bhi Ltd
DSP Noise Cancelling Modules
With AGC action Preamplifier
PO Box 318, Burgess Hill, RH15 9NR England.
Tel: +44 (0)1444 870333 Fax: +44 (0)845 217 9936
info@bhi-ltd.com, www.bhi-ltd.com
NEDSP1068-PCBA
NEDSP1068-PCBA-MIC
& on-board mic option
Document: HBA09-001-70 Issue: 7 Date: 20-08-2012
HBA09-001-70 HB-NEDSP1068
INDEX
1. Introduction
2. Basic Specifications
3. Installation
4. External Overload Indicator
5. Use with On-board Microphone
6. External Microphones or signals
6.1.1. Connecting and external Microphone Capsule or signal
6.1.2. Dynamic Microphones
6.1.3. Condenser Electret
6.1.4. External Signal Sources
6.1.5. Using external Signal Sources
6.1.6. Setting the preamplifier gain
6.1.7. AGC Settings
6.1.8. Setting the output level
6.1.9. Noise Cancellation Settings
Appendix A – Block Schematic and PCB Outline
Appendix B – Connection pin outs and Jumper settings
Appendix C – PCB Dimensioned Drawing
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HBA09-001-70 HB-NEDSP1068
1. Introduction
The NEDSP1068 module is a small DSP noise cancellation module, designed mainly
for cleaning up noisy voice communications in the microphone path. It is based on the
proven bhi NEDSP1061 pcb module, but has the added feature of an on-board AGC
mic preamplifier with various options. This enables the module to be fitted easily into
the microphone path of a target audio system, as well as in the receive path for use with
line level audio signals. There are two options: The basic module is the NEDSP1068PCBA, which has an adjustable AGC mic pre-amplifier circuit, and is able to operate
with either an external electret or dynamic microphone. The NEDSP1068-PCBA-MIC
has an on-board SMD microphone, and the adjustable AGC MIC pre-amplifier circuit.
2. Specifications
DSP Noise cancelling 9 to 35dB
Input Power 5 – 18v D.C.
Input Current 30mA
Audio input level Before Preamp and AGC - 4mV to 100mV p-p dependent on
preamp gain setting - Line Input - 800mV p-p
Input Impedance Pre-amplifier -100KOhm
Line in direct - 10KOhm
Audio Output level Adjustable from 0v to 2.0v p-p
Output Impedance 10 K Ohm
Preset Controls DSP input level, DSP Output level
Optional PIC ‘beep tone’ level control (Tx)
Settings Pre-amplifier Gain
AGC Attack and Release Times
Internal/External Microphone
Noise Cancelling Level
Indications On-board Overload LED
Off -board Overload LED Drive capability
Board size 30mm x 36.5mm *
Mounting Hole pitch 24mm x 27.5mm *
Connectors Pitch 0.1” *
Options On board controller for use with additional keyboard circuitry
*- See Appendix C – PCB Dimensioned Drawing
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HBA09-001-70 HB-NEDSP1068
3. Installation
IMPORTANT NOTE: ESD – When handling this module, please take all possible
precautions to ensure that the risk of accidental Electrostatic Discharge is
minimised. Use of a personal grounding strap and static dissipative mat is
recommended.
The NEDSP1068-PCBA module can be used two ways. One is to treat it as a subassembly board and mount it directly to a mother board assembly using 0.1” pitch PCB
Header pins. A drawing showing the PCB dimensions and header locations is provided
in Appendix C of this document. The Header pins and the integrated plastic spacer bar
provide a very good securing medium for the PCB, especially if both the main
connections and the external microphone pins are used. Alternatively, the main
connector may be used and the two M3 securing holes at the other end. The other way
of mounting is to connect the module in-line with the microphone path by wires to followon circuits, then all the mounting holes may be used to mount the pcb in a suitable
location or housing. When using hardware to secure the PCB take care to note the fact
that there are tracks very close to the mounting holes and therefore nylon washers may
be required under the heads of any screws of nuts used. Please note that the securing
holes are not part of the board grounding mechanism.
Note: Both module versions are supplied with all jumpers and links open. Before
powering up the module, ensure that all the jumper links have been set correctly for
your application and microphone type. In all cases, except when using the line level
input from pin 9 J2, the PRE jumper link must be soldered.
4. External Overload Indicator
The module has connections that enable the Overload LED mounted on the PCB to be
duplicated for use on a front panel, or used for monitoring by other circuitry. The
connections for this can be found on J4 (Ovr). Pin 5 Ovr needs to be connected to the
LED cathode, while the LED anode needs to be connected to Pin2 ( V+) via a 180R
resistor.
