f Errors associated with the use of adaptive differential pulse code modulation in the compression of isometric and dynamic myo-electric signals A. D. C. Chan D.F. Lovely B. Hudgins Institute of Biomedical Engineering, University of New Brunswick, PO Box 4400, Fredericton, New Brunswick, Canada E3B 5A3 Abstract--Muscle activity produces an electrical signal termed the myo-electric signal (MES). The MES is a useful clinical tool, used in diagnostics and rehabilitation. This signal is typically stored in 2 bytes as 12-bit data, sampled at 3 kHz, resulting in a 6 kbyte s - storage requirement. Processing MES data requires large bit manipulations and heavy memory storage requirements. Adaptive differential pulse code modulation (ADPCM) is a popular and successful compression technique for speech. Its application to MES would reduce 12-bit data to a 4-bit representation, providing a 3:1 compression. As, in most practical applications, memory is organised in bytes, the realisable compression is 4 : 1, as pairs of data can be stored in a single byte. The performance of the ADPCM compression technique, using a real-time system at 1 kHz, 2 kHz and 4 kHz sampling rates, is evaluated. The data used include MES from both isometric and dynamic contractions. The percent residual difference (PFtD) between an unprocessed and processed MES is used as a performance measure. Errors in computed parameters, such as median frequency and variance, which are used in clinical diagnostics, and waveform features employed in prosthetic control are also used to evaluate the system. The results of the study demonstrate that the ADPCM compression technique is an excellent solution for relieving the data storage requirements of MES both in isometric and dynamic situations. Keywords--Real-time signal processing, Data compression, Myo-electric signal, Biological signals, Intelligent instrumentation Med. Biol. Eng. Comput., 1998, 36, 215-219 1 Introduction DURING THE contraction of skeletal muscle, there is an asso- ciated movement of sodium and potassium ions in the indivi- dual muscle fibres. This electrical activity can be detected on the skin surface directly overlying the muscle using electrodes. The resultant signal is the sum of the muscle fibre action potentials in the vicinity of the electrodes, termed the myo- electric signal (MES). The MES can be used in a variety of ways, for example as a diagnostic tool or rehabilitation aid. The amplitude of the surface-recorded MES is in the order of tens of millivolts, with a bandwidth of "10 Hz-1 kHz although most of the signal energy is concentrated below 250 Hz. In converting the MES into a binary stream or pulse code modulation (PCM) signal, a 3 kHz sampling rate is often used. In addition, because of the wide dynamic range of the MES, a 12-bit resolution has become the de facto standard. Assuming the 12-bit samples are stored in byte pairs, then the storage requirement for a single channel is 6 kbytes s -l. Correspondence should be addressed to Prof. Lovely First received 17 February 1997 and in final form 6 August 1997 9 tFMBE: 1998 Medical & Biological Engineering & Computing March 1998 It is highly desirable to reduce this large data storage requirement to lower the demands placed on memory resources. Adaptive differential pulse code modulation (ADPCM) is a compression scheme that is widely used in telephony applications to compress speech. This compression applied to MES would be able to reduce the data into a 4-bit representation. Pairs of 4-bit ADPCM data can be conveni- ently stored in bytes, providing an overall 4:1 compression ratio, i.e. the storage requirement for a signal channel is reduced to 1.5 kbytes s -1. Earlier work carried out by NORRIS and LOVELY (1995) has proven the feasibility of using ADPCM compression on MES. In this earlier study, harmonic distortion and percent residual difference (PRD) for the compression of sinusoidal signal were used as performance measures. The system was implemented in real time using dedicated hardware. A brief discussion of the errors associated with the compression of MES data for an isometric contraction of arbitrary intensity was included. The purpose of this research is to address the following questions: 9 How does the level of myo-electric activity affect perfor- mance? 9 What effect does the sampling rate have on performance? 215