of Microprocessors

714 MUSICAL ApPLICATIONS OF MICROPROCESSORS
the extremely short development schedules of most synthesizers don't allow
10-20 weeks' delivery time for masked ROMs.
Besides a small amount of scratch RAM for computation and temporary
storage, a larger CMOS RAM is normally used to hold patching configurations,
sound parameters, and other information that specifies how different
sounds are synthesized. Each such configuration is called a program (not to be
confused with the ROM program that controls the microprocessor), and
typically there are 50-100 or more requiring 2-4K to hold them all. An
important feature is a backup battery to retain the RAM's content when the
synthesizer is off ln fact, it is common to memorize the control panel state
when power is removed so that it will "come up" in the exact same state when
power is restored. Another common feature is an audio cassette interface
similar to that on early personal computers for saving and restoring the
parameter memory content. Manufacturers often have parameter tapes
available for sale, and serious players can build a library of synthesizer voice
tapes.
The various console buttons, switches, and keypads are connected to
the microprocessor either individually or in a matrix arrangement if they are
numerous. Generally, software performs switch debouncing and even makes
simple momentary action pushbuttons act like alternate action latching
switches in conjunction with a light. The lights may also be directly driven
by register bits; however, multidigit seven-segment displays are usually
multiplexed by software. For extensive alphanumeric panels, which may have
20~40 characters, "intelligent displays" may be used, which automatically
refresh themselves. Then the synthesizer processor need only store bytes in a
small RAM in the display.
The music keyboard is generally interfaced just like the one described
in Chapter 9. Velocity sensing is performed by measuring the time
differential between two contact closures. A more recent keyboard design has
each key press two conductive rubber pads against patterns on a printed
circuit board. One pad touches when the key is about one-third depressed,
and the second touches at the three-quarter mark. The advantage of this
approach is somewhat less contact bounce and significantly lower manufacturing
cost in large volumes. One manufacturer even uses LEDs and
photocells for zero bounce key position sensing. Because the keyboard must
be scanned very rapidly and uniformly for accurate velocity sensing, it is
sometimes given its own independent microprocessor rather than burdening
the synthesizer's main processor with this task.
The actual sound-generating and control elements are on the right side
of Fig. 19-1. Typically, each synthesizer voice is produced by a moderately
small voice module that may either plug into an internal "synthesizer bus" or
be part of a large synthesizer board. Such a module usually has two VCOs,
two to four VCAs, and a VCF plus several analog switches for reconfiguring
these elements in a number of useful, but not totally general, ways. The