Proceeding of the 2013 IEEE International Conference on Space Science and Communication (IconSpace), 1-3 July 2013, Melaka, Malaysia
Data Compression Technique for High Resolution Wireless Photoplethysmograph Recording System K. S. Chong
K. B. Gan
Department of Electrical, Electronic & Systems Faculty of Engineering & Built Environment, University Kebangsaan Malaysia, 43600 Bangi, Malaysia. Email:
[email protected]
Institute of Space Science University Kebangsaan Malaysia, 43600 Bangi, Malaysia. Email:
[email protected]
M. A. Mohd. Ali
E. Zahedi
Institute of Space Science University Kebangsaan Malaysia, 43600 Bangi, Malaysia. Email:
[email protected]
Department of Electrical, Electronic & Systems Faculty of Engineering & Built Environment, University Kebangsaan Malaysia, 43600 Bangi, Malaysia. Email:
[email protected] and School of Electrical Engineering Sharif University of Technology Tehran, Iran
INTRODUCTION Photoplethysmography is a low cost and convenient physiology measurement technique from the point of view of patient comfort. It is an optoelectronic method for measuring and recording changes in the volume of body parts such as finger and ear lobes caused by the changes in volume of the arterial oxygenated blood, associated with cardiac contraction [1]. When light travels through a biological tissue (earlobe or finger), it is absorbed by different absorbing substances [2]. Primary absorbers are the skin pigmentation, bones and the arterial and venous blood. The characteristics of the photoplethysmogram (PPG) pulses are influenced by arterial ageing and arterial disease [3]. The simplest PPG probe consists of a light emitting diode (LED) and a photo-detector. The LED transmits light with usually a constant intensity, which is adjusted to the amplitude of the signal collected. The photo-detector is usually a silicon photodiode that produces a current proportional to the incident light. Improved technology in photodiode and LED allow the LED and photodiode to be small enough to fit in small fingertip probes using
Abstract—Multi-site photoplethysmography is an optoelectronic technique that measures changes in blood volume associated with cardiac contraction. Photoplethysmogram (PPG) recording enables researchers to study the vascular and hemodynamic properties of human subjects. Currently, there is no commercial system available in the market to perform multi-channel PPG recording. The measurements can be obtained from fingertips, ear lobes and toes due to their low absorption and high degree of vasculature. The main objective of this project is to develop a suitable data compression algorithm for two-channel simultaneous high resolution wireless PPG recording system. MATLAB software was used during the algorithm development phase to ensure the rapid prototyping. The result showed that delta modulation (DM) is able to compress PPG signals with a compression ratio (CR) up to 16 with an PRMSE of 3.92×10-4. This system is expected to be a portable, wearable and battery operated device for clinical applications.
I.
Keywords-component; Photoplethysmograph; pulse code modulation; delta modulation; multisides; compression ratio
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transmission or reflection probes. It has been established that the characteristic of the PPG signals is body site specific, with pulses from the various peripheral sites showing difference in pulse transit time, amplitude, shape and variation of each over time [4].
problem [10, 11]. However, when using multiple-site, high resolution (16-bit) PPG recording, large amount of data is produced, especially for long term recording system (24 hours recording). Such a requirement for memory size may be a concern when the objective is to store the data, or to the channel bandwidth shall the multi-channel PPG data be transmitted to a remote system. One solution to this problem is to use a data compression technique to reduce the data size and eliminate the redundancy of the physiological data [12].
Multiple-site PPG signals may provide physiologically important information from these observed differences, especially related to cardiovascular related performance [3, 4]. Results show that pulse timing, amplitude and shape of high frequency components of PPG waveforms between right and left sides of bodies in three different sites of fingers, toes and earlobes are expected to be similar since the anatomical structure are similar [4,5]. Pulse timing is characterized by the PWTT (Pulse Wave Transit Time), the time for a pulse to travel between two arteries sites. Any difference in vessel properties can affect the time and shape of the rising edge (anacrotic) and falling edge (catacrotic) of the PPG signal leading to important clues about pathological changes [4, 5].
In general, data compression techniques can be categorized into two groups: lossless and lossy data compression. Lossless data compression is able to reconstruct original signal without any loss of signal characteristic while for lossy data compression, some error is introduced when reconstructing the original signal due to some loss of information of the signal. The selection of a particular data compression technique depends on the physiological recording system’s application.
The time delay between two PPG signals from left and right arm is the evidence of increases in vascular mechanical properties. As an example, the delay between pulses from right and left toes between healthy and patients with some occlusion in the legs were also investigated [4, 6, 7] and were found to be 20 to 80 ms for patients with arterial stenosis in one leg [6]. It was also noticed that there was increased PWTT due to higher vascular resistance, as the speed at which the arterial pressure wave travels is directly proportional to the blood pressure. An acute rise in blood pressure causes the vascular tone to increase and arterial wall become stiffer causing the PWTT to be shortened. The main problem of the multiple sites PPG is the complexity of the data acquisition process and data storage. Such instruments are not available commercially, therefore they have always been custom made, application specific and developed by the researchers [2, 5-7].
Figure 1. The apparatus of two channels PPG recording system using two independent systems. Notice the amount of cabling required for the two systems [13].
