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Using a microphone with NEDSP01901-PCB module - NEDSP1901-PCB-MIC versionDescription
The NEDSP1901-PCB module has increased sensitivity down to a few millivolts of input. Following
the DSP processor is a buffer amplifier that allows the overall input to output gain to reach up to
36dB and drive directly into a 600 Ohm load. In its normal configuration, the
NEDSP1901-PCB
is set
such that the module throughput gain is a nominal 1. The gain can be increased by about 6dB using
RV2 (Output Level) and should high input levels be encountered RV1 can be adjusted to attenuate
high level input signals.
The
NEDSP1901-PCB
module also has some improved functionality which also allows it to increase
the gain further up to about 30dB for use with very small signals and under normal circumstances
these selectable adjustments are hidden from view by the module itself.
Not only does the module contain the ability to increase the gain by quite a lot, but it also contains
an AGC (Automatic gain Control) facility that selects the gain setting continuously during operation,
lending the module for use as a communications microphone Noise Reduction Module. The
NEDSP1901-PCB-MIC
has been created for this purpose. The NEDSP900 module is fitted on headers
giving access to the relevant jumpers that set the microphone level parameters depending on what
type of microphone is being used.
Using with Microphones
The
NEDSP1901-PCB-MIC
version has as separate plug in DSP module so the following types of
microphones or inserts can be used with this version:
Magnetic or Dynamic inserts ,
MEM’s Capsules
, Condenser Electret (or just Electret) Capsules
Dynamic or Magnetic Microphones
These usually have a very low impedance (between 30 Ohms and 600 Ohms) and provide signal
levels of between -55 dBV/Pa and -40 dBV/Pa. As a word of caution, because of the variability of the
more domestic versions of this type of microphone, the sensitivity can be much greater even
between the same device. Apart from Amateur and CB communications microphones, this type can
be found on aircraft and military headsets where the microphone sits on a boom arm placing it close
to the user’s mouth. This ensures that a reasonable audio signal is gained by the microphone while
extraneous sounds are quite low level. Some versions also have two units wired back to back. This
allows one to pick up the user
’s voice and external noise, while the other picks up mainly the noise
and very little of the user’s voice. Used correctly, these types can improve the user to background
noise signal. The fact that some of these microphones are very low impedance, normally the
industrial and military types, helps to reduce their susceptibility to RF and electrical interference,
however caution should be observed to keep the interconnection leads short and preferably
screened.
Condenser Electret Microphones
These are very common now-a-days and present a cheap reliable device with very good sound re-
production. Working on the principle of capacitance change, a diaphragm is used as one plate of a
capacitor. The capacitor is biased such that a voltage change may be detected as the capacitance
changes with any sound detected. The signal is generally very small which means that a simple
amplifier is normally placed in the capsule to increase the signal level to around -44 dBV/Pa.
Most Electret microphones need a small bias current to power their internal amplifier.
