As figure 1.5 shows, one way to make smooth amplitude changes
in a signal without clicks is to multiply by an envelope generator; one is
invoked in block diagram form in figure 1.6. This may be
implemented in Pd using the line~ class:
:
envelope generator.
The output is a signal which ramps linearly from one value to another over
time, as determined by the messages received. The inlets take messages to
specify target values (left inlet) and time delays (right inlet). Because
of a general rule of Pd messages, a pair of numbers sent to the left inlet
suffices to specify a target value and a time together. The time is in
milliseconds (taking into account the sample rate), and the target value is
unitless, or rather, its units should conform to whatever input it may be
connected to.
Patch A03.line.pd demonstrates the use of a line~ object to control the
amplitude of a Sinusoid. The active part is shown in figure 1.11(b).
The six message boxes are all connected to the line~ object, and are
activated by clicking on them; the top one, for instance, specifies that the
line~ ramp (starting at wherever its output was before receiving the
message) to the value 0.1 over two seconds. After the two seconds elapse,
unless other messages have arrived in the meantime, the output remains
steady at 0.1. Messages may arrive before the two seconds elapse, in which
case the line~ object abandons its old trajectory and takes up a
new one.
Two messages to line~ might arrive at the same time or so close together
in time that no DSP computation takes place between the two; in this case, the
earlier message has no effect, since line~ won't have changed its output
yet to follow the first message, and its current output, unchanged, is then
used as a starting point for the second segment. An exception to this rule
is that, if line~ gets a time value of zero, the output value is
immediately set to the new value and further segments will start from the
new value; thus, by sending two pairs, the first with a time value of
zero and the second with a nonzero time value, one can independently specify
the beginning and end values of a segment in line~'s output.
The treatment of line~'s right inlet is unusual among Pd objects in that
it forgets old values; thus, a message with a single number such as ``0.1" is
always equivalent to the pair, ``0.1 0". Most Pd objects will keep the
previous value for the right inlet, instead of filling in zero.
Patch A04.line2.pd shows the line~ object's output graphically, so that
you can see the principles of Figure 1.5 in action.