Conclusion
Our results demonstrate REAL can recover the speech signal by exploiting the back-scattered intensities from vibrating surfaces. With strong resistance to acoustic noise and the ability to collect specific audio signals over long distances, REAL provides a feasible solution to tackle the cocktail party problem in the optical channel. It is demonstrated that REAL could direct ‘hear’ the voices from masks and throats in a noisy environment, where the noise characteristics are fully considered in the hardware and the neural networks could help in signal recovery. Further work could include utilizing additional sensing modalities to enhance the overall detection accuracy such as the audio-visual cues and microphone array. With the high signal quality, simple construction, affordability and miniaturization readiness, we anticipate the REAL system will foster a new way in human-robot interaction, benefiting applications in speaker identification, speech understanding and accelerating the development of voice-guided home and field robots.
Materials and methods
Components and construction of REAL
A low-power collimated laser (such as a laser pointer) can be used as a REAL laser. The detection field of view of the telescoping lens system must align well to cover the surface laser spot. A color filter should be added before the APD to isolate the background light noise, and the laser power and spot size need to be adjusted to ensure laser safety according to IEC 60825-1:2014. Choose an appropriate gain of APD to ensure that shot noise dominates other noises such as electronic noise, while not introducing much excess noise. A low-noise transimpedance amplifier (TIA) and a secondary amplifier might be necessary to amplify the signal. The entire system needs to have a bandwidth beyond doubling the desired highest audio frequency.
SNR model of REAL
The signal of REAL is caused by the varying detection power when the vibrating surface displaces. The signal model is expressed as
\[\Delta U = U - U' = \frac{{e\eta }}{{hv}}{P_0}MR[\frac{1}{{{r^2}}} - \frac{1}{{{{(r + \Delta r)}^2}}}] = A[\frac{1}{{{r^2}}} - \frac{1}{{{{(r + \Delta r)}^2}}}]\]