replace in opening quotations

node41.html

@@ -115,7 +115,7 @@ distinguishes it from the simpler <IMG
  ALT="$f(t)=1$">.  But intuition tells us that
 the constant function is in the <I>spirit</I> of digital audio signals,
 whereas the one that hides a secret between the samples isn't.  A function 
-that is ``possible to sample" should be one for which we can use some reasonable
+that is "possible to sample" should be one for which we can use some reasonable
 interpolation scheme to deduce its values on non-integers from its values on
 integers.
 
@@ -228,7 +228,7 @@ integers at least, to one with frequency between 0 and <IMG
  WIDTH="13" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
  SRC="img41.png"
  ALT="$\pi $">; you simply can't 
-tell the two apart.  And since any conversion hardware should do the ``right"
+tell the two apart.  And since any conversion hardware should do the "right"
 thing and reconstruct the lower-frequency sinusoid, any higher-frequency one
 you try to synthesize will come out your speakers at the wrong
 frequency--specifically, you will hear the unique frequency between 0 and <IMG