Difference Between Digital Optical Terminal (Uncompressed) and Analog Optical Terminal

**Difference Between Digital Optical Terminal (Uncompressed) and Analog Optical Terminal** Currently, optical terminals for video images, audio, data, Ethernet, and telephones are being widely applied in highways, transportation, electronic police, surveillance, security, industrial automation, electric power, customs, water conservancy, banking, and other fields. In the early stage, analog frequency modulation (FM), amplitude modulation (AM), and phase modulation (PM) optical terminals occupied a considerable share of the market. Their transmission method involves modulating baseband video, audio, and data signals onto a certain carrier frequency, transmitting them through the transmitting optical terminal over optical fiber, and then demodulating them at the receiving optical terminal at the other end to restore the corresponding baseband video, audio, and data signals. In contrast, digital optical terminals, which are now being widely adopted in domestic and international projects, perform high-resolution digitization of multiple analog baseband video, audio, and data signals to form high-speed digital streams. These multiple digital streams are then multiplexed, transmitted by the transmitting optical terminal, received by the receiving optical terminal at the other end, demultiplexed, restored to individual digital signals, and finally converted back to analog video, audio, and data through digital-to-analog conversion. From the current market situation, analog optical terminals are gradually exiting the market, while digital optical terminals are rapidly proliferating in the market. Digital replacing analog is also a significant historical development trend in optical fiber communication technology. Because digital optical terminals offer high transmission signal quality and do not suffer from the serious intermodulation interference when multiple signals are transmitted simultaneously, susceptibility to environmental interference, poor transmission quality, and poor long-term operational stability that are inherent in analog FM, PM, and AM optical terminals, many large and key projects have now widely adopted digital optical terminals. What exactly is the difference between analog video optical terminals and digital optical terminals? This is also a question of great concern to many users. This article discusses the topic from the following aspects: 1. Different signal transmission methods over the optical fiber As the name implies, the optical signal emitted by the optical transmitter of an analog optical terminal is an analog optical modulated signal. Depending on whether the amplitude, frequency, or phase of the optical signal changes with the amplitude, frequency, or phase of the input analog carrier signal, the terminals are respectively called AM, FM, or PM optical terminals. In contrast, the optical signal emitted by the optical transmitter of a digital optical terminal is a digital signal, i.e., 0 or 1, corresponding to the strong or weak state of the optical signal. Different combinations of 0s and 1s represent video, audio, and data signals of different amplitudes. 2. Different input and output processing methods for analog signal transmission Regardless of whether it is an analog or digital optical terminal, the input baseband video, audio, and data signals must be processed. For analog AM optical terminals, the processing method is to modulate the amplitude of a high-frequency carrier signal with the amplitude of the video, audio, and data signals, so that the amplitude of the high-frequency carrier signal changes with the amplitude of the video, audio, and data signals. For analog FM optical terminals, the processing method is to modulate a high-frequency carrier signal with the amplitude of the video, audio, and data signals, so that the frequency of the high-frequency carrier signal changes with the amplitude of the video, audio, and data signals. For analog PM optical terminals, the processing method is to modulate a high-frequency carrier signal with the amplitude of the video, audio, and data signals, so that the phase of the high-frequency carrier signal changes with the amplitude of the video, audio, and data signals. For digital optical terminals, the input baseband video, audio, and data signals undergo high-resolution analog-to-digital conversion. For example, an amplitude signal in the range of 1 Vp-p is represented by a 12-bit digital signal, with 1 V divided into 4096 levels. Therefore, the maximum voltage amplitude error caused by analog-to-digital conversion is 1/4096 V (approximately 2.5 mV). This error voltage is called the quantization error voltage. Voltage values of various amplitudes, from 0 V, 1/4096 V, 2/4096 V... up to 1 V, correspond to digital codes of 000000000000, 000000000001, 000000000010...111111111111, respectively. The digitally encoded signal directly controls the strong or weak state of the optical signal emitted by the optical transmitter (corresponding to 0 or 1). The receiving optical terminal then performs digital-to-analog conversion on the digital code to restore the original baseband video, audio, and data signals. 