Datasheet
SLAS412− DECEMBER 2003
www.ti.com
22
See Table 2 for various AGC programming options.
Input
Signal
Output
Signal
AGC
Gain
Decay Time
Attack Time
Target Gain
Figure 18. AGC Characteristics
Table 2. AGC Settings
MIC INPUT
BIT CONTROL REGISTER
AGC enable D0 01H
Target gain D7−D5 01H
Time constants (attack and decay time) D4−D1 01H
Noise threshold D5−D4 06H
Noise threshold flag D11 04H
Hysteresis D10−D9 1DH
Debounce time (normal to silence mode) D8−D6 1EH
Debounce time (silence to normal mode) D5−D3 1EH
Max input gain applicable D15−D9 1EH
Gain applied by AGC D15−D8 01H
Saturation flag D0 04H
Clip stepping enable D3 06H
NOTE
:
All settings shown in Table 2 are located in Page2 of the control registers.
STEREO AUDIO DAC
Each channel of the stereo audio DAC consists of a digital audio processing block, a digital interpolation filter, digital
delta-sigma modulator, and an analog reconstruction filter. The DAC is designed to provide enhanced performance at low
sample rates through increased oversampling and image filtering, thereby keeping quantization noise generated within the
delta-sigma modulator and signal images strongly suppressed within the audio band to beyond 20 kHz. This is realized
by keeping the upsampled rate constant at 128 x Fsref and changing the oversampling ratio as the input sample rate is
changed. For Fsref of 48 kHz, the digital delta-sigma modulator always operates at a rate of 6.144 MHz. This ensures that
quantization noise generated within the delta-sigma modulator stays low within the frequency band below 20 kHz at all
sample rates. Similarly, for Fsref rate of 44.1 kHz, the digital delta-sigma modulator always operates at a rate of 5.6448
MHz.
Digital Audio Processing
The DAC channel consists of optional filters for de-emphasis and bass, treble, midrange level adjustment, or speaker
equalization. The de-emphasis function is only available for sample rates of 32 kHz, 44.1 kHz, and 48 kHz. The transfer
function consists of a pole with time constant of 50 µs and a zero with time constant of 15 µs. Frequency response plots
are given in the Audio Codec Filter Frequency Responses section of this data sheet. The de-emphasis filter can be enabled
or bypassed depending on bit D0 of register 05H/Page2.