of Microprocessors

OrHER INPUT METHODS
333
INPUT
SEQUENCE
MAIN FEEDBACK PATH
AUXILIARY FEEDBACK PATH
r-----------------------------------------,
I :
MIX
AMP
~
I
OUT
Fig. 10-8. SAH module feedback sequence generator
late the same sequence. If the SAH modules were perfect, the sequence
would repeat indefinitely, but in reality analog errors would accumulate and
the sequence would evolve in perhaps interesting ways. If, instead of removing
the external input, it were fed a constant 1/12 V, the sequence would
shift upward a halfstep on each interaction. If the feedback gain were greater
or lesser than unity, the pitch intervals in the sequence would progressively
increase or decrease, respectively. Enabling a second feedback path would
create such complex patterns that they may be difficult to predict beyond the
first repetition. The use of slew-limited SAH modules adds yet another
dimension of possibilities.
Digital Feedback Techniques
A digital feedback system can be set up using flip-flops connected as a
shift register. Figure 10-9 shows such a system. The input summer that
drives the register is a parity generator that actually computes the "modulus
2 sum" of all its inputs. The switches enable a particular feedback path if
closed. A low-frequency veo provides trigger pulses to drive the system.
Fig. 10-9. Feedback shift register sequence generator