Gear Talk: The Skinny on HD
With Mediacorp‘s Channel 5 going the high definition (HD) route come next month, I thought it’d do well to give a quick summary on what HD is, and what this means to us television and filmmakers.
High Definition in Singapore means more than just television programming â€“ almost all local films screened this year and last were shot in HD resolutions (you can check out which ones by clicking the drop-down tab on the right listing films “Made in Singapore 2006 – Current”).
In fact, HD begun a good 38 years ago in Japan, as developed by the Nippon Hoso Kyokai (NHK), but never really took off until the late 90’s and in later, more successfully, in the 21st century. At this current point in time, there exists a number of standards for TV broadcast: 1080i, 1080p and 720p, all of which supports the brilliant 5.1 Dolby Surround Sound mix.
Before I subject you to more technical details, I probably should explain how HDTV is noted in writing, while in the process explaining how it works. In this example, we shall look at the Mediacorp standard â€“ 1080i:
1080 refers to the number of vertical display lines. Since all HD footages are in 16:9 aspect ratio, the width of the screen will compute to 1920 pixels. The letter ‘i’ denotes the format. In this case it is Interlaced, as opposed to Progressive (p).
Progressive scans are usually considered the better of the two, since it involves refreshing the entire picture in one frame. Interlacing involves two passes â€“ the first for even fields, and the second for odd fields. The two scans are then pieced together. While this method gives you a smaller file size â€“ it often produces artifacts, especially where there is a lot of movement.
PROGRESSIVE: The entire frame is scanned as one field.
INTERLACED: Frame is made up of two fields pieced together.
[Images courtesy of Originasian Pictures LLP.]
In addition to the different resolutions (1920 x 1080 VS 1280 x 720) and the different formats, there are also differing frame rates. When shooting for theatrical release, the norm is often 24 frames per second (fps). Depending on the available bandwidth and the amount of detail and movement in the picture, the optimum format for video transfer is thus either 720p24 or 1080p24. When shown on television in countries using PAL, film must be converted to 25 frames per second by speeding it up by 4.1 percent. In countries using the NTSC standard (30fps), a technique called 3:2 pulldown is used. One film frame is held for three video fields, (1/20 of a second) and then the next is held for two video fields (1/30 of a second) and then the process repeats, thus achieving the correct film rate with two film frames shown in 1/12 of a second.
This year’s broadcast tradeshows (such as NAB, IBC etc) also saw the introduction of many 60p HD cameras from major manufacturers such as Panasonic. Besides the visual feast that is HD, high-definition also means more channels of audio for a fuller surround sound effect. Always the poorer cousin to video, HD audio is still a dark art in the Asia region â€“ few production houses are able to correctly record and mix a 5.1 Dolby sound. However, nifty tools such as the Holophone looks set to minimise the hassle of recording 6 channels.
While I have painted nothing but a rosy picture of HD, there are, as always, some complaints about new technology. Since almost all HD transmissions are digital (be it via satellite, terrestrial or cable), any bit of interference will render the signal unwatchable. While analogue SD transmissions present a ‘snowy’ image, digital ones skip, freeze or black out entirely. And since HD means more data, more compression is applied on the broadcaster end, effectively allowing for more chances of failure/in.
As a content producer though, this is, fortunately, not of concern. What you should be interested, therefore, are the methods of capturing. Due to the enormous bandwidth that uncompressed HD consume, most cameras have codecs built in to compress the footage at source. Below is a list of common capture compression schemes in a handy table format for comparison:
|Format||Max Resolution||Bit Depth||Colour Sampling||Compression||Data Rate|
|HDCAM SR||1920 x 1080||10,12||4:4:4 / 4:2:2||DPCM / DCT/ Mpeg-4||440Mbit/s|
|D5 – HD||1920 x 1080||8,10||4:2:2||DCT||235Mbit/s|
|DVVCPRO HD||1440 x 1080||8||4:2:2||DCT||100Mbit/s|
|HDCAM||1440 x 1080||8||3:1:1||DCT||140Mbit/s|
|XDCAM HD||1440 x 1080||8||4.2.0||MPEG â€“2 (VBR)||18, 25, 35 Mbit/s|
|HDV||1440 x 1080||8||4:2:0||Long GOP MPEG-2||25Mbit/s|
Not included in the list is the new AVCHD and AVC-Intra codecs, which are relatively new. You can still check out our previous articles here and here on these two codecs to understand a little more. Interestingly, the new Channel 5 HD will be transmitted via Mpeg-4 AVC, the standard which the BBC are looking to adopt as well.