DTV over Optical Fibers
DTV over Optical Fibers
Long distance transmission without amplification is one of DTV's five very attractive features. However, due to the coaxial cable loss, the long distance still has a physical limit. The “long” mentioned here is around 1~2 km. Over this distance, one can choose to amplify the RF signal or convert the RF signal to optical signal to transmit over optical fibers. The use of fiber can extend the distance to tens of kilometers.
This article starts with general introduction to fiber optical communications, and then the system architecture for DTV over optical fibers.
Types of Optical Fibers
In terms material, there are plastic fibers and glass fibers. Plastic fibers are used for high data rate transmission over short distance, while glass fibers are used for high data rate for long or ultra-long distance transmission from several kilometers to thousands of kilometers. For surveillance application, glass fibers that are often used.
In terms of transmission characteristics, there are single-mode fibers (SMFs) and multimode fibers (MMFs). SMFs have small radius and support only one light wave propagation mode, while MMF have larger radius and support multiple propagation modes. Either type has its own pros and cons. For example, the dispersion of SMF is smaller because there is no “mode dispersion”, while the alignment requirement of MMF is more relaxed because of its larger radius. The SMF-28® made by Corning® is a world-famous SMF product that are widely installed.
Optical Fiber Loss
Low loss is no doubt the major feature of optical fibers. Table 1 lists two wavelengths [[1]] commonly used for optical fiber communication. From the table, we see that the wavelength range around 1550nm [[2]] has the smallest loss of 0.18 dB/km. The wavelength range around 1310nm is the next, with loss of 0.32 dB/km. Chronologically speaking, 1310nm products are available earlier in the market. The development of 1550nm products happens later. Today, products in the two wavelength ranges, including optical transmitters, receivers, and other active and pass components, are readily available. Although the loss around 1310nm is relatively higher, for surveillance applications, in which the transmission distance is no greater than, say 20km, the effect results from loss difference is limited.
Table 1. The wavelengths and loss of optical fiber [[3]]
|
Wavelength (nm) |
Maximum Value (dB/km) |
|
1310 |
£0.32 |
|
1550 |
£0.18 |
In another technical article, we have mentioned the loss of RG6 coaxial cable is 0.066 dB/m, or 66dB/km. In terms of dB, the loss per kilometer of coaxial cable is 66/0.18 ~ 367 times larger than of optical fiber. In other word, it takes 367km fiber to attenuate the same loss in 1km coaxial cable. Considering only loss, optical fibers are absolutely more advantageous than coaxial cables.
[1] Fixing velocity v, the relation between wavelength l and frequency f is one-to-one, i.e., l = v/f. However, in optical communications, people use “wavelength” to describe a wave, while in the RF transmission, people use “frequency” instead.
[2] The light wave used in optical fiber communications is not “visible” light, but infrared, which is invisible, and has wavelength larger than the visible light.
[3] Corning® SMF-28® Ultra Optical Fiber Product Information
https://www.corning.com/media/worldwide/coc/documents/Fiber/SMF-28%20Ultra.pdf
Optical Transmitters: Optical Modulation
The choices of optical source and modulation method are important for optical transmitters. The modulation method is application dependent. We will only discuss modulation methods related to surveillance applications in this article.
There are several types of optical transmitters readily available in the market for surveillance applications. They include:
(1) For CATV (cable television)
CATV uses sub-1GHz frequencies, which is the same as DTV. The optical transceivers for CATV can be directly used for DTV.
(2) For CCTV (closed-circuit television, or analog television)
In the specification of such optical transmitters, one may see a description like “10-bit digitally encoded broadcast quality video”. The “10-bit” here means that the transmitter will sample the input analog video and convert the sampled signals to pulsed-code modulated (PCM) optical signals. At the optical receivers, the optical signals are converted back to electrical signals, and then to the analog video waveform with DAC. An alternative way is to directly modulate video waveform to optical signals. The reason why the analog video is sampled at transmitters and reconstructed at receivers is because the amount of analog video distortion can be controlled even with these EO (electrical to optical) and OE (optical to digital) conversions.
However, because analog video is very sensitive, the process of sampling and reconstruction, and the quantization error due to pulsed-code modulation may still cause problems such as cross-color and cross-luminance.
The sampling rate of such transmitters (or transceivers) is optimized for CCTV applications. At most 100MHz is large enough to cover one CCTV channel. Because DTV’s carrier is normally hundreds of MHz, this types of TXs or TRX’s cannot be used for DTV.
(3) For AHD/TVI/CVI (analog HD surveillance systems)
This type of optical TXs is similar to that for CCTV, but with higher sampling rate because analog HD signals occupy wider bandwidth than CCTV signals.
(4) For Ethernet
The input to this type of TXs is RJ45 connector. The baseband Ethernet signal are used to directly modulation the laser current and generated two optical power levels corresponding to 0 and 1, the so-called on-off keying. Because Ethernet is bi-directional, the products for this application are normally in transceivers (TRXs) form, with a pair of optical connectors, one for each direction.
In on-off keying, only two levels 0 and 1 are relevant. In other words, the linearity requirement of the TRXs is very low. Some receivers even use limiting amplifier to improve SNR. Although DTV uses digital modulation, the RF output is COFDM modulated signal. Therefore, optical TRXs for Ethernet is also not suitable for DTV as well.
Optical Receivers
Optical receivers use photodiodes (PDs) to convert optical signals back to electrical signals. The material of the PDs determines the input wavelength range. The market available PDs in general cover both 1310 nm and 1550 nm.
Architecture
There may be several possible scenarios, and they are discussed below.
(1) One DTV camera (one channel) over one optical fiber. Referring to Figure 1, connect camera output to the optical transmitter input. As mentioned earlier, the choice of 1310 nm or 1550 nm light source does not make much difference.
(Figure 1)
(2) Multiple cameras (multiple channels) over one optical fiber. Referring to Figure 2, combine the RF signals with an RF combiner, and use the combined signal to modulate the light source. At the DVR side, the optical receiver converts the optical signals back to RF signals. The channels are separated and received at the electrical domain.
.png)
(Figure 2)
(3) One camera with return channel. There may be several ways to achieve bi-directional transmission in optical fibers. For example, one may use two different wavelengths for the two directions in one optical fiber. A practical implementation is 1310 nm for one direction, and 1550 nm for the other. The other possible way is to use two optical fibers, one for each direction.
In any case, both the camera and DVR ends need one optical transmitter and one receiver. For the one-fiber solution, it requires a passive optical component, the WDM combiner, at both ends. The “WDM” here stands for “Wavelength Division Multiplexer”. The function of this component is to combine or separate optical signals with different wavelengths.
(Figure 3)