Video and Signal Conversion, Part 1 Converters Must Keep Pace with Video Format Changes

PRODUCTION ROOM DIGITAL HANDBOOK Video and signal conversion, part 1 Converters must keep pace with video format changes. BY RENAUD LAVOIE

n previous articles, I have cov- modular, or glue, products. While output currents that bias the laser for ered multiviewers, routers and they can require time to reconfigure the correct average power output and some specific topics about AC- cables and firmware settings, they are modulate the laser to generate the I coupling capacitors, the evolu- easier to upgrade than rack-based digital 1s and 0s. The video pathologi- tion of SFP modules and fiber-optic products and usually cost less as well. cal signal has a large DC component, theory. One important aspect I As requirements change for video so it is crucial to have a laser driver haven’t covered yet is conversion. production and processing, the fea- that supports this unbalanced power In a perfect world, interfaces and tures for conversion can change as pattern. (See Figure 2.) standards would interact fluidly and perfectly. Unfortunately, this is not the case, and with history as a guide, new advances in technology will perpetu- ate this situation. A key component requirement is that an SFP enable ab- solute interoperability. A simple SFP Figure 1. Electrical-optical transceiver–converter. Images courtesy Embrionix cage allows easy interchange between multiple modules that bridge a wide well. Upgrading to better features and • Laser or LED. This device converts number of standards. functions is a never-ending process; electrical currents into photons, or This article is dedicated to the func- nothing is ever perfect. light. Optical output power is in direct tion of conversion, a broad topic. proportion to the current level. An ar- There are many options for these func- Optical converter ticle covering the detailed physics of tions. There are rack-based products, The optical converter is usually this conversion will be published later card-based products, dongle products targeted towards video signals run- in Broadcast Engineering. and, of course, SFP products. ning at 270Mb/s, 1.5Gb/s and 3Gb/s, For the optical-to-electrical con- Because this topic has great depth, and soon 6Gb/s and 11.88Gb/s (10G- verter, the blocks are: it will be divided into two parts. This SDI), but they can support data rates • Photodiode. It receives the photons first section includes an introduction from 15Mb/s to 11.88Gb/s depending and converts them to an electrical to signal conversion as opposed to on configuration of the internal signal current output. format/video conversion. We’ll look processing blocks. (See Figure 1.) • Trans-impedance amplifier (TIA). It at techniques to convert between This is the simplest optical-to- converts the photodiode current to a electrical and optical signals. This ar- electrical and electrical-to-optical voltage output, which feeds a limiting ticle will also review basic video stan- converter. The basic blocks for the amplifier. The photodiode current is dards conversion techniques. electrical-to-optical converter are: small; it might only be in the pico-amp In part two, we’ll examine imple- • Laser driver. Electrical digital sig- range. The TIA converts this current to mentations of graphic format con- nals feed the laser driver generating a voltage and amplifies the signal, but verters; VGA, DVI, HDMI and oth- ers; as well as the familiar up/down/ Unique 1s crossconversion systems. 19 0s 19 0s

Diversity There are so many options for Unique 0s converter features, performance and 19 1s 19 1s packaging that it is impossible to de- scribe all the functions and details in one article. A quick market sur- Figure 2. Pathological signal (See “Understanding blocking capacitor effects,” vey would indicate that users prefer Broadcast Engineering magazine, August 2011)

