GA3227 Datasheet, PDF (8/13 Pages) ON Semiconductor – Pre-configured DSP System

CONSOLIDATOR GA3227
high cut filter, one bi−quadratic filter followed by volume
control, and a low−distortion AGC−O.
All AGC loops use a feed−forward topology to ensure
system stability. Detectors in the AGCi path and AGC−O
path are twin fast/slow detectors with independently
adjustable attack and release time constants. The squelch
detector, residing in the AGCi path, is implemented as a
single fast/slow detector.
FUNCTIONAL BLOCK DESCRIPTION
A/D and D/A Converter
The system’s A/D converter is a 2nd−order sigma−delta
modulator, operating at a 1.536 MHz sample rate. The
system’s input is pre−conditioned with anti−alias filtering
and a programmable gain pre−amplifier. The analog output
is over−sampled and modulated to produce a 1−bit pulse
density modulated (PDM) data stream. The digital PDM
data is then decimated down to pulse−code modulated
(PCM) digital words at the system’s sampling rate of
24 kHz.
The D/A is comprised of a digital 3rd−order sigma−delta
modulator and an H−bridge. The modulator accepts PCM
audio data from the DSP path and converts it into a 32−times
over−sampled, 1−bit PDM data stream, which is then
supplied to the H−bridge. The H−bridge is a specialized
CMOS output driver used to convert the 1−bit data stream
into a low−impedance, differential output voltage waveform
suitable for driving zero−biased hearing aid receivers.
Channel Processing
Figure 6 represents the I/O characteristic of the AGC
channel processing.
• 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 AGC 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)
During Parameter Map creation, constraints are applied to
the compression parameters to ensure that the I/O
characteristics are continuous. In both Extended
Programmable mode and Trimmer mode, predefined
parameter adjustments support two popular styles of
compression ratio adjustment:
• The compression region of the I/O curve pivots about
the upper threshold. As the compression ratio trimmer
is adjusted, high−level gain remains constant while the
low−level gain changes.
• The compression region of the I/O curve pivots about
the lower threshold. Low−level gain remains constant
as the compression ratio trimmer is adjusted.
Parameters such as low level gain, lower threshold and
compression ratio can be combined and controlled by a
single trimmer.
The squelch region within each channel implements a low
level noise reduction scheme (1:2 expansion) for listener
comfort. This scheme operates in quiet listening
environments (programmable threshold) to reduce the gain
at very low levels.
Single−channel compressor operation is supported by
disabling the band split filter and one of the channel
compressors. The remaining compressor can be configured
as a Wide−Dynamic−Range Compressor (WDRC), or as an
Input Compression Limiter (ICL).
HRXt Head Room Expander
The Consolidator GA3227 has an enhanced Head Room
Expander (HRX) circuit that increases the input dynamic
range of the Consolidator GA3227 Digital without any
unwanted audible artifacts. This is accomplished by
dynamically adjusting the preamplifier’s gain and the
post−A/D attenuation depending on the input level.
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
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