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
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.
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 standardThis 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 standardReceivers 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 standardThis 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.
Actual copper performance
Copper cables require special driver chips at the send end to generate the waveforms defined in the SMPTE specifications and dedicated equalizer chips at the receive end to compensate for the frequency-dependent losses in the cable. SDI equalizers contain feedback loops, so they automatically work with any cable length up to their particular limit. This limit is defined by the maximum gain available in the chip.
The equalizers are designed to compensate for skin effect loss, which is proportional to the square root of the frequency. This is why it is important to use cables with minimal dielectric loss, because the equalizer does not have the correct characteristic to compensate for this kind of loss. If the equalizer is a poor match to the loss, the overall frequency response is not flat, and the data symbols become distorted (intersymbol interference), causing a reduction in the eye height and eye width and leading to bit errors. When the 3G equalizers were being designed, one of the specifications was for them to work with the same family of cables that are used for HD signals, so they are compatible with the installed base.
Some of the latest generation of 3G equalizers provide a maximum gain of more than 40dB, so they can achieve error-free operation on up to 140m of the appropriate cable. (See Figure 1.)
Implementation guidelines for copper links
As has been shown, the use of high-quality transmit and receive components with adequate gain and a correctly defined frequency response allow cable lengths of up to 140m to be compensated for reliably.
In addition to signal distortion due to high-frequency loss, digital errors can also be induced if the data eye closes horizontally, because the times of the transitions become distorted. There are several causes of this eye closure:
- Intersymbol interference (ISI), caused by imperfect equalization of the attenuated signal from the long cable. Reflections due to suboptimum transmitter and receiver return loss may also contribute to ISI.
- Duty cycle distortion (DCD), caused by differences in timing for rising and falling edges of the signal. ISI and DCD combine to make up deterministic jitter.
- Noise generated in the receive electronics, because it amplifies the attenuated signals from the cable, also known as random jitter.
- Crosstalk from adjacent signals in the sensitive equalizer's input.
These jitter effects are not corrected by the equalizers. The absolute value of these jitter effects, measured in picoseconds, is largely independent of the data stream. But as the data eye becomes narrower with higher data rates, the relative value, measured in data periods (referred to as Unit Intervals [UI]), increases. A 3G UI is half the width of an HD UI, and so is twice as vulnerable to jitter.
SMPTE 424M, the 3G standard, has a looser specification for transmit jitter than SMPTE 292, the HD specification. It specifies .3UI at the transmitter compared with .2UI for HD. This recognizes the difficulty of achieving the .2UI figure at 3G, where 1UI is only 330ps. While this makes things easier for the transmitter,it means there is less jitter budget left for the remaining elements in the link. SMPTE 424M does make a strong recommendation for .2UI transmit jitter, and modern 3G capable SDI transmitters and receivers do achieve this figure. So when calculating a jitter budget, it is recommended to check the actual specification of 3G equipment and not just that it is SMPTE-compliant.
In practice, this means that, to be safe, jitter needs to be removed from the signal each time a long cable has been traversed or a significant piece of signal processing has taken place, e.g., signal selection in a crosspoint. Jitter is removed using a reclocker, which regenerates the serial clock using a phase-locked loop (PLL) locked to the incoming data. The outgoing data are resampled using the regenerated clock, and the jitter budget is thus reset. Alternatively, if the signal has been transformed from the serial to the parallel domain for processing, the deserializer should have good tolerance to incoming jitter and be able to generate error-free data from the incoming stream. Similarly, the serializer should include a serial clock generator with low jitter, so the new serial data stream is clean as it is launched.
In a reclocker, the specifications to look for are input jitter tolerance (IJT), which specifies the amount of jitter on the incoming signal that can be tolerated without data errors, and output intrinsic jitter (OIJ), which specifies the amount of jitter remaining on the signal at the reclocker's output. A good reclocker has an IJT of .7-.8UI and OIJ of less than .1UI.
Deserializing receivers and serializing transmitters
In the receiving deserializer, the important specification to look for is IJT. In the transmitting serializer, it is OIJ. For 3G signals, IJT should be at least .7UI, and OIJ should be an absolute maximum of .2UI. This allows for a further .1UI to creep in between the serializer and the equipment output without jeopardizing SMPTE compliance. If a lower OIJ can be achieved, then the output can better the SMPTE specification and has the potential for longer cable lengths. To achieve low OIJ, the system designer must ensure that the clock used by the serializer has low jitter, either by using a serializer with a built-in clock cleaner or by cleaning up the clock used with an FPGA serializer by using a dedicated clock cleaner. The clock-cleaning process is performed in both cases by using a clock generator with low intrinsic jitter, combined with a low loop-bandwidth PLL to reject jitter from the parallel reference clock.
Multipass and jitter
Jitter removal using reclockers is effective for jitter at frequencies above the loop bandwidth of the reclocker's PLL. This is almost all of the jitter and is effective for many passes. At the 2009 NAB Show, one vendor showed 101 passes through a cable, equalizer and reclocker combination, with no measured errors.
Low-frequency jitter reduction
The loop bandwidth is a compromise between the need to track the dynamic characteristics of the incoming data stream and the need for low output jitter. In practice, a loop bandwidth of around 1MHz is used. This rejects jitter at frequencies above 1MHz, but passes on lower-frequency jitter. It is possible in a high-complexity system, where the signal remains in the serial domain for many cable runs, that low-frequency jitter accumulates to a point where it approaches the SMPTE 424M specification of .2UI at 10Hz. In this case, the signal should be deserialized and reserialized. The deserializing process transfers the data to the parallel domain, in which the .2UI at 3Gb/s represents only .15UI at the 148.5MHz parallel clock rate. The parallel data are then reserialized using a PLL with a very low loop bandwidth, and the jitter budget is reset over the complete spectrum. (See Figures 2 and 3.)
3G systems can be safely implemented using coaxial cable runs of up to 140m. It is important that:
- The transmitted signals meet or exceed the SMPTE specification;
- The cable equalizers are of high quality;
- The signals are equalized and reclocked after every cable run;
- The connectors are 3G-qualified; and
- The appropriate foam dielectric cable is used.
For longer links, fiber-optic cables are recommended. For these:
- It is important to use send and receive interfaces with video capability;
- Single-mode fiber is required for links longer than 400m;
- For long links, ensure that the send power and the receive sensitivity are correct for the fiber loss;
- For short links, ensure that the receiver is able to accept the input power without overload; and
- The signals should be reclocked after every fiber run.
Nigel Seth-Smith is a member of the Strategic New Product Definition and Business Development team at Gennum.
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