Soft Modem
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SOFTWARE MODEM
M.SASIKALA
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What is modem?
A modem is a device or program
that enables a computer to transmit
data over, for example, telephone orcable lines.
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Difference
A cable modem is a rather broad
term and may vary in technical
specification depending on supplier.In general cables modems connect to
a dedicated cable and offer internet
services, possibly along side a cable
TV service.
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Definition:
Software modem:
A modem implemented entirely
in software. Software modems rely on
the computer's processor to modulate
and demodulate signals.
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Theory of operation
FSK technique is used in 1200 band
modems.
It transmits sinusoidal tones
- 0s and 1s of different frequencies.
The 01 bit pattern creates thechirping sound characteristic of
modems. 2008 Wayne Overheads for
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Theory of operation
Higher-speed modems are backward
compatible with the 1200 band FSK
scheme and begin a transmission witha protocol.
2008 Wayne Overheads for
time
0 1
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FSK encoding
Generate waveforms based on current
bit:
bit-controlledwaveformgenerator
0110101
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FSK detection scheme
A/Dc
onve
rter
zero filter
one filter
detector0 bit
detector1 bit
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FSK detection scheme
The analog input is sampled and the
resulting bit stream is sent to two
digital FSK filters
- one passes frequencies in the range of0 and rejects 1-band.
-the other one does the reverse.
The output of the filter is sent to
the detectors.
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FSK detection scheme
It computes the average value of the
signal over the past n samples.
When the energy goes above the
threshold value, the appropriate bit
is detected.
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Transmission scheme
Send data in 8-bit bytes. Arbitrary
spacing between bytes.
Byte starts with 0 start bit.
Receiver measures length of start bit to
synchronize itself to remaining 8 bits.start (0) bit 1 bit 2 bit 3 bit 8...
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Requirements
Inputs Analog sound input, reset button.
Outputs Analog sound output, LED bit display.
Functions Transmitter: Sends data from memoryin 8-bit bytes plus start bit.
Receiver: Automatically detects bytesand reads bits. Displays current bit onLED.
Performance 1200 baud.
Manufacturing cost Dominated by microprocessor andanalog I/O
Power Powered by AC.
Physicalsize/weight
Small desktop object.
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Specification
2008Wayne Wolf
Overheads forComputers as
Receiver
sample-in()
bit-out()
1 1
Transmitter
bit-in()sample-out()
Line-out*
output()
1 1
Line-in*
input()
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System architecture
It consists of one small
subsystem(interrupt handler for
input and output) And two major
subsystem(transmitter andreceiver).
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Transmitter
Waveform generation by table
lookup.
float sine_wave[N_SAMP] = { 0.0, 0.5,0.866, 1, 0.866, 0.5, 0.0, -0.5, -0.866,
-1.0, -0.866, -0.5, 0};
2008Wayne Wolf
Overheads forComputers as
time
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Receiver
Filters (FIR for simplicity) use circular
buffers to hold data.
Timer measures bit length.
State machine recognizes start bits,
data bits.
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Hardware platform
Hardwar architecture is relatively
simple.
-A/D converter.-D/A converter.
-Timer.
The amount of memory required to
implement algorithm is relatively very
small.
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Component design and
testing
Easy to test transmitter and receiver on
host.
Transmitter can be verified withspeaker outputs.
Receiver verification tasks: start bit recognition;
data bit recognition.
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System integration and
testing
Two ways to test modem system:-by having the modems transmitter send
bits to its receiver.
-by connecting two different modems. Use loopback mode to test components
against each other. Loopback in software or by connecting D/A
and A/D converters.
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THANK YOU!!!