DV Format & Compression

DV25 refers to a compression algorithm (often called a CODEC). Various manufacturers have made slight adjustments to the method of recording and storing the DV25 data (hence the variations in names as noted above), but overall, the format is the same. By the way, if anyone ever says to you, “I’m using uncompressed DV,” you have permission to laugh. All DV files are compressed. We’ll cover more on compression in a bit. In fairness, if someone does refer to “uncompressed DV,” they’re probably trying to say that no additional compression

DV25 Specifications
Compression Ratio: DV is compressed at a ratio of 5:1 Unlike certain other formats, DV compression is fixed (you can’t scale the rate of compression).
Data Rate: 25 Mbps Now, you know why we call it DV25. The great thing about using DV25 on a computer is that you can predict how much hard drive storage you’ll need. Note the following (approximate) examples:
1 Second = 3.5 MB
1 Minute = 215 MB
4 Minutes, 40 Seconds = 1 GB
1 Hour = 13.2 GB

DCT Compression: DV25 uses a form of compression called Intraframe Discrete Cosine Transform (DCT). It is fairly similar to MPEG compression but is made up entirely of I-frames. In addition, each frame stands along – there is no reliance on other frames for color or other data. As a result, DCT compression is ideal for nonlinear editing.

Color Sampling: Color Sampling could take up an entire chapter in a book on video. We’ll spare you the technical blah-blah and just let you know that DV uses a color sample rate of 4:1:1 This number is a ratio of luminance sampling vs. color sampling. The initial number – 4 – refers to the baseline for sampling. In this case, all digital formats sample luminance at 13.5Mhz. If you see a format with color sampling rates of 4:4:4, you’ll know that there is nothing missing from the color compared to the luminance. 4:2:2 sampling is used by many “professional” formats and in the analog world that would include Betacam SP. 4:2:2 color sampling in the digital video world typically is used for DV50 formats (hence the 50 in the name). We’ll cover more on this in a moment.

DV25 vs. DV50 and other video formats
I realize some of you may be starting to nod off, but the functions and relationships of video formats is critical information for a motion graphics designer. DV25 will be thrown at you in an ongoing manner, but if you don’t know its strengths and weaknesses, especially when compared to other formats, you could be complicating your project results – so a little background can go a long way.

Standard definition digital video signals are typically 4:2:2 YUV component resolution. This includes DigiBeta, D9, and DV50 formats. The chrominance (color) components and Chroma resolution are one-half of the luminance resolution. This reduction in information allows the data to be smaller –the average human eye is far more aware of luminance than chrominance.
DV25 further reduces the chrominance components and Chroma resolution to one-quarter the luminance resolution. In most applications, the human eye does not notice this reduction – and in fact, most images contain relatively low saturation colors, so DV25 is capable of producing remarkable images – in some cases, better than professional analog formats of old.

DV25 was more difficult to work with when creating composited layers – as you do in Photoshop and After Effects, due to the lower spatial color resolution. The result can be aliased images – they look like stair-steps on diagonal lines. After Final Cut Pro, Adobe Premiere, and After Effects and other applications now have filters and other tools to help overcome this problem – but you need to know the structure of the data, or you won’t know how to solve the problem. In this case, you can achieve a quality matte edge by slightly blurring the RGB channel that is related to the key channel (such as green or blue). It’s also very important that you apply the proper amount of light to your subject – and if you’re using a green or blue screen, make sure you create enough separation and use a reverse key or backlight. Blue or green “oversplash” on clothing or hair can ruin all attempts at creating an appropriate matte, especially on 4:1:1 signals.

If you have the opportunity to work with DV50 footage, you will have more flexibility than you will with DV25. You’ll also be dealing with higher bandwidth video, which means you’ll need a bigger pipe to transfer video data and most probably, a third-party video card to process and manage the data. There is a wide range of third-party solutions for both Mac and PC available. Whenever possible, use the component connections (SDI) to connect DV50 edit or player decks to your workstation.

