American Audio UCD-200 - REV 8-10 Specifications download pdf (Page 25)

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SLAS495A− JUNE 2006 − REVISED OCTOBER 2007

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upon power down, the PGA soft-steps the volume to mute before shutting down. A read−only flag (D7 control

soft-stepping control can be disabled by the programming D12=1 in register 1DH of Page 2. When soft-stepping

is enabled and ADC power down register is written, MCLK should be running to ensure that soft-stepping to

mute has completed. MCLK can be shut down once Cell PGA power down flag is set.

Delta-Sigma ADC

The analog-to-digital converter has a delta-sigma modulator with a 128 times oversampling ratio. The ADC can

support maximum output rate of 53 kHz.

Decimation Filter

The audio ADC includes an integrated digital decimation filter that removes high frequency content and

downsamples the audio data from an initial sampling rate of 128 times Fs to the final output sampling rate of

Fs. The decimation filter provides a linear phase output response with a group delay of 17/Fs. The –3 dB

bandwidth of the decimation filter extends to 0.45 Fs and scales with the sample rate (Fs).

Programmable High Pass Filter

The ADC channel has a programmable high-pass filter whose cutoff frequency can be programmed through

control register. By default the high pass filter is off. The high-pass filter is a first order IIR filter. This filter can

be used to remove the DC component of the input signal and offset of the ADC channel.

Automatic Gain Control (AGC)

The TSC2111 includes Automatic gain control (AGC) for Microphone Inputs (MICIN_HED or MICIN_HND) and

Cell-phone input (CP_IN). AGC can be used to maintain nominally constant output signal amplitude when

recording speech signals. This circuitry automatically adjusts the PGA gain as the input signal becomes overly

loud or very weak, such as when a person speaking into a microphone moves closer or farther from the

microphone. The AGC algorithm has several programmable settings, including target gain, attack and decay

time constants, noise threshold, and max PGA applicable that allow the algorithm to be fine tuned for any

particular application. The algorithm uses the absolute average of the signal (which is the average of the

absolute value of the signal) as a measure of the nominal amplitude of the output signal.

Target gain represents the nominal output level at which the AGC attempts to hold the ADC output signal level.

The TSC2111 allows programming of eight different target gains, which can be programmed from –5.5 dB to

–24 dB relative to a full-scale signal. Since the TSC2111 reacts to the signal absolute average and not to peak

levels, it is recommended that the target gain be set with enough margin to avoid clipping at the occurrence

of loud sounds.

Attack time determines how quickly the AGC circuitry reduces the PGA gain when the input signal is too loud.

It can be varied from 8 ms to 20 ms.

Decay time determines how quickly the PGA gain is increased when the input signal is too low. It can be varied

in the range from 100 ms to 500 ms.

Noise threshold determines level below which if the input speech average value falls, AGC considers it as a

silence and hence brings down the gain to 0 dB in steps of 0.5 dB every FS and sets noise threshold flag. The

gain stays at 0 dB unless the input speech signal average rises above noise threshold setting. This ensures

that noise does not get gained up in the absence of speech. Noise threshold level in the AGC algorithm is

programmable from −30dB to −90 dB for microphone input, and from −30 dB to −60 dB for cell-phone input.

When AGC Noise Threshold is set to −70 dB, −80 dB, or −90 dB, the microphone input Max PGA applicable

setting must be greater than or equal to 11.5 dB, 21.5 dB, or 31.5 dB respectively. This operation includes

debounce and hysteresis to avoid the AGC gain from cycling between high gain and 0 dB when signals are near

the noise threshold level. When noise threshold flag is set, status of gain applied by AGC and saturation flag

should be ignored.

Max PGA applicable allows user to restrict maximum gain applied by AGC. This can be used for limiting PGA

gain in situations where environment noise is greater than programmed noise threshold. Microphone input Max

PGA can be programmed from 0 dB to 59.5 dB in steps of 0.5 dB. Cell-phone input Max PGA can be

programmed from −34.5 dB to −0.5 dB in steps of 0.5 dB, as well as +12 dB.

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American Audio UCD-200 - REV 8-10 Specifications Page 25

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