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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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