Without doubt, DV50 images have more data to work with – and as such, typically retain more of it. In fact, most DV50 recorders utilize two DV25 codecs to compress and decompress DV50 material. The variations of formats within the DV50 range are also extensive and we won’t deal with that here, except to say that you can work with either YUV or RGB native, lossless or uncompressed video and 10, 12, or 16-bit processing power. No matter the solution, it will offer higher DV25.

At the same time, DV25 offers a better image than many professional analog formats (some of them still in use today). The key is to have a quality camera, a good shooter, strong high-quality connections and a good DV codec in place. There are many general distribution motion pictures that include sequences, scenes or even the entire film that was shot using DV25. Welcome

Compression
This is also important when it comes to dealing with video. We’ve already mentioned a few codecs, but let’s briefly go over some compression issues that may come into play as you work on various DV projects.

There are many specific types of compression, but two general processes. The first type is typically called intraframe compression. When manipulating video using intraframe compression, each individual frame of video is treated separately. As an example, Motion JPG (M-JPG) video uses intraframe compression – typically in the range of 8:1 to 40:1. The 8:1 video will look pretty good. The 40:1 video is really just to give you an idea of the image when working with a nonlinear editing system.

Interframe compression is unlike intraframe compression in that it refers to changes in the video data and only carries forward those changes, allowing for highly increased compression. Various forms of MPEG video use interframe compression. It is possible to get signals with 200:1 compression rates and a decent picture.

Codecs
Early codecs were used for very specific purposes. There was a codec for use in offline nonlinear editing, a different one for use in online nonlinear editing – plus codecs for CD-ROM, distribution, broadcast and so on. Of these, the M-JPG codec may have been the most versatile as it could be cranked up or reduced, depending on the application. This was very convenient in early third-party video cards used in both Macs and PCs and it was how many early generation digital video productions were created.

As I mentioned a few moments ago, DV25 is not scalable (and DV100, DV50, DV25 are not really scalable video as much as different formats). Certain nonlinear applications such as Final Cut Pro can render low-resolution proxy files from source material – but this is essentially a workaround (although a very acceptable

YUV vs. RGB
This is a critical part of preparing for professional work using DV nonlinear editing solutions. When using any number of image management applications (Photoshop, After Effects, etc.), you have the choice of selecting either YUV or RGB workspace. Understanding the how and why is important, because translating from one format to the other will absolutely degrade the overall quality of the original material.

YUV is the format video professionals are used to working with. RGB is the format computer designers are used to working with. If you’re going to stay within the computer environment, you can get away with RGB. If you’re going to output to tape, you’ll need to use YUV. Some very expensive systems for editing or compositing video are RGB native, but convert the signal to YUV on export – the cost of the conversion gear keeps the reduction in quality to an acceptable (read that as negligible) level.

Uncompressed vs. Lossless vs. Compressed
Again, you could write a book about video formats and the technical wizardry that goes on behind the post-production scenes. But for our purposes, let’s look at one more key issue: the number of pixels being pushed around.
Uncompressed video (4:4:4) is a lot of data. A native Mac or PC can’t even dream about moving 2,000 by 2,000 pixels per frame around on the screen. High-end nonlinear editors and compositing workstations have support hardware and software to move the data for the designer or editor. Oh – and by way, uncompressed doesn’t mean flawless. Every company that offers “uncompressed” video management is still using a technology of some sort to move the video data.

Typically, in the personal computer world, you’ll deal with some form of compressed video. Most systems use M-JPG or DV compression, which will range from about 3:1 all the way up to 40:1. There are a few MPEG-2 nonlinear systems that use a ratio of 2:1, but those typically will not be in your line of work – although you will output DV projects to MPEG-2 (for DVD work).

If your film reels say 25 ft on the box, this is the reason.
Your small reels are typically 50 ft in length. [We charge per 50 ft, even if it is less than 50 ft.]

When you bought the film, it would be in 25 ft rolls for 8mm Double Roll Cameras as shown below.

This is what it says on the back of the box:

Eastman Kodak Company provides 25 feet of double 8mm film on this roll which is intended to yield 50 feet of 8mm movies. The additional 4 feet of film at each end which serves as leader and trailer for light protection and cameral loading, is not warranted for picture taking purposes.