Video standards converters
Video standards converters cover a broad range of formats: NTSC, PAL, interlaced and progressive, and computer graphics formats in addition to various signal transports such as DVI, HDMI, USB, DisplayPort and Thunderbolt. And even this list is a subset of existing equipment. Technology advances and the convergence of the worldwide video market have made it necessary to repurpose video assets that could have originated from any part of the world, from a video camera or a computer, for an application that is going to a different part of the world, being viewed by a different monitor, being stored on a computer disc, or any combination thereof.
One good example is the NTSC or PAL composite video baseband signal (CVBS) to digital decoder, sometimes called a video A/D converter, or simply a converter. The CVBS format has been used for more years than other current formats, decades before SDI, or any other digital format. The original digital transition started nearly 20 years ago, but some studios and production facilities still have large numbers of CVBS feeds, signals and tape assets.
Today, nearly every video signal is distributed digitally to the final viewer. Video production is digital, and playout is digital. But these legacy CVBS signals and tape assets must be converted to digital for compatibility with the new digital infrastructure.
What parameters are important in an analog to digital converter? Based on the application, some are more important than others. Typically, choices are made for resolution (8, 10 or 12 bits), linearity (differential gain and differential phase), chroma, luma, gain, hue and brightness are important in nearly every application to preserve the original picture quality. A long or faulty coaxial cable can seriously degrade CVBS signal picture quality as can poor filters and converter chips. Today, digital transmission for video creates far fewer picture artifacts than analog modulation did. But, accurate anti-alias filters, linear converters and little or no differential delay between luma and chroma are still requirements for high-quality on-air pictures.
Today’s semiconductor technology provides really small converters. For example, one SFP for CVBS includes two independent converters in a single SFP package. (See Figure 3.) These fully featured converter chips integrate more features into less space with less power than some modular products and card-based converters.
The basic blocks are:
- A\D converter. This critical block includes analog signal filtering. Resolution, linearity and analog noise are also controlled with this chip. A poorly designed semiconductor has a negative impact on overall performance. Composite NTSC or PAL conversion is still a common function. While component RGB signals are still used, computer graphics formats have rapidly diminished their use. Simultaneously, DVI, HDMI and other digital video-specific transports have replaced the three coaxial cables of RGB systems with a single cable, albeit with multiple twisted pairs inside.
- Image processing. This block could accomplish functions such as audio de-embedding, audio analysis, video analysis, time base correction, etc.
- SDI serializer. This block converts the parallel data from the A/D to serial data. It receives 10-bit or 20-bit video words as well as necessary synchronization and ancillary information such as audio data, or signal-specific metadata such as TRS, EAV and SAV. It adds framing data and converts the parallel data to serial. As the serial data is clocked out, a data scrambler is used to reduce DC content, and then NRZI channel encoding is used to ensure that data is invertible.
- D/A converter. This block receives the digital data in parallel in addition to video timing and framing signals. It converts it to an analog composite or component signal. It could use 8, 10 or 12 bits of resolution. The video D/A conversion, or encoding, is far easier to implement than the A/D, so this block is less critical than in the decoder.
- SDI deserializer. This is the opposite of the serializer. It receives the serial signal, recovers framing, deserializes the data and outputs the data in parallel words (10 or 20 bits). The sophistication and miniaturization of semiconductors allows this block to de-embed audio signals, provide output sync signals, analyze for CRC errors, extract other metadata and provide status flags and information about the video signal itself.
This concludes part one of this series. Be sure to visit the Web version of this article as it links to previous digital video signal tutorials. The second part of this article will appear in the April issue of Broadcast Engineering.
—Renaud Lavoie is president and CEO of Embrionix.