HBA09-001-70
HB-NEDSP1068
6
6.1.5 Using External Line Level Sources
Both NEDSP1068 module versions have the same pin-outs for the noise cancellation
level, power and audio in/out. To input a signal source that bypasses the Preamp/AGC
signal, connect to pin 9 of J2, with the ground connected either to the MIC Ground or to
the J2 Pin 7. The
PRE
jumper needs to be removed using a good quality solder braid,
and the pads should be inspected to ensure that they are not connected together.
6.1.6 Setting the Preamplifier Gain
Dependent on the what sort of signal source is being used, the Pre-amplifier gain needs
to be set appropriately such that the AGC system is capable of accommodating the
complete signal level range without causing the output to be continuously compressed
or distorted (when the AGC is unable to cope). See the table below for the relevant
gain settings.
Gain A
B
40 dB
Soldered
Open
50 dB
Open
Soldered
60 dB
Open
Open
6.1.7 AGC Settings
Attack and Release Time Setting
The AGC has an attack time set by C27 & C31 which can be selected with the
‘AT’
link.
The
AT
link is not soldered for standard the factory setting, and the
A/R
links will be left
open, setting the Attack Time to 0.11mS, and the Release Time to 55Ms, which should
be adequate for normal use. However, for more noisy communication requirements a
setting of 0.276 mS Attack time (
AT
Link soldered) and 550mS Release time (
A/R
Link
1 soldered) will provide a better solution. For link pad locations, refer to the board
layout shown on page 10 of this document. The Attack and Release times may be
altered by changing the jumper settings as given below:
Link Settings
‘AT’ link
Attack Time
A/R Link 1
& 2 Open
A/R Link
1
soldered
A/R Link
2
soldered
Open 0.11mS
440mS
220mS
55mS
Soldered 0.276mS 1140mS
550mS
138mS
Because the DSP algorithm take up to one second to discriminate between noise and
voice signals, it can be seen that the release time increments being smaller may not
make a great deal of difference.
