Bug 1275 - No preview in Spectral edit effects

Introduces a new type 'spectral' t Nyquist plug-ins

plug-ins/SpectralEditMulti.ny

@@ -1,13 +1,13 @@
 ;nyquist plug-in
 ;version 4
-;type process
+;type process spectral
 ;name "Spectral edit multi tool"
 ;action "Filtering..."
 ;author "Paul Licameli"
 ;copyright "Released under terms of the GNU General Public License version 2"
 
 ;; SpectralEditMulti.ny by Paul Licameli, November 2014.
-;; Updated to version 4 by Steve Daulton November 2014.
+;; Updated by Steve Daulton 2014 / 2015.
 ;; Released under terms of the GNU General Public License version 2:
 ;; http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 
@@ -22,6 +22,7 @@
   (let* ((f0 (get '*selection* 'low-hz))
         (f1 (get '*selection* 'high-hz))
         (fc (get '*selection* 'center-hz))
+        (bw (get '*selection* 'bandwidth))
         (tn (truncate len))
         (rate (snd-srate sig))
         (transition (truncate (* 0.01 rate))) ; 10 ms
@@ -30,14 +31,27 @@
         (breakpoints (list t1 1.0 t2 1.0 tn))
         (env (snd-pwl 0.0 rate breakpoints)))
     (cond
-      ((not (or f0 f1)) (throw 'error-message "Please select frequencies."))
-      ((and f0 f1 (= f0 f1)) (throw 'error-message
-                      "Bandwidth is zero.\nPlease select a frequency range."))
+      ((not (or f0 f1)) ; This should never happen for a 'spectral' effect.
+        (throw 'error-message "Please select frequencies."))
+      ((and f0 f1 (= f0 f1))
+        (throw 'error-message
+          (format nil "~aBandwidth is zero (the upper and lower~%~
+                       frequencies are both ~a Hz).~%~
+                       Please select a frequency range."
+                  p-err f0)))
+      ;; Biqud filter fails if centre frequency is very low and bandwidth very high.
+      ;; 'Magic numbers' 10 Hz and 10 octaves are experimental.
+      ((and f0 (< f0 10) (or (not bw)(> bw 10)))
+        (throw 'error-message
+          (format nil "~aNotch filter parameters cannot be applied.~%~
+                      Try increasing the low frequency bound~%~
+                      or reduce the filter 'Width'."
+                  p-err)))
       ;; low pass frequency is above Nyquist so do nothing
       ((and (not f1) (>= f0 (/ *sound-srate* 2.0)))
           nil)
       ;; notch frequency is above Nyquist so do nothing
-      ((and (and f0 f1) (>= fc (/ *sound-srate* 2.0)))
+      ((and f0 f1 (>= fc (/ *sound-srate* 2.0)))
           nil)
       ;; high-pass above Nyquist so fade to silence
       ((and (not f0)  (>= f1 (/ *sound-srate* 2.0)))
@@ -46,4 +60,5 @@
                (prod (diff 1.0 env) sig))))))
 
 (catch 'error-message
+  (setf p-err "Error.\n")
   (multichan-expand #'result *track*))