Half-band Filter
As previously described, every second filter coefficient for a half-band filter with an odd number of terms is zero. When specifying the filter coefficient data for this filter class, the zero value entries must be included in the coefficient file. For example, the filter coefficient file that specifies the filter impulse response in Figure 3-43 is shown in Figure 3-44.
如前所述，具有奇数个项的半带滤波器的每第二滤波器系数为零。 指定此过滤器类的过滤器系数数据时，零值条目必须包含在系数文件中。 例如，图3-44中显示了指定滤波器脉冲响应的滤波器系数文件，如图3-44所示。
滤波器系数集由FIR编译器解析。 如果不存在交替的零条目或者系数组不是偶对称的，则将该条件标记为错误并且不生成滤波器。 在这些情况下，会出现一个对话框以指示问题。
从技术上讲，半带滤波器的零值项可以出现在滤波器脉冲响应极值处，如图3-45所示。但是，注意这些值对结果没有贡献。
This condition is detected when the filter is specified. If the number of taps is such that the zero-valued coefficients form the first and last entry of the impulse response, the filter length is reported as an invalid value. The number of taps N for a half-band filter must obey N=3 + 4n, where n=0,1,2,3,…. For example, a half-band filter can have 11, 15, 19, and 23 taps, but not 9, 13, 17, or 21 taps.
指定过滤器时会检测到此条件。 如果抽头的数量使得零值系数形成脉冲响应的第一个和最后一个条目，则将滤波器长度报告为无效值。 半带滤波器的抽头数N必须服从N = 3 + 4n，其中n = 0,1,2,3，.... 例如，半带滤波器可以具有11,15,19和23个抽头，但不具有9,13,17或21个抽头。