5. Use With The On-board Microphone
The on-board Knowles SPM0204HD5 SMD microphone (NEDSP1068-PCBA-MIC)
makes this PCB extremely useful for many products. However, some consideration
should be given to the way in which the PCB is mounted within the equipment to ensure
that the pcb mounted microphone doesn’t pick up mechanical noise and vibration from
surrounding parts, such as power supplies and fans etc.
6. External Microphones or Signals
The standard NEDSP1068-PCBA does not have the on-board a microphone built onto
it, and can be used with a variety of external microphones.
To select the external mic option, ensure that the PRE jumper link is soldered, and the
jumper marked MIC
solder braid.
is open. You may need to remove the solder link carefully using de-
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HBA09-001-70 HB-NEDSP1068
6.1.1 Connecting an external microphone capsule or line signal
Locate and identify the jumper J4 (see diagram in appendix A). See the table below for
the correct connections for the different types of microphone.
Pin Designation Use Operation
1 I or In External Microphone External Microphone or low level audio
input
2 + or V+ Microphone Supply Separate Bias Supply 3v3 (3 pin
electrets)
3 0 or Gnd Microphone Ground Ground
4 S or Scr Microphone Shield Ground
5 O or Ovr External Overload
LED
6.1.2 Dynamic Microphones
Dynamic microphones tend to have a lower output level than active Condenser Electret
devices, but can have a much more rounded audio reproduction.
To use this type of device, resistor R7 needs to be removed. This can be found on the
underside of the PCB as shown in photograph in appendix A on page 11. Ensure that
the PRE jumper is soldered and the MIC jumper is open. The Pre-amplifier Gain setting
needs to be set to 60 dB (links A & B open, see section 6.1.6).
The microphone connections are then made to MIC IN (J4 pin 1) and MIC GND (J4 pin
3) respectively.
6.1.3 Condenser Electret Microphones
Electret microphones need to be biased either via the third pin or via the microphone
input lead. Both types of microphone have been catered for on the 1068 module. For
self biased devices R7 needs to be fitted. This is nominally factory fitted for 2.2K Ohms,
but may be adjusted as per the following formula:
R7 = 3.3v – 2 v (Mic voltage)
0.5 mA
External biased capsules will need to use the regulated voltage available on J4 pin 2.
This supplies 3v3 for the microphone bias.
The pre-amplifier gain should be set to 50dB for most electret inserts (see section
6.1.6).
6.1.4 External Signal Sources
The microphone input will also accept low level signals up to about 500mV p-p.
However, the pre-amplifier gain should be set to its lowest (40dB) and R7 needs to be
removed.
External Overload LED facility
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HBA09-001-70 HB-NEDSP1068
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
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.
A/R Link
1
soldered
A/R Link
2
soldered
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HBA09-001-70 HB-NEDSP1068
Disabling the AGC Action
For certain applications the AGC system may need to be disabled such that the preamplifier is not limited in any way. To disable the AGC circuit, locate and remove R16
on the underside of the 1068 PCB as shown below.
If the AGC system is disabled the user should be aware that too higher gain setting
could give rise to overdriving of the DSP device, which may then cause it to malfunction.
The Overload indicator should operate under these circumstances to help prevent this
happening, but prevention is preferable and will make the module more reliable.
To disable AGC remove R16
Underside of 1068 PCB showing location of R16
6.1.8 Setting the output level
Whilst the modules have been preset during manufacture, you will probably need to
carry out minor adjustments to ensure that you have the required output level and the
best response under all signal conditions for your application.
With the module connected to your system, apply power and check the audio
performance.
Before going any further, reduce the noise cancelling level or switch the noise
cancellation facility off – see Noise Cancelling settings (para 6.1.9).
Using the maximum audio level that your system is likely to encounter, check that the
Overload LED is illuminating. Adjust potentiometer P2 (Audio in) to ensure that the LED
illuminates and then back it off until there is just the odd flash on the LED.
Adjust potentiometer P1 (Out) for the required audio level into your system.
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HBA09-001-70 HB-NEDSP1068
Check to make sure that the audio is clear and distortion free.
If there seems to be some spitting or distortion heard under sudden loud noises, then
consider adjusting P2 (Audio in) again, counter clockwise very slightly and re-check
under the same conditions.
If you have an oscilloscope to hand you can check the audio levels at the output and at
the junction of R3/C13 (see page 11 for the top side of the PCB layout). The signal at
this point should be no more than 2.0 V p-p maximum. Failure to observe this level can
lead to the DSP IC shutting down, this then requires power to be removed from the
board and then re-applied to cause a reset.
