Dr Didier LeGall....................
tvbeurope.com
Tape Formats: A Matter Of Compression
by David Fox
The choice of which tape format to record on has always been a key question for broadcasters buying new equipment. Previously the big decision was analogue or digital. Now that everything is digital it is a matter of compression, which goes beyond tape to disks and transmission systems. Can that tape compression format you shoot on live the whole way through the post production and transmission chain?
"Formats should no longer be a consideration. The only consideration should be what problem I want to solve," says Dr Didier LeGall, chief technology officer, C-Cube. Admittedly, he'd be happy if everyone used several formats, because C-Cube makes chips which allow formats like DVCPRO and MPEG-2 to exist in the same station.
"There are multiple problems to be solved by broadcasters, for each of which there might be a different solution," he explains. For an ENG crew, a small, lightweight camera may be the key requirement, so they will use DV. "From a pure quality prospective, MPEG 4:2:2 at 50Mbps and DVCPRO-50 have been designed with very similar characteristics." As far as he's concerned, only one format is without merit: "To me, Motion-JPEG has no benefits at all. Everyone does it differently. It is a loser."
Other manufacturers, even those who make the formats, have also generally lost out in the format wars. They have had to cater for every format broadcasters use, which increases development costs. Tektronix is one of those which has most loudly bemoaned the lack of standards, although its president, Tim Thorsteinson, says: "We're not in the format business. We'll do any format the broadcasters want."
In order to deliver 'complete format independence' Tektronix now has three models in its Profile range, the latest being a DVCPRO version (PDR400), added to the JPEG PDR200 and MPEG-2 4:2:2 PDR300, and claims to be the only manufacturer which can do this. The PDR400 allows four-times real time transfers, and has up to six video channels, scrub audio and an optional mix/effects board. Existing versions can be upgraded to DVCPRO. The first product of the Avid/Tektronix alliance is the bundling of the DVCPRO Profile with Avid's DVCPRO NewsCutter.
The choice of DVCPRO was because it was the only format "fully open today. We're opposed to closed proprietary formats. We feel it is the wrong step for the broadcast industry," says Thorsteinson.
Sony is also opposed to closed proprietary formats, because broadcasters don't want them, and has recently put its considerable weight behind MPEG-2. "MPEG is quickly becoming one of the major technology issues for European broadcasters, because it gives real cost savings and offers true interoperability," says Miles Flint, president Sony BPE.
He predicts it will end the format wars and will increase innovation and competition "as manufacturers compete on a more level playing field than ever before." Flint explains Sony's rush to adopt MPEG as "quite simply because it's the right thing to do. It makes sense."
This effectively leaves two main compression systems competing for your approval, but it's not going to be as simple as that. You'll probably want both of them, and this worries Tektronix, whose director of product marketing, Ray Baldock, believes there will need to be a lot of work done on improving DV/MPEG interoperability. "It's our belief that these compression formats will exist in the real world in one facility. So, we have to deal with that in our product line. We already have customers using DV in the news environment, yet when they move into transmission, users like the efficiency of MPEG."
Format choices
From the DV camp you can pick standard DV, DVCAM and DVCPRO, while Betacam SX and Digital-S represent the main MPEG-based acquisition formats. There are about four million DV camcorders in use worldwide. Although the majority have been bought for domestic use, it does mean that tapes are readily available almost anywhere. Coupled with their low price, light weight andsmall size, they are also widely used by broadcasters for documentary and news programmes.
Sony's professional DVCAM range has sold more than 50,000 units, and more than 55,000 units of DVCPRO have been delivered worldwide. DVCAM can be used for post production, but Sony has positioned SX for this market, which leaves DVCPRO as the DV-based format with most support as a complete production system. Panasonic has an alliance of 26 manufacturers, including Avid, Tektronix, C-Cube, Deutsche Telekom and Quantel, offering DVCPRO-compatible editing systems, servers, news production systems, and ATM networking. Although it can't, yet, offer a complete end-to-end production system, Panasonic promises this will be possible soon. Ireland's RTE is using DVCPRO for current affairs initially, followed by news, but Panasonic insists that DVCPRO is not just a news and current affairs format, even if that is the fastest growing area. However, "it is a fairly demanding area to go in to. It pushes the system to its limits, especially in terms of robustness and reliability," says Chris Daubney, managing director, Panasonic Broadcast Europe.
Indeed, DVCPRO News Automation (DNA) is being promoted as "the total digital solution for news," including servers and archive, says Bob Mueller, president, Panasonic (US). Available next year, it will offer faster than real-time transfer of files, giving a first generation signal, on Silicon Graphics Origin 200 servers, which can support up to 16 QuickCutters and multiple incoming and playout channels. "In future, the principles of DNA can be applied to production and post production, and later to HDTV," he adds.
Committed to MPEG
"Our future is MPEG. Our future is open," says Sony's Miles Flint. Its MPEG strategy is a response to customer demands and he points out that SX is already MPEG compliant. It has also just launched an MPEG-based server, the MAV-70.
