DV Demystified

A Comparison of DV Camera Formats

"No, I didn't shoot it on DV, I shot it on DVCAM!"

A while back I heard a friend say that. I understood perfectly what he meant, but for just a moment I was tempted to respond "Hey, DVCAM is DV."

It reminded me that there's still a lot of confusion about the various "DV" formats. In order to set the record straight, here's an overview of the various digital tape formats along with a basic technical explanation of what makes them similar and different.

First off, the catch all term "digital video" with a lowercase "d" and a lowercase "v," describes a process, not actually a format. It's simply taking a stream of video and audio data that conforms to the technical standards of your part of the world, and "encoding" that data into a stream of digital zeros and ones. And if there was only one simple way to do that, we'd ALL know exactly what everyone means by "digital video," and then we wouldn't need this article.

But alas, it's not that simple. The reality is that "digital video" isn't just one thing, it's a catch all term for lots and lots of variations of how people have come to take something basic — like moving pictures on a screen and the soundtracks that accompany them — and transform those pictures into a stream of digital bits and bytes.

Digital Video (capital "D" and capital "V") or DV is a camcorder format. There are different tapes and different types of DV and that's what we'll discuss here.

Today, the vast majority of camcorder manufacturers support the consumer DV data format, also known as DV25 — which is a common DV video and audio signal usually written to and from a Mini DV cassette.

But while Mini DV is the "typical" consumer configuration, DV is such a flexible video architecture that it's also at home in a variety of manufacturer specific variations. Some designed for consumer use, and others designed for industrial and even high-end professional users.

Before we get to a list of the common digital camera formats and how they differ, we should briefly define a few terms. Sample rate is a number describing the amount of digital data used to represent a stream of video, measured in megabits per second (Mbps). Color sample is a comparison of the relative amounts of video data assigned to each portion of a video stream. The first number represents Y, or the black-and-white portion of the stream; the second two numbers represent the relative amounts dedicated to R-Y and B-Y, the two color portions of the stream (see sidebar for more on this). Compression is a simple ratio that describes how much smaller the video file becomes after it's been through the compression/decompression scheme, or codec. Thus, if an uncompressed video stream is 1 gigabyte in size, it'll be reduced to only 200 megabytes after you run it through the DV codec, which compresses video at 5:1. If all this sounds confusing to you, hang in there–we'll define these terms in more detail later in the article.

One of the first things you'll notice is that at the lower end of the digital video spectrum, the numbers are exactly the same, even as you move up to more "professional" formats.

What's going on? Isn't there any difference between, say a Digital8 Handycam ($300) and a Sony DVCAM DSR-450 ($18,000)?

Yes, there are certainly differences. The higher priced rigs will have a more robust housing, better lens options, more dependable battery systems, and even some technical differences.

At their heart, the basic data stream written to tape is exactly the same whether your rig is writing to Digital8, Mini DV, DVCAM or DVCPRO. In other words, the quality of the data is the same.

Does Your Data Rate?

When the technicians planned DV for consumers, they settled on a data rate of right about 25 megabits per second (commonly written as 25Mbps.)

When the first consumer Mini-DV camcorders came out, consumers and professionals alike got a big surprise. This 25Mbps basic digital video stream looked really good! In fact, it can look so good, that early users immediately jumped on it as a higher quality alternative to the S-VHS, Hi8 and 8mm camcorder formats of the past.

And then something really surprising started happening. Upper end users started casting an eye towards it as an acceptable industrial and corporate video format.

And suddenly "upscale" flavors of this simple home DV standard started appearing. Sony introduced DVCAM which uses the same exact 25Mbps signal flow featured in its camcorders, with some refinements including wider track pitches on its DVCAM camera heads, locked audio capabilities, and a line of gear built to more rugged "industrial" equipment standards.

This path also describes Panasonic's DVCPRO format. A more robust line of hardware — reading and writing exactly the same 25Mbps signal that a common consumer camcorder uses.

But remember we said that DV isn't just one format. It's up to manufacturers to determine how much data to pack into their particular flavors of digital video. So it should come as no surprise that some manufacturers — even some with a heavy investment in basic DV — also produce other kinds of digital video gear that is aimed at a higher end user.