This article originally appeared in the February 2013 issue of Broadcast Engineering magazine. Copyright 2013, Broadcast Engineering. Reprinted with permission. PRODUCTION ROOM DIGITAL HANDBOOK the gain is designed so TIA output is exactly the point where any failure (CVBS) to digital decoder, sometimes distortion-free. The high-gain limiting might occur. called a video A/D converter, or sim- amplifier converts the low-level TIA ply a converter. The CVBS format has output into digital 1s and 0s. Video standards converters been used for more years than other This processing is necessary to en- Video standards converters cover a current formats, decades before SDI, sure distortion-free operation even broad range of formats: NTSC, PAL, or any other digital format. The origi- with the video pathological signal. To interlaced and progressive, and com- nal digital transition started nearly reduce jitter, advanced optical convert- puter graphics formats in addition to 20 years ago, but some studios and ers integrate reclockers just before the various signal transports such as DVI, production facilities still have large laser driver or right after the limiting HDMI, USB, DisplayPort and Thun- numbers of CVBS feeds, signals and amplifier. This reduces jitter in order to derbolt. And even this list is a subset tape assets. achieve the best signal quality and the of existing equipment. Technology Today, nearly every video signal is lowest possible bit error rate. advances and the convergence of the distributed digitally to the final view- Diagnostic functions such as jitter worldwide video market have made er. Video production is digital, and measurement, or eye diagram analy- it necessary to repurpose video as- playout is digital. But these legacy sis, can be included along with im- sets that could have originated from CVBS signals and tape assets must be age and signal processing capability. any part of the world, from a video converted to digital for compatibility These advanced features are available camera or a computer, for an applica- with the new digital infrastructure. as options in some SFPs, providing tion that is going to a different part of What parameters are important in engineers nonintrusive, real-time sig- the world, being viewed by a different an analog to digital converter? Based nal performance monitoring and sta- monitor, being stored on a computer on the application, some are more im- tus at the edge of their signal distribu- disc, or any combination thereof. portant than others. Typically, choices tion network. Engineers can view all One good example is the NTSC or are made for resolution (8, 10 or 12 the signals in the studio and know PAL composite video baseband signal bits), linearity (differential gain and

This article originally appeared in the February 2013 issue of Broadcast Engineering magazine. Copyright 2013, Broadcast Engineering. Reprinted with permission. PRODUCTION ROOM DIGITAL HANDBOOK

differential phase), chroma, luma, gain, hue and brightness are important in nearly every application to preserve Figure 3. CVBS A/D and D/A converter with SDI I/O the original picture quality. A long or faulty coaxial cable can seriously de- diminished their use. Simultaneously, implement than the A/D, so this block grade CVBS signal picture quality as DVI, HDMI and other digital video- is less critical than in the decoder. can poor filters and converter chips. specific transports have replaced the • SDI deserializer. This is the opposite Today, digital transmission for video three coaxial cables of RGB systems of the serializer. It receives the serial creates far fewer picture artifacts than with a single cable, albeit with multiple signal, recovers framing, deserializes analog modulation did. But, accurate twisted pairs inside. the data and outputs the data in paral- anti-alias filters, linear converters and • Image processing. This block could lel words (10 or 20 bits). The sophis- little or no differential delay between accomplish functions such as audio tication and miniaturization of semi- luma and chroma are still require- de-embedding, audio analysis, video conductors allows this block to de- ments for high-quality on-air pictures. analysis, time base correction, etc. embed audio signals, provide output Today’s semiconductor technol- • SDI serializer. This block converts sync signals, analyze for CRC errors, ogy provides really small converters. the parallel data from the A/D to serial extract other metadata and provide For example, one SFP for CVBS in- data. It receives 10-bit or 20-bit video status flags and information about the cludes two independent converters in words as well as necessary synchro- video signal itself. a single SFP package. (See Figure 3.) nization and ancillary information This concludes part one of this se- These fully featured converter chips such as audio data, or signal-specific ries. Be sure to visit the Web version of integrate more features into less space metadata such as TRS, EAV and SAV. this article as it links to previous digital with less power than some modular It adds framing data and converts the video signal tutorials. The second part products and card-based converters. parallel data to serial. As the serial data of this article will appear in the April The basic blocks are: is clocked out, a data scrambler is used issue of Broadcast Engineering. BE • A\D converter. This critical block in- to reduce DC content, and then NRZI Renaud Lavoie is president and CEO cludes analog signal filtering. Resolu- channel encoding is used to ensure of Embrionix. tion, linearity and analog noise are also that data is invertible. controlled with this chip. A poorly de- • D/A converter. This block receives signed semiconductor has a negative the digital data in parallel in addition The following are available on the impact on overall performance. Com- to video timing and framing signals. It Broadcast Engineering website: posite NTSC or PAL conversion is still converts it to an analog composite or a common function. While compo- component signal. It could use 8, 10 • Video processing nent RGB signals are still used, com- or 12 bits of resolution. The video D/A • 3-D standards conversion puter graphics formats have rapidly conversion, or encoding, is far easier to • Signal conversion

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This article originally appeared in the February 2013 issue of Broadcast Engineering magazine. Copyright 2013, Broadcast Engineering. Reprinted with permission.