replace in opening quotations

node76.html

@@ -200,7 +200,7 @@ component of zero frequency, for which
  ALT="$a$">--without dividing by two.
 (Components of zero frequency are often called
 <I>DC</I><A NAME="5614"></A>
-components; ``DC" is historically an acronym for ``direct current").
+components; "DC" is historically an acronym for "direct current").
 These conventions for amplitudes in spectra will simplify the mathematics later
 in this chapter; a deeper reason for them will become apparent in
 Chapter <A HREF="node104.html#chapter-delay">7</A>.
@@ -248,7 +248,7 @@ the signal's momentary behavior.
 <P>
 This way of viewing sounds is greatly oversimplified.  The true behavior of
 audible pitch and timbre has many aspects which can't be explained in terms of
-this model.  For instance, the timbral quality called ``roughness" is sometimes
+this model.  For instance, the timbral quality called "roughness" is sometimes
 thought of as being reflected in rapid changes in the spectral envelope over
 time.  The simplified description developed here is useful nonetheless in
 discussions about how to construct discrete or continuous spectra for a wide