3. Different processing methods result in different signal distortion, deformation, and interference in video, audio, and data signals Because analog optical terminals require AM, FM, and PM modulation, whether the amplitude changes of the analog signal have a one-to-one linear relationship with the amplitude, frequency, or phase changes of the carrier signal caused by modulation becomes the key to the quality of the analog optical terminal. To date, it is very difficult to achieve true linear modulation, and nonlinearity inevitably causes signal distortion. In addition, the modulated carrier signal must further modulate the optical signal, and the nonlinearity of the optical signal is also a very important factor. As is well known, the nonlinearity of optical devices is greatly affected by ambient temperature changes, operating voltage stability, and optical transmission power. Therefore, optical devices must undergo 7-10 days of thermal cycling aging during production, along with process screening, aging, and testing, which can only control this variation within a certain range. During long-distance transmission over optical fiber, the optical signal suffers from power attenuation, transmission frequency drift, and phase distortion. The dispersion effect of the optical signal can also cause signal deformation. When the optical signal reaches the receiving end, the receiving optical device still introduces nonlinear distortion. The analog signal after optoelectronic conversion enters demodulation, and demodulation, like modulation, produces nonlinear distortion. Therefore, considering the entire analog optical terminal, the five processes from input signal modulation, electro-optical conversion, optical transmission, optoelectronic conversion, to demodulation all introduce nonlinear distortion. These signal deformations and distortions are inherent and therefore cannot be eliminated. As a result, it is very difficult for analog optical terminals to achieve satisfactory results in transmitting video images, audio quality, and data. Digital optical terminals have only the quantization error of analog-to-digital conversion (e.g., only 2.5 mV for a 1 V video signal with 12 bits), which is insufficient to cause signal distortion. 4. Intermodulation distortion caused by simultaneous transmission of multiple signals In field surveillance applications, users may have various signals, such as video images, audio, data, Ethernet, telephones, or other user-defined signals. Transmitting each signal with a separate pair of optical terminals would inevitably be expensive. Therefore, to improve optical fiber utilization efficiency and reduce costs, various signals must be multiplexed in the optical terminal so that they can be transmitted over one pair or one single optical fiber. For FM, AM, and PM optical terminals, transmitting 10/100M Ethernet signals or multiple telephone channels and other high-speed signals is difficult to achieve. Mixing multiple video or audio signals through FM, AM, or PM modulation onto a single carrier inevitably causes various image and intermodulation interference. Therefore, it is not uncommon in the current market for many well-known foreign brands of FM, AM, and PM optical terminals to experience mutual interference when transmitting multiple video, audio, and data signals simultaneously. These unstable phenomena are inherent shortcomings that analog modulation technology has long possessed. Therefore, the signal transmission capacity of analog optical terminals is limited, generally not exceeding 4 channels of simultaneous transmission. Digital optical terminals, on the other hand, transmit digital signals, making it easy to achieve large-capacity multiplexing without mutual interference. Some fully digital optical terminals can achieve transmission of up to 128 channels of video, audio, data, Ethernet, and telephone over a single optical fiber without any intermodulation interference. 5. Different stability Because analog modulated optical terminals adopt the carrier modulation method, the carrier and optical transmitter are easily affected by ambient temperature, resulting in the drawback that transmission quality varies with environmental changes. Due to this drawback, for large and important projects, the maintenance of analog optical terminals has become a very troublesome issue, which has also caused great dissatisfaction among many engineering contractors and end users. Therefore, choosing digital optical terminals for important projects is a wise choice. 6. Different prices Due to different system architectures, the prices of analog optical terminals and digital optical terminals are slightly different. Generally speaking, single-channel video, audio, and data digital optical terminals are slightly more expensive than single-channel analog optical terminals, while digital optical terminals with four or more channels of video, audio, and data are, on the contrary, much cheaper than analog optical terminals. Therefore, the cost-performance ratio of digital optical terminals is higher than that of analog optical terminals.