ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 77

4.3 Scheme of ECG Transmission

In this section, an ECG transmission technique is illustrated which may be utilized
for remote offline monitoring of patients between two fixed stations. The devel-
oped system utilizes portable embedded system-based modules, placed at the two
end of the transmission link. The transmit-end module collects the ECG samples
from the ECG source and communicates them using a suitable modulation tech-
nique using an active media for remote-end reception. At the receive end, another
embedded system coupled with the receiver unit decodes the samples and delivers
them to a desktop computer for storage and analysis using state of the art software.
Thus, the entire system constitutes by the following components:

1. Portable tele-cardiology kit (PTK) as the collecting device from the ECG
source, encoding, and data packaging with error checking protocol for

transmission;
2. Transmitters and receivers;
3. Healthcare End Embedded System (HES) for the decoding of ECG received
from receiver unit, modification of packets for bit error rate (BER) and packet
error (PE) computation and serial delivery to the PC;
4. PC-based ECG acquisition and processing system for the serial reception of
data, error estimation, saving in time-stamped format, and data processing for
ECG feature extraction.

4.3.1 Use of Bi-phase Modulation for ECG Encoding

ECG is a low-frequency signal, primarily in the band 0.05-100 Hz. However, the
clinical information is mostly confined within the 30 Hz range. Moreover, except
the QRS regions of a typical ECG cycle, the signal change in medium (P and T
waves) and very low (equipotential segments like PR, ST, and TP). Bi-phase
encoding is a digital encoding scheme suitable for the transmission of such low-
frequency signal. It utilizes two distinct frequencies (normally one double of the
other) to encode the digital output of the input binary bit stream. In a BPM scheme,
as shown in the Fig. 4.2 , the output bit always toggles its state after a fixed interval
of time. This interval of time, in other words, determines the bit width of the
output bit. Again, depending on the input data bit status, the output level may
toggle once in the middle of the bit-width interval. If the input bit is '1', the output
undergoes one state transition at the middle of that interval, while for '0' bit, there
is no such transition. This implies that the frequency to represent a '1' bit is
exactly double to that of '0' bit. So, in general, a bi-phase modulation is a special
type of FSK (Frequency Shift Keying) modulation [ 27 ]. For successive occurrence
of same bit at input, there will be a state transition between two intervals at the
output. By this way, during transmission, dc like nature of the original ADC