參數(shù)資料
型號： TLV320AIC36IZQE
廠商： TEXAS INSTRUMENTS INC
元件分類： 消費家電
英文描述： SPECIALTY CONSUMER CIRCUIT, PBGA80
封裝： 5 X 5 MM, GREEN, PLASTIC, VFBGA-80
文件頁數(shù)： 85/165頁
文件大?。?/td> 1895K
代理商： TLV320AIC36IZQE
SBAS387A – MAY 2009 – REVISED JUNE 2010
www.ti.com
5.7.3
Fine Digital Gain Adjustment
Additionally, the gains in each of the channels is finely adjustable in steps of 0.1 dB. This is useful when
trying to match the gain between channels. By programming Page 0, Register 82 the gain can be adjusted
from 0 dB to -0.4 dB in steps of 0.1 dB. This feature, in combination with the regular digital volume control
allows the gains through the left and right channels be matched in the range of -0.5 dB to +0.5 dB with a
resolution of 0.1 dB.
5.7.4
AGC
The TLV320AIC36 includes Automatic Gain Control (AGC) for ADC recording. AGC can be used to
maintain a nominally-constant output level when recording speech. As opposed to manually setting the
PGA gain, in the AGC mode, the 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 parameters, 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.
Since the gain can be changed at the sample interval time, the AGC algorithm operates at the ADC
sample rate.
Target level represents the nominal output level at which the AGC attempts to hold the ADC output
signal level. The TLV320AIC36 allows programming of eight different target levels, which can be
programmed from –5.5 to –24 dB relative to a full-scale signal. Since the TLV320AIC36 reacts to the
signal absolute average and not to peak levels, it is recommended that the target level 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 output signal
level exceeds the target level due to increase in input signal level. Wide range of attack time
programmability is supported in terms of number of samples (i.e. number of ADC sample frequency
clock cycles).
Decay time determines how quickly the PGA gain is increased when the output signal level falls below
the target level due to reduction in input signal level. Wide range of decay time programmability is
supported in terms of number of samples (i.e., number of ADC sample frequency clock cycles).
Noise threshold determines the level below which if the input signal level falls, the AGC considers it
as silence, and thus brings down the gain to 0 dB in steps of 0.5 dB every FS and sets the noise
threshold flag. The gain stays at 0 dB unless the input speech signal average rises above the noise
threshold setting. This s that noise is not 'gained up' in the absence of speech. Noise threshold level in
the AGC algorithm is programmable from -30 dB to -90 dB of full-scale. 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 hysteresis and
debounce to avoid the AGC gain from cycling between high gain and 0 dB when signals are near the
noise threshold level. The noise (or silence) detection feature can be entirely disabled by the user.
Max PGA applicable allows the designer to restrict the maximum gain applied by the AGC. This can
be used for limiting PGA gain in situations where environmental noise is greater than the programmed
noise threshold. Microphone input Max PGA can be programmed from 0 dB to 63.5 dB in steps of 0.5
dB.
Hysteresis, as the name suggests, determines a window around the Noise Threshold which must be
exceeded to either detect that the recorded signal is indeed noise or signal. If initially the energy of the
recorded signal is greater than the Noise Threshold, then the AGC recognizes it as noise only when
the energy of the recorded signal falls below the Noise Threshold by a value given by Hysteresis.
Similarly, after the recorded signal is recognized as noise, for the AGC to recognize it as a signal, its
energy must exceed the Noise Threshold by a value given by the Hysteresis setting. In order to
prevent the AGC from jumping between noise and signal states, (which can happen when the energy
of recorded signal is very close to the Noise threshold) a non-zero hysteresis value should be chosen.
The Hysteresis feature can also be disabled.
26
APPLICATION INFORMATION
Copyright 2009–2010, Texas Instruments Incorporated
Product Folder Link(s): TLV320AIC36
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