How does a modem translate the signals?

Answer 1

This is a both a simple and complicated question at the same time. The digital side doesn't require much understanding of digital circuits to comprehend how a bit stream can be created on the pin of a chip. The analog side requires a basic understanding of analog electronics, transceivers (transmitters/receivers), and/or carrier MOdulation/DEModulation (MODEM).

We will limit the conversation to a single hardwired connection between two computers, isolated from all outside influence; separate transmit and receive carriers (full duplex), and all MODEM handshaking completed (carriers synchronized). In such a controlled sandbox, all users that contribute to this explanation can explore the details of the communication process in this fixed reference frame. As questions are presented, we can peel back some of the definitives.

Let's say the digital connection of MODEM 1 has a small text message in its buffer that it wants to send to MODEM 2. The content of the message is "Hello". These five characters have a specific bit pattern (specified in a standard 7 Bit ASCII table) that is translated into a bit stream:

Character >

H

e

l

l

o

ASCII >

4816

6516

6C16

6C16

6F16

Bit Patterns >

0010 1000

0110 0101

0110 1100

0110 1100

0110 1111

Bit Stream >

0010100001100101011011000110110001101111

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Modulation Signal

This bit stream will be sent to the transmitters' modulation stage to be impressed on the carrier one bit at a time. We'll use simple FM, and the bit timing will have an intra-bit voltage level halfway between zero and one that corresponds to the carrier's center frequency. This straight binary to FM technique is over simplified and very wasteful of the capacity of these carrier bandwidths.

MODULATION: If the part of the bit stream present at the modulation stage input is a zero (0), the carrier frequency will be decreased to its lower frequency limit. If the bit stream is at the intra-bit level, the carrier frequency will be at its center frequency. If the bit present at the modulation stage input is a one (1), the carrier frequency will be increased to its upper frequency limit. This sequence of events applied to the entire 40 bit message will amount to the carrier of the transmitting modem shifting from center frequency, to low limit, to center frequency, to high limit, to center frequency, toggling between limits for the entire five character (40 bit) message.

DEMODULATION: At the receiving end, MODEM 2 is listening to the carrier shift through these patterns, and as it does, it rebuilds the shifting carrier into the original digital bit pattern and clocks it into its own buffer.