR$数字相位高通滤波器,假设窗函数长度$N=17$,通带截止频率为$\omega_c$
1试求该线性相位$\rm FIR$数字高通滤波器的单位抽样响应$h(n)$
2该高通滤波器的群延时是多少
解:
17分理想高通滤波器的频率响应为
$$
H_d(e^{j\omega})=
\left\{
\begin{array}{ll}
e^{-j\omega\tau}, & \omega_c \leq |\omega| \leq \pi \\
0, & \text{else}
\end{array}
\right.
$$
则理想高通滤波器的单位抽样响应$h_d(n)$为:
$$
h_d(n)=\frac{1}{2\pi}\int_{-\pi}^{\pi}H_d(e^{j\omega}) e^{j\omega n}\mathrm{d}\omega=\frac{1}{2\pi}[\int_{-\pi}^{-\omega_c}e^{j\omega(n-\tau)}\mathrm{d}\omega+\int_{\omega_c}^{\pi}e^{j\omega(n-\tau)}\mathrm{d}\omega]\\
=
\left\{
\begin{array}{ll}
\frac{1}{\pi(n-\tau)}\{\sin[(n-\tau)\pi]-\sin[(n-\tau)\omega_c]\}, & n\ne\tau\\
\frac{1}{\pi}(\pi-\omega_c)=1-\frac{\omega_c}{\pi}, & n=\tau
\end{array}
\right.
$$
故单位抽样响应$h(n)$为:
$$
h_d(n)=\left\{
\begin{array}{ll}
\frac{1}{\pi(n-\tau)}\{\sin[(n-\tau)\pi]-\sin[(n-\tau)\omega_c]\}, & n\ne\tau\\
\frac{1}{\pi}(\pi-\omega_c)=1-\frac{\omega_c}{\pi}, & n=\tau
\end{array}
\right.
,N=0,1,2,\dots,17
$$
23分高通滤波器的群延时为
$$
\tau=\frac{N-1}{2}=\frac{17-1}{2}=8
$$

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<h1><center>22-23年数字信号处理期末考试</center></h1>
## 一、填空题本大题共18小题每个空格1分共30分)
1. 设某个离散时间系统为$y(n)=T[x(n)]=2x(n)+a\sin(2\pi(n+2))$,其中$a\ne0$为实数,问该系统是否是线性系统?(),是否移不变系统?(),是否因果系统?(),是否稳定系统?()。
答案:线性、移不变、因果、稳定
2. 已知一个因果序列$x(n)$的$z$变换为$X(z)=\frac{0.6+z^{-1}}{1-1.5z^{-1}-z^{-2}}$,求序列$x(n)$的初值$x(0)$为(),终值$x(\infty)$为()。
答案:$0.6$,不存在
3. 一个线性移不变系统的系统函数和该系统的逆系统的系统函数的乘积为()。
答案:$1$
4. 一个线性移不变系统的单位采样响应满足$h(n)=h(N-1-n)$$N$为偶数,采用窗函数设计法,可以设计()(低通/高通/带通/带阻)类型的滤波器。
答案:低通和带通
5. $x(n)=n2^{n}u(n)$,则$X(z)=$(),收敛域为()。
答案:$\frac{2z}{(z-2)^2}$$|z|>2$
6. 一个$\rm IIR$数字滤波器,如果要灵活地调整该滤波器的某些极点,而不影响到其他极点和零点,可以采用()(典范型/级联型/并联型)形式的结构?
