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We may regard the delay-line memory itself as the fixed ``air'' which
propagates sound samples at a fixed speed
(
meters per
second at
degrees Celsius and 1 atmosphere). The input signal
can be associated with a sound source, and the output signal
(see Fig.2.1 on page
) can be associated with
the listening point. If the listening point is
meters away from
the source, then the delay line length
needs to be
where
denotes the discrete-time sampling interval. In other
words, the number of samples delay is the propagation distance
divided by
, the distance sound propagates in one sampling
interval. In practice, rounding
to the nearest integer
causes no audible difference, unless the echo time is so short that
the system is not really perceived as an echo (we'll learn about
``comb filters'' in §2.6 below).
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