Biomedical Engineering Reference
In-Depth Information
from a telephone exchange is redistributed among its users. Thus, a trunk line from
the EPABX to Telephone exchange opens up the possibility of networking with
rest of the World. A schematic diagram is shown in the Fig. 4.7 . The intercom
system has been utilized for ECG transmission. The destination number, to which
ECG is to be sent to, was dialed first and on getting a 'connected' signal the
transmitter started sending the patient ECG data either in real-time mode or 'store
and forward' mode. For the latter case, the data is transmitted from its memory
where the patient ECG is prestored into the memory of thee embedded module.
In order to make the ECG data receiving automatic, an auto answering machine
principle (AMP) was incorporated within the ECG receiver. On getting the ring
tone from EPABX for a period of about 3-4 s, the ECG receiver established the
connection. The receiver then waits for the required header information in order to
start the data recording process if no header appears within a specified time period,
it disconnects the line considering it as a wrong call or the call is initiated for
talking purposes. It may be mentioned that this ECG receiver line must be a
dedicated line.
The basic schematic for the ECG transmission system using standard telephone
system is shown in Fig. 4.8 . The bi-phase formatted output of the encoder unit
MCU was directly connected to the MIC (or speaker) input of the speaker of the
telephone set. In the receiver unit, the output was extracted from the earpiece units
of the telephone set. For bi-phase encoding, the pulse widths used for encoding bit
'0' and '1' were set as 2,500 and 1,250 ls, respectively. Figure 4.9 a provides the
CRO screen shot for encoded ECG and packet generation from the PES. The
output of the receiver is shown in Fig. 4.9 b. The decoded bit pattern was sent to
the desktop computer using serial port using RS-232 protocol. Details of serial
communication between the MCU and MATLAB platform is discussed in detail in
the Sect. 3.5 .
Phone receiver
R1
Local L oop
Phone receiver
R2
Local
Loop
Trunk
Trunk
EPBAX
Local
Exchange or
end offices
Tandem
Offices
Regional
Offices
Phone receiver
Rn
Fig. 4.7
Block schematic of data transfer using PSTN network
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