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Le présent article n'est pas disponible en Français
May 2009

James Morris

Several of Canon’s XH and XL series camcorders feature High Definition Serial Digital interface (HD-SDI) outputs that allow for exporting uncompressed digital video in real time. In this article we examine the key features and benefits of using HD-SDI.

The move from standard definition (SD) to high definition (HD) was a big one, and the five-fold increase in pixels required some compromises. In order to tame the data rate for existing connectivity and storage media MPEG compression was adopted and HDV used an anamorphic 1440x1080 resolution rather than the native 1920x1080 pixels of 1080i and 1080p video. These are worthwhile compromises most of the time, as the quality will still be ‘gob-smacking’ compared to SD. But if you want to maintain your HD in its full glory, there is another option – HD-SDI.

HD-SDI has a much greater bandwidth than more consumer-oriented connections. The original version runs at up to 1.485Gbits/sec - more than three times the maximum throughput of FireWire or USB 2. A dual-link version of HD-SDI is just starting to arrive which provides twice the data rate - this will eventually become 3G-SDI, conforming to the SMPTE 424M standard.

For now, though, the basic HD-SDI is the mature and established standard. Thanks to its massive bandwidth HD-SDI doesn’t need video to be compressed. The data rate is high enough for uncompressed transmission of 1080i50, 1080p25 or 720p50 video signals, although not 1080p50 (hence the move to dual-link HD-SDI). So you essentially get the raw output from your camcorder’s sensor before it has been cut down and compromised by the compression circuitry.

Canon camcorders with HD-SDI

The HD-SDI output on the XH G1S - it's the top output in the middle of this picture.

The HD-SDI output on the XH G1S - it's the top output in the middle of this picture.

The interface is integrated into a number of models in Canon’s XH and XL range. The XH G1, XH G1S, XL H1, and XL H1S offer HD-SDI output so that you can hook these up to an external device to feed uncompressed video for recording on a third-party system, or monitoring and mixing in pure digital form. The connector is a standard BNC attachment, but the cable grade is important as poor quality wiring can degrade the signal over longer distances.

The HD-SDI video stream consists of two 10-bit portions; one for luminance samples and one for chrominance, so the native signal is 10-bit. But the colour encoding uses 4:2:2 sampling, meaning that there are half as many colour samples as luminance. This has important implications when compared to the MPEG-based compression used by most local camcorder storage formats (including HDV). HDV generally employs 4:2:0 colour encoding so has a quarter of the colour samples compared to luminance. Aside from the generally richer colour afforded by 4:2:2 sampling, it is also much easier to work with when chroma keying, as the green or blue background will be reproduced more accurately. So if you do a lot of keying work in your productions, switching to HD-SDI could dramatically improve your results.

Audio data

 Canon’s XL H1S with the HD-SDI output with cover on. The HD-SDI button is to the right on the bottom row of four controls at the back of the camcorder.

Canon’s XL H1S with the HD-SDI output with cover on. The HD-SDI button is to the right on the bottom row of four controls at the back of the camcorder.

Video isn’t the only data sent over HD-SDI - the format also supports up to 16 channels of audio, or eight stereo pairs. This is 48kHz, 24-bit uncompressed sound using PCM encoding and is also an improvement over the compressed audio used by camcorder recording formats.

However, the HD-SDI signal is also convertible to HDMI without any loss of quality, so all you need is a small box to convert between the two. For example, there are boxes that sit in between an HD-SDI-equipped camcorder and HDMI-equipped HDTV, allowing the use of a cheaper domestic model. This may not be studio grade, but it will still be a good indication of how your intended audience is likely to see your footage in the best-case scenario.

External recording hardware

The need for external recording hardware is where HD-SDI becomes a little more complex. Since the signal is uncompressed, and requires around 190MB for every second of video, large amounts of fast storage are a necessity. This pretty much rules out recording the native format when in the field, at least to a camera-mounted device. The best option here is HD-SDI-equipped portable editing PCs and, due to the cabling, they won’t allow the camera to be fully mobile.

For field use, there are some more manageably sized HD/SD recorder/player options that take the HD-SDI signal and compress it at up to 160Mbits/sec, using MPEG-2 with I-frames only. This is very close to the original quality, but the 4:2:2 sampling is maintained and the data rate is still high, so the video will be distinctly superior to HDV. Some HD/SD recorders record to Compact Flash memory, and the files are directly compatible with many popular video-editing applications.

The XH G1S is one of four Canon camcorders that offer the HD-SDI facility.

The XH G1S is one of four Canon camcorders that offer the HD-SDI facility.

The most likely application for HD-SDI is in a studio environment where the signal can be fed straight into a computer-based editing system, monitor or live mixer. However, this doesn’t mean this system needs to be in close proximity. With high quality cabling, HD-SDI signals have a range of as much as 300 feet, and can be routed round a facility using switchers, which are readily available as rack mount or stand alone devices. SDI can also be used in the field and is ideal for live feed productions, such as TV shows, as well as for editing and monitoring video at the highest possible quality.

A distribution amplifier can extend the range of use even further. It also allows you to route the same signal to multiple destinations such as monitors in the studio and machine room, plus hard disk recording systems. HD-SDI is also ideal for live studio mixing. For example, some switchers are capable of mixing between up to eight HD-SDI inputs.

Editing systems

For editing, an HD-SDI interface will be required for the host system, of which there is now a myriad options for Mac and PCs. Acceleration hardware is essential, as is a computer with extremely fast storage. A multi-disk RAID array with terabytes of capacity may be necessary too, as each terabyte will only be sufficient for only about 16 minutes of footage. A multi-processor, multi-core host workstation can be used, however an SDI board and a RAID of three normal SATA hard disks will suffice. So the cost will be slightly higher than for editing compressed formats such as HDV.

However, there are no fundamental differences in the workflow when editing uncompressed HD-SDI compared to compressed HDV. Once the footage has been captured to the workstation’s storage, editing follows the same process, and uses the same software. All of the usual professional editing applications support uncompressed HD-SDI. In other words, the main implication of HD-SDI is the added cost of the supporting hardware. But if your productions need the ultimate in video quality, it’s a major step towards what HD really can be, compared to compressed formats such as HDV.

To find out more about the Canon video products mentioned in this article or to try out or purchase Canon video products, contact your nearest Canon video dealer. For contact details just click here.