TAS3103ADBT datasheet(89/152 Pages) TI

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3−25

data that falls in the subaddress range 0x01 through 0xD1 after issuing an I2C command to change the volume level

on one of the three monaural channels, the busy bit at subaddress 0xFF must be monitored to determine when the

volume activity has ceased and it is safe to resume I2C read activity at subaddresses 0x01 through 0xD1. A value

of 0 in the least-significant bit of the byte output on reading subaddress 0xFF signifies that all volume transition activity

has completed.

3.6.2

Loudness Compensation

Loudness compensation employs a single, coefficient-programmable, biquad filter and a function block

f(volume_setting), whose output is only a function of the volume control setting. The biquad filter input is the channel

processed audio data stream that also feeds the volume gain-control mixer. The biquad output feeds a gain-control

mixer whose other input is the volume-control setting after processing by the function block f(volume setting).

Loudness compensation then allows a given spectral segment of the audio data stream (as determined by the biquad

filter coefficients) to be given a delta adjustment in volume as determined by the programmable function block

f(volume_setting).

The output of the function block f(volume_setting) can be expressed in terms of the programmable I2C coefficients

as:

f(volume_setting) = [(volume_setting)LG

× 2LO × G] +O

where:

LG = logarithmic gain = 5.23-format number

LO = logarithmic offset = 25.23-format number

G = gain = 5.23-format number

O = offset = 25.23-format number

If LG = 0.5, LO = 0, G = 1.0, and O = 0, f(volume setting) becomes:

f(volume_setting) =

√volume_setting

The output of the soft volume/loudness compensation block for the preceding case would be:

audioOut = [audioIn × volume_setting] + [audioIn × F(s)Biquad × √volume_setting ]

Figure 3−17 is a block diagram of the loudness logic. If a given monaural channel is set up as in the preceding

example, a true mute is not obtained when a mute command is issued for the monaural channel. The function block

f(volume_setting) approximates logarithmic arithmetic and a consequence of the mathematical approximations used

is that the function block can output a nonzero value for an input volume setting of 0. For f(volume_setting) =

√volume_setting, a volume_setting value of 0.0 results in the function block outputting a gain coefficient that cuts the

output of the biquad filter by approximately 90 dB. If true muting is required, it can be achieved by setting G = 0 after

the volume control setting has softly transitioned to 0.

Biquad

Audio Out

Audio In

Loudness

Soft Volume

Figure 3−17. Loudness Compensation Block Diagram

TAS3103ADBT Datasheet(PDF) 89 Page - Texas Instruments

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