"Movin' on Up!"

One higher data rate alternative is Panasonic's DVCPRO50 line. In this variant of DV, the engineers doubled the data rate up 50Mbps. What does this gain you? Well, to understand that, we need to talk about 4:1:1 and 4:2:2 color sampling.

Remember, the goal of all digital video systems is to take a video or audio signal and turn it into numbers. That process is called "sampling."

A video picture is composed of four elements that must be stored and reproduced accurately to create a pleasing picture.
The first and most important element is the brightness value of each individual pixel. With that information alone, a black and white picture, complete with shades of gray can be described. The other three elements are the Red, Green, and Blue values for each pixel. (This is why raw TV signals are sometimes referred to as RGB signals.)

When it came time to establish the sampling conventions for digital video, the common camcorder DV system established to use 4:1:1 sampling. In plain English this means that the luminance (or black-and-white) portion of the signal is sampled four times. That's the 4 in the term. Then color is sampled once for a part of the color signal, followed once again by another part of the color signal.

(Ah, I see some of you are wondering why only 2 color samples to describe the 3 colors in RGB? See the Sidebar for details!)
But using only one quarter of the data used for luminance to describe each of the two channels of color information leads to some difficulties.

One of the most famous is in the area of keying. It turns out that while 4:1:1 sampled color can be perfectly pleasing to the eye it's a bit hard to describe and position color accurately enough on screen to provide for good, clear keying.

So in a 4:2:2 color space — like that used in the DVCPRO50 format, there's twice the color information to help make even prettier, more accurate color images.

And when color is really important — like in complex titling, gradient builds, or computer animation where the goal is to re-create worlds with photo-realistic precision, DVCPRO50 might not carry enough data — and so, there's a DVCPRO100 format that doubles the color information again!

The DigiBeta Dream

So where does that other ubiquitous high-end format Digibeta fit in?

We've been discussing sampling rate so far. But that's not the only factor that affects the quality of a digitized picture. Another important area is compression, or more specifically, how much and what kind of data reduction is applied to the original digitized signal in order to fit the digital data onto the tape.

Digital Betacam's key advantage — and the reason it's so popular with high-end production companies – is that it uses milder data compression (at a modest 2:1 ratio) compared to the DV25 and DV50-based formats.

In doing so, it provides a very high quality and robust video capability — but at a hardware cost often many times that of some other kinds of digital video production equipment.

The Digital Dilemma

There will always be disagreements about how much better one digital format's picture is than another. And side issues such as whether Mini DV is "good enough" for a particular type of project will be debated probably until the format becomes obsolete!

But whichever side of the debate you find yourself on, one thing is certain. The DV revolution has provided more quality choices — at a greater range of price points — than the video production industry has ever seen before.

And that's good news for all of us!

Bill Davis writes, shoots, edits, and does voiceover work for a variety of corporate and industrial clients.

Sidebars:

Y-RY-BY

When you first encounter numbers like 4:1:1 and 4:2:2 to describe the luminance and color information of a signal, it's common for the curious to wonder why there isn't another number, one for the red, another for the blue, and a final number for the green?

It's those clever engineers again. Somewhere along the line, they figured out that if they could store the values from the red and the blue pixels, and compare those values to the luminance values of the same pixel, they could calculate the green values "on the fly." And the component color system was born. In it, there are three signals: Y stands for the luminance, R-Y is the Red pixel value compared to the luminance value, and B-Y is the Blue value, also compared to luminance.

Given the values of the three signals, the circuitry is designed to simply calculate the final piece of the puzzle, the green signal. Interestingly, when the math is done, the calculated green signal actually has more raw data than either the red or blue channel in digital video and that's one technical reason why it's common to use green rather than blue for keying DV.

Table 1 lists the formats by data rate,
sample ratio, and compression.

Digital 8 25Mbps 4:1:1 5:1

Mini DV 25Mbps 4:1:1 5:1

DVCAM 25Mbps 4:1:1 5:1

DVCPRO 25Mbps 4:1:1 5:1

DVPRO50 50Mbps 4:2:2 3.3:1

Digital-S (D-9) 50Mbps 4:2:2 3.3:1

Digital Betacam 90Mbps 4:2:2 2:1

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