As broadband digital optical transmission device technology and the rapid development of digital video technology, digital video optical transceiver began to market, and gradually replace the analog optical transceiver market ushered in the era of high-speed digital video optical transceiver development, and presents a mature technology, superior performance, wide application, the situation of high demand.
The principle and coding of optical terminals
The optical transmission system consists of three parts: the light source (optical transmitter), transmission medium, and detector (optical receiver). The light source and detector work are done by the optical transmitter.
Optical transceiver is multiple E1 (a trunk line data transfer standard, usually speed of 2.048 MbPS, this standard for China and Europe adopted) signals into optical signals and transmission equipment, its role is mainly realize the electric - light and light - electric transformation).
The optical terminal mainly includes two kinds of analog optical terminals and digital optical terminals:
1. Analog optical transmitter
The analog optical transmitter adopts PFM modulation technology to transmit image signal in real time.
Transmitter to analog video signals to after PFM modulation (generally have a few kinds of frequency modulation, phase modulation, amplitude modulation way, thus the analog optical transceiver is divided into several optical transceiver frequency modulation, phase modulation and amplitude modulation), and then to the electric - light switch, light signals to the receiving end, light - electric conversion, then PFM demodulation, restore the video signal.
Because of the PFM modulation technology, the transmission distance can easily reach about 30Km, and some products can reach 60Km or even hundreds of kilometers.
Moreover, the image signal is very small after transmission and has high SNR and small nonlinear distortion.
By using the wavelength division multiplexing technique, the two-way transmission of image and data signals can be realized on a single fiber.
2. Digital optical transmitter
Because digital technology has obvious advantages in many aspects compared with traditional analog technology;
So just as digital technology has replaced analog technology in many fields, the digitisation of optical terminals is an inevitable trend.
At present, there are two main methods of digital image optical terminals: one is the MPEGII image compression digital optical terminal, the other is a non-compressed digital image optical terminal.
Image compression digital optical terminals are generally used in MPEGII image compression technology, which can compress the activity image into N by 2Mbps data and pass the standard telecommunication interface transmission or directly through the optical fiber transmission.
Because of the image compression technology, it can greatly reduce the signal transmission bandwidth, so as to facilitate the transmission of image signals with less resources.
At the same time, because of the standard interface of N by 2Mbps, it can be used to transmit and monitor images of the existing telecommunication transmission equipment, which brings convenience to the engineering application.
However, image compression digital optical terminals also have their inherent shortcomings.
Its fatal weakness is the inability to guarantee real-time image transmission.
Since image compression and decompression take a certain amount of time, they usually generate a delay of 1-2s for the transmitted image.
Therefore, this kind of equipment is only suitable for the place where the real time requirement is not high. In addition, the image can be distorted after compression, and the price of the optical end machine is also on the high side.