VLC Based Multi-hop Audio Data Transmission System

VLC Based Multi-hop Audio Data Transmission System Le The Dung1, Seungwan Jo2, and Beongku An2 1

Dept. of Electronics & Computer Engineering in Graduate School, Hongik University, Korea [email protected] 2 Dept. of Computer & Information Communications Engineering, Hongik University, Korea [email protected], [email protected]

Abstract. In this paper, we propose a multi-hop transmission system using visible light communication to transmit audio data. In our proposed transmission system, at the transmitter we encode audio data based on S/PDIF standard – a popular standard for digital audio signal, and transmit the encoded audio signal via general LED. At each relay, digital audio signal is improved and amplified before sending. At the receiver, encoded audio signal from photodiode (PD) is decoded, amplified and coverted to analog audio signal. We evaluate our proposed transmission system in a room with flourescent light source. The audio signals obversed at the receiver show that with the support of relays, our proposed transmission system can provide high quality audio transmission from transmiter to receiver via multi-hop relays at a long distance. Keywords: VLC based system, audio transmission, S/PDIF digital audio signal, multi-hop communication.

1

Introduction

Nowadays, optical communication has been widely used around us in various applications due to its advantages compared with conventional radio frequency communication (i.e. high speed, harmlessness to health). Optical communication can be classified into two categories: wired optical communication (or guided optical communication), in which fiber optic cable is used as communicating channel and wireless optical communication, in which free space is used as communicating channel. Visible Light Communication (VLC) based system belongs to wireless optical communication. The development of LED technology brings new opportunities for energy savings and reduces maintenance cost in illumination system. Unlike incandescent and fluorescent light bulbs, LEDs have significantly low thermal inertia. Moreover, LEDs are able to switch between on and off state at high frequency. This characteristic can be explored to send data at high speed using visible light waves. In [1], the authors develop a prototype for sending data between two devices. However, that prototype only provides moderate data rate in one hop communication. The authors in [2] demonstrate a visible communication link for audio transmission. In that system, transmitter sends audio data to receiver directly in a short distance. Also, digital audio signals in [2] need to be improved. In our previous work [3], we proposed a VLC based multi-hop transmission system for sending text data. To the J.J. Park et al. (Eds.): GPC 2013, LNCS 7861, pp. 880–885, 2013. © Springer-Verlag Berlin Heidelberg 2013

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best of our knowledge, there hasn’t been any implementation and performance evaluation of multi-hop audio data transmission system in practice. In this paper, we propose a multi-hop system to transmit high quality audio data at high speed from transmitter to receiver via several relays. The rest of this paper is organized as follows. Section 2 explains in detail signal format and the functions of all modules used in our system. Section 3 presents experimental setup and results achieved with implemented system prototype. Finally, section 4 concludes the paper.

2

Our Proposed Multi-hop Audio Data Transmission System

2.1

Signal Format

The format of digital audio signal used in our transmission system is S/PDIF. This name stands for Sony/Philips Digital Interconnect Format. SPIDF is a data link layer protocol and physical layer protocol for carrying digital audio signal between various devices. S/PDIF is standardized by International Electrotechnical Commission (IEC) [4] in IEC 60958 as IEC 60958 type II (IEC 958 before 1998). SPDIF has several small differences with AES/EBU [5] and can be considered as minor update of the original AES/EBU. Digital audio signal in S/PDIF format is transmitted over either a coaxial cable with RCA connectors or a fiber optical cable with TOSLINK connector as in Figure 1(a). Nowadays, many modern devices have digital audio output ports which use S/PDIF format as in Figure 1(b).

(a)

(b)

Fig. 1. (a) Types of cables used in S/PDIF digital audio signal transmission, (b) Digital audio output port of a device Clock Data 1 0 0 1 1 0 1 0 Encoded Data ( C) Fig. 2. Bi-phase Mark Coding (BMC) scheme in S/PDIF

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In S/PDIF, original digital data stream is encoded using Differential Manchester encoding, also called Bi-phase Mark Code (BMC) or FM1 as in Figure 2. According to this code, in one data period, two zero crossings of the signal mean a logical 1 and one zero crossing means a logic 0. Therefore, the frequency of the clock is twice the bit rate of original data. The advantages of BMC code are as follows:  

Signal transition happens at least once every bit, allowing receiving device to perform clock recovery and synchronization. To be less error-prone in noisy environment compared with other codes.