答案:级联型或并联型
7. 知$x(n)=A\sin(\frac{2}{13}\pi n+\frac{\pi}{2})$,判断该序列是否是周期性的(),如果是周期性的,试确定其周期()。
答案:是,$13$
8. 设$x(n)=\{1,1,-1,-2;n=0,1,2,3\}$$x_2(n)=x((n+2))_6R_6(n)=$()。
答案:$\{-1,-2,0,0,1,1;n=0,1,2,3,4,5\}$
9. 以下哪种计算是错误的()。
A$W_N^{nk}=W_N^{n(k+N)}$
B$(W_N^{nk})^*=W_N^{-nk}$
C$W_N^{nk}=W_{mN}^{nmk}$
D$W_N^{nk}=W_N^{(n+m)k}$
答案D
10. 假设信号$x_{1}(t)$和$x_{2}(t)$的最高频率分别为$20\rm{kHz}$和$30\rm{kHz}$,对信号$x_{1}(t)\cdot x_{2}(t)$$x_{1}(t)* x_{2}(t)$进行理想抽样,其离散时间信号能不失真恢复原信号的最小抽样频率分别为()、()。
答案:$100\rm{kHz}$、$40\rm{kHz}$
11. 一个有限长序列$x(n)$的傅立叶变换$X(e^{j\omega})$是数字频率$\omega$的()。
A连续的周期函数
B离散的周期函数
C连续的非周期函数
D离散的非周期函数
答案A
12. 设有一谱分析用的信号处理器,抽样点数必须为$2$的整数次幂,要求频率分辨力$\le 10\rm{Hz}$,信号最高频率为$5\rm kHz$,试确定最小记录长度为();抽样周期最大间隔为();在一个记录中的点数为()。
答案:$0.1\rm s$$100\rm \mu s$$1024$
13. 已知一个线性相位$\rm FIR$数字滤波器的一个零点为:$1-2j$,则该系统其他零点为()()()。
答案:$1+2j$$\frac{1}{1-2j}$$\frac{1}{1+2j}$
14. 采用双线性变换法将模拟低通滤波器映射变换为数字低通滤波器,其数字频率$\omega$与模拟角频率$\Omega$是()(线性/非线性)关系。
答案:非线性
15. 一个线性移不变系统的差分方程如下所示,该系统是否具有严格的线性相位性质(),其群延时为()。
$y(n)=x(n)+3x(n-1)+2x(n-2)+4x(n-3)+2x(n-4)+3x(n-5)+x(n-6)$
答案:是,$3$
16. 一个离散时间信号$x(n)$输入到一个单位抽样响应为$h(n)$的线性移不变系统,系统输出信号$y(n)$的$z$变换$Y(z)$与$x(n)$的$z$变换$X(z)$和$h(n)$的$z$变换$H(z)$的关系为()。
答案:$Y(z)=X(z)H(z)$
17. 以下关于线性移不变数字滤波器的陈述,正确的是()。
A数字滤波器输出的频率成分必定比输入的频率成分少
B数字滤波器只能输出低频信号
C数字滤波器输出的频率成分可以多于输入的频率成分
D数字滤波器输出的频率成分可以与输入的频率成分一样多或者少于输入的频率成分。
答案D
18. 一个序列$x(n),0\le n \le N-1$,先将其补零到长度为$L$点($L>N$),问其频谱与原信号的频谱相比是否变化?()
答案:不变化,或者频谱包络没变化
## 二、判断对错题:(本大题共10小题每小题1分共10分)
1. 不管抽样频率的大小如何,将一个连续时间信号抽样变成一个离散时间信号,其离散时间信号的傅里叶变换不一定是一个周期信号。
答案:错
2. 从模拟滤波器设计数字滤波器的数字频域频带变换方法,其频带变换函数一定为一个全通函数。
答案:对
3. 用窗函数设计法设计$\rm FIR$线性相位数字滤波器时,改变窗函数的类型可以改变阻带最小衰减的大小。
答案:对
4. 基-$2$时间抽取法($\rm DIT$$\rm FFT$算法的效果与基-$2$频率抽取法($\rm DIF$$\rm FFT$算法的效果是一样的。
答案:对
5. 将一个连续时间序号抽样变成一个离散时间信号,其模拟角频率$\Omega$和数字频率$\omega$不一定是线性关系。
答案:错
6. 用窗函数设计法设计$\rm FIR$线性相位数字滤波器是在频域中进行设计的。
答案:错
7. 一个离散时间信号$x(n)$,当$n$不是整数时,$x(n)$没有任何意义。
答案:对
8. 一个线性时不变离散系统是稳定系统的充分必要条件是:系统函数$H(z)$的极点在圆内。
答案:错
9. 相位延迟(滞后)系统的系统函数的零点可以在单位圆内,也可以在单位圆外。
答案:对
10. 全通滤波器的系统函数的极点数目和零点数目不一定一样多。
答案:错
## 三、证明题本大题共2小题每小题5分共10分
1. 从模拟滤波器设计$\rm IIR$数字滤波器的变换方法之一是冲激响应不变法,其变换关系为$z=e^{sT}$,其中,$T$是抽样周期试证明该变换关系满足模拟滤波器变换到数字滤波器的二个基本条件1$s$平面的虚轴映射为$z$平面的单位圆2$s$平面的左半平面映射为$z$平面的单位圆内。
证明:
变换关系为$z=e^{sT}$,设:$z=re^{j\omega}$$s=\sigma+j\omega$
