
                           GETTING IN SYNC
 
In MIDI,  synchronization refers to various techniques of making several
time-dependent devices work together as one.  For example,  driving 3 or
4 synths with your favorite keyboard, or "synching-up"  a synthesizer to
keep in tempo with tracks on tape deck.  While all methods incorporate a
master/slave  scheme,  mechanisms such as FSK,  Chase Lock and MIDI Sync
are "tempo relative",  where  one  device  serves  as  conductor  to  an
orchestra of peripherals.  Other techniques such as SMPTE incorporate an
arbitrary  format  or  "fixed  time"  signal that has no relationship to
tempo.  Both timing methods refer  to  "striping",  as  the  process  of
individually  recording  a track of sync signals before other sequences,
songs or sounds are taped.  
 
MIDI  Sync  is  a  "tempo  relative"   technique,   generally  used   by
"sequencer-like"  devices,  such  as  a  computer or rhythm composers to
drive multiple MIDI instruments.  At the heart beat of MIDI sync a  MIDI
clock  message,  referred to as MIDI clocks.  These MIDI Clocks are sent
at a rate of 24 clocks per  quarter  note,  creating  a  relative  tempo
usually expressed as Song Position Pointer (SPP), used to indicate where
a song or sequence begins and/or ends.  
 
Frequency Shift Keying (FSK) is a simple "tempo relative" procedure that
uses  two  distinct  tones  to  form a signal.  While One tone fills the
tape,  the other marks the occurrence of a clock signal.  As the tape is
read,  the  sync  device generates a clock signal every time it sees the
transition from fill signal to  mark  signal.   Due  to  the  nature  of
digital  encoding,  FSK  can  be  written  for  various degrees of clock
resolution.  For example,  a program might use a clock resolution of  24
ticks  per beat to accommodate MIDI sync,  or adjust to various hardware
such as the MPU401's rate of 192  ticks  per  beat.   Unfortunately  the
on/off  nature  of  FSK  disallows  SPP,  means  sync-up can only at the
begging of a song or sequence.  Furthermore the frequency sensitive  FSK
offers   little   reliability   when   up  against  poor  tape  quality,
inconsistent speeds or volume distortion.  
 
Chase Lock Sync (CLS)  can be best  describe  as  MIDI  Sync  for  tape.
Unlike  FSK,  CLS does not rely discrete tones but instead utilizes wave
forms.  Allowing for MIDI Clocks and SSP,  position information  can  be
read  by the sync device,  whereupon the sequencer responds by "chasing"
the current sequence to the location indicated.   This  usually  creates
split  second delay,  requiring that CLS be individually recorded before
other tracks.  Though CLS is limited to a clock resolution of  24  ticks
per  beat,  this  draw back is greatly over shadowed by CLS'  ability to
sequence from any location on the tape.   Furthermore,  the  ability  to
jump  to any position allows for recovery from disturbances such as drop
out.
 
The Society of Motion Picture and Television Engineers offers the "fixed
time"  method SMPTE.  While also using an analog signal,  SMPTE  differs
from  CLS  and  MIDI  Sync as synchronization is built upon "frames"  of
information instead  of  a  tempo  relationship.   The  "frame  address"
describes a group of bits defining a single location.  The rate at which
frames  are written to tape determine which of the SMPTE Format,  listed
below is used.

24 frame used by motion pictures.
25 frame used in Europe for video work because the television scan rate
   is 25 frames per second.
30 frame non-drop used world wide for audio sync.
30 frame drop used in the US for video work as US color television scans
   at  29.97 frames  per second. The term "drop" indicates that selected
   frames  are periodically  dropped  to  yield  a  29.97  frames/second
   scan rate.
 
