The overview of HD-SDI surveillance applications across various products, transmission methods, and storage devices is covered in this piece. However, there may still be some questions about HD-SDI signals and signal distribution. The following Q&As will tackle FAQs.
The overview of HD-SDI surveillance applications across various products, transmission methods, and storage devices is covered in this piece. However, there may still be some questions about HD-SDI signals and signal distribution. The following Q&As will tackle FAQs.
Q: How to arrange SDI transmission distances? Is it right to say that the ideal distance between points is 100 meters? In addition, is there any measuring device can be used to test SDI signal?
A: Generally speaking, HD-SDI could transmit signals within 120 to 140 meters; signal distributors and boosters can be set apart from each other within this range as relays for extending the transmission longer. As for the distance range of 100 meters, it is widely accepted as the normal HD-SDI transmission distance that yields the stable signals. Currently, most SDI distribution amplifiers are designed as 1:4, 1:6, and 1:8 specifications. Some vendors provide solutions of 1:16 multi-distribution patterns. However, SDI chip formats and capabilities vary significantly, so installers should verify carefully the performance before adopting such solutions.
As to the measurement of HD-SDI signal, certainly, any electrical signal generates a dB value, including HD-SDI signals, whose dB value is about 800mV, 15dB at 1.485-1.5GHz, while 3G-SDI has a value of 18dB at 2.97GHz. To measure HD-SDI or 3G-SDI signals, it requires professional dB scales or oscilloscopes, such as Tektronix's WFM7120 digital waveform monitor, which is recommended by many manufacturers.
It supports 3Gb/s HD/SD auto format detection to deliver more comprehensive analysis and processing capabilities with 1080P 50/59.94/60 (3Gb/s) HD video signals for observing the distribution of SDI signals. Tektronix WFM7120 digital waveform monitor also complies with SMPTE 424M and 425M broadcast-grade signal test standards. This type of 3Gb/s SDI signal measurement instruments provide multiple ways to transmit bandwidth-intensive 1080P (Full HD) signals complying with SMPTE 424M and 425M standards in a reliable way. Waveform monitors can also be used to verify if the target complies with 1080P 50/59.94/60 Level A and Level B format standards, and include options to measure eye and jitter to evaluate the physical layer performance of the 3Gb/s interface on a single SDI link. Only one drawback is that those equipments cost a significant amount that a non-SDI manufacturer feels uncomfortable to invest in them.
Q: Do SDI signals have problems of packetized data like IP transmission does? If we adopt mixed transmission approach with coax and fiber optic cables, what should be considered to ensure good transmission quality? Are there any wiring examples that are both installation-friendly and cost-efficient?
A: SDI signals are broadcast-grade digital electrical signals which are transmitted in a form of real-time digital uncompressed voltage outputs instead of IP digitalized signals. Therefore, unlike IP megapixel signals, SDI video signals will not be compressed or packetized during their transmission, which ensures latency-free transmission.
SDI signals can reach certain distances depending on the specific coaxial cables. For example, 270Mb/sec (SD-SDI) signals can be sent over 440 meters with RG6 coax or 330 meters with RG59 coax, while 1.485Gb/sec (HD-SDI) signals can be sent over 230 meters with RG6 coax or 160 meters with RG59 coax. With these allocation points, you can adopt a signal booster to further transmit 300 meters of SD-HDI signals, 100 meters of HD-SDI signals, and 70 meters of 3G-HDI signals. As for signal phases, the best practice is to adopt only one booster, which means only one SDI signal is amplified at a time in order to ensure the stability and quality of signals.
For mixed approach with fiber optic cables, keep in mind that the distances of SDI signals transmitted by fiber optic transceivers depend on the single-mode or multi-mode of fiber optic cables. Also, fiber optic transceivers should support SD/HD/3G-SDI formats, and signals range from 270Mb-1.5 Gb/sec to 2.97 Gb/sec. Failing to verify the capabilities could lead to low performance of transmission and time wasted for repeated maintenance.
Q: In the best practices of cable TV (or community antenna television, CATV) installations, they provide detailed charts explaining wire wattages and amplifying values in various conditions. Are there any similar charts for SDI signals? How to define the related standards?
A: These questions are really difficult to answer. CATV installations require reference values of wire transmission distances and cable impedances to determine their wire lengths because they are related to multiple channels of mixed signals. By contrast, SDI signals are standardized to use a nominal impedance of 75 ohms and signal strength of 800mV and 15-18dB to deliver stable transmission over coaxial cables. Therefore, SDI installations basically don't require any references regarding cable impedances or wattages. If you find any information about SDI wiring references, welcome to share with asmag.com.
Q: Does the resolution need to be reduced to ensure a smooth SDI transmission?
A: Here are some additional clarifications about the definition of SDI signals. For example, ITU-R BT.656 defines digital video interfaces used for broadcast-grade video, and this standard is used for transmission of uncompressed, unencrypted digital video signals (optionally including embedded audio and time code) with a nominal impedance of 75 ohms coax. Most professional video equipment complies with this standard. Its first version as defined by SMPTE 259M is for the transmission of digitalized NTSC and PAL analog signals over serial digital interface, commonly known as SD-SDI. Transferring SD-HDI signals requires bit rates of 270 Mbps. With the introduction of 1080i and 720p HD video standards, those interfaces are enhanced to support bit rates of 1.485 Gbps. 1.485-Gbps serial interfaces are commonly known as HD-SDI, defined by SMPTE 292M, which uses the same nominal impedance of 75 ohms coax. SMPTE 424M is a new standard with enhanced SDI capabilities of 2.97 Gbps on the same type of coax. This new standard is also referred to 3-Gbps, 3G-SDI, supporting 1080p and higher definition image quality of digital cinema. Therefore, being defined as a HD standard, SDI remains the same resolution regardless of any conditions or transmission formats in its transmission process.
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