CODEWITHSUNDEEP
objective-ciosaudiosignal-processing
dormitkon
dormitkon

Reputation: 2526

Detecting heartbeats signals with "Digital heart beat rate sensor (IC)" - iOS

I just bought Digital heart beat rate sensor: http://www.dealextreme.com/p/digital-heart-beat-rate-sensor-3-5mm-data-port-16009 And I'm looking how I can make application for iOS to work with.

Sensor has 3.5mm jack and I can detect signal with audio framework on iOS.

Can you give me some guidelines how to start with detecting these signals into heart beat rates?

Upvotes: 3

Views: 1745

Answers (1)

Max
Max

Reputation: 2131

That sensor looks rather like one I have here in my junk box. If so, it generates a voltage signal which depends on the pressure exerted on it by the skin against which it is pressed. If there is a strong pulse at the point of pressure, I see a signal on an oscilloscope which has a component at the pulse rate: so it is at a frequency of around 1-2Hz.

This is WAY below the audio range, and in most audio interfaces would be filtered out before it ever got to the audio in ADC. I don't have a handy iPhone to check this on, but it would be bad design if the audio input did let such frequencies through. And Mr Jobs (R.I.P.) did not approve of bad design!

There is also a lot of interference at other frequencies: mains hum (50Hz here), and at lower frequencies spurious signals from muscle twitches.

To make this work, you would need some sort of signal conditioning. If it was up to me, I would use a high input impedance amplifier, with about a 0.1Hz - 10Hz passband, followed by a voltage to frequency converter. That would give me a tone, which i could set in the audio band, whose frequency varied up & down as the pressure on the sensor changes. That would let me use fairly simple frequency detection software to recover the pressure waveform, which could then be processed using autocorrelation or similar techniques to recover the heartbeat frequency. A DTMF decoder is not the right tool, though.

I did find when I played about with the senor that it was very touchy, responding to almost everything going, and it wouldn't be easy to pick out the heartbeat. Your sensor may be different, though.

Upvotes: 3

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