DV is here to stay

In September 1993, the top 55 companies in the global electronics industry agreed on the specifications of a new standard for digital video recording. The format adopted by the DVC Consortium seemed to have the potential to become the most widely used video recording system worldwide.

Five years later the market offers at least four variations on the original theme, all descendants of the initial formula. Most manufacturers are producing DV equipment for home use rigorously based on the original agreement.

However, Sony decided to offer an improved format that it baptized as DVCAM, and Panasonic did the same with DVCPRO, which already has two different "flavors" ... Competition continues and the market gets complicated.

Additionally, the rapid development of connectivity solutions based on the FireWire system has made the possibility of transferring digital video to non-linear editing systems a reality, without any loss of quality, within such reasonable costs that they have allowed to completely rethink the technical levels of educational and corporate videography.

However, the explosion of DV has brought with it a certain dose of confusion to the world of video production. Most of the newly emerging digital video recording formats are related to the DV standard, although some of them do not follow the physical specifications of the format.

As a format for recording on videotapes, DV was initially intended for home users or the 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 66x48mm box.

The latter is known as MiniDV, perhaps trying to feed the public with the idea that it is a different format. In fact, the existence of the larger cassettes is something of a secret to the "insiders" who set up their home video studios with the tabletop recorders offered by a few manufacturers.

Sony introduced variations in the physical structure of the cassettes, adding a small non-volatile memory chip that can store information about the shots recorded on the tape, such as timecode addresses and a "good shot - bad shot" mark. This innovation is one of the keys to Sony's proposal for the use of the DVCAM format in news operations.

The original DVC Consortium specification proposed three different ways of recording sound, based on different sampling. On the one hand, there was talk of 12-bit sound, with 32kHz sampling, more than enough for ordinary applications. Using 12-bit sound, DV equipment can handle up to four channels of audio.

When higher quality is required, the format can accommodate uncompressed 16-bit audio at 44.1 or 48Khz, of identical quality to that of professional DAT recorders. The coexistence of these standards is one of the factors that begins to complicate things for unsuspecting users...

Precisely, sound is one of the areas in which DV formats for professional use differ. DVCAM allows you to record 12- or 16-bit digital sound, with an immediate effect on the number of usable channels: by using a lower bandwidth, it allows you to record up to four channels of sound.

The DVCPRO only uses 16-bit sound. However, recorders of any format can seamlessly reproduce 12-bit sound originated by DV equipment of any brand.

But there is a fundamental difference in the way sound is "packaged" between different formats. The original DV standard uses non-synchronous sound. This does not imply that the material has a tendency to lose the synchrony between audio and video, but that when the digital audio is transmitted it can be slightly "messy" with respect to the corresponding video frames.

This is not a problem under normal circumstances, but it can cause very small sound jumps when editing the material. After all, this is equipment for home use... that many of us intend to use for other purposes. On the other hand, professional formats use completely synchronous sound, which can be edited just like analog audio.

However, this brings some differences in the flows of digital information originated by equipment of different brands, which make them partially incompatible with each other. In fact, it can be difficult to copy a DVCAM recording to a DVCPRO recorder while retaining the original encoding. Many times it is necessary to process this material by analog routes. Or copy the digital video and an analog audio output...

This implies that to transfer this material it is necessary to follow the process of playing - decompressing - encoding an analog (or digital) signal - compressing and recording again. It is to reduce itself to using sophisticated digital recorders just like their analog counterparts. The results are not fatal... but they are far from optimal. They may be comparable to those of component analog video recorders, but it is very likely that in a process of this type the defects caused by compression will become more evident.

To do justice to manufacturers, it must be said that these problems tend to disappear. Any professional DV recorder can play home-type DV tapes minimizing audio problems in cuts.

And we're very close to full playback compatibility. A DVCPRO recorder can play a DVCAM tape, and vice versa. Almost always. But, without wishing to harass manufacturers, why is it that the "almost" always fails with the most important jobs?

A factor typical of digital audio systems can complicate things for some filmmakers. In the analog world it is possible to handle high levels in sound signals. In fact, recording formats based on metal tape are especially permissive, as in many cases they can handle levels of up to +3DB. This has led to many technicians getting used to handling their signals at the top of the scale.

If they start operating with digital equipment they will find a small problem: in the moments when the signal is saturated, they begin to hear annoying noises, because when the capacity of the recording system is exceeded the signal is eliminated, and in the signal peaks pops and cracks begin to appear that can disable the sound.

This is not a failure of digital systems. It is the consequence of the bad habits acquired during many years of work in analog environments, in which the characteristic distortion of saturated sound became part of what some consider a "normal" audio. Solution? Respect the levels that can really be handled by the system. It should be remembered that the reference level in digital recordings must be set well below the 0DB of the scale, which correspond to the maximum level that the system can handle before its recordings begin to sound like a plastic container full of stones.

The physical architecture of DV tapes is prone to certain problems. Consider that the 6mm wide tape is constantly in contact with the drum, a 21.7mm rotating drum that rotates at 9000rpm. And the space used on tape to record a video frame is slightly more than two-thirds the diameter of a human hair. What can be the effect of a dust particle that happens to hinder the reading of the tape?

Anticipating these problems, the DV Consortium introduced a robust error correction algorithm in the standard, and it works much better than the dropout correctors of analog recorders, since its basic resource is the "repetition" of portions of the previous table when it is necessary to correct a defect.