代入$z=e^{sT}$,有:$re^{j\omega}=e^{(\sigma+j\omega)T}=e^{\sigma}\cdot e^{j\Omega T}$$r=e^{\sigma T}$$\omega=\Omega T$
1当$\sigma=0$时,对应于$s$平面的虚轴,此时,$r=1$时,对应于$z$平面的单位圆;
2当$\sigma<0$对应于$s$平面的左半平面此时$r<1$对应于$z$平面的单位圆内
故该变换关系满足模拟滤波器变换到数字滤波器的二个基本条件。证毕。
2. 证明系统$H(z)=\frac{z^{-2}-0.1z^{-1}-0.2}{1-0.1z^{-1}-0.2z^{-2}}$是一个全通系统。
证明:
$$
H(z)=\frac{z^{-2}-0.1z^{-1}-0.2}{1-0.1z^{-1}-0.2z^{-2}}=\frac{(z^{-1}-0.5)(z^{-1}+0.4)}{(1-0.5z^{-1})(1+0.4z^{-1})}
$$
系统函数的极点为:$0.5$$-0.4$;系统函数的零点为:$2$$-2.5$
极点均在单位圆内,零点在单位圆外,且零点极点关于单位圆镜像对称,故该系统是一个全通系统。
## 四、计算题本大题共5小题每小题10分共50分
1. 一个稳定的线性移不变系统的差分方程描述为
$$
y(n)-0.5y(n-1)-0.06y(n-2)=x(n)-0.5x(n-1)
$$
1求系统函数$H(z)$,并求出其零极点;
2指出系统的收敛域并判定系统的因果性
3求系统的单位采样响应$h(n)$
4画出系统的级联结构图和并联结构图以一阶基本节表示
解:
12分对差分方程两端进行$z$变换,可得
$$
Y(z)-0.5z^{-1}Y(z)-0.06z^{-1}Y(z)=X(z)-0.5z^{-1}X(z)
$$
经整理后,得
$$
H(z)=\frac{Y(z)}{X(z)}=\frac{1-0.5z^{-1}}{1-0.5z^{-1}-0.06z^{-2}}=\frac{1-0.5z^{-1}}{(1+0.1z^{-1})(1-0.6z^{-1})}
$$
极点为$z_1=-0.1$$z_2=0.6$;零点为$z_1=0$$z_2=0.5$。
22分由于该系统是稳定系统因此收敛域包括单位圆故收敛域为$|z|>0.6$,所以该系统是因果系统。
33分$H(z)$进行部分分式分解后得
$$
H(z)=\frac{\frac{6}{7}}{1+0.1z^{-1}}+\frac{\frac{1}{7}}{1-0.6z^{-1}}
$$
因此可求得该系统的单位抽样响应为
$$
h(n)=\frac{6}{7}(-0.1)^nu(n)+\frac{1}{7}(0.6)^nu(n)
$$
43分系统函数为
$$
H(z)=\frac{1-0.5z^{-1}}{(1+0.1z^{-1})(1-0.6z^{-1})}
$$
故级联结构图为:
<img src="1.4.1.4.1.drawio.svg" alt="1.4.1.4.1.drawio" style="zoom:80%;" />
系统函数为
$$
H(z)=\frac{\frac{6}{7}}{1+0.1z^{-1}}+\frac{\frac{1}{7}}{1-0.6z^{-1}}
$$
故并联结构图为:
<img src="1.4.1.4.2.drawio.svg" alt="1.4.1.4.2.drawio" style="zoom:80%;" />
2. 设两个有限长序列分别为:
$$
\begin{array}{ll}
x(n)=\cos(0.5n\pi)[u(n)-u(n-5)]\\
h(n)=2\delta(n)+\delta(n-1)+3\delta(n-2)
\end{array}
$$
1求序列$x(n)$和$h(n)$具体数值;
2求线性卷积$x(n)*h(n)$
3求循环卷积$x(n)⑥h(n)$
4求循环卷积$x(n)⑧h(n)$
解:
12分$x(n)=\{1,0,-1,0,1;n=0,1,2,3,4\}$$h(n)=\{2,1,3;n=0,1,2\}$
23分利用相乘相加法求线性卷积
$$
x(n)*h(n)=\{2,1,1,-1,-1,1,3;n=0,1,2,3,4,5,6\}
$$
33分长度为$6$的循环卷积:
利用圆周卷积是线性卷积以$6$点为周期的周期延拓序列的主值序列,即
$6$点圆周卷积$x(n)⑥h(n)$为
$$
x(n)⑥h(n)=\left\{\sum_{r=-\infty}^{\infty}y_l(n+6r)\right\}R_6(n)
$$
故有$1$个重叠,即第$1$个点:$2+3=5$
$6$点圆周卷积:
$$
x(n)⑥h(n)=\{5,1,1,-1,-1,1;n=0,1,2,3,4,5\}
$$
42分长度为$8$的循环卷积:
$$
x(n)⑧h(n)=\{2,1,1,-1,-1,1,3,0;n=0,1,2,3,4,5,6,7\}
$$
3. 基$2$频率抽取$\rm FFT$算法中,回答下列问题:
1对输入序列$x(n)$是如何分组的?其特点是什么?
