UM1545 datasheet(25/64 Pages) STMICROELECTRONICS

25 / 64 page

UM1545

APW walkthrough

Doc ID 023225 Rev 1

25/64

gain and the maximum threshold. Then it applies the value as an attenuation, modifying

the device prescaler. By acting in this way, the auto-scaler achieves two key goals:

–

The overall filter’s chain shape is unchanged

–

The dynamic of the filtered audio signal remains within safe boundaries for both

ears and speakers

Figure 22 shows an example of how the auto-scaler works. Two peak filters with a +15 dB

gain each are applied with a +25 dB threshold, thus with a +5 dB in excess. APW auto-

scaler automatically computes the gain in excess and applies the attenuation using the

prescaler. When the total frequency response exceeds the threshold, its plot becomes dark

orange. The prescaler controls (highlighted in red) show the designed filter maximum gain

and the attenuation required to satisfy the requirements. The threshold (marked by an

orange line) is user-definable and can be changed either by editing the Gain limit box or by

manually dragging the marker on the plot.

Figure 22.

Auto-scaler

●

Coefficient scaling: Sound Terminal® products adopt a 24-bit fixed point notation to

handle the filter coefficients. APW is also able to check whether the computed

coefficients fit the notation or not. In this latter case a warning message appears on the

top left of the plot. Exceeding the mathematical notation limits generates a filter

frequency response different from the desired one, to see the real frequency response,

right click on the plot and select Show fixed point plot. A violet graph will appear

showing the real applied filter. Figure 23 shows an example of this behavior.

UM1545 Datasheet(PDF) 25 Page - STMicroelectronics