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Uncompressed vs. Compressed

Aug 1, 2004 12:00 PM, By Bob Turner

Part I: Reasons you should consider using uncompressed HD.

There were several compressed high-definition “mastering” technologies introduced at NAB 2004. There were also several manufacturers that ridiculed the concept of significantly compressed HD mastering.

Which option offers advantages over the other? Is uncompressed HD better than compressed? With more solutions for both coming to the market, the debate about compressed vs. uncompressed is gathering steam.

This article is the first of two I plan to write on this debate. In part one I will focus on the arguments of those in favor of uncompressed HD.

Advocates for uncompressed high-definition video say that for compositing and effects — especially for short-form projects — you do not want to limit your creativity by using a compressed format. They frequently point out that the prices for uncompressed HD applications, very high-quality HD video boards, and storage systems can be startlingly low.

Those arguing for uncompressed HD video point to the known problems with compressed footage. Electronically generated gradient backgrounds, color grading, chromakeying, and special effects image quality are all issues with compressed HD. Uncompressed fans may also mention problems with compression concatenation as the distribution flow goes through various paths. (I should point out that many of these concerns are related to bit-depth rather than compression, but some argue that shallow bit-depth is associated with the concept of compression and the issues are linked. Others will argue that bit-depth and compression are separate issues.)

Manufacturers that offer uncompressed HD editing solutions include: 1Beyond, Adobe, AJA Video, Apple, Aurora Video Systems, Blackmagic Design, Bluefish444, Boxx Technologies, Canopus, Cinegy, Discreet, DVS, Eyeon Software, In-Sync, Interactive Effects, Leitch, Linux Media Arts, MTI, Nucoda, Optibase, Pinnacle Systems, Quantel, Soluciones Graficas por Ordenador, and Sony.

Two pros argue for uncompressed video

Noted compression expert Kevin Marks writes extensively on uncompressed video in his Epeus' Epigone weblog (http://epeus.blogspot.com/2004_01_01_epeus_archive.html #107407444953585472).

In a segment entitled “Media Heresy: Compression is Becoming Redundant,” Marks espouses the theory that as computing power increases and platform and storage costs drop, compression becomes less relevant over time.

“The deeper point is a trend-based one,” Marks writes. “If storage continues to improve in capacity per dollar at 3 times the rate of computing power, compression becomes wholly redundant — the CPU running the bit-manipulation is the bottleneck. The HD editing computers work this way — they have DMA (direct memory access) hardware in the disk interface and the screen interface, and the computer's job is to get out of the way.

“The other reason compression is a bad idea in the long run is precisely because of its success in removing redundancy. If you have uncompressed audio or video, a single bit error will likely go unnoticed. If you are unlucky and it is the high bit of a sample, you will get a transient click in the sound or a brightly colored dot in the wrong place in video, but it will soon pass and be covered by a correct bit.If you have a single bit error in a compressed stream it will make the rest of the frame, or possibly many frames, corrupt. In the worst case it can destroy the rest of the file from then onwards. For archival content this kind of fragility is not what you want.”

A second argument for uncompressed HD is offered by Maurice Patel, Discreet's product marketing manager.

When asked his view of the compressed vs. uncompressed HD issue, he offered this reply: “From Combustion to Fire and Inferno, our assumption is that clients want to add value to the media in postproduction and that they do this by altering the images themselves. However, this is not the case for all types of production (ENG, reality TV, or certain types of documentary) where the main concern is to edit the order/length of image sequences and not to change the images themselves. For this type of work, compressed media is often fine.

“Understanding the relevance of compression means understanding where our clients derive value and where they can succeed [make money] as professionals and as businesses,” Patel continues. “In other words, if the service you sell is purely the service of editing images, then compression is not an issue as long as the images look good. If the service you sell is one of also changing those images [changing the pixels] then compression will be a significant handicap and can cause a rapid decline in image quality [generational loss and artifacting]. Compression reduces the freedom and flexibility to make any change you want to the image, whenever you want to, in the postproduction process. Once you throw data away there is no way of getting it back. …

“For tasks like color correction, keying, compositing, and DVE, you need as much data as possible to create a high-quality result,” Patel says. “For example, in a DVE every time you change the position of the pixels you have to apply a filter [bilinear, bicubic, or other]. The less data you have to calculate the filtered value, the poorer the result will be. A filter uses a kernel that analyzes surrounding pixels. Unfortunately, compression schemes tend to significantly alter those values.

“Compression is often a valuable tool for what I would call pure editorial … because it reduces the amount of data required to store a project online, which is useful when you have large amounts of footage [takes] to store that will not end up in the final product anyway. But once that process is over, compressed images become a huge liability if — and the if is important — you are then required to do anything else with those images, such as visual effects. And today that is almost a given,” Patel says.

“Ultimately, few people, if given the alternative, would prefer to build a digital workflow around compressed media if they could do it for the same cost uncompressed. And today there is less and less cost prohibition to uncompressed workflows — these are within the realm of desktop products.

“So to conclude, use compression where it makes sense and don't let it dictate your workflow. If you want to create high-quality images with any graphics, visual effects, DVE, or color correction treatments, think carefully before using a compressed format for post. The results can be quite ugly.”

Mildly compressed mastering formats

I would like to conclude with a word about the mildly compressed high-definition formats. It is important to state that only hard disk recorders are truly uncompressed at the present time, but many view these mild compression formats as virtually uncompressed.

This category includes the Panasonic AJ-HD3700B D-5 HD “studio quality” 4:2:2, 10-bit videocassette format (1080i/720p); and the 10-bit, MPEG-4 based Sony HDCAM-SR videocassette format (1080i/720p/1080p). The compression ratio for the HDCAM-SR format is 2.7:1 when recording 4:2:2, and 4.2:1 when recording 4:4:4.

Next month I will present the other side of the debate: arguments in favor of compressed HD.

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© 2008 Penton Media, Inc.