2画出$4$点基$2$频率抽取$\rm FFT$流图;
3利用该$4$点$\rm FFT$流图计算$x(n)=\{2,1,-1,2;n=0,1,2,3\}$的$\rm DFT$ $X(k)$
解:
12分基$2$频率抽取$\rm FFT$算法对输入序列是按照前后分组的,即前一半分成一组,后一半分成一组。
其特点是a输入自然顺序输出倒位序b可以原位运算c能极大提高$\rm DFT$的计算速度;
24分$4$点基$2$频率抽取$\rm FFT$流图如下:
<img src="1.4.3.2.1.drawio.svg" alt="1.4.3.2.1.drawio" />
34分$x(n)=\{2,1,-1,2;n=0,1,2,3\}$的$\rm DFT$如下:
$$
\begin{array}{ll}
x_1(0)=x(0)+x(2)=2+(-1)=1\\
x_1(1)=x(1)+x(3)=1+2=3\\
x_2(0)=[x(0)-x(2)]W_4^0=(2+1)\cdot 1=3\\
x_2(1)=[x(1)-x(3)]W_4^1=(1-2)\cdot (-j)=j\\
X(0)=x_1(0)+x_2(1)=1+3=4\\
X(2)=[x_1(0)-x_1(1)]W_4^0=(1-3)\cdot 1=-2\\
X(1)=x_2(0)+x_2(1)=3+j\\
X(3)=[x_2(0)-x_2(1)]W_4^0=(3-j)\cdot 1=3-j\\
\end{array}
$$
$$
\therefore X(k)=\{4,3+j,-2,3-j;k=0,1,2,3\}
$$
1. 线性移不变因果系统的差分方程
$$
y(n)=-0.9y(n-1)-0.18y(n-2)+x(n)-0.4x(n-1)
$$
1求系统的频率响应$H(e^{j\omega})$的表达式
2采用几何作图法判断该系统是什么类型的数字滤波器低通、高通、带通、带阻、全通
解:
14分对差分方程两边求$z$变换,有:
$$
Y(z)=-0.9z^{-1}Y(z)-0.18z^{-2}Y(z)+X(z)-0.4z^{-1}x(z)
$$
系统函数为:
$$
H(z)=\frac{Y(z)}{X(z)}=\frac{1-0.4z^{-1}}{1+0.9z^{-1}+0.18z^{-2}}=\frac{z(z-0.4)}{(z+0.6)(z+0.3)}
$$
系统的频率响应为:
$$
H(e^{j\omega})=\frac{e^{j\omega}(e^{j\omega}-0.4)}{(e^{j\omega}+0.6)(e^{j\omega}+0.3)}
$$
2幅度谱为
$$
|H(e^{j\omega})|=\frac{|e^{j\omega}-0.4|}{|e^{j\omega}+0.6||e^{j\omega}+0.3|}=\frac{AB}{AC\times AD}
$$
如下图所示。
<img src="1.4.4.2.1.drawio.svg" alt="1.4.4.2.1.drawio" style="zoom: 50%;" />
当$\omega=0$时,$A$点位于右横轴与单位圆交点上,$AB$取最小值,$AC$、$AD$取最大值,故$|H(e^{j\omega})|$ 取最小值;
当$\omega=\pi$时,$A$点位于左横轴与单位圆交点上,$AB$取最大值,$AC$、$AD$取最小值,故$|H(e^{j\omega})|$取最大值;
当$\omega$从$0$变化到$\pi$时,$AB$从最小值变化到最大值,$AC$、$AD$从最大值变化到最小值,故$|H(e^{j\omega})|$从最小值变化到最大值;
故:$|H(e^{j\omega})|$体现出高通滤波器的幅度特性,也即,该系统是高通滤波器。
2. 采用窗函数设计法设计一个线性相位$\rm FIR$高通滤波器,抽样频率为$40\rm kHz$,阻带截止频率为$8 \rm kHz$,通带开始频率为$10\rm kHz$,阻带衰减不少于$-40\rm dB$。
| 窗函数 | 表达式 | 过渡带宽$\Delta\omega$ / $(2\pi/N)$ | 阻带最小衰减 / $\rm dB$ |
| :----: | :----------------------------------------------------------: | :---------------------------------: | :---------------------: |