The error correction system finds reading defects by verifying the digital information taken from the tape, which ensures consistent performance in all cases.

Normally this leads to virtually invisible corrections, but it has a relatively serious problem: when a massive error occurs, the correction consists of 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 normalized.

These kinds of problems, however, are really rare. But it is better that users do not implement a very intense tape reuse program. Over time, this can lead to unpleasant surprises.

The issue of dropouts and error correction is intimately linked to one of the biggest differences between DV, DVCAM and DVCPRO: the physical way in which each format uses the tape.

The DV for home use records the information on the tape by recording "stripes" 10 microns wide. This size of the track pitch (spacing of the recorded parts of the tape) creates certain problems to edit by insert, since a great precision of the mechanical and electronic systems of the recorders is needed to achieve successful draws.

The interchangeability of tapes between different machines may vary. For example, if the transport of a DV camcorder has been manipulated to remove a tape that has been locked, excessive pressure may have been exerted on the tape guides and may cause the recorder to be out of standard. Sometimes, the only way to make a correct playback of the recorded material is to use the same camcorder that recorded it.

Interchangeability problems have always existed, and tend to be more acute in cases where equipment is not rigorously maintained, which unfortunately occurs in many production houses in our region. And the size of the mechanisms of this recorder makes it especially dangerous to leave the preventive maintenance of the equipment to the proverbial cameraman armed with a simple kit of watchmaker screwdrivers.

The DVCAM uses a 15 micron track pitch , while the DVCPRO uses 18 microns. The use of more space on the tape allows to implement without major problem conventional editing systems capable of making accurate ties.

Actually the DVCAM is an improvement over the original DVC specification, whose main changes are the track pitch, the audio encoding and the addition of the ClipLink system to record information about the shots on the cassette itself.

Panasonic decided to add attractive features for professional users, such as an analog track control channel, an additional cue track , and an additional analog audio channel. DVCPRO recordings include in the header of the tracks a digital information package that facilitates editing by insert. In addition, it introduced a new cassette size, the "M" type (97.5x64.5mm) and decided not to use the Mini, although it retains playback compatibility with a mechanical adapter similar to the one that allows VHS-C tapes to be played on a current VCR.

Each manufacturer defends its format tooth and nail, although its marketing policies tend to position them in slightly different segments. DVCAM is part of Sony's comprehensive proposal to install digital news production systems, although it provides the compatibility paths to be integrated into conventional installations.

DVCPRO is offered as a general-purpose format, aimed at both the ENG market and general material production and distribution applications.

Panasonic is trying to break the precarious balance of supply by introducing a new product line that allows the DV format to be included in the transition to digital television. This is the DVCPRO 50, a new format that records twice as much digital information on tapes of the same size as ordinary DVCPRO. The bandwidth goes from 25 mbs to 50 mbs, with the parallel use of two DV encoders.

The DVCPRO 50 uses 4:2:2 encoding and a milder compression of just 3.3:1, which puts it in the same range as JVC's Digital-S format. It enables better performance in critical post-production applications and retains all the features of DVCPRO, but with the ability to record 16:9 images and easily integrate into standard-resolution DTV environments (see Uncertainty in transition. TV&Video Latin America, Vol. 4, Ed. 2, March-April 1998, p. 16).

The arithmetic match between the DVCPRO 50 and the Digital S (3.3:1 compression at 50 mbs) is apparently not so casual. In fact, it seems that Digital-S equipment uses DV encoders as part of the hardware of their recorders.

This allows us to predict that the future of digital video recording will be plotted in multiples of 25 mbs. In fact, both Panasonic and JVC have announced "100" products as part of their strategy for the high-definition production market.

The obvious question of the producers is: which format suits me? The visual test is terribly disappointing. For a casual viewer, the playback of footage recorded in any of the DV formats is suspiciously similar. Even the DVCPRO 50 material does not present major differences, apparently.

It is not surprising, because they all use the same compression architecture, although there may be very subtle differences due to the improvements that one or another manufacturer introduces in the encoders of their equipment.

While Panasonic ensures that the operation of DVCPRO equipment is more cost-effective, thanks to mp tape, Sony ensures that DVCAM ME tape performs better and delivers more stable results in the long run. Sony also assumes as an advantage its direct compatibility with the consumer DV format, although for practical purposes Panasonic is not far behind.

To complicate matters, each brand has released its own standard of transporting uncompressed digital signals, which differ basically in the encoding of audio channels. However, both formats support uncompressed digital serial (SDI) signals, which either way allows DV equipment to communicate with the rest of the world. And also among them... although decompressing-transferring-recompressing has become the norm when it comes to copying material from one DV format to the other.

In theory, the FireWire standard would allow cloning of DV material without major inconvenience. In fact, a single FireWire line could carry up to four DV video streams; but, in practice, inconsistencies between formats have left the practical applications of FireWire systems for the scope of users of the basic DV format. While users of high-quality equipment look like cats to be able to make digital copies with sound...

How to decide? You have to consider everything: applications; compatibility; return on investment; performance in difficult conditions; price; future; DTV; service and representation; durability; operating costs; MiniDV support; weight; compression; brand loyalty...

Gentlemen producers, welcome to the most difficult technical decision of the last ten years. We look forward to your feedback.