Digital Video Media & Formats
from ulead.com

When home video taping first became available there were two format - VHS & Betamax, and VHS won. Now, ever since the arrival of camcorders and then digital video, there seems to be an ever increasing array of formats. Which should you use? This tutorial attempts to guide you through the maze.

Digital

Digital video formats are always a trade-off between size and quality. Maximum quality is only achieved by sampling at a high rate and not compressing the data. Doing this with broadcast-quality video requires about 20 MB/sec of digital data (or 1.2 GB/min). For most purposes this is an impractical amount of data, so various compression schemes are used to reduce the amount of data without noticeably reducing quality.

DV

The DV format now universally adopted in consumer camcorders compresses the data to a fixed rate of about 3.5 MB/sec. This preserves visible quality very close to broadcast, and better than any consumer analog tape format. The fact that the video data is recorded on the tape digitally means that it can be copied by Firewire to a computer disk and back to tape, without any of the loss of quality associated with analog copying.

DV and Mini-DV are the same thing, Mini-DV just uses a physically smaller tape cartridge with shorter running times.

M-JPEG

This format is almost exclusively used to capture analog video for digital editing. It's usually implemented by the hardware of a capture/replay card, and different manufacturers tend to implement it slightly differently. Hence, M-JPEG files captured by one card may not be playable on another. The compression, and thus resulting data-rate, is normally adjustable. To retain the quality of Hi8 or SHVS, a data rate of about 3 MB/sec or higher is required. 1 MB/sec is roughly equivalent to VHS quality.

DVD

DVDs are recorded using MPEG-2 compression. The MPEG compression schemes achieve much lower data rates than DV or M-JPEG for the same visual quality, but are more difficult to edit than DV or M-JPEG.

VCD

The VCD format uses MPEG-1 compression, at a data-rate identical to that of an audio CD. The quality is low, slightly lower than VHS, although a good player can sometimes make it appear slightly better. The compression parameters for an MPEG-1 file that can be used to make a VCD are very precisely defined. The current (2.0) standard for VCD also allows still images to be included, enabling such things as a CD photo-album to be created.

SVCD

This is a rather loose standard that allows MPEG-2 format data to be used on a CD medium. Many current DVD players will also play SVCD-format CDs. It's a useful interim format for putting reasonable quality video on recordable CD, until recordable DVD is available at consumer prices. There is much more flexibility in the compression parameters of MPEG files written to SVCD, unlike VCD in which there is none.

MPEG

The MPEG formats are not limited to CD and DVD use. Digital broadcast TV uses virtually the same MPEG-2 format as DVD. Both MPEG-1 and MPEG-2 formats can be used at a variety of frame-sizes, frame-rates, and data-rates. MPEG-1 files can be small enough to be sent over the Internet, especially if a small frame-size is used.

The main differences between MPEG-1 and MPEG-2 are that MPEG-1's maximum frame size is 1/4-frame and that MPEG-1 only allows one field/frame. MPEG-2 supports up to full-frame and two fields/frame, corresponding to TV. MPEG-2 also allows higher quality audio (the popular MP3 format for audio compression is in fact "borrowed" from the MPEG standard).

Other

There are several other compression formats that can be used on a PC, including for example Intel's Indeo and Apple's Quicktime. These can be an appropriate choice for video that is targeted exclusively for viewing on a computer. Microsoft have also recently introduced their Windows Media Format, which is a deliberately low quality, low data-rate format designed specifically for Internet use.

However, none of these are usually relevant to work that involves any sort of external medium (tape, CD, DVD). The two formats that are becoming increasingly predominant in these areas to the exclusion of others are DV and MPEG.

Editing

There is a fundamental difference between the DV and MPEG styles of compression, which affects the editing process. DV (and M-JPEG) stores each frame as a complete image - it's the digital equivalent of a cine film. MPEG only stores some complete frames -- the remainder is stored as differences between adjacent frames. This enables MPEG to achieve its much more efficient compression, but means the editing process is more complex for a computer to handle.

The consequence is that simply making cut-edits on an MPEG file can result in quality loss, since frames have to be re-computed, and the more edits are made the greater the loss. It can also make the editor software sluggish to respond because of the amount of intense computing required. DV & M-JPEG, on the other hand, do not suffer from this problem because frames do not depend on each other. Images only have to be re-computed where transitions, special effects, etc. are used.

Where an MPEG result is required, the best quality is usually achieved by editing in DV or M-JPEG format, then rendering or converting the result to MPEG. Even better is to also save a DV copy as an archive in case subsequent changes are needed.

Devices are available that will capture directly to MPEG. They are appropriate if little or no editing is required. The main advantage is that they save the time required to generate the resulting MPEG file, which can be a slow process even on the latest fast processors.



