An attempt to build a simple electroglottograph. This circuit uses a small (~1mA) high-frequency (here 400 kHz) AC current to measure small changes in the impedance of the larynx, which correspond to the vocal cords vibrating. The data can be used, for example, for speech processing.
The circuit has several distinct parts:
- the oscillator, which generates the AC waveform used to probe the larynx;
- the impedance sensor, which uses a voltage to current converter to generate a constant-amplitude current waveform from the oscillator signal;
- the AM detector, which extracts the impedance signal;
- the filter, which removes the doubled carrier frequency from the signal (which is only very weakly modulated);
- the amplifier, which raises the level of the signal so that it can be captured by a sound card ADC.
Marek Materzok •
05/20/2016 at 18:32 •
At last, I have finally managed to construct a device with a respectable signal quality. The circuit was completely redesigned, with no subcircuit left unchanged. Amongst the many changes are:
- The power supply was changed from single-supply to dual-supply (I'm currently using two 9V batteries). This allows to use high quality, low noise op-amps, and also reduces noise by moving supply currents away from the ground net.
- The oscillator is now implemented using the 74HCU04 and a 1 MHz ceramic oscillator.
- The supply for the oscillator is a low-noise LDO.
- The current excitation circuit was replaced by a JFET-based, cascoded and denoised current source, switched by a SPDT analog switch through a center-tapped transformer.
- The AM demodulator was completely removed! It turns out that the output signal is already present on the transformer's center tap. Therefore, the SPDT analog switch with the transformer serves both as an excitation circuit and an AM detector. As far as I know, the technique is novel - I haven't seen such a circuit in any patent or paper I have read on the subject.
- I'm using an ultralow noise op-amp (currently an AD8597) for the preamplifier.
- I have changed the lowpass filter topology to multiple feedback and added gain to the filter circuit. I currently have a gain of 100 in the preamplifier and gain of 10 in the active filter.
- The common-mode noise from the probes is additionally filtered using two methods: a common-mode choke in series with the probes and a center-tapped inductor with the center tap connected to ground. The technique is inspired by the common-mode filters used in Ethernet connectors. The filter significantly reduced pickup of external noise.
I have assembled the prototype on a perfboard, the required SMD components were mounted on adapters. Even though the prototype looks messy, it seems to work really well. I believe it will work even better when assembled on a well-routed PCB.
Marek Materzok •
10/09/2015 at 13:07 •
I have the electroglottograph circuit prototyped on a breadboard. It works - you can clearly see the signal, but it's very noisy. And that's only when a man's vocal cords are being probed - the signal taken from a woman's vocal cords is much weaker (probably due to the larynx being smaller in women), and it's almost completely hidden by noise.
The circuit shown on the project's page is actually one of the later iterations of the project. Previous versions included:
- A simpler oscillator, based on a CMOS 555, instead of the Colpitts circuit with a ceramic resonator currently used;
- A decoupled voltage divider instead of the TL431 for the mid-supply voltage;
- A rectifier-based AM detector (using two op-amps and two diodes).
All these changes did not do much to reduce the noise. Where does it come from? I don't know. Does it come from the supply (I currently use a 9v battery regulated by a 7805)? Maybe, but in this case better decoupling in key places should make some difference, but it does not. Is it radio noise? Maybe, but the cable used for the probes is already shielded and twisted, and I tried shielding the entire prototype circuit; no real difference. Is the voltage-to-current converter this noisy? I'm not an expert, but I tried calculating the theoretical noise level, and it's lower than what I observe.
I'm currently abroad and do not have access to my scope, so I cannot look for the noise more directly. I also do not have soldering equipment, so I currently can't check if this is just a breadboard issue. If somebody has more experience building these kinds of circuits and has some idea about the possible source of the noise, I would gladly hear about it.
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