Video Formats

Video formats come in a variety of alternatives. Consumer formats are lower quality than professional formats and are stored on lower-capacity devices. While there is a perform-ance difference between them at present, Moore’s Law provides for ever-increasing pro-cessing capability. Eventually there is no economic reason why consumers should not enjoy studio-quality presentation in their own homes.

5.12.1 Transport and Delivery of Video

Broadcast studios and the transmission systems deliver the video in an encoded form that applies some analog compression. The luma value describes a basic black and white TV signal. Subtracting the luma signal from the RGB color values derives two color compo-nents. This color overlay does not have to be maintained at the same resolution because the human eye is better at resolving brightness than color, so some compression is already accomplished.

The delivery and transport mechanisms used to build TV systems are all based on transferring this luma and chroma component information. A camera will rarely produce

Luma samples Chroma samples

Luma samples Chroma samples

Figure 5-32 4:2:0 sampling.

Figure 5-33 Y’UV sampling.

an RGB signal directly, nor will a telecine machine deliver an output in an uncompressed form. While it is possible to design and engineer an RGB transfer system, it would be non-standard. Only large movie-effects companies would deploy the engineering effort to develop special-purpose hardware to do this sort of thing.

The rest of us will at best get a DV50 bit streambut more likely a DV25 bit stream in the studio. Downstream in the consumer or semi-professional world, it is more likely that a component signal in S-video format will be delivered. Otherwise, the best to expect is a PAL or NTSC signal off air. While the off-air quality is reasonably good when viewed directly, it has already been through several compression processes and converted back and forth between analog and digital several times. The analog output from a receiver is somewhat compromised compared to the original studio quality. The source material could be delivered on a VHS tape running in long-play mode that is only just viewable.

These reductions in the quality all contribute to making the compression process more dif-ficult. Many of the practicalities covered in Chapters 31 to 37 describe the remedial work before the actual compression process begins.

5.12.2 Broadcast-Standard Digital Video

If you are working in the broadcast industry, you encounter uncompressed material and serial digital sources that have some compression with invisible artifacts. The equipment manufacturers refer to formats such as DV50 or DV25. Video servers used for broadcast-ing allow these storage densities to be switched. The server will then trade off storage capacity against production quality. It is inadvisable to change this setting while the server is in use, as it will sometimes result in the server completely purging the video storage and reformatting the disk array. Note also that DV25 is sometimes loosely referred to as DVCAM.

5.12.3 Pro-Consumer Digital Video

At the top end of the consumer-grade equipment, in an area that is also populated by pro-fessional small filmmakers or semi-propro-fessional videographers, you will find formats such as DV. DV is slightly lossy compared with uncompressed video but the quality is a lot bet-ter than VHS. It is compressed at a ratio of 5:1 when compared with the raw, uncom-pressed footage. It is good enough for news gathering in many cases and the DVCAM format is widely used. DV is output by digital home video cameras (handycams) such as the Sony DCR PC 105.

These cameras readily connect to your Mac OS computer using the FireWire (a.k.a.

IEEE 1394or iLink) cable and are controllable using iMovie, Final Cut Express, or Final Cut Pro. Other video-editing tools such as Adobe Premier are equally useful. Provided your Intel platform supports FireWire and you have the necessary applications installed, it all works similarly on a PC.

Although you often see DV and FireWire being used together, they are distinctly sep-arate technologies. DV is a video-storage and transfer format and FireWire is a connectiv-ity standard. Protocols that work on the FireWire interface have been developed for the

control and transfer of DV material to and from compatible equipment. So the standards are used cooperatively with one another but are independent.

5.12.4 Amateur Video Formats

If you are dealing with legacy material, a lot of footage will be recorded on VHS or beta-max. You are going to experience some significant quality issues with the compression when it is applied to video that has been taken off a VHS tape.

In ascending order of quality, these home video formats are grouped like this:

● VHS—Long Play

These are the things that go bump in the night, or more likely during that rush job that you have to deliver first thing in the morning. Video is sometimes a royal pain to encode prop-erly. Just getting it off the tape is difficult. That is when you discover some strange noise or artifact that causes problems. Some of these must be dealt with at source because they are much harder to remove later.

Dot crawl is an example that is fixed by using S-video connectors that separate luma and chroma instead of a combined composite connection.

Tracking errors are an issue to do with the quality of recording or the maintenance of your VCR. These and other gotchas are dealt with in Chapter 31 and the chapters imme-diately following it where the practicalities of video-compression processes are addressed.

5.14 Summary

This chapter has examined some fairly complex matters regarding video presentation.

Refer to this chapter when reading about the video-compression algorithms in Chapters 8 to 14. It will give you some insights into why the encoding technology is implemented the way that it is.

The appendices summarize a variety of different formats and codecs for use with video. There is a whole zoo of formats described in Appendix L.

In the next chapter we will look at digital-imaging systems. These are where film and video converge in the same format. Most of the hard work involved with film and video is in trying to deal with their own special properties and wrestle them into a form that fits into a digital-imaging system.

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