replace in opening quotations

node77.html

@@ -192,7 +192,7 @@ of a sound, called
 <A NAME="5641"></A><I>carrier signal</I>, which
 is simply multiplied by the input.  In this context the input is called the
 <A NAME="5643"></A><I>modulating signal</I>.
-The term ``ring modulation" is often used
+The term "ring modulation" is often used
 more generally to mean multiplying any two signals together, but here we'll
 just consider using a sinusoidal carrier signal.  (The technique of ring
 modulation dates from the analog era [<A
@@ -312,7 +312,7 @@ with <IMG
 </DIV>
 
 <P>
-Parts (a) and (b) of the figure show ``general" cases where <IMG
+Parts (a) and (b) of the figure show "general" cases where <IMG
  WIDTH="13" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
  SRC="img7.png"
  ALT="$\alpha $"> and <IMG