Sony says it has sold more than 12,000 plus SX machines. Being compatible with analogue Betacam (of which some 400,000 machines are in use) is a further factor in its favour.
However, Sony has been promising to make an SX chipset available to other manufacturers for a few years. This hasn't happened, due to the complexity of implementing the system. Fortunately, in the last year IBM, Fast and others have brought out MPEG chipsets so Sony has instead concentrated on producing a module which will interface between SX and 601, which will be demonstrated early next year. It will deliver an MPEG compliant codec -- the only thing different in SX is its interface protocol. New SX units can now be fitted with a board which makes it fully MPEG compliant. Existing users will be able to fit a protocol translator for output to other systems. These will use a single chip and should be "mathematically transparent as they involve just a protocol change." They can even be used for high speed transfers, says John Ive, Sony's director of strategic planning.
One problem with SX is it uses the I/B interframe coding sequence, where every second frame is difference data. JVC product manager Colin Green claims this means it can only do eight generations, whereas Digital-S can do 20-30 generations, even if it recodes each time. However, he admits that for news, which is SX's primary application, "the quality loss is irrelevant as they won't be going many generations."
He maintains that JVC is the "dark horse" among manufacturers, as it is ideally positioned to take advantage of 50Mbps-based broadcasting with Digital-S, and is ready for 100 Mbps HDTV whenever that arrives. Although DVCPRO-50 and Digital-S use the same chipset, Green claims that the two are not otherwise comparable. DVCPRO-50 uses 1/4 inch tape running at 66mm per second, giving a maximum tape time using the thinnest, most fragile tape of 90 minutes -- and he believes that any studio format must be able to reliably record two hours per tape for playout.
He also claims that its high track density will result in alignment problems when using tapes on different machines. Where broadcasters choose DVCPRO-50 for its ability to play back DV tapes, he warns there will be added problems because DV uses ME (metal evaporated) tape. As DVCPRO is designed to use MP (metal particle) tape, the heads won't last as long with ME. He believes that Digital-S should be seen as a direct alternative to Digital Betacam, claiming that in EBU and SMPTE quality tests, "Digital-S came extremely close to, or even better than, Digital Betacam. But, of course, we're nowhere near that price."
Let battle commence
"Increasingly, as time goes on, we won't get so hung up by formats, because MPEG is a bitstream which can be recorded on different storage formats," says John Ive. "Obviously we've got DV and MPEG products. There is a place for both of them. There are some occasions when the small cassette and absolute portability is important, and DV is great if you need a standalone solution and don't have to network. But, if the end user wants to get heavily into networked systems, especially via ATM or satellite, they've got to consider MPEG, because being a bitstream it is ready to network," he adds.
"We've long supported the utilisation of MPEG in the transmission stage," says Michael Brinkman, director strategic development, Panasonic. However, he believes some people see a system is MPEG, whether at 50 Mbps or 18 Mbps, and think it will be transparently transcoded directly into an MPEG transmission format, when, instead, the signal must revert to baseband (601) first. So there is no benefit from working in MPEG just because you transmit in MPEG. Instead, "we think people should apply the best form of compression for the job," says Brinkman.
"When MPEG was launched, we first targeted only transmission. But when we developed 4:2:2 we aimed it at production and contribution," says LeGall, (the former chair of MPEG's video committee, who helped develop the MPEG-2 standard). He believes that MPEG 4:2:2 is less expensive for storage for big facilities, but recommends DV for production and post production. Ive disagrees. He points out that none of the DV formats are MPEG compliant in any way, so will always need decoding and recoding. The new DVD recorders will be MPEG-based, and any other disk systems will be able to cope with MPEG.
Sony also intends making Digital Betacam MPEG-friendly via a transcoder, although how transparent it will be will depend on the bit rate used. However, as DigiBeta is 90Mbps, "whatever happens you will be using a higher bit rate than MPEG," which only goes up to 50 Mbps, he says. MPEG is unique in that it can address costs by adapting to bandwidth and other constraints, and it is totally open. "This means it will go a long way in the industry. Its flexibility will bring it to HDTV and to low bit rates. There are other solutions, but I believe that MPEG will triumph," says Ive.
One of the benefits of MPEG in a networked system is its ability to do high speed transfers, although simply carrying a tape from one machine to another may remain the fastest way of transferring longer programming. However, DVCPRO and Digital-S also have this advantage, thanks to SDTI, which allows two-times real time transfers for 50 MHz material and four-times for 25 MHz.
"SDTI is going to revolutionise the industry. It's going to change the way we work," says Green. Because it carries compressed signals, the compression only has to be done once, at acquisition and decompression at transmission, "which gives a vast increase in the number of generations it can work with, making it [Digital-S] comparable to uncompressed D5." Because it is based on the SDI standard, it can use any SDI router and can run alongside SDI signals. All that needs to be added is a chipset at either end, "so it is fairly cheap to upgrade an SDI network to SDTI, which is why this standard is so important," says Green. However, Baldock points out that "there is currently no provision for moving lots of metadata, such as scripts, via SDTI, where ATM would be better."
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