Skip to main content

Format conversion and synchronization

As the conversion to DTV speeds up, format conversion continues to be of increasing importance, while the array of choices available to perform this essential function continues to increase. A variety of equipment available today can route, distribute or display digital signals over a wide range of signal formats and frequencies without the need for external conversion equipment. There are, however, situations requiring conversion to a different signal format for production, storage or transmission. Choosing a format converter to suit your needs requires careful thought. For example, how do you decide between a linear converter and a unit that combines motion compensation to provide superior performance, or a programmable unit to adapt to future requirements as opposed to a converter with a fixed function?

A major requirement with HD-DTV is to upconvert existing or incoming SDI (or analog) material to HD-SDI. Any HDTV program that will include SD footage will require upconversion of this footage for inclusion within the new program. Similarly, downconversion of HD-SDI material into SD material will be required for some time to come to accommodate SD storage, transmission and various forms of SD processing. The need to convert between the progressive and interlaced formats is mainly a concern for those adopting the 720p. For those adopting the 1080i, the material will remain interlaced (for conversion from 480i to 1080i) and only occasionally will conversion to or from 720p be required. The choices available for these conversions range from selectable converters such as Xantus and Star-Up format and standards converters from TeraNex, to the Mach 1 converter from Snell and Wilcox (see Pick Hits, p. 82). These units provide conversion that can be changed to suit the needs of a particular session as required. Fixed converters are available from companies such as YEM with their 720p to 1080i converter, the HFC-2020, or 1080i to 720p with the HFC-4040. These units provide a dedicated solution and save the need for involved user interaction and the complexity of multipurpose units. Individual modular converters are also available to solve other requirements within the SD/HD environment. Monitoring HD signals can be accommodated by converting them to analog with the AJA HD10M or to SDI with the HD10MD. These units require the mode to be preset, typically with dip switches, prior to placing them into service.

If you have a need for conversion between composite analog NTSC, PAL or SECAM and component analog or SDI, you might want to check out the DTC1600 Quattro from Video International. This unit provides conversion between the selected I/O formats with a choice of processing, noise reduction and interpolation control. Modular units are also available to fill the need for preset conversion between composite or component analog and SD digital video. A variety of different solutions are available from companies including AJA, Cobalt, Leitch and Miranda.

A big concern with regard to conversion relates to archive and storage usage. Material needs to be tracked to determine the recorded format and then converted into the desired format. The use of individual conversion equipment is still the norm, as trying to maintain a mix of HD and SD formats in one facility is an expensive, if not impractical, proposition. It generally makes sense to keep everything in HD where needed for HD production or transmission and then downconvert to SD for SD production or for inclusion in SD transmission.

This brings up another concern with format conversion: aspect ratio control. While there are only two valid display aspects available for television, 4×3 and 16×9, care should be used when transferring an image originated in one format into the other. There are many manufacturers that provide the ability to adapt a 4×3 image into 16×9 space and vice versa within SD/HD converters. If this were the only concern it would create no more than a simple production decision. However, the problem is compounded by the transmission of DTV images and the inability to know if the resultant program is being viewed as 16×9 or 4×3. When the receiver in use fixes the image, the viewer has no choice in the matter. Or perhaps even more of a problem, they do have a choice, using the selectable display controls on DTV receivers. The result is that 16×9 images may be viewed with the sides missing on a 4×3 display or, even worse, a 4×3 image may be viewed with the top and bottom missing on a 16×9 display.

When it comes to flexibility in frame synchronization, quite a few manufacturers were showing modular equipment frames that accept SD (and some also HD) modules, including video and audio synchronizers and audio embedders and de-embedders. New frames that include SD frame synchronizer modules include the NEO series from Leitch, the Synapse series from Axon, the Avenue Series from Ensemble Designs, the IQ Modular system from Snell & Wilcox and the Symphonie series from Miranda. The new styles of frames provide the ability to perform remote monitoring and control. If you are looking for a modular frame with this in mind, consideration should be given to the current availability of modules, the planned modules for the product line, including HD, and the interconnectivity between different manufacturers' frames.

A major problem with video frame synchronizers has been with embedded audio. While the video requires a frame add or delete to synchronize to the local reference, the audio would experience a repeat or drop of audio data and subsequent interruption in the embedded audio frame sequence. This is no longer a problem with the SFS20 module in Axon's Synapse system, which provides audio processing to perform a smooth audio transition while video frames are dropped or inserted. This provides one solution to the problem facing users who have chosen to embed the audio wherever possible.

Less care is sometimes taken over audio signal conversion, but care in this area can save problems later. The common sampling rates in current use are 44.1- and 48kHz, with 96kHz becoming more popular. Rate conversion to a common sampling rate is often desirable when a variety of rates coexist and there is a need to synchronize asynchronous streams. ADC's DA4050 can convert asynchronous AES audio over a wide range into synchronous AES audio while maintaining the audio channel phasing.

Computer video often comes into play in production when screen shots or Web pages need to be included in news programs or within training videos. Producing a high-quality, low-resolution image (525/60 or 625/50) in analog or digital from a high-quality XGA or SVGA source can be achieved with products such as the XTD 820R from Analog Way. This unit also allows zooming or panning of the image in real time to make the output more useful.

Whatever your conversion need is in today's mixed analog and digital, multiple-resolution world, there is probably a solution to your problem.

Mike Betts is the senior partner of Broadcast Training Partners.