Chapter 7: Digital Video Interfaces

In this chapter the various standardized interfaces for component and composite video will be detailed along with the necessary troubleshooting techniques.

7.1 Introduction

Of all the advantages of digital video, the most important for production work is the ability to pass through multiple generations without quality loss. Digital interconnection between such production equipment is highly desirable to avoid the degradation due to repeated conversions.

Video convertors universally use parallel connection, where all bits of the pixel value are applied simultaneously to separate pins. Disk drives lay data serially on the track, but within the circuitry , parallel presentation is more common because it allows slower, and hence cheaper, memory chips to be used for timebase correction. Reed-Solomon error correction depends upon symbols assembled from typically eight bits. Digital effects machines and switchers typically operate upon pixel values in parallel.

The first digital video interfaces were based on parallel transmission. All that is necessary is a set of suitable driver chips, running at an appropriate sampling rate, to send video data down cables having separate conductors for each bit of the sample, along with a clock to tell the receiver when to sample the bit values. The complexity is trivial, and for short distances this approach represented the optimum solution at the time.

Parallel connection has drawbacks too; these come into play when longer distances are contemplated. A multicore cable is expensive, and the connectors are physically large. It is difficult to provide good screening of a multicore cable without it becoming inflexible . More seriously, there are electronic problems with multicore cables. The propagation speeds of pulses down all of the cores in the cable will not be exactly the same, and so, at the end of a long cable, some bits may still be in transition when the clock arrives, whilst others may have begun to change to the value in the next pixel.

Where it is proposed to interconnect a large number of units with a router, that device will be extremely complex because of the number of parallel signals to be handled. In short, parallel technology could not and did not replace the central analog router of a conventional television station. The answer to these problems is the serial connection. All of the digital samples are multiplexed into a serial bitstream, and this is encoded to form a self-clocking channel code which can be sent down a single channel. Skew caused by differences in propagation speed cannot then occur. The bit rate necessary is in excess of 200Mbits/s for standard definition and almost 1.5Gbits/s for HD, but this is well within the capabilities of coaxial cable.

The cabling savings implicit in serial systems are obvious, but the electronic complexity of a serial interconnect is naturally greater, as high speed multiplexers or shift registers are necessary at the transmitting end, and a phase-locked loop, data separator and deserializer, as outlined in Chapter 3, are needed at the receiver to regenerate the parallel signal needed within the equipment. The availability of specialized serial chips from a variety of manufacturers meant that serial digital video would render parallel interfaces obsolete very quickly.

A distinct advantage of serial transmission is that a matrix distribution unit or router is more easily realized. Where numerous pieces of video equipment need to be interconnected in various ways for different purposes, a crosspoint matrix is an obvious solution. With serial signals, only one switching element per signal is needed. A serial system has a potential disadvantage that the time distribution of bits within the block has to be closely defined, and, once standardized, it is extremely difficult to increase the word length if this is found to be necessary. The serial digital interface (SDI) was designed from the outset for 10-bit working but incorporates two low-order bits that may be transmitted as zero in eight-bit applications. In a parallel interconnect, the word extension can be achieved by adding extra conductors alongside the existing bits, which is much easier.

The third interconnect to be considered uses fibre optics. The advantages of this technology are numerous: the bandwidth of an optical fibre is staggering, and is practically limited only by the response speed of the light source and sensor, and for this reason it has been adopted for digital HDTV interfacing. The optical transmission is immune to electromagnetic interference from other sources, nor does it contribute any. This is advantageous for connections between cameras and control units, where a long cable run may be required in outside broadcast applications. The cable can be made completely from insulating materials, so that ground loops cannot occur, although many practical fibre- optic cables include electrical conductors for power and steel strands for mechanical strength.

Drawbacks of fibre optics are few. They do not like too many connectors in a given channel, as the losses at a connection are much greater than with an electrical plug and socket. It is preferable for the only breaks in the fibre to be at the transmitting and receiving points. For similar reasons, fibre optics are less suitable for distribution, where one source feeds many destinations. The bi-directional open -collector or tri-state buses of electronic systems cannot be implemented with fibre optics, nor is it easy to build a crosspoint matrix.

The high frequencies involved in digital video mean that accurate signal termination of electrical cables is mandatory. Cable losses cause the signal amplitude to fall with distance. As a result the familiar passive loop-through connection of analog video is just not possible. Whilst much digital equipment appears to have loop-through connections, close examination will reveal an amplifier symbol joining the input and output. Digital equipment must use active loop-through and if a unit loses power, the loop-through output will fail.



Digital Interface Handbook
Digital Interface Handbook, Third Edition
ISBN: 0240519094
EAN: 2147483647
Year: 2004
Pages: 120

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