170 lines
9.1 KiB
Plaintext
170 lines
9.1 KiB
Plaintext
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GETTING IN SYNC
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In MIDI, synchronization refers to various techniques of making several
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time-dependent devices work together as one. For example, driving 3 or
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4 synths with your favorite keyboard, or "synching-up" a synthesizer to
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keep in tempo with tracks on tape deck. While all methods incorporate a
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master/slave scheme, mechanisms such as FSK, Chase Lock and MIDI Sync
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are "tempo relative", where one device serves as conductor to an
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orchestra of peripherals. Other techniques such as SMPTE incorporate an
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arbitrary format or "fixed time" signal that has no relationship to
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tempo. Both timing methods refer to "striping", as the process of
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individually recording a track of sync signals before other sequences,
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songs or sounds are taped.
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MIDI Sync is a "tempo relative" technique, generally used by
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"sequencer-like" devices, such as a computer or rhythm composers to
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drive multiple MIDI instruments. At the heart beat of MIDI sync a MIDI
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clock message, referred to as MIDI clocks. These MIDI Clocks are sent
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at a rate of 24 clocks per quarter note, creating a relative tempo
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usually expressed as Song Position Pointer (SPP), used to indicate where
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a song or sequence begins and/or ends.
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Frequency Shift Keying (FSK) is a simple "tempo relative" procedure that
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uses two distinct tones to form a signal. While One tone fills the
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tape, the other marks the occurrence of a clock signal. As the tape is
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read, the sync device generates a clock signal every time it sees the
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transition from fill signal to mark signal. Due to the nature of
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digital encoding, FSK can be written for various degrees of clock
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resolution. For example, a program might use a clock resolution of 24
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ticks per beat to accommodate MIDI sync, or adjust to various hardware
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such as the MPU401's rate of 192 ticks per beat. Unfortunately the
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on/off nature of FSK disallows SPP, means sync-up can only at the
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begging of a song or sequence. Furthermore the frequency sensitive FSK
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offers little reliability when up against poor tape quality,
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inconsistent speeds or volume distortion.
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Chase Lock Sync (CLS) can be best describe as MIDI Sync for tape.
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Unlike FSK, CLS does not rely discrete tones but instead utilizes wave
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forms. Allowing for MIDI Clocks and SSP, position information can be
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read by the sync device, whereupon the sequencer responds by "chasing"
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the current sequence to the location indicated. This usually creates
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split second delay, requiring that CLS be individually recorded before
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other tracks. Though CLS is limited to a clock resolution of 24 ticks
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per beat, this draw back is greatly over shadowed by CLS' ability to
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sequence from any location on the tape. Furthermore, the ability to
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jump to any position allows for recovery from disturbances such as drop
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out.
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The Society of Motion Picture and Television Engineers offers the "fixed
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time" method SMPTE. While also using an analog signal, SMPTE differs
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from CLS and MIDI Sync as synchronization is built upon "frames" of
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information instead of a tempo relationship. The "frame address"
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describes a group of bits defining a single location. The rate at which
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frames are written to tape determine which of the SMPTE Format, listed
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below is used.
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24 frame used by motion pictures.
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25 frame used in Europe for video work because the television scan rate
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is 25 frames per second.
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30 frame non-drop used world wide for audio sync.
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30 frame drop used in the US for video work as US color television scans
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at 29.97 frames per second. The term "drop" indicates that selected
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frames are periodically dropped to yield a 29.97 frames/second
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scan rate.
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In the US, both 30 frame drop and non-drop are generally used, drop for
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video/audio sync, and non-drop for audio sync. Both implement an 80 bit
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frame, written at a rate of 2400 bits per second, each frame containing
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a time stamp address and frame number. A time stamp of 01:37:22:19
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would identify the frame address as frame 19, at 1 hour, 37 minutes, 22
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seconds.
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Being an established standard SMPTE allows a tape written by a SMPTE
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writer can be read by any SMPTE reader that supports the given format,
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whereas FSK and CLS are subject to manufacturer's proprietary format;
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making tape interchange impractical. While offering the same advantages
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of random positions starts and error recovery as CLS, MIDI's acceptance
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of SMPTE as MIDI Time Code messages allows SMPTE frame addresses to be
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sent over MIDI cables in real time, making SMPTE a powerful triggering
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mechanism. Being a fixed format, SMPTE has the added advantage of tempo
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changes at any time without the hassle of rewriting the sync track.
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With all of these advantages the only disadvantage is not with SMPTE
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itself, but the lack of SMPTE applications for the IBM PC. This however
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is has changed as developers such as LTA Productions and Twelve Tone
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Systems have been hard at work upgrading applications for the newly
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released Yamaha C-1 Music Computer and Music Quest MQX-32 MIDI Interface
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Allow me to Interrupt
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Imagine 8 mail boxes in a row each with their own unique address, and a
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mail clerk that goes around raising the flags whenever information needs
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to be exchanged. This analogy roughly describes how the PC uses
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hardware interrupts (IRQ) to pass information to and from various
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interface cards. Though most PC users are unaware that their MIDI
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applications use IRQ2 in this way, the ever expanding selection of PC
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interfaces has produced situations where more than one card is
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contending for same IRQ.
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One can quickly tell if there is an interrupt address problem when
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software hangs during sequencing, often locking up the entire system so
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even CTRL-ALT-DEL won't work. You may be spared such frustration if
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during installation or execution your software informs there is no MPU
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card is present, even if you know one has been installed.
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While most PC MIDI vendors at least offer fixes, users may find
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applications and interfaces that offer alternate interrupts easier to
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affect. In either case, if you are about upgrading your computer, or
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about to purchase new MIDI hardware or software, call the vendor first
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and find out if they know how to handle the interrupt problem.
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The following table summarizes how interrupts are generally used in the
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PC/XT/AT type machines (Industry Standard Architecture) :
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IBM PC/XT (8086/88) IRQ Use IBM AT (80286/386) IRQ Use
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0 Timer 0 Timer
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1 Keyboard 1 Keyboard
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2 Reserved 2 Orred summary of IRQ8-IRQ15
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3 Secondary Async (COM2) 3 Async port 2 (COM2)
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4 Primary async (COM1) 4 Async port 1 (COM1)
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5 Hard disk controller 5 Parallel port 2 (LPT2)
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6 Floppy disk controller 6 Floppy disk controller
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7 Printer (not used by most) 7 Parallel port 1 (LPT1)
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9 acts as IRQ2
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If your PC/XT or compatible is having IRQ2 problems :
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1.IRQ2 may be already in use by another interface such as a multi-
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function card or an VGA or EGA video adapter;
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2.installation of a hard disk renders IRQ5 unavailable;
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3.if COM2 port is assigned, then IRQ3 cannot be used;
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4.though IRQ7 is assigned for printer, the number of printer
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adapters using them are very, very rare, making IRQ7 a good
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alternative to IRQ2.
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If you have IRQ problems and own an 80286 (IBM AT) or 80386 micro :
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1.IRQ2 and/or IRQ9 may be occupied by another interface such as a
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multi-function card or an VGA or EGA video adapter;
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2.you cannot use IRQ3 if COM2 port has been installed;
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3.if a second parallel printer port is enabled, IRQ5 is unavailable;
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4.again, only a handful of printer adapters use IRQ5 or IRQ7. In
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fact, the MS-DOS print spooler doesn't even support them, making
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IRQ5 or IRQ7 good alternatives.
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