matlab命令之FIR滤波器的设计
命令并不是具体的讲解，只是把常用的记录下来，用的时候再具体的查，主要是知道有哪些命令，大体有什么功能。若有后续的解释会再添加文字链接。
1.fdatool 图形化设计数字滤波器
2.dfilt 生产成离散时间滤波器对象，
Construct a discrete-time, filter object （是生成滤波器对象，而不是滤波器）
用法：dfilt.STRUCTURE(滤波器系数)。
根据STRUCTURE的不同类型生成不同类型的滤波器对象，有很多类型
3.firpm FIR数字滤波器的最优化设计方法
FIRPM Parks-McClellan optimal equiripple FIR filter design. B=FIRPM(N,F,A) returns a length N+1 linear phase (real, symmetric coefficients) FIR filter which has the best approximation to the desired frequency response described by F and A in the minimax sense. F is a vector of frequency band edges in pairs, in ascending order between 0 and 1. 1 corresponds to the Nyquist frequency or half the sampling frequency. A is a real vector the same size as F which specifies the desired amplitude of the frequency response of the resultant filter B.
B是返回的滤波器系数，N是滤波器阶数,F是频带的边界，例如带通滤波器{低通起点 低通终点 高通起点 高通终点]，低通终点到高通起点是过渡带，A是表示幅度的，跟F的长度相同，
用法：h = firpm(30,[0 .3 .4 1],[1 1 0 0]) 低通滤波器
4.fir1 用窗函数的方法设计脉冲响应滤波器
b = fir1(n,Wn)
b = fir1(n,Wn,'ftype')
b = fir1(n,Wn,window)
b = fir1(n,Wn,'ftype',window)
b = fir1(...,'normalization')
4.fir2 设计基于频率采样的FIR滤波器 （没怎么遇见过）
5.fdesign 生成一定类型的滤波器对象
Create a filter design object for designing discrete-time filters
生成的对象是用来设计滤波器的，看来“对象”并不是指滤波器，可以看成对滤波器的规格参数的描述(specification)
Hs = FDESIGN.TYPE(SPECTYPE,SPEC1, SPEC2,...)
TYPE的类型可以有：
fdesign.bandpass - Designs bandpass filters.
fdesign.bandstop - Designs bandstop filters.
fdesign.decim - Designs decimators.
fdesign.halfband - Designs halfband filters.
fdesign.highpass - Designs highpass filters.
fdesign.interp - Designs interpolators.
fdesign.lowpass - Designs lowpass filters.
fdesign.nyquist - Designs nyquist filters.
fdesign.src - Designs sample-rate converters.
6.firls
Design filters using filter specifications from a filter design object and a least-square minimization technique
用滤波器对象来设计滤波器，这里的specification就是上面fdesign生成的滤波器对象。
d = fdesign.interp(2,'pl,tw',60,.04); % 60 is the polyphase
% length
hm = firls(d);
查看fdesign生成的滤波器对象：
>> d
d =
ResponseType: 'Halfband with filter order and transition width'
SpecificationType: 'PL,TW'
Description: {2x1 cell}
InterpolationFactor: 2
NormalizedFrequency: true
Fs: 'Normalized'
PolyphaseLength: 60
TransitionWidth: 0.0400
>> get(d,'responsetype')
ans =
Halfband with filter order and transition width
查看firls生成的滤波器
>> hm
hm =FilterStructure: 'Direct-Form FIR Polyphase Interpolator'
Numerator: [1x120 double]
InterpolationFactor: 2
ResetBeforeFiltering: 'on'
States: [59x1 double]
NumSamplesProcessed: 0
7.window 各种窗函数的设计
WINDOW(@WNAME,N) returns an N-point window of type specified
by the function handle @WNAME in a column vector. @WNAME can
be any valid window function name, for example:
@bartlett - Bartlett window.
@barthannwin - Modified Bartlett-Hanning window.
@blackman - Blackman window.
@blackmanharris - Minimum 4-term Blackman-Harris window.
@bohmanwin - Bohman window.
@chebwin - Chebyshev window.
@flattopwin - Flat Top window.
@gausswin - Gaussian window.
@hamming - Hamming window.
@hann - Hann window.
@kaiser - Kaiser window.
@nuttallwin - Nuttall defined minimum 4-term Blackman-Harris window.
@parzenwin - Parzen (de la Valle-Poussin) window.
@rectwin - Rectangular window.
@tukeywin - Tukey window.
@triang - Triangular window.
其中@符号后面的单词是窗的类型，每个都是一个函数，可以用来设计对应的窗函数，不同的窗主瓣宽度，衰减不同，一般是根据阻带衰减来确定使用哪类窗函数，
8.freqz
Compute the frequency response of discrete-time filters, adaptive filters, and multirate filters
用法：
[h,w] = freqz(ha)
[h,w] = freqz(ha,n)
[h,w] = freqz(hd)
[h,w] = freqz(hd,n)
[h,w] = freqz(hm)
[h,w] = freqz(hm,n)
h保存频率响应，复数形式，包含幅频响应和相频响应
9.fvtool
Open the Filter Visualization Tool (FVTool)
用法：
fvtool(b,a)
fvtool(b1,a1,b2,a2,...bn,an)
fvtool(Hd1,Hd2,...)
fvtool(..跟上面类似...,'TYPE');
TYPE包括phase,magnitude