Abstract

With the increasing demand for efficient data transmission, Visible Light Communication (VLC) emerges as a promising alternative to traditional Radio Frequency (RF) communication. Leveraging the vast spectrum of visible light, VLC offers higher data rates and supports more users in close proximity. Among its many applications, the transmission of audio signals via VLC presents unique opportunities and challenges, particularly in assessing audio quality. This paper evaluates the quality of audio signals transmitted through a short-range analog Intensity Modulation/Direct Detection (IM/DD) VLC link. Modern approaches for enhancing audio signal quality, such as echo cancellation and noise suppression, are reviewed, and key objective metrics like Total Harmonic Distortion (THD), Signal-to-Noise Ratio (SNR), and the Perceptual Evaluation of Audio Quality (PEAQ) are analyzed. An experimental setup using OpenVLC1.3 RevA capes was implemented to measure audio signal quality in real-time. The results indicate that VLC can transmit audio with high fidelity, achieving THD levels below 1%. Future research could explore improvements in modulation techniques, channel coding, and digital audio VLC transmission, with the potential to extend the system’s range and performance. By focusing on both analog and digital transmission methods, this study provides a foundational assessment of audio signal quality in VLC and highlights avenues for further enhancement.

Keywords: Visible Light Communication (VLC), Audio Signal Quality, Intensity Modulation/Direct Detection (IM/DD), Total Harmonic Distortion (THD), Perceptual Evaluation of Audio Quality (PEAQ), Audio Transmission, OpenVLC, Digital Audio.

Introduction

The continuous increase in the amount of traffic in the RF spectrum requires the exploration of new spectrum parts that can provide faster, more reliable, and more efficient data transmission. In comparison to RF communications, VLC has the potential to accommodate more users and provide significantly higher data rates per user due to its enormous bandwidth available for data transmission. The frequency range of visible light (VL) is between 400 and 800 THz; therefore, VL has 10,000 times the range of radio waves. The VLC implies the emission of highly directed and limited VL, thus enabling the coexistence of many non-interfering communication links in close proximity.

In the ever-expanding range of possible VLC applications, the transmission of audio signals remains a fundamental type of communication with a broad range of specific and interdisciplinary research areas. Audio signals include all types of sounds in frequency range from 20 Hz to 20 kHz. They carry significant and relevant information and play an important role in our daily communication (interpersonal communication, entertainment, perception of the environment, etc.). Modern approaches for acoustical signal enhancement include echo canceling, acoustic feedback and active noise control, dereverberation, noise suppression, spatial filtering, and audio-visual signal enhancement [1].

The assessment of audio quality is an important step in modern communication systems design. Subjective and objective measures are available for that purpose. Since subjective assessments are time-consuming and expensive, objective measures gained more attention in research studies. Some of the most significant methods include:

• Total harmonic distortions (THD)

• Inter-modulation distortions (IMD)

• Signal-to-noise ratio (SNR)

• Signal-to-noise and distortion (SINAD).

The most favorable measure that gives an estimate of the subjective difference grade of an audio signal is the perceptive evaluation of audio quality (PEAQ). The main purpose of PEAQ is to give an estimate of the audio quality of a tested audio device. PEAQ achieves this by comparing the input and output audio signals of the device, resulting in a quality score that reflects the audio quality of the output signal, as depicted in Fig. 1. This comparison focuses entirely on perceptual differences, while imperceptible distortions are neglected.

