TAS3103 datasheet(79/146 Pages) TI

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3−19

There are several techniques of volume management for a linear volume control process.

•

Precise calculations involving logarithms can be employed.

•

A high-resolution gain table, with entries for every 0.5-dB step, can be employed.

•

A more coarse gain table (entries in 3 to 6-dB steps with linear interpolation between entries) can be

employed.

•

Or approximations involving very simple calculations can be employed.

As an example of using approximations, equations for increasing a linear 5.23 gain setting X by 0.5 dB that involve

only simple binary shift and add operations and the accuracy of these equations are given below.

20log10X + 0.5 dB ≈ X + 2−4X gives 0.52657877-dB steps

20log10X + 0.5 dB ≈ X + 2−4X − 2−8X gives 0.49458651-dB steps

20log10X + 0.5 dB ≈ X + 2−4X − 2−8X + 2−11X gives 0.49859199-dB steps

20log10X + 0.5 dB ≈ X[1 + 2−4 − 2−8 + 2−11 + 2−12 − 2−13 + 2−14 − 2−16 + 2−18] gives

0.4999997332 dB−steps

Approximations can also be found for decreasing a linear 5.23 gain setting X by 0.5 dB that involve using only simple

binary shift and add operations, but the equations differ slightly from those used to increase the gain in 0.5-dB steps.

The approximations to decrease the volume by 0.5 dB and the accuracy of these approximations are given below.

20log10X − 0.5 dB ≈ X − 2−4X gives −0.56057447 dB-steps

20log10X − 0.5 dB ≈ X − 2−4X + 2−7X gives −0.48849199-dB steps

20log10X − 0.5 dB ≈ X − 2−4X + 2−7X − 2−10X gives −0.49746965-dB steps

20log10X − 0.5 dB ≈ X[1 − 2−4 + 2−7 − 2−10 − 2−12 − 2−15 − 2−21] gives −0.5000006792 dB-steps

Repeated use of a set of the above approximations results in an accumulation of the errors in the approximations.

For example, if an application started at 0-dB volume, and repeatedly used the approximations to increase and

decrease the volume, the exact reference point of 0 dB would be lost. If an application does require the maintenance

of accurate reference points, it is necessary for the application to establish a set of exact gain reference settings and

command these exact settings in place of a computed gain setting whenever the current gain setting and the next

computed gain setting straddle an exact gain setting.

Table 3−4 lists the I2C coefficient settings to adjust volume from 24 dB to −136 dB in 0.5-dB steps. For each volume

setting, the gain in dB is presented in one column, the same gain in a floating point number

float

+ 10

Gain

presented in the adjacent column, and the same gain formatted in the 32-bit hexadecimal gain coefficient format

required to enter the value into the TAS3103 via the I2C bus is presented in a third column.

TAS3103 Datasheet(PDF) 79 Page - Texas Instruments