Envelope-to-Gate Converter

The simplest way to convert an envelope follower output to a gate signal is with a simple comparator. This is trivial to build, but suffers from a couple of problems. When the threshold voltage is set at a low level, long, sustained notes will hold for nearly their full value, as desired:

              """""""""""""""""""""""""
              "                          "
              "                             "
              "                                "
              "                                   "
Threshold ----"--------------------------------------"-------
Envelope """"""                                         """""

              +--------------------------------------+
              |                                      |
Gate Output --+                                      +-------

But when picking fast notes on the guitar, the GR-300's envelopes never go all the way down to zero. With a low threshold, the gate signal will not re-fire on rapid passages:

              """"""""      ""      ""   
              "       "    "  "    "    " 
              "        "  "    "  "        "
              "         ""      ""            "       
              "                                  "   
Threshold ----"-------------------------------------"--------
Envelope """"""                                        """"""

              +-------------------------------------+
              |                                     |
Gate Output --+                                     +--------

The threshold can be raised for fast repeated notes:

              """"""""      ""      ""   
Threshold ----"-------"----"--"----"----"--------------------
              "        "  "    "  "        "
              "         ""      ""            "       
              "                                  "   
              "                                     "
Envelope """"""                                        """"""

              +-------+    +--+    +----+
              |       |    |  |    |    |
Gate Output---+       +----+  +----+    +--------------------

But then the gate signal goes low too early during long sustained notes:


              " " " " " " " " " " " " "
Threshold ----"--------------------------"--------------------
              "                             "
              "                                "
              "                                   "
              "                                      "
Envelope """"""                                         """"""

              +--------------------------+
              |                          |
Gate Output --+                          +--------------------

An additional problem: if the threshold is set so that loud re-picked notes re-set the gate signal, then soft notes and re-picks will not fire the Gate signal at all:

Threshold -----------------------------------------------
              """""   """"""   """""""""""""               
              "    " "      " "               "      
              "     "        "                   "   
Envelope """"""                                     """"""


Gate Output ---------------------------------------------

A good solution to all these problems is one I actually came up with myself. I call it the Threshold Envelope circuit. The gate comparator threshold is derived from the output of an AD envelope generator. The AD is fired from a comparator set at a very low level. A Max Sample and Hold circuit samples the maximum value that the envelope follower output reached during the attack of the current note. The maximum value that the AD envelope reaches is a percentage of the Max S/H value as set by an adjustment pot. 70-80% seems to work pretty well. The decay time of the AD is set to be slightly faster than the decay time of the guitar string. The result is that the threshold envelope follows closely underneath the envelope follower voltage during the entire cycle of the note, regardless of whether the note is played loudly or softly. If fast re-picking is done, the envelope follower voltage will very quickly fall below the AD output and cause the Gate signal to go low.