Digital Video Details

Just a few years ago, the work of amateur and professional videographers was separated by a vast difference in image quality. This was primarily due to the limitations of consumer tape formats. But times have changed. Digital video for consumers encompasses all of the 25Mbps DV formats. So, if you have a DV camcorder, be it Mini DV or Digital8, it uses the same recording scheme that professional DVCAM (Sony) and DVCPRO (Panasonic) camcorders employ. That means your $600 Mini DV camcorder records the same image data as a $10,000-plus DVCAM or DVCPRO model that the pros use. Consider the following similarities:

  • Mini DV, Digital8, DVCAM and DVCPRO all use the same 25Mbps data rate.
  • They all use the same 5:1 DCT intra-frame compression scheme.
  • They all use the same 4:1:1 sampling scheme (NTSC).
  • They all deliver the same 720×480 resolution (NTSC) and frame rate.
  • IEEE 1394 (FireWire) is supported in all four video formats.

    Compression

    Visual and audio information that comes into a camcorder is encoded into a digital format. Raw or uncompressed, full-frame size, full-frame rate NTSC video data might occupy 30MB (megabytes) for every second of video, which is a huge amount of space. For pragmatic reasons, therefore, the video is usually compressed. This compression can result in a loss of image quality and can be very computationally intensive, depending on the complexity of the compression scheme. Digital Video (DV – with a capital "D" and a capital "V") uses a compression scheme that is relatively lossless and is not difficult for a computer to work with. The data compression ratio for DV is about 5:1, which results in one second of video taking up approximately 3.6MB of disk space. Expressed more appropriately in terms of megabits per second, the data rate for DV is 25Mbps.

    While there are many other digital video formats, such as DVCPRO50, which has a 50Mbps data rate, when we talk about DV we are talking about a 25Mbps data rate. Digital8, Mini DV, DVCAM and DVCPRO all have the same data rate and, therefore, the same potential level of quality. DV compression happens only within individual independent frames (intra-frame) and uses a Discrete Cosine Transformation (DCT) algorithm, which is the mathematics used to compress the data.

    Additionally, since the human eye is more sensitive to variations in brightness than it is to color (there are more black-and-white sensitive rods on the retina than color sensitive cones, among other factors), more data is used to encode brightness information (luma) than is used to encode color information (chroma). This can be expressed in a color sampling ratio of 4:1:1 (Y:U:V) for NTSC, where Y is luma (brightness), U is blue minus the luma information (B-Y) and V is red-luma (R-Y). Green information can be retrieved from the luminance channel.

    Comparing the 25Mbps DV format to other formats is a very complicated exercise. For example, you can’t objectively say that DVCPRO50 is twice as good as DV. Like DV, DVCPRO50 video is the same frame size, same frame rate and uses DCT data compression, but it samples the color in a 4:2:2 ratio and has a 50Mbps data rate. MPEG-2 video is the same frame size, same frame rate and samples the video in a 4:2:2 ratio, but it uses a completely different compression algorithm that compares data from blocks of adjacent frames (instead of just within a single frame). The result is that MPEG-2 compression can result in smaller files (say 8Mbps, variable), but it can be more lossy than DV. HD formats use different compression schemes, color sampling and have different frame sizes (resolution) and, of course, different data rates. In the end, it is like comparing apples and oranges, and quality is largely a matter of subjective human perception. And we haven’t even mentioned audio (we will save that for another article).

    Optics and Imaging

    All camcorders record footage using two subsystems. The optical system focuses the subject onto an image sensor that converts the image into an electrical signal and the recording system that records the signal onto the storage medium (usually videotape).

    The main differences between low-cost consumer DV camcorders and professional models aren’t in the recording scheme, because the images are recorded in the same DV format. Pro gear has an optical system designed to suit the needs of professionals, while consumer gear has optical features tailored to the needs of hobbyists.

    Pros demand better optics than casual shooters and are willing to pay the high prices that better optics demand. This means bigger, longer, heavier lenses. Additionally, they need balanced inputs for audio, so they can connect their camcorders with professional sound equipment. They don’t need a lot of in-camera special effects, fades, wipes and such, because post-production will handle those things.

    For the hobbyist, size and weight are important considerations. Small camcorders with light lenses are preferred to big heavy ones, and the trade off in image quality is a small price for most casual shooters. For the pro, a big video camera is usually a plus. Bulk means steadier shots, especially when shooting on the move. Big batteries, booms and lots of other equipment are a normal part of a pro’s baggage, so the weight and size of a large camcorder is not a problem. Consumers simply have different needs than the pros. In fact, many consumer camcorders deliver sufficient quality for professional applications. Today, we find consumer-level DV camcorders being pressed into professional use at almost every level.

    CCDs

    Another difference between consumer and pro camcorders is that pro camcorders often have three CCDs, while most consumer camcorders have only one. How important is this difference to the image quality, and are 3-CCD consumer camcorders worth the extra cost and size? These are tough questions, for several reasons. First, it involves subjective evaluations of relative image quality, but more importantly, there are variables that cannot be isolated.

    Owning both a Sony VX-1000 and a Sony TRV-9, I can report that the 3-CCD camcorder does indeed yield much better images, but it’s not possible to say with certainty how much of the difference is due to the CCDs or to differences in the optical systems. You don’t have to look very closely to see that the VX-1000 has a massive lens compared to the tiny TRV-9, and certainly, there is a big difference in the quality of the two optical systems. Identifying which parts make the VX-1000 superior is not as important as whether or not the total package delivers a meaningful improvement. Rest assured, it does. If I want the best possible images, I always choose the VX-1000 over the TRV-9.

    DVCAM and DVCPRO Formats

    The other major differences between pro and consumer DV camcorders are unrelated to image quality, but those differences can justify the high cost of pro video gear.

    As stated, all DV formats are identical in data structure: 25Mbps. This means that Digital8 = Mini DV = DVCAM = DVCPRO in terms of quality. So, why are DVCAM and DVCPRO touted as professional? The only important characteristic of these formats that may make them more suited to professional applications is the physical construction of the tape, which is sturdier and more robust.

    Consumer vs. Professional

    It’s clear that the major differences between consumer and professional DV camcorders lie in the quality of the construction, the quality of the lenses and the presence of manual controls, rather than on the technical capabilities of the DV format. The differences in image quality between pro and consumer gear come from decisions and compromises that are made during the design process to tailor the product to meet the broad market’s needs.

    Professionals use top-of-the-line 3-CCD consumer camcorders in the field every day. The top Canon model (XL1S, $4,699) even offers an interchangeable lens system for those who need such things. So, for many consumers and professionals alike, the Mini DV format is the most cost-effective solution. Once you start traveling with a video crew, however, you might need the durability of DVCAM or DVCPRO.

    Conclusion

    Mini DV and Digital8 wrap professional quality DV recording systems in packages well tailored to the needs of hobbyist and semi-professional videographers. Although your budget- or moderate-priced digital camcorder might not have all the bells and whistles that a $10,000 pro model has, it sees and records the same image data to tape.

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