Audio Engineering

468 Chapter 15

(a)

4.0

Height (metres)

Spring tide

High water

Sample

number Value

1 1.55

1.8

1.7

1.6

1.5

12345

Time interval

678910

2 3 4 5 6 7 8 9

10

11

12

1.64

1.56

1.59

1.55

1.75

1.50

1.69

1.75

1.79

1.84

1.85

Hours after high water

 6  5  4  3  2  1  1  2  3  4  5

3.6

3.3

2.8

2.4

2.0

1.6

1.2

0.8

0.4

Data curve Windowed average

Unwindowed average

Figure 15.15(a) : To explain the ideas behind digital fi ltering, we review the shape of the

tidal height curve (Portsmouth, UK, spring tides) for its underlying detail. The pen Plotter

trace would also record every passing wave, boat, and breath of wind; all are overlaid on the

general shape of the curve.