217 lines
10 KiB
Plaintext
217 lines
10 KiB
Plaintext
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How Dolby Stereo (aka Dolby Surround) works:
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(version 2.01, 4/22/94)
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Don Munsil (don@munsil.com)
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2.01 changes:
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- Better explanation of inverting a sample.
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2.0 changes:
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- Added responses from Otto Chrons and Adrian Kwong.
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- Cleaned up text considerably.
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- Added more tips on practical aspects.
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** Introduction:
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Dolby Stereo is a system designed by Dolby Labs in the '70s for creating
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better movie sound, including surround-sound. The other benefits are
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increased directionality of front sounds and improved localization of
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on-screen sounds like dialog for the people sitting on the edges. The
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system is totally separate from Dolby noise-reduction systems like Dolby B
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and C. Remember that there is only one encoding process: Dolby Stereo,
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which can be decoded by a Dolby Surround, Dolby Pro-Logic, or THX
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decoder, for varying degrees of home-theatre "quality."
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There are four output channels in Dolby Stereo: Left front, Right front,
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Center front, and Surround. These are encoded on two channels in such a
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way that the soundtrack can be played back on a standard stereo device, a
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mono device, or a Dolby decoder. (Or a THX system, which is a Dolby
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Pro-Logic decoder with specifications licensed by Lucasfilm).
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A word is in order about THX. It is NOT different from Dolby Pro-Logic, at
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least not precisely. THX is a licensing and quality-control wing of
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LucasArts with the stated aim of improving movie sound and picture. It
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licenses the name THX for movie equipment, movie theatres, home equipment,
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and laserdiscs. The name means that the THX engineers certify it as good
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quality. There are a few processing steps in home THX that are not
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typically present in a plain Pro-Logic system, but they are merely
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enhancements to Dolby Pro-Logic decoding, not a different system.
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The center channel is the most used one in a film. It contains all of the
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dialog, and most of the on-screen sound effects. It is important to have a
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center channel so that people on the edges of the theatre (or your living
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room) still hear the on-screen sound from the direction of the screen, not
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panned to one side or another. Without the center channel, people hear
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most of the sound coming from the nearest speaker. In addition, dialog
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tends to be clearer when it is localized in the center channel.
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The surround channel is used for "ambient" effects: sounds that should
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envelop the listener. It is not a rear channel, and should not be used for
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directional sounds. In a properly setup system, the listener should not be
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able to tell where the surround speakers are located. In a movie theatre,
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there are generally many surround speakers around the back of the hall. In
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the home, this is impractical, so two speakers facing in such a way as to
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maximize reflected sound are used.
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Any device capable of playing back two distinct channels can produce Dolby
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Stereo-compatible signals. This includes any stereo soundcard, stereo vcr,
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or even CD player or record (there are Dolby Stereo encoded CDs).
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** Here's how:
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I'll refer to the four output channels as LO, RO, CO, and SO, for left,
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right, center, and surround, respectively. The input channels (the ones
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coming from the source and into the decoder) I'll call LI and RI. (If you
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don't like acronyms, feel free to use search-and-replace to make this more
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readable.) :-)
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The LO channel and RO channel are recorded normally on the LI and RI tape
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channels (we'll assume tape recording for example purposes). The CO
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channel is recorded on BOTH the LI and RI channel at exactly the same
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volume, in-phase (i.e., no special processing). The SO channel is recorded
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on LI and RI at the same volume, with inverted phase (i.e. every peak on
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one channel is a trough on the other).
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There are a few interesting nuances here. Because the CO channel is
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recorded normally on LI and RI, it will still image in the center on any
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stereo playback system, as long as the listener is basically in the
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center. As the listener moves further left or right, the image moves in
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the same direction, which is a little distracting, but acceptable. This is
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what one gets with a simple "surround" decoder (in addition to the
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surround channel, of course). Without the Center channel, the imaging is
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fine for one or two people, but not so great for groups. Dolby Pro-Logic
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adds (among other things) the center channel.
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Note that because of the inverted encoding scheme for the surround
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channel, it will disappear when played back on a mono system. Each peak is
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precisely canceled by a trough on the other channel, and all surround
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information is lost. This is another reason to put only ambient,
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non-essential sounds in the surround channel. Many systems are still mono
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(most VCRs, for example) and will not reproduce them.
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The Dolby decoder reproduces a fairly good semblance of the original four
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channels from the two input channels, though there will always be leakage
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and crosstalk. Dolby Pro-Logic and THX circuitry have special processing
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that minimizes perceived crosstalk.
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When the Dolby Stereo is mixed, the engineers listen to it running through
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a Pro-Logic decoder exactly like the one in the theatre or your home.
