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Breaking Down the Document: The Scanning Stage
Everything starts with the physical document you want to send. The fax machine needs to convert the image on that paper – text, pictures, diagrams – into a digital format. Think of it like taking a very specific type of black and white photograph. Inside the sending fax machine, the paper is fed past a scanner. This scanner typically uses a bright light source, often a thin fluorescent lamp or an array of Light Emitting Diodes (LEDs), to illuminate the document strip by strip as it moves through. Opposite this light source is a sensor, usually either a Charge-Coupled Device (CCD) or a Contact Image Sensor (CIS). The CCD is similar to what you might find in older digital cameras. It uses lenses to focus the reflected light from the paper onto a line of tiny light-sensitive sensors. White parts of the paper reflect a lot of light, while black parts reflect very little. These sensors measure the intensity of the reflected light for thousands of tiny points along each scanned line. The CIS is a more compact and often cheaper alternative. It places the sensor bar very close to the paper, sometimes using a row of LEDs directly integrated with the sensors. It works on the same principle: detect the amount of light reflected from each tiny spot on the page. Whether using a CCD or CIS, the sensor converts the varying light levels into electrical signals. These analog signals are then processed by an analog-to-digital converter (ADC). The ADC essentially decides whether each tiny spot (pixel) is primarily black or white. It converts the image into a stream of binary data – ones and zeros. A ‘1’ might represent white, and a ‘0’ might represent black (or vice versa, depending on the standard). The level of detail captured depends on the machine’s resolution, measured in dots per inch (dpi). Standard fax resolution was often around 200×100 dpi or 200×200 dpi.Talking the Telephone’s Language: Modulation
Now we have the document represented as a long string of digital ones and zeros. But here’s the catch: standard telephone lines, the Public Switched Telephone Network (PSTN), were designed to carry analog signals – the smooth, continuous waves of human voices, not the sharp, distinct pulses of digital data. Sending raw digital data down an analog phone line is like trying to shout binary code and expecting someone to understand. This is where the modem (modulator-demodulator) inside the fax machine comes in. The modulator takes the digital stream of ones and zeros and encodes it onto an analog carrier wave. It changes characteristics of an audible tone (like its frequency or phase) to represent the digital data. This process is called modulation. The infamous screeching and beeping sounds you hear when fax machines connect are actually these modulated analog signals – the sound of digital data disguised as audio tones suitable for the phone line.Key Conversion: The core of fax transmission involves converting the scanned document’s visual information into digital data (bits). Then, a modem modulates this digital data into analog sound signals suitable for transmission over traditional phone lines. This digital-to-analog conversion is essential for compatibility with the PSTN.Before sending the actual document data, the sending and receiving fax machines perform a “handshake.” This is a negotiation phase where they exchange information using specific tones. They agree on parameters like the transmission speed (baud rate), resolution, and compression method (like Modified Huffman or Modified READ) to ensure they are compatible and can understand each other. This handshake ensures the receiving machine knows how to interpret the incoming analog signals.
Traveling the Wires: Transmission
Once the handshake is complete and the digital data is modulated into analog tones, the signal is sent out over the connected telephone line, just like a regular phone call. It travels through the local exchange, potentially across long distances via switches and routers within the global telephone network, eventually reaching the phone line connected to the receiving fax machine. The reliability of this transmission depends heavily on the quality of the phone line connection. Noise or interference on the line can corrupt the analog signal, leading to errors or dropped lines during the fax transmission. This is one reason why faxes sometimes fail or have streaks or missing lines on the received copy.Decoding the Signal: Reception and Demodulation
When the receiving fax machine answers the call, its modem listens for the incoming analog signals. The demodulator part of the modem takes these screeching tones and performs the reverse process of modulation: demodulation. It translates the changes in the analog carrier wave back into the original stream of digital ones and zeros that represent the scanned image. Any compression applied by the sending machine during the handshake phase is now reversed. The machine reconstructs the raw bitmap data – the pattern of black and white dots – that represents the original document page by page, line by line.Recreating the Document: The Printing Stage
The final step is to take this reconstructed digital data and turn it back into a physical copy. The most common printing technology used in traditional, standalone fax machines is thermal printing.Thermal Printing Explained
Thermal fax machines use special heat-sensitive paper, called thermal paper. This paper is coated with chemicals that turn black when heated. Inside the fax machine, there’s a thermal print head, which is essentially a row of tiny heating elements spanning the width of the paper. There’s one heating element for each potential pixel in a horizontal line. As the thermal paper feeds past the print head, the fax machine’s processor reads the incoming digital data (the ones and zeros representing the image). For each ‘black’ dot in the data stream, it activates the corresponding tiny heating element on the print head for a fraction of a second. This heat causes the coating on the paper directly underneath it to darken, creating a black dot. Where the data indicates ‘white,’ the heating element remains off, and the paper stays white. Line by line, the print head recreates the image by heating the correct spots on the paper as it moves. This method is simple, requires no ink or toner cartridges, and makes the machines relatively cheap and reliable. The main downside is that thermal paper can fade over time, especially when exposed to heat or sunlight, and it has a distinct feel.Thermal Paper Limitations: While cost-effective, documents printed on older thermal fax paper are not archival. The heat-sensitive coating can degrade, causing the image to fade significantly over months or years. For important documents received via thermal fax, making a photocopy onto plain paper is often recommended for long-term preservation.While thermal printing was dominant, especially in older or more basic models, some later or multifunction fax devices incorporated other printing technologies like inkjet or laser printing, using plain paper instead of thermal rolls. These work identically to their standalone printer counterparts, simply using the demodulated fax data as the source image to print.
