In September 1993, 55 most important companies in the global electronics industry agreed specifications of a new standard for digital video recording. The DVC format adopted by the Consortium seemed to have the potential to become the video recording system most widely used worldwide.
Five years later the market offers at least four variations on the original theme, all descendants of the original formula. Most manufacturers are producing equipment for home use DV strictly based on the original agreement.
However, Sony decided to offer an improved format which he named as DVCAM and Panasonic did the same with DVCPRO, which already has two different "flavors" ... The competition continues and the market is complicated.
In addition, the rapid development of connectivity solutions based on the system FireWire It has made the possibility of transferring digital video systems of non-linear editing without any loss of quality, within such reasonable costs that have enabled completely rethink the technical levels of the educational and corporate videography reality.
However the explosion of DV has brought a certain amount of confusion to the world of video production. Most digital recording formats emerging video are related to the DV standard, although some of them do not follow the physical format specifications.
As recording format DV videotape was intended to start for home users or so-called prosumers. The initial specification, known as DVC, proposes two cassette sizes: the "L," which supports up to four hours of recording with a size of 125x78mm, and the "Mini", which can handle up to an hour of material in a box 66x48mm.
The latter is known as MiniDV, perhaps trying to feed the public the idea that it is a different format. In fact, the existence of the tapes bigger is something of a secret to the "initiated" riding their home studios video recorders table with offering a few manufacturers.
Sony introduced changes in the physical structure of the cassettes, by adding a small chip of non-volatile memory that can store information about taps recorded on the tape, such as time code addresses and a "good pick - bad pick" mark. This innovation is one of the keys to Sony's proposal for the use of the DVCAM format in news operations.
Audio problems begin
The original DVC Consortium specification proposed three different ways of recording sound, based on different samples. On the one hand there was talk of 12 bits sound, with sampling of 32kHz, more than enough for ordinary applications. When using 12 bit sound, DV equipment can handle up to four audio channels.
When higher quality is required, the format can accommodate uncompressed audio bit 16 44.1 or 48Khz, quality identical to that of DAT recorders professionals the. The coexistence of these standards is one of the factors that begin to complicate things for the unwary users ...
Indeed, the sound is one of the areas in which the DV formats differ for professional use. The DVCAM digital sound recording 12 or 16 bits, with an immediate effect on the amount of usable channels: by using a lower bandwidth, can record up to four channels of sound.
DVCPRO uses only 16 sound bits. However, any format recorders can play without problems 12 bit sound caused by DV equipment of any brand.
But there is a fundamental difference in how to "package" the sound between different formats. The original DV standard uses non-synchronous sound. This does not imply that the material has a tendency to lose synchrony between audio and video, but when digital audio is transmitted may be slightly "messy" with respect to the corresponding video frames.
This is no problem under normal circumstances, but it can cause tiny breaks sound when the material is published. After all, this is equipment for domestic use ... many intend to use for other purposes. Moreover, professionals use completely synchronous sound formats that can be edited as analog audio.
However, this brings some differences in the flow of digital information originating from different brand equipment, which make them partially incompatible. In fact, it may be difficult to copy a DVCAM recording on a recorder DVCPRO preserving the original coding. Many times it is necessary to process this material for analog routes. Or copy the digital video and analog audio output ...
This means that to transfer this material is necessary to continue the process of play - decompress - encode an analog (or digital) signal - compress and record again. It is reduced to use sophisticated digital recorders as their analog counterparts. The results are not fatal ... but far from optimal. They may be comparable to analog video recorders component, but it is very likely that in a process of this type defects caused by compression become more evident.
In fairness to manufacturers, I must say that these problems tend to disappear. Any professional DV recorder can play DV tapes minimizing household-type audio problems in the courts.
And we are very close to full playback compatibility. A DVCPRO recorder can play back a DVCAM tape, and vice versa. Almost always. But non of harassing manufacturers, why is it that the "almost" always fails in the most important jobs?
Factor own digital audio systems can complicate things some filmmakers. In the analog world can handle high levels of sound signals. In fact, the recording formats based on metal tape are especially permissive, since in many cases can handle levels up to + 3DB. This has led many technicians are accustomed to manage their signals at the top of the scale.
If they begin to operate digital equipment will find a small problem at times when the signal is saturated, you begin to hear noises because when the capacity of the recording system is exceeded the signal is removed, and the signal peaks begin to appear pops y cracks they can disable the sound.
This is not a malfunction of digital systems. It is the consequence of bad habits acquired during many years of work in analog environments in which saturated distortion characteristic sound became part of what some consider a "normal" audio. Solution? Respect levels that can actually be handled by the system. Remember that the reference level for digital recordings should be set well below the 0DB of the scale, corresponding to the maximum level that the system can handle before you start your recordings sound like a plastic container filled with rocks.
The physical architecture of the DV tape is prone to certain problems. Consider that the tape width 6mm is constantly in contact with the drumA rotary drum rotating at 21.7rpm 9000mm. And the space used on the tape to record a video frame is slightly more than two-thirds the diameter of a human hair. What could be the effect of a particle of dust, by chance, hinder reading the tape?
Anticipating these problems, the DV Consortium introduced a robust error correction algorithm in the standard, and works much better than correctors dropouts analog recorders, as their basic resource is the "repetition" of portions of the above table when you need to correct a defect.
