BHI NEDSP900 User manual

bhi Ltd
Document: HB07-003-10 Issue: B
PO Box 318, Burgess Hill, RH15 9NR England.
Tel: [phone]
[e-mail], www.bhi-ltd.com
(Picture of a basic assembled unit)
Handbook for
NEDSP900 Development
Board

NEDSP900 Development Board
2
Contents
1.0 Introduction
1.1 Recommendation
1.2 Board Mechanics
1.3 General Specifications
2.0 The inputs
2.1 SK6 Speaker/Headphone/Line Signals
2.2 SK7 Microphone Input Signals
2.2.2 Electret Bias
2.3 Balanced Input Pre=amplifiers
2.4 Signal Input Selection
2.5 Overload Indicators
3.0 Gain Settings
3.1 Setting the gain for SK7 (Microphone input)
4.0 Noise Reduction Settings
5.0 Outputs
5.1 Line Drive
5.2 Headphone/Speaker Output
6.0 Other Facilities on the Board
6.1 Audio Monitor Points
6.2 Noise Reduction Control
6.3 Algorithm ‘Hold’ (SF)
7.0 Connector Specifications
Appendix A –PCB Schematic Diagrams, Component Positions and BOM
Appendix B –Balanced Input Amplifiers
Appendix C –Fitting the board to an enclosure

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1.0 Introduction
The NEDSP900 is a dual channel audio DSP Noise Suppression pcb module for improving the speech
quality in Voice communication systems used in noisy environments. It is capable of handling a wide
range of audio inputs and providing up to 40dB of noise suppression. The module operates on a single
supply rain with on‐board voltage regulation and clock oscillator. It has low power consumption which
makes it suitable for portable applications.
The module has a 26 pin Dual-in-line format with 2mm pin spacing allowing it to ‘plug-in’ to a standard
2mm pitch socket strip.
The NEDSP900 development board was to aid the design-in of the NEDSP900 module. The board allows
various signals to be input to the module and then provides both a Line Out and a Headphone/Small
speaker output to evaluate the processed signal.
Parts of the board have not been populated, but Circuits, Bills of Material (BOM) and information as to
what components fit in these areas are given for the user to add to whatever level they require. This
information can be found throughout this document, but specifically in Section 7.0 and the various
Appendices.
The board has been created such that it can be mounted on standard M3 type pillars for ease of use.
However the format also allows a standard extruded case to be purchased and modified to may this test
board into a robust unit for field testing of the boards’properties.
1.1 Recommendation
We would like to suggest that you just take a few minutes to read through this document first
prior to using the eval board. Some comments have been highlighted in bold,which you should
consider before you connect this device up to your system.
1.2 Board Mechanics
The PCB is approximately 99.5 mm x 159.5 mm and is supplied with M3 x 10mm standoffs as
supports (in 6 positions, see diagram overleaf). These allow components on the underside of the
board to clear the bench. If the PCB is to be fitted into an enclosure, these standoffs may be
used for this purpose or exchanged for shorter ones.
An extruded case can be used to house this board. This provides plenty of room for the board
plus protection for initial system integration when carrying out larger projects. Information
regarding this case and an alternative, if other supplementary circuitry is to be built in, is given
in Appendix C. Drilling patterns for both the front and back covers can also be found in
Appendix C.

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1.3 General Specifications
Operating voltage: 8 to 16v d.c.
Note: While Current and Reverse Polarity protection has been built into the Power circuitry on this
PCB, please check that the power lead is connected with the correct polarisation before applying
power.
Audio Input Acceptance: Input –Stereo (Audio 1 –Plug Tip/ Audio 2 –Plug Ring)
8 Ohms at 2Watts RMS
32 Ohms at 32mW
10K Ohm at 600mV p-p
Balanced Audio Input amplifiers with Phantom Power facility have
been provisioned, but not populated on the PCB (See Appendix B)
Audio Outputs: Headphones/small speakers –0.5 + 0.5 Watts into 8 Ohms
Line Out - Both channels 600 Ohms at up to 0dBV
Connections: Audio Input
Mono Microphone Input
Line Output
Headphone/ Low power Speaker Output
Power 2.1mm Coaxial Power Jack
Various other connections using 0.1” connectors, mainly Molex KK,
have been provisioned for monitoring and control purposes
Switches 4 off 4 Pole DIL Switches have been provided for the purpose of Gain
and Noise Reduction control
Indicators: Power ON
Audio 1 Overload
Audio 2 Overload
Important: Take appropriate Electro-Static precautions before handling the PCB and while
making any additions or changes to the PCB components.
2.0 The Inputs
There are two main input sockets SK6 and SK7 that allow various levels
of signal to be accommodated. Also, there are two balanced audio
input amplifiers with Phantom Power capability. These amplifier
circuits are included on the PCB, but not populated. The circuitry plus
some explanation and the parts list are provided in Appendix B.

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2.1 SK6 Speaker/ Line signals
This socket allows for stereo speakers/headphone signals to be input, matched and attenuated.
The input connection Tip is connected to Channel 1 of the NEDSP900 module, while the first
Ring is connected to Channel 2 of the module. Each channel has attenuation/matching facilities
that can be selected using Jumpers JP 1 through to JP4 as given in the table below:
Low Impedance
Speaker Level
(8 Ohms/3 Watts)
Low Impedance
Headphone Level (33
Ohms/30mW)
High Impedance
Line level (10k
Ohm/ 0.707 v p-p)
Input 1- Ring
JP1 (Not JP3)
JP3 (Not JP1)
None
Input 2 - Tip
JP4 (Not JP2)
JP2 (Not JP4)
None
Below is a picture to assist in the location of the above jumpers.
Above: Jumpers set for Speaker Level Inputs Above: Jumpers set for Headphone Level inputs
Note: For Line levels, the Jumpers are removed altogether.
2.2 SK7 Microphone Input signals
A standard 3.5mm stereo jack socket is provided such that a Condenser Electret or other
microphone may be connected to channel 1 only as this has higher gain settings and an AGC
function that can be used if required.
When using SK7, ensure that JP8 (Mic) is removed
(this ensures no speaker signal is fed to Channel 1).
Speaker level signals will only be taken from the
Ring connection of SK6.
2.2.2 Electret Microphone Bias
Most Electret microphones require some form of
electrical bias. In some this is provide by an