An Introduction to Tape/MIDI Sync and the MQX-32

                                  Dave Hocker

                    Copyright (c) 1988 - All Rights Reserved
                               Music Quest, Inc.


  Overview

  Since the introduction of the Music Quest MQX-32 Enhanced MIDI Co-processor
  Card, we here at Music Quest have fielded a large number of questions about
  the features of the MQX-32 and the various different forms of tape/MIDI
  sync.  The purpose of this paper is to discuss the most common forms of
  sync in use by computer/music/MIDI systems, with particular emphasis on
  tape sync.  In doing so, we will look at how each sync method is
  implemented.  Also, we will consider the relative strengths and weaknesses. 
  After you complete reading this paper, you will have a basic understanding
  of these different sync methods, and you will have a good perspective on
  the sync features of the Music Quest MQX-32 PC MIDI interface.  Hopefully,
  we will have answered many of your questions, while simultaneously
  providing enough information so you can ask new ones.


  Types of Sync

  We're going to discuss four different types of sync.

     MIDI sync
     FSK (Frequency Shift Keying)
     CLS (Chase Lock Sync)
     SMPTE (Society of Motion Picture and Television Engineers)

  In our discussions, we will treat MIDI sync differently than the other
  three methods, mainly because it is not a tape sync technique, per se. 
  Near the end of this paper we will discuss various converters that can
  convert from one sync source to/from another.


  Some Basic Concepts

  Before getting into the details of sync methods, we need to get a few
  basics under our belt.  First let's make sure we all understand the general
  reason for sync.  The term sync is used to refer to various techniques that
  allow multiple, time dependent devices to work together.  For MIDIists,
  this usually means that you want to sync multiple MIDI devices together or
  that you want to sync a tape recorder with one or more MIDI devices.  The
  second reason is probably the most common circumstance, as it is used to
  overcome a shortage of sound generators or synthesizers.  It allows you to
  create a finished performance by reusing sound generators on separate
  tracks.  That is, you make several passes on the tape, recording a new
  track on each pass.

  When you are synchronizing, you always have one master and one or more
  slaves.  A good analogy is a conductor and an orchestra.  The conductor is
  the master sync source who keeps all of the orchestra's musicians together.



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  There is another sync basic that we can draw from the conductor analogy, 
  One of the roles of the conductor is to set the tempo for the orchestra. 
  The conductor uses his/her baton as a "clock".  The faster the baton moves,
  the faster the tempo, and vice versa.

  Another concept we need to understand is the difference between sync
  methods that are "tempo relative" and ones that are "fixed time".  With a
  tempo relative sync method, the rate of the sync signals is directly
  proportional to the tempo.  That is, the sync signals are generated at a
  rate that is determined by the beats-per-minute tempo.  As we shall see,
  MIDI sync, FSK, and CLS are tempo relative sync methods (our orchestra
  conductor is also a tempo relative sync source).

  A fixed time sync method uses signals that are not proportional to tempo. 
  The sync signals are generated at a fixed rate.  Of the four sync
  techniques we will discuss, SMPTE is the only one that uses fixed timing.

  When we are synchronizing a tape recorder and MIDI devices, we use one of
  the tape tracks to store the sync information.  The process of "striping a
  tape" refers to the task of writing the sync information on the sync track. 
  After the tape is striped, it becomes the master source of sync signals for
  the system.


  MIDI Sync

  MIDI sync is most commonly used to allow one or more "slave" MIDI devices
  to be synchronized to a "master" MIDI device.  Typically, these devices are
  sequencers or devices with "sequencer like" capability (e.g. drum
  machines).  MIDI sync uses a combination of MIDI messages to accomplish its
  goal.