process_essay/main.typ

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Marius Drechsler\
Process Essay\
May 25th, 2025

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  *The Digital Journey of a Message*
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// Have you ever wondered what happens with your voice when you are talking to someone on the phone?
When speaking over the telephone, voice signals undergo complex transformations before reaching their intended recipient.
From the instant the sound waves are produced by the vocal cords until they reach the recipient,
a series of analog and digital processes collaborate to carry the message.
This transformation process can be broken down into three major steps -- sampling, quantisation and modulation.
These steps systematically convert analogue voice signals into digital information, allowing a reliable transmission over communication networks.
This essay will investigate each of these steps in more depth to understand how modern
communication works on a technical level.

// To start, we will take a closer look at the analogue signal that reaches your phone's microphone.
To begin, it is necessary to examine the analogue signal that reaches the microphone of a telephone.
Every sound wave, such as human speech or musical tones, is called time and value continuous.
That means that such a signal has an infinitely accurate value at each imaginable point in time.
However, an electronic device, for example a computer or a phone, cannot understand such an analogue signal, thus, it must first be transformed into an electrical signal the device can process.
//In general, we can assume that an electrical device can only process time and value discrete signals.
Generally, electronic devices can only process signals that are discrete both in time and amplitude.
To transform the original continuous signal into its discrete or rather digital representation, sampling and
quantization steps are used in the communication process.

In the sampling process, the analogue signal is transformed into a time-discrete and value-continuous signal.
Conceptually, an analog-to-digital converter (ADC) takes rapid snapshots of the amplitude of the input signal at uniform intervals and records each reading.
The rate at which these snapshots occur is called sampling frequency.
Ideally, it is desirable to maximize the interval between each of these snapshots, using the lowest possible sampling frequency.
The Nyquist Frequency defines this lowest possible sampling frequency as double the frequency of the originating signal @shannon.
For example, if the frequency of the original signal is 1 MHz, the analog-to-digital converter will need to take a snapshot of the signal at a frequency of at least 2 MHz.
At this moment, the signal is time-discrete and value-continuous.
To convert these time-discrete values into quantized digital values a quantizer will be used in the next step.

Quantization describes the operation of transforming a continuous value into its discrete form.
Consider the random placement of dots in a row on a piece of paper.
A simple quantization process would be to take a ruler and for each point record the next highest marking on the ruler.
Analogously, the quantizer of an electronic device maps the continuous input values to predefined codewords -- a collection of bits (for example "000", "110", or "011").
// Note: Maybe expand more what happened up until now
The analogue message has now been fully digitally encoded.
During the next steps, the digital signal will be further processed and prepared for transmission.

A digital signal in its raw form is inefficient to transmit because of the limited underlying bandwidth.
Since the message has to be transported over some kind of communication channel, the amount of information that can be transported in a fixed period of time is limited.
The first step in solving this issue is using compression by removing irrelevant and redundant information from the signal.
A popular example for a compression method is called Huffman Coding @huffman.
Conceptually, the Huffman Code consists of multiple codewords of varying length where symbols with a higher probability of occurrence are assigned to the shorter codewords, thus reducing the overall size of the message.
Since the assignment of symbols to codewords is based on their probability of occurrence, this method of compression requires information about the statistics of the incoming symbols.
If these statistical information is not known, other compression methods such as the Lempel–Ziv–Welch algorithm can be used.
Furthermore, compression algorithms specifically tailored for different signal sources can be used, for example PNG for pictures, MP3 for audio or MPEG for video signals.
With the analogue message digitized and compressed for easier transport over the communication channel, the message must now be prepared on a physical level for it to be transmitted using a radio wave or a data cable.

To prepare the digital and compressed message for transmission over a physical channel, digital modulation -- such as amplitude modulation -- is used.
Currently, the message to be transmitted can be represented as a set of codewords like "00 01 10 11".
Modulation works by defining a specific signal amplitude for every possible codeword, which is called Amplitude-Shift Keying (ASK) @modulation.
The simplest form of ASK is On-Off Keying (OOK), in which a binary 1 is represented by the transmission of a wave, and a binary 0 by the absence of transmission.
Signal modulation is not limited to changing the amplitude of the transmission signal, thus it is also possible to alter the frequency or phase.
In the example above, four sine functions with varying amplitudes may be defined as the modulated signal that is being transported over the physical communication channel.
Because four different Amplitude-Shifts were defined, this type of modulation is called "4 ASK" @modulation.
Depending on the type of communication channel, a different modulation type may be used, using either one or a combination of different modulation parameters.
For example, a popular modulation type that uses a combination of amplitude shifts and phase shifts is called "Amplitude-Phase-Shift Keying (APSK)" @modulation.
Using modulation, the signal has been prepared on a physical level to  instruct a communication interface -- such as an antenna or an optical transmitter -- to finally transmit the message.

As final step, the receiver of the message has to process the received signals in exactly the reverse order to create a comprehensible message.
The most important prerequisite for this to work is that both sender and receiver have agreed on the same transmission and reception conditions.
The receiver will first need to use the correct modulation type to convert their received signal back to a set of codewords.
Proceeding, they will decompress the message based on the used compression algorithm and use a Digital-to-Analog Converter (DAC) to transform the digital message back into sound waves which will be output by the speaker of their phone.
This whole process now happens at such a high speeds that real-time communication across the globe is made possible.

//Essay has a total of #total-words words.

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#bibliography("./bibliography.bib", style: "ieee", title: "References")