In the US,  both 30 frame drop and non-drop are generally used, drop for
video/audio sync, and non-drop for audio sync.  Both implement an 80 bit
frame, written at a rate of 2400 bits per second,  each frame containing
a  time  stamp  address  and  frame number.  A time stamp of 01:37:22:19
would identify the frame address as frame 19, at 1 hour, 37 minutes,  22
seconds.
Being  an  established  standard  SMPTE allows a tape written by a SMPTE
writer can be read by any SMPTE reader that supports the  given  format,
whereas  FSK  and  CLS are subject to manufacturer's proprietary format;
making tape interchange impractical.  While offering the same advantages
of random positions starts and error recovery as CLS,  MIDI's acceptance
of  SMPTE  as MIDI Time Code messages allows SMPTE frame addresses to be
sent over MIDI cables in real time,  making SMPTE a powerful  triggering
mechanism.  Being a fixed format, SMPTE has the added advantage of tempo
changes at any time without the hassle of rewriting the sync track.  
 
With  all  of  these  advantages the only disadvantage is not with SMPTE
itself, but the lack of SMPTE applications for the IBM PC.  This however
is  has  changed as developers  such as LTA Productions and  Twelve Tone 
Systems  have been  hard  at  work upgrading  applications for the newly 
released Yamaha C-1 Music Computer and Music Quest MQX-32 MIDI Interface 

                         Allow me to Interrupt
 
Imagine 8 mail boxes in a row each with their own unique address,  and a
mail clerk that goes around raising the flags whenever information needs
to be exchanged.   This  analogy  roughly  describes  how  the  PC  uses
hardware  interrupts  (IRQ)  to  pass  information  to  and from various
interface cards.  Though most PC  users  are  unaware  that  their  MIDI
applications  use  IRQ2 in this way,  the ever expanding selection of PC
interfaces  has  produced  situations  where  more  than  one  card   is
contending for same IRQ.  
 
One  can  quickly  tell  if  there  is an interrupt address problem when
software hangs during sequencing,  often locking up the entire system so
even  CTRL-ALT-DEL  won't  work.   You may be spared such frustration if
during installation or execution your software informs there is  no  MPU
card is present, even if you know one has been installed.  
 
While  most  PC  MIDI  vendors  at  least  offer  fixes,  users may find
applications and interfaces that offer alternate  interrupts  easier  to
affect.   In either case,  if you are about upgrading your computer,  or
about to purchase new MIDI hardware or software,  call the vendor  first
and find out if they know how to handle the interrupt problem.  
 
The  following table summarizes how interrupts are generally used in the
PC/XT/AT type machines (Industry Standard Architecture) : 
 
IBM PC/XT (8086/88) IRQ Use     IBM AT (80286/386) IRQ Use
0 Timer                             0 Timer
1 Keyboard                      1 Keyboard
2 Reserved                      2 Orred summary of IRQ8-IRQ15
3 Secondary Async (COM2)        3 Async port 2 (COM2)
4 Primary async (COM1)          4 Async port 1 (COM1)
5 Hard disk controller          5 Parallel port 2 (LPT2)
6 Floppy disk controller        6 Floppy disk controller
7 Printer (not used by most)    7 Parallel port 1 (LPT1)
                                9 acts as IRQ2
 
If your PC/XT or compatible is having IRQ2 problems : 
 
1.IRQ2 may be already in use by another interface such as a multi-
  function card or an VGA or EGA video adapter;
2.installation of a hard disk renders IRQ5 unavailable;
3.if COM2 port is assigned, then IRQ3 cannot be used;
4.though IRQ7 is assigned for printer, the number of printer 
  adapters using them are very, very rare, making IRQ7 a good 
  alternative to IRQ2.
 
If you have IRQ problems and own an 80286 (IBM AT) or 80386 micro :
 
1.IRQ2 and/or IRQ9 may be occupied by another interface such as a 
  multi-function card or an VGA or EGA video adapter;
2.you cannot use IRQ3 if COM2 port has been installed;
3.if a second parallel printer port is enabled, IRQ5 is unavailable;
4.again, only a handful of printer adapters use IRQ5 or IRQ7. In 
        fact, the MS-DOS print spooler doesn't even support them, making 
        IRQ5 or IRQ7 good alternatives.





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