Technology: 3D Standards Conversion Using the Snell Alchemist Ph.C-HD

By Paola Hobson, Snell

Although it might be a few years away from widespread, commercial adoption, 3D broadcasting is upon us. With each season, a new batch of 3D cinema titles draws excited moviegoers who are gradually warming up to the possibility that they’ll someday have the same viewing experience in their homes. Consumers’ appetites are further being whetted by continuing innovation in display screen technology, a growing library of 3D games and 3D content on Blu-ray disk, and announcements by many of the largest networks that they’ll soon be launching home 3D TV services.

For these broadcasters, and any organisation contemplating delivery of 3D TV services, standards conversion is an important issue since they will be buying external content from international sources. A few examples of 3D content that might require international standards conversion include content from major live events such as the Olympic Games or the FIFA World Cup, studio material such as 3D movies originally products for cinema screenings, and content from other 3D television channels.

High-quality, frame-rate standards conversion, therefore, is no less important for 3D services than for today’s HD and SD content produced in 2D. The added twist is that standards conversion for 3D content must respect precise vertical alignment of the left and right eye images as well as exact horizontal disparity, which thereby preserves the depth perspective, and maintain precisely matched colour and resolution in order to deliver the most comfortable and best-quality results to viewers. Therefore, as with any other downstream processing, converting 3D content from one frame rate to another requires equipment that will apply identical processing to the L and the R channels.

An example of a conversion system that addresses these challenges is Snell’s Alchemist Ph.C-HD, which provides motion-compensated frame-rate conversion for both side-by-side formats and discrete left-right signal paths. The product of extensive research into the exacting demands of 1080p and HD frame-rate standards conversion, Alchemist Ph.C-HD yields converted images of such high quality and clarity that even the most dynamic content is delivered in the sharpest detail, free from artefacts and virtually indistinguishable from the original input.

The Snell Alchemist Ph.C-HD standards converter.

Two-channel standards conversion: tape to tape

For users producing 3D material on HDCAM SR tape, one option is to record the L and R streams onto one tape using the SR dual-stream mode. The tape can then be played out using the dual outputs into a 3D display or 3D projector/screen combination which will enable users to enjoy the stereoscopic effect using passive or shuttered polarised glasses. For standards conversion of this content, a typical tape-to-tape workflow could involve two Snell Alchemist Ph.C-HD motion-compensated frame-rate converters set with the same parameters and externally referenced to a video source compatible with the chosen output format. This approach is most appropriate for non-live productions where it is essential to preserve downstream quality, e.g. for cinema viewing or downstream editing.

By careful use of the Alchemist Ph.C-HD ‘Sync-Auto’ timecode mode, the exact same relationship between input frame rate and output frame rate can be maintained for both left eye and right eye conversions. The two Alchemist Ph.C-HDs can feed any downstream device that supports L and R streams on a dual-link input.

Another example of a two-channel tape-to-tape workflow involves 3D content originating in 1080 23.98psf and requiring a 1080 59i output with 2:3 cadence. This conversion can be supported using two Alchemist Ph.C-HD units with the Snell FilmTools option fitted. Again, the ‘Sync-Auto’ timecode mode can be used to ensure identical 2:3 sequences are applied to both Left eye and Right eye content, with respect to timecode.

A key benefit of the Alchemist Ph.C-HD Sync-Auto timecode feature is the ability to carry out a two-channel 3D conversion using only one Alchemist Ph.C-HD unit. This means that the user can effectively achieve the equivalent of the two-unit conversion just described, but using only one Alchemist Ph.C-HD. The conversion requires two passes, but as long as the temporal phase is locked to the same reference point, the user is guaranteed consistent cut position and movement between the two independent conversions of the Left and Right streams. Without the Alchemist’s Sync-Auto mode, such a two-channel conversion would not be possible.

Example of a conversion without using sync mode.

Example of a conversion using sync-auto mode.

Two-channel standards conversion: live workflow

In some situations, broadcasters might need to perform 3D standards conversion in live workflows; for example, a major sports event being broadcast live in 3D. As with the tape-to-tape example described above, this conversion can be accomplished with two Alchemist Ph.C-HD units integrated into a live transmission workflow (note that the broadcaster might not have control of the 3D production if it is taking a feed from an international supplier). The difference here is that the content is converted to 720 59p for transmission. The quality of the pictures viewed at the home display depends largely on the bandwidth used to compress the pictures at the contribution and transmission stages, but standards conversion using Alchemist Ph.C-HD is virtually transparent.

Single-channel standards conversion

For practical reasons, some broadcasters favour a single-channel 3D distribution path, where the L and R images are horizontally sub-sampled and placed side by side into a frame before the content is compressed and transmitted. This has the advantage of bandwidth efficiency, as only a single transmission channel is required. Other single-channel approaches include vertical sub-sampling (a top-over-bottom picture configuration) and a time-multiplexed approach in which odd fields carry the L signal and even fields carry the R signal.

If a broadcaster receives a 3D contribution feed which is already in the side-by-side format, the standards conversion appears towards the end of the processing workflow. Here, the end quality of the programme to be delivered depends critically on the combiner process, which may introduce unnecessary bandwidth limitations when horizontally sub-sampling. However, the final Alchemist Ph.C-HD standards conversion from 1080 50i to 720 59p is generally artefact-free.

Standards conversion in a 3D workflow where L and R 1080 50i images are combined side by side before being converted to 720 59p for cinema presentation or transmission.

Conclusion

While each of these approaches for 3D frame rate and format conversion has been tested by Snell in real situations with customers, the recommended choice is the two-channel approach. This offers the best-possible quality for the end result, since the full available bandwidth of the L and R signals is preserved throughout the workflow.

As broadcasters and content producers accept the inevitability of 3D, they will require technologies for downstream standards conversion that will produce the highest-quality results and deliver 3D output that is as close to the original content as possible. The good news is that advances in conversion technologies are helping make the job easier and playing a large role in advancing 3D towards true consumer acceptance.

Paola Hobson is senior product manager for conversion and restoration at Snell, provider of a comprehensive range of solutions for the creation, management, and distribution of content, as well as the tools necessary to transition seamlessly and cost-effectively to digital, HDTV, and 3Gbp/s operations.

www.snellgroup.com

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