Kurzweil K2000 - K2000 Audio;Power Board Theory - Part 10; Effects Section and A;D Converter Theory; Effects D;A Conversion and Filtering Theory

SECTION 5
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5.1.5 K2000 AUDIO/POWER BOARD THEORY SCHEMATIC PAGE
5.1.5.1 EFFECTS SECTION
The Effects Section is mono-in, stereo-out and can perform reverberation, echo, delay, frequency
equalization, chorusing, and flanging. Although the processing happens digitally, the input and
output data format is not compatible with the KISS format produced by the Engine Board; thus it
is necessary to convert the analog audio signal to be processed into digital form, have the
effects section process it, and convert it back into analog audio. The Effects Section could be
removed with little or no effect on the remaining circuitry.
Virtually all of the effects circuitry is licensed from DOD Digitech and is basically contained
in the DSP-256 stand-alone effects processor. One significant difference from the DSP-256
standard is the clock frequency has been reduced from 36MHz to 32MHz which is available from the
Engine Board.
An official circuit and software description from DOD is not available, thus the following
discussion represents Kurzweil engineering's analysis.
The Effects Mixer and Effects Input Low-Pass Filter circuits are described in 9.4.2 and 9.4.3
connected with schematic page 4.
5.1.5.2
EFFECTS A/D CONVERTER
The single channel of A/D conversion and two channels of D/A conversion needed would normally re-
quire one AID converter and two D/A converter chips. However in the DOD circuit, one l6-bit
parallel input D/A converter, U16 (G-3 to G-4), is time-shared among the three functions. Analog
switches in UIO and U14 (C-5 to C-7) do the switching among the three functions of U16. The S&HL
and S&HR (Sample and Hold left and right) signals from the Effects LSI chip (U50, C-l to 0-4)
processed through U49, control these switches. Although each switch IC has 3 sections and only 3
switches are needed in all, the AID function switch (U10) was separated from the D/A function
switches (U14) to reduce crosstalk and the resulting distortion. Refer to figure 6 in 9.3.6 (for
schematic page 3) for simplified details on the analog switch operation.
The "A" switch section of U10 along with op-amp U12-7 (0-5) and C43 (0-6) perform as a sample-
and-hold circuit which holds the signal voltage stable while the AID conversion is taking place.
Placing an oscilloscope probe at TP1 will show a sampled version (stair-step approximation) of
the input signal. Voltages higher than 8.0 volts peak at this point will be clipped by the A/D
converter.
U50 performs A/D conversion by sending trial values to U16 which converts them into analog
currents which are converted into voltages by op-amp U13. comparator U11 then tells U50 whether
the trial value is less than or more than the analog input signal at TPI. Based on that result,
U50 sends out another trial value. Sixteen trials are performed to determine the signal voltage
to an accuracy of one part in 65,000 and the whole process requires 24 microseconds
(1.5uS/trial).
There are three adjustments around U16 which are required for quiet, distortion-free performance.
R63 (H-4) adjusts the linearity of U16 by slightly changing the most sifnificant bit value. It
is typically adjusted to minimize visible distortion of a 20mv sine wave. R122 (G-3) adjusts the
DC offset and is normally set for +50mv at TP6 with no signal to minimize residual distortion of
small signal amplitudes. R69 (G-3) adjusts the full scale signal level and is normally set for 8
volts full scale at TP6.
Throughout the Audio/Power Board, Digital Ground (Ognd) and Analog Ground (Agnd) are kept
separate. Since U16 has a single ground pin for both its analog and digital section, the two
ground systems are connected together under U16. This connection is called GJl in the schematic
and is actually two pads under U16 with a trace connected across them on the solder side of the
board. Plugging in the optional hard disk drive may also connect the two ground systems together
at J18 because most hard drives tie the +12 Return and +5 Return pins together at the drive's
power connector. Typical DC power cables however should have sufficient impedance to prevent
serious ground100p problems. If not, it might be necessary to insert a choke in series with the
+12 return lead from the hard disk drive.
5.1.5.3
EFFECTS D/A CONVERSION AND FILTERING
Two analog switch sections of U14 are used for the two channels of D/A output. One at a time is
turned on by signals from U50 through U49 and the corresponding voltage from U16 and U13 is held
on capacitors C45 and C48. The held voltage then appears in low impedance form at U15-l (D-5 to
0-7). As with the sampled input voltage, these two pOints should show stair-stepped
approximations of the processed output signals.
U50 cannot provide settling time for smooth transition between new data being presented to U16
and the output analog switch closures to begin passing the converted voltage to the output. The
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