Alternatively, probe the wiper of P2 (Audio in) and check that the signal doesn’t peak
over 625mV peak/1.25volts p-p. This will ensure that the output signal won’t spit on
peaks or cause the DSP device to shut down until power is removed and re-applied.
6.1.9 Noise cancelling settings
Jumpers J5 through to J7 select the level of noise cancellation (see the table below).
For most operations level 4 is quite adequate, but for more severe conditions, the level
may be increased to 5, 6 or 7.
It will be found that under certain high noise conditions, and with poor speech quality,
levels 7 and 8 can cause the audio quality to be rather mechanical with musical tones
being present with the audio. This may be an acceptable trade off because as you may
now be able to clearly hear the speech, whereas before the high noise prevented this.
To inhibit the noise cancelling algorithm completely, pin 8 of J2 should be grounded.
Audio will be passed through un-processed. It is recommended that a switch be fitted to
perform this function, so that you can check the audio quality and the filter performance.
Level Tone
Reduction
(dB)
1 4 9 X X X
2 5 11 - X X
3 6 13 X - X
4 8 15 - - X
5 16 17 X X 6 21 20 - X 7 25 24 X - 8 65 35 - - -
X – Jumpers require fitting to obtain the filter level. Remove all jumpers if the module is
to be used with an external controller or via J2 pins 1, 2, 3 and 8.
White Noise
Reduction (dB)
N0 N1 N2
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HBA09-001-70 HB-NEDSP1068
Appendix A – Block Schematic and PCB outline
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HBA09-001-70 HB-NEDSP1068
PCB outline showing top side of PCB and user link pads
R7 may be found on the reverse side of
this PCB as shown opposite.
On-Board microphone
R7 location
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HBA09-001-70 HB-NEDSP1068
Appendix B - Connections and Jumper settings
J2 – General Input/Output Port
Pin Designation Use Operation
1 N0 Noise Cancelling level
Bit ‘1’
2 N1 Noise Cancelling level
Bit ‘2’
3 N2 Noise Cancelling level
Bit ‘4’
4 TX or T Data TX/Beep out Control data or Audio Beep
5 Vin or + +Ve Supply +5 v to +18v DC
6 RX or R Data RX/Control
Button
7 0v or - Ground Ground
8 NOFF Noise cancel ON/OFF Selected when High
9 Audio In or I External Line Input Maximum 900mV input
10 Audio Out or O Audio Output Variable output up to 2.5v p-p
Selected when High, weak pull up
applied
Selected when High, weak pull up
applied
Selected when High, weak pull up
applied
Control data or Control button input
J4 – External Microphone and indicator Port
Pin Designation Use Operation
1 In or I External Microphone External Microphone or low level audio
input
2 V+ or + Microphone Supply Separate Bias Supply 3v3 (3 pin
electrets)
3 Gnd or - Microphone Ground Ground
4 Scr or S Microphone Shield Ground
5 Ovr or O External Overload LED External Overload LED
Jumper Settings
Pre-amplifier gain
Gain A B
40 dB Soldered Open
50 dB Open Soldered
60 dB Open Open
AGC Attack & Release Times
‘AT’ link Attack Time A/R Link 1
& 2 Open
Open 0.11mS 440mS 220mS 55mS
Soldered 0.276mS 1140mS 550mS 138mS
A/R Link 1
soldered
A/R Link 2
soldered
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HBA09-001-70 HB-NEDSP1068
Internal Microphone to Pre-amplifier Routing
Solder the ‘MIC’ Jumper, note: that this has to be removed for any external microphone
use.
Pre-amplifier to DSP Routing – Solder the ‘PRE’ Jumper, note that this has to be
removed for any LINE level signal input to the DSP device.
DSP Level Settings
Level N0 N1 N2
1 X X X
2 - X X
3 X - X
4 - - X
5 X X 6 - X 7 X - 8 - - -
X – Jumpers require fitting to obtain the level. Remove all jumpers if the module is to be
used with an external controller such as the –KBD version or via J2 (pins 1, 2, 3 and 8)
Note: By grounding Pin 8 of J2, audio at the input to the DSP will be passed directly
through the DSP to the output without being processed by the Noise Cancellation
Algorithm.
Optional Connector – J1
For use with boards fitted with microcontrollers for push button or remote control similar
to NEDSP 1061 – KBD.
Pin Number Use
1 ICSP Data
2 Vdd
3 ICSP Clock
4 MCLR/Vpp
5 Vss
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HBA09-001-70 HB-NEDSP1068
Appendix C - PCB Dimensioned drawing
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