The error correction system defects is read to verify digital information is taken from the tape, which ensures consistent performance in all cases.
Normally this leads to corrections virtually invisible, but has a relatively serious problem: when a massive error occurs, the correction is a slight jump caused by the total repetition of the previous frame. And if it is a very serious problem, the image freezes until the reading is normal.
Such problems, however, is really rare. But it is better that users do not implement an intense program for reusing tapes. Over time, this can lead to unpleasant surprises.
The use of tape
The issue of dropouts and error correction is intimately linked with one of the biggest differences between DV, DVCAM and DVCPRO: physical way each format uses the tape.
The household DV records the information on the tape recording "strips" of 10 microns wide. This size track pitch (Spacing recorded sections of the tape) creates certain problems editing by insert, for high precision mechanical and electronic systems recorders for successful draws needed.
You can vary the tape interchangeability between different machines. For example, if a transport camcorder DV has been manipulated to remove a tape that has been locked, it may have been exerted undue pressure on the tape guides and can make the recorder is out of standard. Sometimes the only way to make a playback correct the recorded material is to use the same camcorder that you recorded.
Interchangeability problems have always existed, and tend to be more acute in cases where teams do not have a rigorous maintenance, which unfortunately happens in many production houses in our region. And the size of the mechanisms of this recorder makes it particularly dangerous to leave the preventive maintenance of equipment to the proverbial armed cameraman simple Kit Screwdriver watchmaker.
The DVCAM uses a track pitch of 15 microns, while DVCPRO uses 18 microns. Use more space on the tape can implement without further ado conventional editing systems capable of accurate draws.
DVCAM is actually an improvement over the original DVC specification, the main changes are the track pitch, Audio coding and the addition of ClipLink system to record information about jacks on the cassette itself.
Panasonic decided to add attractive features for professional users, such as a channel control track analog, one cue track additional channel and an additional analog audio channel. DVCPRO recordings include in the head of the tracks a package of digital information that facilitates insert editing. It also introduced a new size cassette, Type "M" (97.5x64.5mm) and decided not to use the Mini, while retaining compatibility playback with a similar can play VHS-C tapes in a current VCR mechanical adapter.
Each manufacturer defends his swashbuckling format, although its marketing policies tend to position them in slightly different segments. DVCAM is part of a comprehensive proposal to install Sony digital news production systems, but provides compatibility routes to be integrated into conventional installations.
DVCPRO is offered as a commonly used format, aimed at both the market and the general ENG production applications and distribution of material.
Panasonic tries to break the precarious balance of supply by introducing a new product line that allows you to include the DV format in the transition to digital television. This is the DVCPRO 50, a new format that records digital information twice on tape the same size as the DVCPRO currents. The bandwidth goes from 25 50 mbs mbs to, using two DV parallel encoders.
The DVCPRO 50 4 uses coding: 2: 2 and a milder compression, just 3.3: 1, which puts it in the same range as the JVC Digital-S format. Allows better performance in critical applications postproduction and retains all the features of DVCPRO, but with the ability to record images 16: 9 and easily integrated into environments DTV standard resolution (see Uncertainty in the transition. TV & Video in Latin America, Vol. 4, Ed. 2, 1998 March-April, p. 16).
The arithmetic coincidence between the 50 and DVCPRO Digital S (compression 3.3: 1 50 to mbs) apparently is not so casual. In fact, it seems that Digital-S teams use DV encoders as part of the hardware their recorders.
This allows us to predict the future of digital video recording will be drawn in multiples of 25 mbs. In fact, both Panasonic and JVC have announced products "100" as part of its strategy to market production in high definition.
The war continues
The obvious question for producers is: what format should I? The visual test is terribly disappointing. To a casual viewer, the playback Engraving in any of the formats DV material is suspiciously similar. Even DVCPRO 50 no major differences, apparently.
Not to wonder, because they all use the same architecture compression, although there may be subtle differences improvements enter one or the other manufacturer in the encoders their teams.
While Panasonic ensures that the operation of DVCPRO equipment is more profitable, thanks to the MP tape, Sony ensures that the DVCAM tape ME performs better and provides more stable long-term results. Sony also assumes as an advantage its direct compatibility with consumer DV format, although for practical purposes Panasonic is not far behind.
To complicate matters, each brand has launched its own standard transport uncompressed digital signals, which differ mainly in coding audio channels. However, both formats support non-compressed digital signals of serial type (SDI), which in any way DV allows computers to communicate with the rest of the world. And between them ... but unpack-transfer-recompress has become the norm when it comes to copy material from a DV format to another.
Theoretically, standard FireWire It would allow the cloning of DV material without major inconveniences. In fact, only one line FireWire It may carry up to four streams of DV; but, in practice, inconsistencies between formats have left practical applications systems FireWire for the scope of users of basic DV format. While computer users are high quality hands and knees to make digital copies sound ...
How to decide? Must consider all applications; compatibility; ROI; performance under difficult conditions; price; future; DTV; service and representation; durability; operating costs; MiniDV support; weight; compression; brand loyalty ...
Producers gentlemen: welcome to the most difficult technical decision over the last ten years. We wait for your comments.