NEDSP900 Development Board
8
higher input setting. If on the other hand you find that you have an insufficient audio level try
increasing the source level or change J1 through to J4 settings for the next lower input level.
When using a Line input make sure that the Jumpers are removed all together.
3.0
NEDSP900 Gain Settings
The NEDSP900 Gain is set using SW2 for Channel 1 and SW3 for Channel 2.
Channel 1 has a gain setting capability of up to 30dB, while Channel 2 has gain settings up to 18dB only.
The reason for this, is that Channel 1 also has the AGC algorithm making use of the full 30 dB gain range
which makes it useful for Microphone operation, especially electret modules without any pre-
amplification. The following is the Gain Settings for this channel:
Channel 2 has no AGC action and is designated as a Speaker or high level audio channel, therefore high
gain settings are not required.
Gain (dB)
SW1 Gain
Bit 1
SW1 Gain
Bit 0 (LSB)
0
OFF
OFF
12
OFF
ON
15
ON
OFF
18
ON
ON
Note: SW1 – Bit 2 (MSB) is the Special Facility control and therefore has no association with the
Channel 2 Gain setting
As a general idea of what to expect from the gain setting on either channel, the following are some
typical input signals/results obtained while trying maintain an output of 600mV P-P at the different
module gain settings. This was a single tone (1 kHz) test the DSP noise reduction was switched OFF.
You can see that the actual gain of the DSP module isn’t actually what was set and therefore a certain
amount of experimentation may be required to achieve the required settings for your application.
Gain (dB)
SW4 Gain Bit 2
(MSB)
SW4 Gain Bit
1
SW4 Gain Bit
0 (LSB)
0
OFF
OFF
OFF
12
OFF
OFF
ON
15
OFF
ON
OFF
18
OFF
ON
ON
21
ON
OFF
OFF
24
ON
OFF
ON
27
ON
ON
OFF
30
ON
ON
ON
