Compared to single-ended signaling, differential signaling offers many benefits: less electromagnetic interference (EMI), less distortion, lower supply voltagei, and lower costii. These advantages have prompted the adoption of differential signaling in many applications, including digital (low-voltage differential signaling, or LVDS) and analog (audio). Similar benefits should accrue to analog videoiii. But for reasons that may no longer be valid, most video applications continue to use a single-ended 75- coax connection.
By leveraging differential signaling and low-cost CAT5 networking cabling, analog video can be distributed over significant distances, reducing the cost of video interconnects and distribution. CAT5 cabling could be cost-effective in common applications like security systems, closed-circuit television (CCTV), and automotive systems.
If a system configuration has a lot of sources like cameras, especially for security or CCTV, video over Internet Protocol probably will be a better choice. That’s because the total cost of the system can amortize the cost of network hardware, video compression hardware/software, and software necessary to control and maintain all systems. Yet in systems with fewer sources and when the total cost of the system is important, analog video interconnects deliver the best cost/performance tradeoff since such systems don’t require compression or any software control.
There hasn’t been a lot of momentum toward the use of differential signaling. One reason for this lack of inertia may be a lack of understanding of how differential circuits operate and are specified. To help remedy that deficiency, the concept of balance can provide an intuitive understanding of differential circuits and help compare them to more familiar single-ended types.
The concept of balance has various names, depending on the part of a differential circuit in which it’s used. It isn’t specified at all in some cases, like most analog differential drivers. But the term is necessary, and other parts of the differential circuit can modify its importance.
An impediment to the implementation of differential analog video has been the absence of a low-cost transmitter-receiver IC that converts, transmits, and receives video over a differential cable while also providing an interface to existing 75- single-ended video circuits. This capability requires a lot of circuitry, including (at minimum) an active balun circuit with amplitude equalization to compensate for the cable loss, as well as clampingiv or biasing to stabilize the video black level. This IC also would need I/O protection, single-supply operation, fault diagnostics, and low cost.
Such an IC is now available, so next, we’ll look at a typical application. To illustrate what can be accomplished, we’ll consider several practical circuits for sending video over several hundred meters of CAT5 cable while also considering the enhancements necessary for handling specific application problems. No formal design standard specifies the design and performance of a differential analog video system, but you can specify such systems using a standard 75- interface at each end. You also can test them using existing standards and test equipment.
Basics of differential signaling One of the best ways to understand differential circuits is by an analogy with the more familiar single-ended (SE) form (Fig. 1). Although the differential circuit looks complex, it’s simply two SE circuits configured to produce equal and opposite signals with respect to a common pointv. The common point is designated as ground in Figure 1, but it could just as well be a dc bias voltage. In any case it’s a virtual ground, and it joins the two SE circuits that make up a differential one. Why should we use an obviously more complicated circuit?
The answer is subtle, and it hinges on the one or more lines that connect the various parts of a differential circuit to the common point shown as ground in Figure 1. We assume these lines have no electrical properties, but nothing could be further from the truth. At a minimum, they exhibit resistance. At higher frequencies, their distributed inductance and capacitance also become important. These parasitic elements are one of the reasons an actual circuit acts differently from a simulation based on its Spice model. Differential and SE circuits address this problem in different ways. Differential circuits rely on the property of balance, and SE circuits use the property of structure.
Structure is the mechanical construction of a component, as is evident in a coaxial cable. Balance, as the word implies, is a state of equilibrium between two quantities. In Figure 1b, currents IA and IB are equal and opposite and are therefore in equilibrium. They will cancel each other if they flow in the same wire, as will the ones shown connected to the ground symbol. The consequences of this cancellation are profound!
Differential circuits are relatively immune to the effect of parasitic elements, because any currents with a tendency to flow in those elements are cancelled. In turn, that effect reduces the need for structure (shielding) because it is extremely difficultvi for signals to couple into or out of a balanced system. As a result, the signal lines designated for CAT5/6 cables are low-cost, unshielded twisted pairs (UTPs).
In contrast, an SE circuit passes signal current through the lines that connect to the ground symbol. Additionally, it’s very susceptible to the presence of parasitic elements in those lines. To address this problem, SE circuits include a heavy ground structure made of wire braid to connect source and load, ensuring that the ground resistance is much lower than the signal-conductor resistance. Wrapping the braid around the signal conductor produces a form that later evolved into coaxial cable, which combines the requirements of ground and shielding. Compared with UTP, this arrangement requires higher precision and more material. It’s also more costly.
Although SE and differential circuits have similar circuitry, they rely on different methods to achieve performance. The performance of differential circuits depends on balance, but what determines balance?
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