State of the art

Although VLC is primarily intended for broadband access, some applications use an audio message signal modulated in light intensity in the short-range VLC scenario. Evaluation of audio quality transferred via VLC is important for assessing performance, ensuring user satisfaction, optimizing system design, establishing standards, and ensuring regulatory compliance. These can contribute to the advancement and widespread adoption of the VLC technology in various audio communication applications, such as air traffic control systems, cabin crew communication systems, the music industry, to name a few. However, to the best of our knowledge, no previous research has thoroughly examined the quality of the audio signal transferred via VLC. State-of-the-art research studies in audio transferred over VLC are primarily focused on communication in aircraft cockpits [2] and underwater voice communication systems [3]. Recently, the assessment of perceptual speech quality in VLC transmission has been explored using the perceptual evaluation of speech quality (PESQ) and the Virtual speech quality objective listener (ViSQOL) metrics [4]. In [5], a portable device for optical wireless transmission of audio signals in a VLC-based underwater voice communication system is presented. The Tx includes a signal processing circuit, Tx driver circuit, LED, and optical antenna. The Rx contains a PD, photoelectric conversion, and amplification circuit. To send location information to aid the visually impaired, authors in [6] have implemented audio multicasting using VLC. The study has been executed using LEDs, which can transmit fast light pulses. The implemented system can function in both dark and bright lighting environments. This study examines the quality of audio signal in an IM/DD short-range analog VLC link. The impact of open hardware and open software platforms for research and experimentation has gained momentum in recent years. OpenVLC represents an open-source, flexible, and low-cost communication system platform for embedded VL networking. In our case, OpenVLC1.3 RevA capes are utilized to evaluate the quality of audio signals in IM/DD VLC indoor communication [7].

The circuit diagrams of the Tx and the Rx are depicted in Fig. 2. The Tx comprises a voltage regulator and a high-power LED (high-density in warm white illumination). To ensure the proper polarization of the LED, the input signal is biased by a DC offset voltage. On the other hand, the Rx consists of a PD, a trans-impedance amplifier, and an amplifier. Since the PD generates a current directly proportional to the light intensity, the transimpedance amplifier serves as a converter, transforming the current into a voltage signal. Additionally, an auxiliary amplifier provides additional amplification of 20 dB. The specific values of the used components are provided in Table I.

The experimental testbed is shown in Fig. 3, consisting of the Tx, the Rx, the voltage source, and the audio analyzer. The advantages of an exploited transceiver are simplicity and non-coherent transmission. The obtained results suggest that the transmission of audio signals with faithful quality (the THD level below 1%) can be accomplished using VLC technology.

The challenges and potential works

The domain of speech and audio signal processing experiences a growing interest with a broad range of specific and interdisciplinary research and development. In many modern audio and multimedia networks and devices, it is required to ensure a precise and reliable assessment of the quality of broadband audio signals. Today, when audio streaming services are among the significant consumers of internet capacity, it is of great importance to deliver high-quality audio signals. To assess the quality of the delivered audio signal, subjective but also objective methods of quality assessment are used, which model the psychoacoustic characteristics of the human auditory system.

In this paper, we provided an evaluation of the quality of the audio signal transmitted by the VLC link. Objective methods such as PEAQ, PESQ, and ViSQOL, as well as the common objective parameters of THD, SMPTE IMD, SNR, and SINAD, were considered. The currently implemented system has scopes to be developed further. The transmission range of this system can be increased while maintaining the audio quality.

Future research may include consideration of other modulation and coding techniques, as well as different modeling of the simulated VLC systems. Alternative modulation/detection schemes can be exploited, possibly combined with channel coding techniques (forward error correction). On the other hand, in the existing experimental real environment, an objective PEAQ method can be considered to assess the quality of an audio signal transmitted via an analog IM/DD VLC link. Potential future research may include examining the performance of digital audio VLC transmission. Digital communication systems provide a more faithful quality of signal regeneration after the transmission process. Common encoding strategies to convert analog audio signals into digital ones include pulse width modulation and delta modulation. Furthermore, the study will also explore the impact of the communication distance and the PD field of view on the reconstructed sound signal quality.

Conclusion

In this study, we investigated the transmission and quality assessment of audio signals over a short-range analog VLC link using Intensity Modulation/Direct Detection (IM/DD). The experimental results demonstrated that VLC technology is capable of delivering high-fidelity audio, with Total Harmonic Distortion (THD) levels remaining below 1%, ensuring minimal distortion in the transmitted signal. Objective metrics, such as THD, Signal-to-Noise Ratio (SNR), and the Perceptual Evaluation of Audio Quality (PEAQ), were used to evaluate the performance, confirming the potential of VLC for high-quality audio transmission. Despite the successful transmission of audio signals, several challenges remain. The line-of-sight requirement, susceptibility to ambient light interference, and limitations in transmission range need to be addressed for broader application. Future work could focus on improving these aspects through advanced modulation techniques, error correction, and the exploration of digital VLC systems to enhance signal robustness and fidelity.