Pocket digital camcorders
Camcorder codecs
Camcorder recording formats are a lot more complicated than in the days of DV. Where DV relied on one unified compression system across all camcorder models, there are now numerous options. But they all revolve around various flavours of MPEG, which reduces the data rate required by video by compressing changes and variations within frames.The choice of MPEG sub-type varies greatly, and this can be confusing.
Generally, MPEG-2 is used for standard definition. The compression levels are adequate to fit a usable amount of video onto an 8cm DVD, and the use of MPEG-2 makes the discs compatible with consumer DVD players. Hard disk and Flash-based camcorders also employ MPEG-2 compression, which again means transferring footage to DVD only requires repackaging, not time-consuming recompression. With the top data rates about the same as commercial DVDs (approximately 8Mb/sec), a couple of hours of footage can be recorded onto a dual-layer DVD disc.
High definition video is much more data-hungry. At full 1920 x 1080 resolution, HD has five times as many pixels per frame as standard definition at 720 x 576. As a result, the most popular format for high definition camcorders is now AVCHD, which uses MPEG-4 H.264 AVC compression. This is more efficient than MPEG-2, and with it you can achieve the same visual quality at half the data rate.
However, not all HD camcorders use AVCHD compression. JVC has stuck with MPEG-2 for its HD models, using a much higher data rate than any AVCHD model yet launched. There are also a few camcorders that use different flavours of MPEG-4 and MPEG-4 AVC, such as Sanyo’s Xacti camcorders.
The plethora of formats is not only bewildering, but also poses a problem for editing software. All applications will capture and edit from DV tape, and most now support HDV, too, but AVCHD has taken a while to achieve widespread support. The latest versions of most apps can now import and edit AVCHD, including Ulead VideoStudio 11, Pinnacle Studio 11, and CyberLink PowerDirector 7. But Windows Movie Maker won’t let you edit with AVCHD, nor will Adobe Premiere Elements 4.
Also, the top-end Full HD and progressive scan modes of AVCHD on some camcorders may cause problems. The MPEG-4 formats used by digital still cameras and the Xacti camcorders are also less widely supported, making such footage harder to edit.
Sensors explained
Ten years ago, when DV tape was just beginning to become the dominant camcorder format, consumer models had one image sensor and professional ones had three – and they all used charge coupled devices (CCDs). Using three sensors together has traditionally been considered better for image quality, because a separate one is used for each colour channel. So colour resolution is the same as the overall video resolution. With a single sensor, a patterned mask separates the colours and divides them between the pixels, meaning the colour resolution is lower than the overall video resolution.
But whether it’s one sensor or three, the size and the pixel count of each is also very important. The larger the sensor, the more light will fall on it from the lens. This improves sensitivity and, therefore, performance in low light. This is a major reason why entry-level camcorders, typically with small 1/6in sensors, tend to perform badly in low light. |
| Adobe Premiere Elements 4 isnt able to edit footage captured in the AVCHD format used by the majority of the camcorders on test |
You might also think that more pixels will help, too. But this is only partly true, as video works at fixed resolutions. In Europe, regular TV operates at 720 x 576. High Definition is limited to two main options – 1280 x 720 and 1920 x 1080 – the latter being Full HD – used for HDTV transmissions, although 1440 x 1080 is also regularly used in camcorders. So if the sensor has more pixels than the video resolution used, the extra will primarily benefit still image capture.
This has led manufacturers to take different approaches for their higher-end models. Some, such as Panasonic and JVC, favour three smaller sensors for the colour benefits. But Sony and Canon now use a single larger unit over 1/3in in size, while all manufacturers are increasing the pixel count to make their camcorders’ integrated digital photography features more appealing.
If the sensor’s resolution is greater than the video resolution, the extra pixels can be used to make electronic image stabilisation more effective, as the actual video resolution doesn’t need to be reduced. Canon has also recently introduced an Advanced Zoom, which uses the sensor’s extra pixels to provide a digital zoom that doesn’t reduce resolution.
In the last couple of years, the CCD has also begun to be supplanted by active pixel sensors manufactured using a complementary metal oxide semiconductor (CMOS) procedure similar to microprocessors. These are usually known just as CMOS sensors, and there are a number of advantages over CCDs. They consume less power, are cheaper to produce, and can also integrate image processing features. For example, Sony’s CMOS technology integrates ClearVid and most recently Exmor image processing.
But the CMOS system has disadvantages too. The earliest examples had noticeably inferior light sensitivity compared with CCDs, which became obvious in the low-light performance of cheap digital stills cameras and camera phones. They are also progressively scanned, which can lead to video shearing in fast pans. These problems have mostly been overcome in the latest camcorders, fortunately. As a result, CMOS sensors have started to appear in professional models, and could take over the entire market in a few years.
This article appeared in the October, 2008 issue of PC Authority.
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