Portable HD storage
Video acquisition is fast becoming tapeless, as camcorder manufacturers move toward alternative media formats. The optical disk, solid-state drive and conventional hard drive are all options that can replace videotape. The formats have advantages and disadvantages, and each has its proponents among camera manufacturers.
The lower cost camcorders that find applications in news, low-budget products or second units predominately record at 25Mb/s or lower. This data rate was adopted for the SD DV format; to support HD, they must use long-GOP coding and possibly MPEG-4 rather than MPEG-2 compression. The consumer HDV and AVCHD formats are representative. These camcorders usually include a 1394 or USB2.0 interface and can record to hard drives or Flash memory.
In contrast, broadcast HD cameras must record at higher data rates than 25Mb/s to deliver improved resolution and 4:2:2 color sampling. This means a move away from consumer recording formats to proprietary recording media, usually supplied by the camera manufacturer. The P2 and SxS solid-state media, plus the Professional Disc optical format, are representative of this class. To ensure the high levels of record reliability expected by broadcasters, the camcorders are designed as a system including the storage medium. Camera manufacturers supply a range of field recorders and drives to support viewing and file transfer at the shoot.
There is a third route: using portable storage with a separate camera rather than the integrated camcorder. This approach is best when high-quality recordings are needed, especially where extensive video processing is used in post production, including color grading and compositing.
To scope the requirements for portable HD recording, consider videotape. The longer S-size HDCAM-SR stores around 130GB of video data, and the full-size cassettes store up to 400GB of data.
The current optical disks and SSDs have a capacity of 50GB to 64GB but cannot match the 880Mb/s write speed of the tape. HDDs in compact and ruggedized formats for camera-top use are around 100GB, with record rates up to 100Mb/s. This will change radically over the next few years as the tapeless technologies evolve. The issues that constrain portable storage are size, weight, power consumption, resistance to vibration and, of course, cost.
There are two approaches to camcorder storage. The storage can be integral to the design, with a slot to insert the storage. The other approach is to attach a small drive between the battery and the camera, on top of the camera, where the microphone would sit, or underneath as part of the mount. This way allows tapeless recording to be used with existing DV format camcorders.
There are many third-party suppliers of this add-on storage. Some record DV-style at 25Mb/s, while others use codecs like JPEG2000 to offer high bit rates more suited to HD.
The Professional Disc that Sony's XDCAM product line uses incorporates similar technology to Blu-ray, but Sony has adapted the technology to suit the requirements of the broadcast camcorder. The Professional Disc is supplied in a caddy to protect it from handling. The disk carries a low-res proxy file in addition to the broadcast resolution file. This can be used in the field for logging and preview.
For HD recording, the XDCAM format uses MPEG 4:2:2 or 4:2:0 encoding at data rates up 50Mb/s. Using the dual-layer 50GB disk at 50Mb/s, the record time is 95 minutes.
The original broadcast format is Panasonic's P2 card. This can record at data rates up to 100Mb/s. The card capacity grows year by year, with 64GB cards available in 2009. P2 camcorders record HD using the DVCPRO HD codec, or more recently in the AVC-Intra format, which offers higher quality for a given bit rate. The record time is one minute per GB, or 64 minutes for the 64GB card.
Sony now has a solid-state format, the SxS card, which it developed with SanDisk. It is based on the footprint of the ExpressCard 34 PC card for laptops. With a current capacity of 32GB, it is used by the XDCAM EX cameras, recording MPEG at 35Mb/s. JVC also has adopted the format.
Ikegami has developed a Flash memory pack for its GFCAM HD camcorder. The GFPAK memory is available in capacities up to 64GB, and it uses a SATA interface.
The Grass Valley Infinity is based on the RevPro removable magnetic media, but it can also record to CompactFlash. The camera operator can choose what is appropriate for the project. RevPro disks have capacities up to 65GB, and the camera records with a wide range of codecs, including 10-bit JPEG2000 at up to 100Mb/s.
Card or drive?
There are two families of solid-state memories. One is aimed at the HDD replacement market as a solid-state drive (SSD). A typical commodity SSD has SATA connections so that it can be used as a drop-in replacement for an HDD. Designed for laptops with the same low power and small size requirements as a camcorder, they are naturally rugged. Some video SSDs may have 1394 connectors, so they can directly connect to HDV camcorders.
The other family is the memory card, typified by CompactFlash (CF) and Secure Digital (SD). Aimed at the consumer market, these products do not generally have the performance required for reliable HD capture.
Many of the portable formats designed for use in camcorders use long-GOP compression for HD to reach the low data rates that can be continuously written to current Flash memory technology. The P2 card uses striped memory modules to reach 100Mb/s data rates, sufficient to support I-frame encoding of HD.
If you want to shoot 4:2:2 or 4:4:4, with minimal compression, possibly at 10-bit resolution, the benchmark is the HDCAM-SR, with a record data rate of 880Mb/s in the high-speed mode. The alternative to a tethered VTR is to use a portable hard drive array. Several manufacturers supply such systems for the digital cinematography market. Such units may be DC powered, with the ability to record dual link HD-SDI as well as 4K signals via an optical connector. The drives are encased in rugged caddies and can be delivered to post in much the same way as a videotape.
Which medium is best?
Each recording technology has pros and cons. (See Table 1.) Solid-state cards and drives are rugged and shockproof, but currently are expensive. They are best suited to use as a temporary cache, rather than an archive format. Optical disks are low cost and can be used for the long-term archive of a production. Hard drives are low cost, but not ideal for use as a permanent archive (more than five years).
Other considerations relate to workflow. What equipment do you need to preview rushes? If you need to archive expensive cards for reuse, what is needed at the shoot to perform the backup? What about copies for escrow to meet the requirements of production insurance?
What may be the optimum medium for news acquisition is not necessarily best for a drama shoot. Tapeless recording presents a wide choice, with price points for every budget.
|Media ||Technology ||Suppliers ||Capacity ||Typical data rate ||Record duration |
|CompactFlash ||Flash ||many ||32GB ||100Mb/s ||32 minutes |
|GFPAK ||Flash ||Ikegami/Toshiba ||64GB ||100Mb/s ||60 minutes |
|P2 ||Flash ||Panasonic ||64GB ||100Mb/s ||64 minutes |
|Professional Disc ||Optical ||Sony/TDK ||50GB ||50Mb/s ||95 minutes |
|Rev Pro ||HDD ||Thomson Grass Valley/Iomega ||65GB ||100Mb/s ||60 minutes |
|SxS ||Flash ||Sony/SanDisk ||32GB ||35Mb/s ||100 minutes |
Table 1. A comparison of tapeless onboard storage formats. Note: The capacity of the media is constantly increasing, so the figures here were representative in August 2009.