Analog

When working with video on a computer, analog formats are of interest when they are the source and/or target of the editing process. For example, if you intend to capture from an analog camcorder and record the result of your editing on an analog VCR, it's important to understand the properties and limitations of these media.

The main issue with an analog medium is the quality it can achieve, which mostly concerns its resolution - i.e. the amount of detail it can store and display. And, the golden rule with every analog format is every time you copy you lose. This means the copy will always be lower quality than the original, even if the target medium is capable of higher quality than the source. This rule applies even if you capture from one analog source into digital format, and then replay the digital format and record back to analog.

Consequently, if you intend to record your work to standard VHS tape, you must start with a source that is higher quality than VHS if you want a reasonable result.

Formats

The popular analog formats can be grouped into two levels, low-quality and high-quality. VHS and 8mm are low quality, SVHS and Hi8 are high-quality. The VHS-C and SVHS-C variants are identical quality to their standard equivalents, but simply use a smaller cassette with a shorter running time.

Digitizing

Analog video is converted to digital format using a video capture device. This may be built into the computer's display adaptor, be a card that fits internally into the computer, or be an external device that connects to a parallel or USB port.

Some devices also provide an analog output so that digital video can be replayed back to analog format. This is essential if you want to record your edited results to VHS tape, but you won't need it if all you want to do is create VCDs or MPEG files for use on the Internet.

Capture format

The two principal capture formats are M-JPEG and MPEG.

Capture/replay devices almost exclusively use M-JPEG, since this is the optimum format for editing at best quality. They are intended for use where both source and target are analog. Whenever possible capture using a data rate that will preserve the quality of the source, even if the quality of the target is lower. For Hi8 and SVHS sources, this means a data rate of about 3 MB/sec, for 8mm and VHS sources use at least 1 MB/sec.

Inexpensive capture-only devices are available that capture in VCD-compatible MPEG-1 format via a parallel or USB port. These are simple to install and good, if the target is VCD or Internet and only limited editing is required. They can also be appropriate even if the source is DV, since all DV camcorders have analog outputs.

MPEG-2 capture devices are now available. Some actually do the encoding largely in software and require a powerful processor (usually PIII-500 minimum). They could be used where the target is SVCD (or ultimately DVD), but better results would be achieved by capturing and editing in M-JPEG, then converting to MPEG-2. If the processor is capable of encoding MPEG-2 in real time, then the conversion should not take very long.

Connectors

Some people are a little confused by analog video connectors. There are two basic analog connectors: composite and S-video. Composite is normally an RCA ("phono") style coaxial connector, while S-video is a small round 4-pin.

With composite video, all the information is encoded into a single signal, but with S-video the luminance and chrominance are carried on separate pairs of wires. Using S-video connections should result in less degradation, but is only detectable, if at all, where the material is at least Hi8/SVHS quality.

If S-video connections are available, it's generally best to use them. But if not, just use composite. S-video connections are not directly related to the SVHS format. In particular, you don't have to use S-video connections to get the benefit of SVHS's higher resolution. You may loose a small amount of quality by using composite, but you certainly don't reduce SVHS to VHS by doing so.

Scart connectors include pins for both composite and s-video signals. An S-video capable device with a Scart connection should use the s-video signal, in preference of composite if it is present.

Sources & Targets

The table below shows combinations of the most popular source and target formats that are not computer-dependent and the usual methods of getting from any source to any target. Sources are listed down the left, targets across the top.

Note: There is no entry for converting analog tape to DV. If this really is a requirement, the best solution is simply to use a DV camcorder or VCR with analog inputs and copy the source tapes directly to DV tapes. Then work entirely in DV.

  VHS/SHVS DV VCD/Net SVCD/DVD
8mm
VHS
M-JPEG capture/replay X
  1. Capture to MPEG-1
  2. Capture to M-JPEG, render to MPEG-1
  1. Capture to MPEG-2
  2. Capture to M-JPEG, render to MPEG-2
Hi8
SVHS
M-JPEG capture/replay X
  1. Capture to MPEG-1
  2. Capture to M-JPEG, render to MPEG-1
  1. Capture to MPEG-2
  2. Capture to M-JPEG, render to MPEG-2
DV Firewire/DV, take analog out from DV camcorder to record VHS/SVHS Firewire
/DV
  1. Analog capture to MPEG-1
  2. Firewire/DV,
    render to MPEG-1

Firewire/DV,
render to MPEG-2

Source-to-target conversion table
These are not the only possible methods of getting from one format to another, but are likely to be the most effective. New products are, of course, being constantly developed, so things can change, but theknowledge of the above will also help you understand how useful a new product you encounter in future is likely be.

Conclusion

Although there are now many video formats available, the choices when it comes to editing work are not really too complicated. We hope this article has helped clarify the options.