Due to the above advantages, in our system, we will use BMC code for transmitting digital audio signal through visible light communication channel. 2.2

System Descriptions

Our proposed multi-hop audio data transmission system is illustrated in Figure 3. In this system, audio data source is fed to transmitter module through USB port. The original digital audio signal is encoded to S/PDIF format, shifted, amplified, and sent to LED. At each relay, digital audio signal is corrected and amplified before sending to next relay. At the receiver, the encoded audio signal received at photodiode (PD) is amplified, removed shifted DC component, decoded to original digital audio signal, and converted to analog audio signal. Finally, analog audio signal is sent to speakers. USB cable Transmitter

Relay 1

Relay 2

Receiver

Coaxial cable

Fig. 3. Our proposed multi-hop audio data transmission system

We will describe in detail transmitter module and receiver module by showing their block diagrams and explaining the function of each block. A. Transmitter Module The main functions of transmitter module are to get audio signal (e.g. analog signal or digital signal) from a device, convert to S/PDIF audio digital signal and transmit to next hop. Figure 4 shows the block diagram and real design of transmitter module. USB port from PC

S/PDIF encoder

Signal shift (up)

Amp LED

Audio output from PC

ADC

Fig. 4. Transmitter module in our proposed VLC based multi-hop audio data transmission system

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If the audio signal from a device is analog one, then it will go through ADC block to be converted to digital signal before encoding to S/PDIF digital signal. Since S/PDIF signal is a bipolar signal and LED will clip this bipolar signal, S/PDIF digital audio signal should be shifted up by adding appropriate DC component before sending to LED. Then the signal is amplified to drive LED. B. Relay Module The main function of relay module are to improve the quality of digital audio signal due to interference from external light source and to compensate the power loss of signal due to long distance transmission via visible light. The block diagram and real design of relay module are shown in Figure 5.

Signal correction Photodiode (PD)

Amp LED

Fig. 5. Relay module in our proposed VLC based multi-hop audio data transmission system

Signal correction block improves the quality of S/PDIF square signal received from photodiode. Then the improved S/PDIF signal is amplified before sending to LED to transmit to next hop. C. Receiver Module The main function of receiver module is to convert received digital audio signal to analog audio signal. Figure 6 shows the block diagram and real design of receiver module. To speakers Signal correction

Amp

DC filter

S/PDIF decoder

DAC

Photodiode (PD)

Fig. 6. Receiver module in our proposed VLC based multi-hop audio data transmission system

First two blocks in receiver module have the same function as those in relay module. Since the digital audio signal was shifted up at transmitter module by adding DC component to avoid signal clipping, at receiver module the DC part of this digital signal should be filtered to obtain S/PDIF digital signal. Then S/PDIF signal is sent to DAC block to reconstruct analog audio signal.

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Performance Evaluation

We evaluate the performance of our proposed VLC based multi-hop audio transmission system in a laboratory in the present of normal room illumination from fluorescent lamps on the ceiling as in Figure 7 (a). The audio data is sent from a PC to speakers at long distance via two relays. The distance between each module is 225 cm and the total distance from transmitter to receiver is 705 cm. To test the degree of signal distortion in our proposed multi-hop audio data transmission system, we use sample sounds (i.e. sine waveform with different frequencies) sending from PC as input analog audio signal to transmitter. Figure 7(b) – Figure 7(e) show the S/PDIF digital audio signal that we measure by oscilloscope at the output of transmitter and receiver, relay 1, and relay 2, respectively. As we can see in those figures, all digital signals still remain square shapes and their frequencies are unchanged because received signals are improved at each module before sending to the next one. As in Figure 7(b) – Figure 7(e), S/PDIF digital audio signal is shifted by adding DC component of around 1.8V to prevent the signal from clipping by LEDs. At the receiver, DC component of that digital audio signal is filtered out as in Figure 7(c) before converting to analog audio signal. Transmitter Relay 1

Relay 2

225 cm

(b)

Receiver

(a)

(c)

(d) (e) Fig. 7. (a) Setup of our proposed VLC based multi-hop audio transmission system; S/PDIF digital audio data signal observed on oscilloscope at (b) transmitter, (c) receiver, (d) relay 1, (e) relay 2

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(b)

Fig. 8. (a) 4 kHz and (b) 10 kHz analog sample sound observed on oscilloscope at transmitter/receiver

Figure 8(a) and Figure 8(b) show the shapes of 4 kHz and 10 kHz audio analog signals observed at transmitter and receiver, respectively. The results confirm that our multi-hop transmission system has good frequency response. Thus, it can provide high quality audio transmission.

4

Conclusions

In this paper, we propose a VLC based multi-hop audio data transmission system. With the support of relays, our proposed system can provide audio data transmission at a long distance. The results of experiment show that audio signals remain good quality without signal distortion when traveling through each hop. The received audio sound is clear when playing on speakers. Therefore, we believe that our proposed multi-hop audio data transmission system can be used to build VLC based networks for sending high quality audio data among devices. Acknowledgement. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012-0007119).

References 1. T.D.C. Little, P. Dib, K. Shah, N. Barraford, B. Gallagher: Using LED Lighting for Ubiquitous Indoor Wireless Networking. In: IEEE International Conference on Wireless & Mobile Computing, Networking & Communication, pp. 373-378. (2008) 2. Do Ky Son, Eun Byeon Cho, Chung Ghiu Lee: Demonstration of visible light communication link for audio and video transmission. In: Photonic Global Conference (PGC 2010), pp. 1-4. (2010) 3. Seungwan Jo, Le The Dung, Beongku An: LED Communication-based Multi-hop Wireless Transmission Network System. In: The Journal of The Institute of Webcasting, Internet and Telecommunication (IWIT), vol. 12, no. 4, pp. 37-42. (2012) 4. http://www.iec.ch/standardsdev/publications/is.htm 5. http://en.wikiaudio.org/AES_EBU