AM 334-01-502
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The processing circuits for the reverb work similarly. All the
signals which are to be reverberated are sent via IC5c to the
reverb unit DH 10 (Fig. 9). The processed signals-divided
into two channels-return via pins 12 and 13 and are applied
to control amplifiers lC6. This controls the reverb volume
via a control current from QS, driven by IC5b. The
reverberation signals go to the summing buses via R99 and
R104 to be mixed with the "regular" signals.
4. Plug-in Board DH 10 - Digital Reverb
Principles of Operation
Fig. B shows the component layout of the DH 10 digital
reverberation board; Fig. 9 is its schematic diagram. Refer to
Fig. 9 for the following description.
The input signal on pin 5 of the edge connector goes to ICl4a
via C31; the latter serves to emphasize the higher
frequencies; ICl4a and ICl6a and their associated com-
ponents form a dynamic compression and limiting circuit.
Zener diodes D7 and D8 limit the signal to a maximum 5-
Volt level. lCl6b, c and d form a lowpass filter, limiting the
audio signal bandwidth (4 kHz: appr. -60 dB).
Sample-and-hold circuit 1C13 holds the amplitude of the
signal constant for the analog-to-digital conversion cycle in
IC 11. During this cycle, IC11 converts the analog input
signal to a 1 2-bit digital signal. This signal is divided into
two bytes which, via 1C5 and 1C6 are sequentially switched
to IC1, the central processing unit (CPU). The CPU is
controlled by programs (short and long reverb, fast and slow
echo) contained in 1C2, a 2-kilobyte erasable programmable
memory (EPROM) chip.
The digital information (data) from the CPU feeds via a 16-
bit bus into 1C7, a 2K x 8 random-access memory (RAM),
from which it is periodically read out; the RAM then
receives additional data from the CPU, reads it out when
required, and so on. This continuous feed-and readout cycle
achieves a delayed feedback of the digital signal; i.e., the
delayed signal is added to the 12-bit output of the analog-to-
digital converter Cl 1: In this way, reverb delays of up to 4
seconds are achieved.
The processed (reverberated) signal, again in 12-bit form, is
again divided into 2 bytes and fed in sequential steps to
digital-to-analog converter 1C12; the latter produces an
analog signal corresponding to the digital data input. This
signal is further processed in IC15c; however, the output
signal here contains amplitude spikes and undesired
frequency components resulting from the digitalization.
IC15a, b and d form a lowpass filter which removes the
undesired. components from the signal. C40 and lCl4b serve
as deemphasis and bandspreading circuits respectively,
reversing the high frequency emphasis and band limiting
described previously.
The reverberated and delayed output signal goes via Jul to
pin 6 in single-channel instruments. In stereo instruments,
1C17 develops a second-channel output. It achieves this via
defined phase shifting in certain frequency ranges and
applies the second output to pin 9. Further stereophonic
audio processing results in an impressive room-filling
reverberation effect.
The switching of the various operational modes (short and
long reverb, fast and slow echo) is accomplished by applying
voltage impulses to the NMI input of the CPU via pin 12.
Applying a reset to the CPU via pins 10 and 11 starts the
CPU's program run.
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