| 矩形窗 | $R_{N}(n)$ | $0.9$ | $-21$ |
| 三角窗 | $w(n)=\left\{\begin{array}{ll}\frac{2 n}{N-1}, & 0 \leq n \leq \frac{N-1}{2} \\2-\frac{2 n}{N-1}, & \frac{N-1}{2} \leq n \leq N-1\end{array}\right.$ | $2.1$ | $-25$ |
| 汉宁窗 | $w(n)=\frac{1}{2}\left[1-\cos\left(\frac{2\pi n}{N-1}\right)\right]R_{N}(n)$ | $3.1$ | $-44$ |
| 海明窗 | $w(n)=\left[0.64-0.46\cos\left(\frac{2\pi n}{N-1}\right)\right]R_{N}(n)$ | $3.3$ | $-53$ |
解:
12分求对应的数字频率
阻带截止频率:
$$
\omega_p=\frac{\Omega_p}{f_s}=2\pi \cdot \frac{\Omega_p}{\Omega_s}=2\pi \cdot \frac{8}{40}=0.4\pi
$$
通带开始频率:
$$
\omega_{st}=\frac{\Omega_{st}}{f_s}=2\pi \cdot \frac{\Omega_{st}}{\Omega_s}=2\pi \cdot \frac{10}{40}=0.5\pi
$$
阻带衰减:
$$
\delta_2=40\rm dB
$$
24分理想高通滤波器的通带开始频率
$$
\omega_c \approx \frac{1}{2}(\omega_p+\omega_{st})=0.45\pi
$$
故,理想低通滤波器的频率响应为:
$$
H_d(e^{j\omega})=
\left\{
\begin{array}{ll}
e^{-j\omega\tau}, & \omega_c \leq |\omega| \leq \pi \\
0, & \text{else}
\end{array}
\right.
$$
$$
h_d(n)=\frac{1}{2\pi}\int_{-\pi}^{\pi}H_d(e^{j\omega}) e^{j\omega n}\mathrm{d}\omega=\frac{1}{2\pi}[\int_{-\pi}^{-\omega_c}e^{j\omega(n-\tau)}\mathrm{d}\omega+\int_{\omega_c}^{\pi}e^{j\omega(n-\tau)}\mathrm{d}\omega]\\
=
\left\{
\begin{array}{ll}
\frac{1}{\pi(n-\tau)}\{\sin[(n-\tau)\pi]-\sin[(n-\tau)\omega_c]\}, & n\ne\tau\\
\frac{1}{\pi}(\pi-\omega_c)=1-\frac{\omega_c}{\pi}, & n=\tau
\end{array}
\right.
$$
33分由阻带衰减确定窗形状、过渡带确定$N$
由于阻带衰减:
$$
\delta_2=40\rm dB
$$
查表可知,选用汉宁窗满足要求。
汉宁窗:
$$
\Delta\omega=3.1\cdot\frac{2\pi}{N}
$$
而过渡带宽:
$$
\Delta\omega=\omega_{st}-\omega_{p}=0.5\pi-0.4\pi=0.1\pi
$$
求出:
$$
N=3.1\cdot\frac{2\pi}{\Delta\omega}=\frac{6.2\pi}{0.1\pi}=62
$$
延时:
$$
\tau=\frac{N-1}{2}=\frac{62-1}{2}=30.5
$$
故:
$$
W(n)=\frac{1}{2}[1-\cos(\frac{2\pi n}{N-1})]R_N(n)=\frac{1}{2}[1-\cos(\frac{2\pi n}{61})]R_N(n)
$$
$$
h_d(n)=\frac{1}{\pi(n-30.5)}\{\sin[(n-30.5)\times\pi]-\sin[(n-30.5)\times0.45\pi]\}
$$
41分求出$\rm FIR$滤波器的单位冲激响应$h(n)$
$$
h(n)=h_d(n)\cdot W(n)
$$
51分验算