What is DV?

There are three tape formats that are known as DV formats: MiniDV, DVCAM, and DVCPRO. All three utilize the same compression method called DV25 (which is sometimes just referred to as DV compression). The same data is recorded onto each format, with the difference between the formats being how the data is physically recorded onto the tape (see below for a more detailed explanation). Video compressed using DV25 does not have to be recorded onto tape; video files on a computer can be compressed into DV.

Note that confusion can arise since many people refer to the MiniDV format as DV, while others use the term DV to refer to all three formats (among which there is some incompatibility).


DV25 Specifications

As stated above, DV25 is the codec used to compress all video that is recorded onto a MiniDV, DVCAM, and DVCPRO tape. This compression occurs when the information is written on the tape. People often refer to "uncompressed DV," which is a bit of a misnomer. DV is always compressed; it's just a very light compression. There is no way to record onto a DV tape and not have the information compressed into the DV25 format. "Uncompressed DV" usually means that no additional compression is added during the capture process. A better term that is often used is "raw DV."


DV25 Compresssion Specs

Compression Ratio: 5:1 This ratio seems rather high when compared to the fact that analog video usually had to be compression at a 3:1 or 2:1 ratio using Motion-JPEG to be of acceptable quality. Yet DV25's 5:1 quality is about comparable to 3:1 Motion-JPEG quality. This ratio is fixed.

Data Rate: 25 Mbps This is why it is called DV25. This data rate is both fixed and constant. It does not matter if the video is a high action sequence or is totally black, it's data rate will always be 25Mbps. While this can be inefficient, it also makes it easy to predict how much space an amount of video will take up on a hard drive. Here are some useful figures (sizes are approximate):

1 Second = 3.5 MB
1 Minute = 215 MB
4 Minutes, 40 Seconds = 1 GB
1 Hour = 13 GB

Compression Method: DV25 uses an Intraframe Discrete Cosine Transform (DCT). It is similar to MPEG, yet it is composed entirely of I-frames, meaning that there are no predicted frames. There is compression within the frame, yet no information is used from other frames so each frame is a self-contained unit. While this method is not as efficient as other methods (like MPEG) that uses predicted frames, its necessary because when editing, edits can only be made on I-frames, so having only I-frames enable you to edit on any frame.

Color Sampling: 4:1:1 This number is a ratio of how often the luminance is sampled compared to how often the color is sampled. Virtually all digital formats sample the luminance at 13.5 MHz. This is noted as the "4" in the above number. A format that has a 4:4:4 color sampling samples the color at the same rate as the luminance, yet it is rarely used except for high resolution RGB graphics. 4:2:2 sampling is used by most high-end tape formats and provides excellent color quality. DV25 uses 4:1:1, which samples the color once for every four luminance samples. Visually, this still yields excellent color (better than most analog formats). The only time when 4:1:1 sampling may be a hindrance is when making graphics and special effects that require very sharp colors (for example, it is more difficult to use DV for chroma keying).


DV Formats

MiniDV: Also referred to as "Regular DV" "Consumer DV" or just "DV," MiniDV is the most common DV tape format. MiniDV provides the most universal playback compatibility. MiniDV tapes can be played in any DV device (including DVCAM and DVCPRO), while many MiniDV devices can play all three formats. MiniDV, as it's name implies, uses only the small DV tapes. Note that while the small DV tapes made by Sony are often marked DVCAM and the tapes made by Panasonic are often marked DVCPRO, it is the device that determines which format is recorded; the tapes are identical. So if a Sony tape that is marked DVCAM is used in a MiniDV camera, the DV data will be recorded as MiniDV.

DVCAM: DVCAM is developed and supported by Sony. DVCAM uses the same DV25 codec as MiniDV, so it has identical picture quality, yet it has a faster tape speed and a wider track pitch. This means that the data is recorded over a larger area, which reduces dropouts (dropouts are defects in the picture caused by the physical loss of the particles used on the tape to record). This makes DVCAM a much sturdier and dependable format that has found favor with higher-end productions. Also, a MiniDV deck, if used in an analog suite, cannot perform frame-accurate edits, while DVCAM can. This makes it a good choice for post-production houses that want to simply add DV to their existing set- up, but is of no importance to those editing over Firewire. DVCAM can only be recorded in specific DVCAM decks (made by Sony), yet it can be played back in most MiniDV and DVCPRO decks.

DVCPRO: DVCPRO is Panasonic's entry into the DV format arena. DVCPRO has an even wider track pitch than DVCAM (18 microns) and uses a metal-particle tape (as opposed to MiniDV's and DVCAM's metal-evaporated tape) for even better durability. DVCPRO was aimed mainly at the TVs news market, were it has gained some acceptance. Among digital editors it is less used, and its adoption was hampered by an initial short-sighted decision to not include Firewire on DVCPRO equipment, although nowadays DVCPRO equipment has Firewire.


For much much more info and technical goodness about DV, check out the DV FAQ written by Adam Wilt, the web's foremost authority on DV. In fact, it's where I learned most a lot of what I know about DV.