Beyond HD 1080p
Mar 1, 2010 12:00 PM, Nigel Seth-Smith
Switch to 3G-SDI seamlessly using existing HD cabling.
Figure 1. Insertion loss of Belden 1694A cable
Select figure to enlarge.
The SMPTE and ITU-R 3Gb/s SDI standards have launched a new generation of interface and processing equipment. We all like the higher data rate, but there is a natural nervousness as to whether it will work reliably, particularly over infrastructure installed for 1.5 Gb/s HD-SDI.
This article shows how 3G products can operate with existing HD cabling, and how users can ensure an error-free system.
To guarantee reliable performance over long cable runs, the system designer needs to know:
when to equalize to compensate for high-frequency loss;
when to reclock to compensate for jitter buildup; and
when to use copper and when to use fiber.
The rules are backed up by analysis of the sources of errors in a high-speed data system and descriptions of how to avoid errors using techniques for budgeting loss and jitter in the system.
Characteristics of coaxial cable and optical fiber
Figure 2. A multipass 3G-SDI system
Select figure to enlarge.
Signals travelling over cables suffer losses. In the case of coaxial copper cables, these losses are caused by a combination of the resistance of the conductors and absorption by the insulating dielectric. Both of these effects are worse at high frequencies. The resistive losses are influenced by the “skin effect,” which makes the losses increase in proportion to the square root of the transmitted frequency. Dielectric losses are directly proportional to frequency. Cables should be selected such that the dielectric loss is not a significant factor over the frequency range to be transported. For HD and 3G-SDI, this means the use of foam dielectrics. Both resistive loss and dielectric loss are directly proportional to cable length.
Fiber-optic cables are made of glass that is almost perfectly transparent, at least at the wavelengths of light used for data carriage. There is, however, some absorption loss over long distances. This absorption is not particularly dependent on the data rate of the link, because the bandwidth of the data is tiny compared with the carrier frequency. There is a second way in which the accurate carriage of information over fiber-optic cables is distorted: dispersion. The light travelling through the cable may take a number of different paths, all of slightly different lengths. Over some distance, this leads to a spreading of the data, eventually leading to complete mixing of adjacent bits. This effect is worse for high data rates, because the bits are closer together. Dispersion is much less of an issue with single-mode fiber, in which the data-carrying part of the fiber has such a small radius that the light can effectively only take a single path. Both absorption loss and dispersion are directly proportional to cable length.
Standards
One area of difficulty for system builders is the lack of a standard for the performance of digital video transport interfaces. The data protocols, voltages, impedances, etc., are all standardized, but the standards bodies, SMPTE and ITU-R, do not dictate the cable length for reliable operation. This is a deliberate decision, so as technology improves, the performance can improve with it rather than being frozen at any particular value. It also recognizes that different applications require different performance, and it makes sense to allow a mix of equipment with different performance and pricing.
There are, however, informative statements in the SDI standards for the coaxial copper interface, which are as follows:
- SMPTE 259M, SD-SDI standard
This standard describes a serial digital interface for 525/60 and 625/50 DTV equipment operating with either 4:2:2 component signals or 4fSC composite digital signals. This standard has application in the TV studio over lengths of coaxial cable where the signal loss does not exceed an amount specified by the receiver manufacturer. Typical loss amounts would be in the range of 20dB to 30dB at one-half the clock frequency with appropriate receiver equalization. Receivers designed to work with lesser signal attenuation are acceptable.
- SMPTE 292, HD-SDI standard
Receivers operating with input cable losses in the range of up to 20dB at one-half the clock frequency are nominal; however, receivers designed to work with greater or lesser signal attenuation are acceptable.
- SMPTE 424M, 3G-SDI standard
This standard is a transport defining a bit-serial data structure for 3Gb/s (nominal) component digital signals or packetized data. This standard specifies a coaxial cable interface suitable for applications where the signal loss does not exceed an amount specified by the receiver manufacturer. Typical loss amounts would be in the range of up to 20dB at one-half the clock frequency.
Note that the reference to “typical” cable losses is included within the scope of the document. This is the part of the document that sets the scene for the standard, but does not include any actual standard requirements. However, a quick reading of the standard can easily lead to the mistaken conclusion that a loss of 20dB at a frequency equal to half of the data rate should used for systems budgeting.
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