GA3219 Datasheet, PDF (8/14 Pages) Gennum Corporation – Venture-TM Digital DSP System

VENTURE GA3219
FrontWave Directionality
The FrontWave block provides the resources necessary to
implement directional microphone processing. The block
accepts inputs from front and rear microphones and provides
a synthesized directional microphone signal as its output.
The directional microphone output is obtained by delaying
the rear microphone signal and subtracting it from the front
microphone signal. Various microphone response patterns
can be obtained by adjusting the time delay.
The FrontWave circuit also provides a fixed filter for
compensating the sensitivity and frequency response
differences between microphones. The filter parameters are
adjusted during product calibration.
A dedicated biquad filter following the FrontWave block
has been allocated for low frequency equalization to
compensate for the 6 dB/octave roll−off in frequency
response that occurs in directional mode. The amount of low
frequency equalization that is applied can be determined
during product calibration.
ON Semiconductor recommends using matched
microphones with FrontWave.
Channel Processing
Figure 6 represents the I/O characteristic of independent
AGC channel processing. The I/O curve can be divided into
the following main regions:
• Low input level expansion (squelch) region
• Low input level linear region
• Compression region
• High input level linear region (return to linear)
0
−10
−20
−30 _{Low Level}
−40 ^Gain
−50
−60
Compression
Ratio
Lower
Threshold
High Level
Gain
Upper
Threshold
−70
Squelch
−80
Threshold
−90
−100
−120 −110 −100 −90 −80 −70 −60 −50 −40 −30 −20
INPUT LEVEL (dBV)
Figure 6. Independent Channel I/O Curve Flexibility
The I/O characteristic of the channel processing can be
adjusted in the following ways:
• Squelch threshold (SQUELCHTH)
• Low level gain (LLGAIN)
• Lower threshold (LTH)
• High level gain (HLGAIN)
• Upper threshold (UTH)
• Compression ratio (CR)
To ensure that the I/O characteristics are continuous, it is
necessary to limit adjustment to a maximum of four of the
available five parameters. During Parameter Map creation,
it is necessary to select four parameters as user adjustable,
or fixed, and to allow one parameter to be calculated.
The squelch region within each channel implements a low
level noise reduction scheme (1:2 or 1:3 expansion ratio) for
listener comfort. This scheme operates in quiet listening
environments (programmable threshold) to reduce the gain
at very low levels. When the Squelch and AFC are both
enabled, it is highly recommended that the Squelch be
turned ON in all channels and that the Squelch thresholds be
set above the microphone noise floor. For details, refer to the
AFC description.
The number of compression channels is programmable in
ARKonline^® and can be 1, 2 or 4.
Telecoil Path
The telecoil input is calibrated during the Cal/Config
process. To compensate for the telecoil/microphone
frequency response mismatch, a first order filter with
500 Hz corner frequency is implemented. Through
ARKonline, it is possible to implement a telecoil
compensation filter with an adjustable corner frequency. To
accommodate for the gain mismatch, the telecoil gain is
adjusted to match the microphone gain at 500 Hz or 1 kHz
(default) and is selectable in ARKonline.
There is also a telecoil gain adjustment parameter that can
be enabled in ARKonline and set in IDS, enabling manual
adjustment of the telecoil gain compensation.
Graphic Equalizer
Venture GA3219 has an 8 band graphic equalizer. The
bands are spaced logarithmically, and each one provides up
to 24 dB of gain adjustment in 1 dB increments.
Biquad Filters
Additional frequency shaping can be achieved by
configuring generic biquad filters. The transfer function for
each of the biquad filters is as follows:
H(z)
+
b0 ) b1
1 ) a1
z^−1 ) b2
z^−1 ) a2
z^−2
z^−2
Note that the a0 coefficient is hard−wired to always be ‘1’.
The coefficients are each 16 bits in length and include one
sign bit, one bit to the left of the decimal point, and 14 bits
to the right of the decimal point. Thus, before quantization,
the floating−point coefficients must be in the range
−2.0 ≤ x < 2.0 and quantized with the function:
round ^ǒx 2^14Ǔ
After designing a filter, the quantized coefficients can be
entered into the PreBiquads or PostBiquads tab in the
Interactive Data Sheet. The coefficients b0, b1, b2, a1, and
a2 are as defined in the transfer function above. The
parameters meta0 and meta1 do not have any effect on the
www.onsemi.com
8