Wired data acquisition systems (Fig. 1 and Fig. 2) will definitely restrict the patient’s mobility and comfort level, especially during sleep [8, 9]. Fig. 2 shows the developed data acquisition system replacing the complex setup shown in Fig. 1 [9]. Thus the idea of using a wireless protocol such as Bluetooth to solve the
Figure 2. Simultaneous two channels PPG recording system with USB cable [9].
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Lossy data compression can be further classified into three major categories: direct data compression, transformation methods and parameter extraction. It was proven that direct data compression is more efficient compared to other categories when employed in compression of electrocardiographm (ECG) signal [12]. The objective of direct data compression is to reduce the redundancy in a data sequence by examining a successive number of neighboring samples. One of the examples is Delta Modulation (DM) data compression.
PPG Probe
PC
16 bits ADC
Microcontroller
Figure 3. Block diagram of PPG signal acquisition system.
The DM compression algorithm uses a single bit per sample. The ‘1’ and ‘0’ represents a greater than or less than condition, respectively as compared to the previous sample. In this paper, the performance of the DM data compression algorithm is investigated using PPG signals. The 16-bit Pulse Code Modulation (PCM) was selected as a reference. MATLAB (The MathWorks, Inc.) software was used to simulate the data compression algorithm. The performance of the DM method in compressing PPG signals was analyzed by calculating the compression ratio (CR) and percentile root mean square errors (PRMSE). II.
Figure 4. Four seconds sample of the original photoplethysmogram sampled at 16-bits, 1000 Hz.
B. Delta Modulation
The Delta Modulation (DM) algorithm involves only a single bit for each sample. A bit ‘1’ represents the case when the signal is greater than the previous state, whereas a bit '0' represents the case when the signal is greater than the previous state. An integrator converts the stored bit stream into an analog signal. Fig 5 shows about 1 cycle of the original PPG signal and decompressed PPG signal. In order to avoid the slope overload problem at the start, the first value of the PPG was transmitted using 16-bits. From there on, only 1 bit is sent for each sample.
METHODOLOGY
A. Data acquisition
The PPG signal was recorded from a healthy person 25 years old. The raw PPG signal was captured using a Nellcor equivalent probe, a custom made analog front-end and converted into digital form by using a 16-bit analog to digital (ADC) converter (Texas Instruments, ADS1198). The digitized PPG signal was sent to a 16-bit microcontroller (Microchip, 30F4011) and transferred to a personal computer (PC) via USB cable for further analysis. The overall acquisition system is shown in Fig. 3. The PPG signal was sampled at 1000 Hz and four seconds of the PPG signal was displayed on the PC screen (Fig. 4). The total data size with these 4000 samples was 64000 bits (8000 bytes) for one channel. Pulse Code modulation (PCM) algorithm was used to convert the analog PPG signal directly into a multi-bit digital code using a 16 bit ADC. The code was stored in the memory and subsequently used for plotting and comparison at later stages.
Figure 5. One second sample: Original PPG signal (Top); Decompressed PPG signal after delta modulation (Bottom)
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C. Performance evaluation
DISCUSSION AND CONCLUSION From the simulation results (Table 1), DM is able to compress the PPG signals with a CR of up to 16 with a small PRMSE of 0.0392%. The result showed that DM compression technique can be used to compress the PPG signal before data transmission suitable for low data rate and low power wireless protocol. It can solve the problem of storing data and also memory in the PPG instrument in the later development stages. IV.
We recall that 16-bit PCM is used as the reference, therefore the compression rate (CR) is defined as the ratio of the size of uncompressed data in compressed data (1): Compression Ratio
(1)
Another interesting index is the percentile root mean square error (PRMSE). It is used to evaluate the percentage error between original signal and decompressed signals. ∑
. 100%
∑
The PRMSE of delta modulation is affected by the selection of step size during compression. Inappropriate selection of step size will increase the PRMSE and alter the characteristic of the PPG signal.
(2)
Due to the fact that in this work the step size of DM was chosen to be a constant value, it may not be able to minimize the reconstruction error. This problem can be solved by introducing Adaptive Delta Modulation (ADM) where the step size is continuously changed in order to better adapt to the changes in signal variations.
Where хi is the original signal data and хi' is the decompressed signal, n is the total number of samples. n is equal to 4000 in this study. RESULTS The results of our simulation study are shown in Table 1. Each second, the PCM algorithm produces a data volume of 16000 bits (1000 Hz×16 bits) while the DM algorithm only requires 1000 bits. From Eq. (1), the CR of DM is very close to 16. The normalized step size of this simulation was fixed to 1/216. At this step size, the PRMSE of DM is about 3.92×10-4. III.
ACKNOWLEDGMENT The authors would like to thank Universiti Kebangsaan Malaysia for sponsoring this work under the Research University Grant: GGPM2011-074 & INDUSTRI-2012-018.
Table 1. Performance of DM in PPG data compression
PCM (reference)
DM
PRMSE (%)
0 (by definition)
0.0392
Encoded data size (Bit)
16000
1000
CR
1
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≈16
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Table 1. shows that the CR of DM is approximate to 16. The reason CR is not exactly 16 is that the first sample is a 16 bits to avoid the slope overload at the beginning.
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