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patterns to the converter. The low-pass filter cutoff frequency is about 7KHz with a cutoff slope of 12dB/octave so an update
rate of 20KHz or more is advisable. This method of producing audio cassette signals allows rounded-off waveforms of
known spectral content to be used which can minimize the adverse effects of phase distortion in typical cassette recorders.
For best results, the modulation scheme should restrict the signal bandwidth to about 500-2000Hz.
The cassette readback circuit is a simple zero-crossing detector connected to bit 3 of the byte at $03400B. When the
instantaneous signal input is negative, the bit will read as a zero; otherwise it will be read as a one. Low- and high-pass
filters minimize the effects of low frequency thumps and high frequency transients and hiss but let the 500-2000Hz band of
interest through unaltered.
GENERAL PURPOSE PARALLEL INTERFACE
Circuitry for the general purpose parallel interface is present on the CPU board but the required integrated circuits and cable
connector are not normally soldered in place. The parallel port is actually a tiny bus expansion with a 256 byte address space
and the capability of reading and writing bytes to any selected address within that space.
MISCELLANEOUS 1/0
One 4-bit status register and 6-bit control register are used for miscellaneous functions. The Miscellaneous Status Register is
at address $03400B and has the following bit assignments:
BIT 3
Cassette read input
BIT 2
Low battery detector
BIT 1
Left pedal
BIT 0
Right pedal
The 6-bit Miscellaneous Control Register is at address $03400F and is write-only. Its bit assignments are:
BIT 5
1=Enable normal MIDI I/O, O=disable
BIT 4
1=Enable internal MIDI loopback, O=permit normal operation
BIT 3
Unused
BIT 2
O=Normal operation, 1=Set SRAM SAFE flip-flop
BIT 1
Unused
BIT 0
1=Illuminate the LED mounted on the CPU board
All bits of this register are zeroed on reset.
FREQUENCY UNITS CONVERTER
The Frequency Units Converter is actually a large lookup table implemented in hardware that converts a frequency specified
in octaves to frequency control words suitable for use with the sound generator. It is used by writing a 15-bit positive pitch
value expressed in units of 1/2048 octave to address $03C000 and then immediately reading the equivalent 15 bit positive
frequency parameter from the same address. Thus it replaces what would otherwise be a 64K byte table or lengthy
computation. The octaves are relative to D9 (9397.243Hz) thus $0000 is 9397.24Hz, $0800 is 4698.62Hz (D8), $1000 is
2349.31Hz (D7), etc.
SOUND GENERATOR
The sound generator consists of an array of 240 identical digital oscillators. Their outputs are summed together and appear at
the output jack. Each of these oscillators is controlled by a set of four 16-bit read/write registers. The 68000 program
exercises complete control over the sound produced simply by writing appropriate data in the appropriate sequence to the
appropriate registers. Readback capability is provided for diagnostics and so that software can find out where an oscillator
currently "is" in amplitude and phase if necessary. Note that 68000 CPU access is permitted only once every 3.2uS so there
will be a variable number of wait states associated with accessing these registers. The four registers are described below.
Phase Register
K150FS Programmer's Model
0=negative, 1=positive
1=battery voltage is low
0=tip of pedal jack connected to sleeve
0=ring of pedal jack connected to sleeve
5
Rev. A 26-APR-88
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