Image Processing Reference

In-Depth Information

Logarithmic

scale

Linear increments

Figure 7-7
Logarithmic scale.

The value measured in dB is a ratio and therefore it only has meaning when

described with respect to a reference value. Taking our 1 mW reference, any values for sig-

nal levels relative to that would be measured in dBmW, which is shortened to dB(m).

The formula for power ratios applies to power measurements in watts. Audio mon-

itoring also measures voltage. Virtually the same formula is used but the result must be

squared. Squaring a value that is converted to a logarithm is accomplished by doubling

the result. Here is the formula for expressing voltage ratios in dB.

()

Voltage1

Voltage2

Voltage Ratio = 20 * log

Computing the dynamic range of the signal for a given sample word size is accom-

plished by adapting the voltage ratio formula like this:

(

)

2
word-size

Dynamic range = 20 * log

So for different word sizes in our sampling scheme a table of dynamic ranges can be

computed as shown in Table 7-1.

So, the dynamic range describes the difference between the quietest sound and the

loudest sound that is accommodated within the range of possible values.

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