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Thus, they design the four channels specifically so they will decode
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properly. It is not a good idea to mix four distinct channels, then do the
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Pro-Logic encoding "blind" as a post-process, because the results will not
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be exactly the same as the original four distinct channels.
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** Doing it on a soundcard (kinky as it sounds):
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On a soundcard, sending signals to the center channel is a simple as
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playing the sound in dead-center (equal volume on both channels). Surround
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is a bit tougher. The sound must be played back inverted on one channel.
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One way would be to have two samples, with one pre-inverted. Another would
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be to invert one side on the fly. I have been told that DMP, a MOD music
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player by Otto Chrons, does just this. I haven't heard it myself.
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Inverting a sample simply consists of negating it. If the sample values
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are unsigned, the negation will still give the desired result because of
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the nature of two's complement representation (though you should make
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sure your compiler is actually doing the appropriate negation -- in C,
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cast the value to signed, then negate it, then cast to unsigned to be
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absolutely sure.)
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>>New addition: Otto Chrons (c142092@cc.tut.fi), author of DMP, emailed
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me the following:
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You wrote an excellent article about Dolby Surround sound at csip.soundcard
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but I wanted to comment on few things. DMP does work with surround channel
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and the technique I use is extremely simple. Dolby standard state that
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you should do +90 and -90 phase shifting on left & right channels, but I've
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found out that doing a 0 and +180 shift works as well. So basically I
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put the original sound data out of the left speaker and negate the data
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on the right speaker. In my mixing routines this is accomplished by
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using ADD and SUB instructions respectively. So doing surround sound is
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as easy as doing mono!
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>>End Chrons
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** A few other issues to keep in mind:
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>>Adrian Kwong (akwong@alfred.carleton.ca) had this to add:
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You might want to add that the surround channel in a normal Dolby
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Surround setup is placed through Dolby A type noise decoding. It has
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something to do with making it 3dB larger following a specific
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frequency envelope. To make the surround appear at the correct
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volume, you'll need to "double" the signal to the surround channel.
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(+3dB is about 2x the signal)
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>>End Kwong
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One thing to keep in mind is that the Surround channel is typically not
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designed to handle a lot of bass. The speakers are generally small, and
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the amplifiers are lower wattage. On a film soundtrack, the low bass is
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generally filtered out of the soundtrack to avoid clipping.
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In addition, the high frequencies are generally filtered out of the
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surround channel because higher frequencies are easier to localize, and
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the surround channel is not supposed to be directional. The end result is
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that the surround channel is only "supposed" to carry about 200-8000Hz,
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which is a fairly narrow spectrum.
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It is difficult to get a sound to play on all four channels at once.
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Generally, on a film soundtrack, a "big" sound, like an explosion, is sent
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mainly to the surround channel. Since this will be lost in mono, a similar
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sound is sent to the left and right channels as well.
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A technique often used with thunder effects is to put the main sound in
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the surround channel, followed by an echo in the front channels (or
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sometimes vice-versa). The Dolby stereo listeners hear the two-part
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thunder, and the mono listeners hear just the second bit (or a scaled-down
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version of the first).
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Another technique is to slightly pitch-shift or delay sounds going to the
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different speakers, but results can be iffy. YMMV. The important thing is
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to do testing on a real Pro-Logic setup, and experiment until a good
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balance is reached.
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One can also send frequency band-limited chunks of the sound to each channel,
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which will decode rather well. It requires that the bulk of each channel
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be using a different band of the frequency spectrum, which is not
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practical in some cases.
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Keep in mind that there are many tricks of the trade that are used in film
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mixing that only Dolby really knows. They don't tell how they do it,
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because they want film companies to buy their technology, not the
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competition (e.g. UltraStereo, Chase Surround, StereoSurround). If anyone
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reading this has useful techniques (that are not trade secrets of Dolby)
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they would like to share, please email the author.
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Fortunately, in computer sound, you don't have to make the soundtrack
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mono-compatible. You can ask the user if the sound system is Dolby
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Surround, and place sounds in the surround channel as needed. On a mono
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system, the sounds can all be sent to the single channel.
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** Disclaimers:
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All of this information is from me, and any errors are my own darn fault.
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None of this information has been endorsed by Dolby Labs. Dolby is a
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trademark of Dolby labs, and should not be used on a product without
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getting their permission. (Although I think if you called it "surround"
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people would get the idea.)
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Please email suggestions/criticisms/additions/subtractions/
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multiplications/corrections to don@munsil.com.
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This text is copyright 1993 by Don Munsil. It may be distributed freely,
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as long as modifications are attributed and marked clearly.
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--
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------------------------------------------------------
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Don Munsil | I respect faith, but doubt is
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don@munsil.com | what gets you an education.
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| -- Wilson Mizner
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