From a technical point of view, digital substitution of analog is an inevitable trend, not to mention that some of the bottlenecks constraining analog optical transceivers can only be broken through digitalization. High-quality, long-distance, ultra-long-distance transmission, multi-functional, multi-service requirements and complex networking structures can only be achieved with digital optical transceivers. 1. The signal transmitted on the fiber is not the same
As the name suggests, the optical signal emitted by the analog optical transceiver is an analog optical modulation signal. It changes the amplitude, frequency, and phase of the optical signal with the amplitude, frequency, and phase of the input analog carrier signal and is referred to as amplitude modulation, frequency modulation, and phase modulation optical transceiver respectively. . The optical signal transmitted by the optical head of the digital optical transceiver is a digital signal, that is, 0 or 1 corresponds to the strong or weak state of the optical signal. Different combinations of 0 and 1 represent video, audio, and data signals of different amplitudes.
2. Analog signal transmission input and output processing method is not the same
Regardless of analog, digital optical transceivers, the input baseband video, audio, and data signals must all be processed. For analog AM optical transceiver, the processing method is to modulate the amplitude of video, audio and data to a high-frequency carrier signal so that the amplitude of the high-frequency carrier signal varies with the amplitude of video, audio and data; and the digital optical transceiver is used for inputting. The baseband video, audio, and data are subjected to high-resolution analog-to-digital conversion. For example, a 1Vp-p amplitude range amplitude signal is represented by a 12-bit digital signal, and 1V is divided equally into 4096, so that the maximum voltage amplitude after analog-to-digital conversion is caused. The error is 1/4096V (about 2.5mV). This error voltage is called the quantization error voltage. The voltage values â€‹â€‹of various amplitudes are coded from 0V, 1/4096V, 2/4096V, and 1V maximum respectively to 000000000000, 000000000001. , 000000000010...111111111111. The digitally encoded signal directly controls the strong and weak states of the optical signal emitted by the optical head (corresponding to 0 or 1). The receiving optical unit then performs digital-to-analog conversion of the digital coding and restores the original baseband video, audio, and data signals.
3. Different processing methods cause video, audio, and data signal distortion, distortion, and interference
Since the analog optical transceiver needs to perform amplitude modulation, frequency modulation, and phase modulation, whether the amplitude of the analog signal and the change of the amplitude, frequency, or phase of the carrier signal due to the modulation is a one-to-one linear relationship becomes the key to the quality of the optical transceiver. So far, it is difficult to achieve true linear modulation, non-linear shape will inevitably cause signal distortion; at the same time the modulated carrier signal must also modulate the optical signal, the nonlinearity of the optical signal is also a very important factor, as we all know, the optical device is not Linear and ambient temperature changes, operating voltage stability, light emission power have a great impact, so the optical device needs 7-10 days of thermal cycle aging in production process screening, aging, testing can only be done The conversion is controlled in a certain range; the optical signal is transmitted over a long distance in the optical fiber, which causes the power of the optical signal to be attenuated, the transmission frequency shifts, and the phase distortion. The dispersion effect of the optical signal also causes distortion of the optical signal; the optical signal arrives at the reception At the end, the receiving optical device still needs to cause nonlinear distortion, and the photoelectrically converted analog signal enters the solution. Tuning, demodulation produces the same nonlinear distortion as modulation. Therefore, the synthetic analog optical transceiver, from the input signal modulation - electro-optical conversion - optical output - photoelectric conversion - demodulation of the five processes, will cause non-linear distortion, and these signal distortion distortion is inherent, so it must be inevitably, so It is difficult to achieve very satisfactory results for the analog optical transceivers to transmit video images, audio quality, and data. The digital optical transceiver only has quantization error of analog-to-digital conversion (such as 2.5mv for a 1V video signal at 12bits), which is not enough to cause signal distortion.
4. Intermodulation distortion caused by simultaneous transmission of multiple signals
In field monitoring applications, users may have a variety of signals, such as video images, audio, data, Ethernet, telephone or other user-defined signals. Each signal is transmitted by a pair of optical transceivers, which is necessarily expensive. In order to increase the utilization efficiency of the optical fiber and reduce the cost, necessary signals are multiplexed in the optical transceiver for transmission on one pair or one optical fiber. For FM, AM and PM optical transceivers, it is difficult to transmit high-speed signals such as 10/100M Ethernet signals or multi-channel telephones. Multiple-channel video or audio signals are mixed in frequency modulation, amplitude modulation, and phase modulation on a single carrier. Inevitably will cause all kinds of mirroring, intermodulation interference. Therefore, there are many phenomena of mutual interference between FM, AM, and phase-shifting optical transceivers, audio, and data at the same time on the market. These unstable phenomena have always been inherent in analog modulation technology. Difficult to solve problems and shortcomings. Therefore, the analog optical transceiver has a limited capacity for transmitting signals and generally does not exceed the simultaneous transmission of four signals. The digital optical transceivers transmit digital signals, which are easy to use for large-capacity multiplexing and do not interfere with each other. Such as Shenzhen Nanhua Weiye Technology Co., Ltd. all digital optical transceivers, can achieve up to 128 video, audio, data, Ethernet, telephone transmission on the same fiber without any crosstalk interference.
5. Different stability
Since the analog-modulated optical transceiver adopts the carrier modulation method, the carrier and the optical head are easily affected by the ambient temperature. There are disadvantages of the transmission quality changing with the environment. Because of this shortcoming, the maintenance of analog optical transceivers has become a headache for large-scale and important projects, which has also caused great dissatisfaction for many engineering contractors or owners. Therefore, choosing digital optical transceivers for some important projects is a wise choice.
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