RHYTHM R3910
www.onsemi.com
13
Figure 6. Independent Channel I/O Curve Flexibility
−100
−90
−80
−70
−60
−50
−40
−30
−20
−10
0
−120 −110 −100 −90 −80 −70 −60 −50 −40 −30 −20
OUTPUT LEVEL (dBV)
INPUT LEVEL (dBV)
Low Level
Gain
Compression
Ratio
High Level
Gain
Squelch
Threshold
Lower
Threshold
Upper
Threshold
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
last 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 (see Adaptive Feedback
Canceller).
The number of compression channels is programmable in
ARKonline
®
and can be 1, 2, 4, 6 or 8.
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 the Interactive Data
Sheet (IDS), enabling manual adjustment of the telecoil gain
compensation.
Automatic Telecoil
R3910 is equipped with an automatic telecoil feature,
which causes the hybrid to switch to a specific memory upon
the closing of a switch connected to MS2. This feature is
useful when MS2 is connected to a switch, such as a reed
switch, that is open or closed depending on the presence of
a static magnetic field. Memory D can be programmed to be
the telecoil or mic+telecoil memory so that, when a
telephone handset is brought close to such a switch, its static
magnetic field closes the switch and causes the hybrid to
change to memory D. However, it is possible that the hearing
aid wearer may move his or her head away from the
telephone handset momentarily, in which case it is
undesirable to immediately change out of telecoil mode and
then back in moments later.
R3910 has a debounce circuit that prevents this needless
switching. The debounce circuit delays the device from
switching out of memory D when MS2 is configured as a
static switch in ‘D−only’ mode. The debounce time is
programmable to be 1.5, 3.5 or 5.5 seconds after the switch
opens (i.e., the handset is moved away from the hearing aid)
or this feature can be disabled.
DAI Path
The DAI input can be adjusted using a first order filter
with a variable corner frequency similar to the telecoil
compensation filter. Through ARKonline, it is possible to
implement this DAI filter to set either a static or adjustable
corner frequency.
The Mic plus DAI mode mixes the Mic1 and DAI signals.
The Mic1 input signal is attenuated by 0, −6 or −12 dB before
being added to the DAI input signal. The DAI input also has
gain adjustment in 1 dB steps to assist in matching it to the
Mic1 input level.
Graphic Equalizer
R3910 has a 16−band graphic equalizer. The bands are
spaced linearly at 500 Hz intervals, except for the first and
the last band, 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 z
−1
) b2 z
−2
1 ) a1 z
−1
) a2 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
ǒ
x 2
14
Ǔ
