【 FPGA 】FIR 濾波器之 Half-band Filter 的係數資料（Filter Coefficient Data）
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，具有奇數個項的半帶濾波器的每第二濾波器係數為零。 指定此過濾器類的過濾器係數資料時，零值條目必須包含在係數檔案中。 例如，圖3-44中顯示了指定濾波器脈衝響應的濾波器係數檔案，如圖3-44所示。
濾波器係數集由FIR編譯器解析。 如果不存在交替的零條目或者係數組不是偶對稱的，則將該條件標記為錯誤並且不生成濾波器。 在這些情況下，會出現一個對話方塊以指示問題。
從技術上講，半帶濾波器的零值項可以出現在濾波器脈衝響應極值處，如圖3-45所示。但是，注意這些值對結果沒有貢獻。
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.
指定過濾器時會檢測到此條件。 如果抽頭的數量使得零值係數形成脈衝響應的第一個和最後一個條目，則將濾波器長度報告為無效值。 半帶濾波器的抽頭數N必須服從N = 3 + 4n，其中n = 0,1,2,3，.... 例如，半帶濾波器可以具有11,15,19和23個抽頭，但不具有9,13,17或21個抽頭。