ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 73

remained the same. Following his study, there were many applications of patients'
physiological data transmission, mainly over public switched telephone network
(PSTN) lines. The earlier tele-cardiology systems used analog signal processing
systems and frequency modulation (FM) for single-channel ECG acquisition or
frequency division multiplexing (FDM) system for multilead acquisition [ 1 ]. With
the introduction of computer programs for ECG interpretation, most of these
applications were centered on transferring of patients' ECG data from hospitals to
computer laboratories. In [ 2 ] a comparison between analog FDM transmission and
digital time, division multiplexing (TDM) is described in the context of technical
(such as real time or offline transmission, bandwidth, accuracy etc.) and economic
(like overall cost) considerations for a transmission length of 20 miles. For analog
transmission, three centre frequencies of 1.075, 1.935, and 2.365 kHz were used.
The functional block diagram of such analog transmission system is shown in
Fig. 4.1 . Analog transmission approaches suffered from corruption of the signal
due to poor quality of the transmission lines, low immunity to line impulse noise,
inadequate processing and incompatibility with computer communication proto-
cols at the receiving end. This raised a question of the reliability of the signal for
clinical use, specially for computer-assisted diagnosis. In addition, limitation of
the then PSTN bandwidth (300-3,400 Hz) was also a hindrance toward trans-
mission of low-frequency ECG signal. Initially, the major problem with digital
transmission was found to be with low data rate (around 9,600 bps in UK) for
3-lead multiplexed ECG transmission. However, with the development of high-
end communication gadgets, these problems were gradually overcome later on.
With the introduction of digital technology, viz., easy computer interfacing
without demodulation, multiplexing and AD conversion and improved signal
quality due to error detection capability, communication gadgets (like synchronous
modems) were used at the transmitting end for digital ECG transmission. How-
ever, to accommodate 3-lead multiplexed ECG over switched network using low-
cost modems for a sampling rate of 200 sps, the acquired ECG data had either to
be compressed (or encoded) or transmitted offline. An application of numerical
ECG encoding by delta encoding using Intellec-8 MOD08 microcomputer is
described [ 3 ]. The method used a low-cost modem using 2,400 bps coupled with a
telephone network. Although digital transmission crept in additional characters as
control frames, the overall accuracy and performed obtained were better than
analog FM transmission. From the advent of wireless communication techniques
in 1970s, there was a gradual shift in choice of transmission media. The first
instance of mobile tele-cardiology was reported in 1970, which provided remote
means of acquisition and transmission of patients' ECG using conventional radio
equipment. Added with this, advent of VLSI technology enabled use of high-speed
processors for complex digital processing of data. A major application of the
microcomputer-based systems was evidenced in medical equipments, communi-
cation systems, and personal computer (PC) technology [ 4 ].