When choosing an HDV camera you'll notice the HDV format comes in a variety of resolutions, scan modes and frame rates. Understanding what these designations mean however, can be very confusing. For instance, the Sony FX1 shoots in 1080i or 1920x1080 60i, where the JVC GR1 shoots in 720p or 1280x720 24p. Both are considered HDV, but how are they different? What is 720p? What is 60i?
To be considered HD, video must follow certain criteria. It must have a 16:9 aspect ratio, varying frame rates, and resolutions of 1080 or 720 horizontal lines. The chart below breaks down these constraints and will show you the main variations of the HDV format.
Above: This chart displays the specifications for the main HD formats. Despite variations in these formats these specs represent the criteria of most High Definition video.
What is the Image Size or Resolution of HDV?
Or what do they mean by 1280x720 and 1920x1080?
HDV can be divided into two categories based on resolution or how many pixels wide and how many pixels high the video footage is. 1920x1080 and 1280x720 are the standard HDV resolutions. Each resolution is within the 16:9 aspect ratio and can be abbreviated as 1080i or 720p. The 1080i image is a higher resolution than the 720p, so why the difference and why would you want the seemingly lower res 720p camera? This brings us into our next topic, scan mode.
What do they mean by Scan Mode of HDV?
Or what is Interlaced vs. Progressive?
When you see the abbreviation 1080i or 720p the "i" and the "p" refer to the video scan mode. A scan mode is how a video camera or television creates or scans an image. There are two scan modes in HDV: interlaced "i", and progressive "p".
Interlaced video is produced when two fields of video data are combined to make one video frame. Essentially, your camera is writing two halves of one video frame making 60fps (frames per second) of interlaced video or 60i, but when combined you actually see 30fps or 30i. You can see evidence of interlacing when you pause video and see jagged edges within the footage. Many feel that of the two HDV formats 60i video is the best choice for sports and natural motion. This is because 60 frames of motion information are being recorded. This creates the illusion of proper motion that cannot be captured when shooting 24p.
Above: This graphic shows how one complete frame of interlaced video is actually made up of two independent fields. The two fields are then combined to make one complete frame.
In contrast, progressive video is a single image, or field, per frame, just as film is a single image per frame. Cameramen used to working with film often prefer shooting in 24 frames per second progressive video (24p) as it resembles the feel of film. Also, because progressive video is an actual image per frame, it allows for better slow motion and freeze frame effects. You don't get the jagged look of paused or slow motion interlaced video. As we'll discuss in the next section, progressive video also has the ability to shoot in a variety frame rates allowing you to capture motion better. This ability is only available on the professional HD cameras, or the newest high end HDV cameras like the Panasonic HVX-200.
What is a frame Rate?
Or what is 60i, 24p, 60p etc.
A frame rate is the number of frames a camera shoots per one second of time (fps). It's important to note, however, if the frame rate is in progressive or interlaced mode. In progressive mode 24p or 60p is truly 24fps or 60fps. In interlaced mode 60i does not mean 60fps, but 60 interleaved fields, as discussed above. When joined together two fields make one frame and are therefore 60i technically becomes 30i. This means that all interlaced video is 30 fps. Progressive video, on the other hand, can come in a whole variety of frame rates. So if you wanted to do slow motion video you'll be able to shoot at 60p and then slow it down to 30p without any loss in image quality.
How is 1440x1080i the same as 1920x1080i
I'd like to mention one final thing concerning the resolution of 1920x1080i HDV footage. Some of you may have noticed that all1080i cameras state a resolution of 1440x1080i and not 1920x1080i. This is because the 1080i HDV format uses non-square pixel dimensions to record video images, but when played through an HD television the HDV video is stretched out to fill a standard 1920x1080 frame. Why do 1080i cameras do this? Full-resolution 1920x1080 video would mean a bandwidth that would exceed the mini DV tape's capabilities. It would simply be too large a file.
Above: This image shows how a 1440x1080i image is stretched to be displayed on a 1920x1080i television.