Introduction
This is the first article of the second phase of the project development: the implementation of the diagnostic probes.
The first implementation described below if the digital microphonic stethoscope based on the usage of the Cirrus Logic Audio Card.
The principle
Apparently the technical principle is almost simple. A small condenser microphone should replace the traditional stethoscope earpieces. But - as always occurs - things are not so simple as they seems.
The microphonic stethoscope replaces the standard earpieces with a small condenser microphone as shown in the images below. Here it was used a common commercial device of 5mm diameter. The microphone should fit in about 20 cm of the stethoscope head. This gives the better sound than put the microphone too near to the stethoscope head. In-line with the microphone there is the cable that is plugged in the audio card. Using a good quality cable with copper shield (leaving it unconnected) I got a good quality/noise ratio. The length of the audio cable is about 2 meters.
The following images shows the microphone in detail
As a matter of fact with the audio card installed and working with those "simple" commands available with the installation you can do nothing, ear nothing and no documentation is provided at all. The solution is inside the commands themselves:
- As the usage is in a headless system there is no reason to involve the mplayer to hear the tests, with the further difficult that without a user interface this command is very difficult to manage. So it has been ignored.
- As the acquisition from the audio card is driven by the arecord linux command and the settings are managed by the amixer command the play features has been set using the aplay command part of the same toolset.
- Based on the low frequencies rom the internal of the human body, a fine tuning has been done to set the better audio acquisition. The attached test file is an example that I have compared with the traditional hearing level and sound quality.
The images below shows the complete probe compared with the traditional one (one hearth, two devices 
)
Sampling and software
To hear the test.wav file you should use - if from a PC - a couple of headphones reporting good bass sounds else it is very low or impossible to hear. Also put the volume to the max level. The following image shows how the acquired sound curve appears:
The file stethoscope.sh is a modified version of the audio card settings with an average quality set for the acquisition / playing of the hearth and general internal human body sounds.
The original sound has been processed (on a fast Mac laptop) using the iZotope software. The following images shows the selective equalisation settings (the noise and unwanted frequencies has been almost completely removed) and the obtained wav file that can be found in attach to this post (the test-processed file)
The definitive approach - that will be discussed in a further article - for the automated processing of the microphonic stethoscope will be done on the dedicated Raspberry PI hosting the sound card. The audio curve optimisation and processing is done with a headless python interface (the real time data are sent to the master device that store them, display etc.) using the snack library that demonstrated to be the open source most reliable and precise project in this field.
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