04 Video - Technical Aspects

8/4/2019 04 Video - Technical Aspects

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Unit 6Digital Video Editing

Unit 314VCreating A Digital Video Sequence

Video TechnicalAspects


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Overview

Frame Rate

Video Resolution

Recording Formats

Aspect Ratios

Audio File formats

Codecs


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All videos are made up from a sequence of stillimages - like photos - played very quickly. Yourbrain is fooled into filling in the gaps and thinks it isseeing real movement.

If we show 15 still pictures per second, we call that 15Frames per Second (fps). That's actually about the ratethat our brain works, so a video has to be at least 15fpsin order to fool your brain into thinking it is realmovement. Anything filmed below that (like somemobile phone footage) looks choppy and low qualitybecause it can be as low as 8fps.

TVs and DVDs in the UK normally use the PAL format,which displays video at 25fps. In the US, however,NTSC is used instead. This format displays video at30fps (actually 29.97, but most people refer to it as 30).Both of these formats are called Standard Definition

Television (SDTV).

High Definition Television(HDTV) is a much newerformat that is used in both the US and the UK. HDTVvideos can be set at 25fps or 30fps as well as 50fps or60fps.

Finally, cinemas uses a frame rate of 23.976fps (or

Fram

Rate


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Video ResolutionEvery video frame, or image, is made up of a series of tiny dots(called pixels). The more pixels you have, the better quality the

picture.

PAL - the British format of TV, displays video at 720 x 576pixels. On the diagram below that's the red and yellow areascombined. That means the screen has 576 squares or dots alongeach row, and 480 going top to bottom (that's 415,000 pixels - or

0.4MP - much less than your digital camera).

Fig: PAL, NTSC andHD formats and theirresolutions in pixels.


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Video Resolution II

Fig: Popular video standards, their

names, resolutions and aspect ratios.

This graph gives moreof a complete view ofdigital videoresolutions. Many ofthe smaller resolutionswere popular sizes formobile devices and forInternet transmissionof video.


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Recording Formats I

1. Standard Definition (SD)The standard format for TV. In the UK andmost of the world the picture is 720 x 576(PAL). Pictures are 720 x 480 (NTSC) in theUS. France use a different format (SECAM)with a picture size of 720 x 576.

Youll see that quality of video every time youturn on the TV as its what the most TVprogrammes are currently transmitted on. Ifyou want decent quality videos, you shouldmake sure that whatever machine you buyproduces at least this quality of video.

It used to be that all camcorders recorded their images to tapes. There were a couple of different formats, butthey all worked in pretty much the same way. Over the last few years, however, there has been an explosionof different formats and qualities aimed at TV, the Internet, and the new High Definition TV standard.


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Recording Formats II

2. High Definition (HD)The best quality video you can get. Picturesare shot at anything up to 1920 x 1080 pixels(but most often 1440 x 1080) which

effectively means theyre good enough to beshown on a full size cinema screen. In order toplay back HD video you need a HD Television(or you can watch them on your computerscreen). In order to output them from yourcomputer, you need a special DVD writer

(called a blu-ray or HD-DVD writer) and a blu-ray or HD-DVD player to play them. Manymodern smartphones and handheld camerasare now also recording in a variety of HDresolutions. YouTube now supports HD filesand professional video editing software now

specifically support exporting HD videooptimised for YouTube and the Internet.


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3. VGA or QVGAThe standards that were for multimediaand the Internet. Video was recorded at

either 320 x 240 or 640 x 480 pixels. Its

only non-standard video recorders (likedigital stills cameras, mobile phones ormemory card camcorders) that recordedto these standards and they do tend tofit more video into their often small

memories. You can still use video fromthese devices in your productions, andsometimes it will look okay however, ifyoure buying a device specifically forvideo, look for a higher recordingstandard. Modern smartphones andcompact cameras support filming invarious HD standards, so VGA and are

Fig. VGA video displayed on television.

Recording Formats III


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Aspect Ratios I

Most monitors and older TVs used to befairly square (not quite square, butsquarer than the following threestandards).

2. Widescreen TV wider than theold fashioned type. The ratio is 16:9 (soif our screen which is 30cm tall will nowhave to be 53cm wide). It is alsosometimes called 1.78:1.

3. Cinemascope is what is used in the cinema. Cinemascope uses a ratio of2.35:1 (there isn't a nice 'whole number' version). This means that our 30cm tall TV isnow a massive 70.5cm wide.

1. Standard Definition (SD)has a ratio of width to height is 4:3. Thismeans that if we have a TV that is 30cmtall, it will need to be 40cm wide. It is alsosometimes called 1.33:1.


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Aspect Ratios II

PillarboxingIf you watch (4:3) video on a widescreen(16:9) TV you might two black bars at thesides (this is called Pillarboxing - think of an

old fashioned post box).

You could zoom in a bit and chop the topand bottom off. Or you could stretch thevideo sideways - but that might not lookright.

If you want to watch a widescreen (16:9)DVD on an older (4:3) the picture will betoo wide to fit.

Solutions: Chop the two ends off (andhope there is nothing important going onthere) or shrink the video down to fit, andbe left with two black bars at the top andbottom of the video (this is called

Letterboxingbecause it looks kindof like a letterbox.


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Audio

There are three main types of audio used for video: Mono, Stereo and SurroundSound.

Monomeans one. One piece of audio isrecorded and then played back through the

speakers.Fig: Both the Left and Right channels are identical

Stereomeans two. There are two separate tracks, orchannels. The left and the right. They might be very similar

for most of the time, but the sound of approaching footstepsmight be slightly louder in the right channel, making it soundlike someone is just slightly to our right. The footsteps mightbe a lotlouder in the right channel, making it seem like theymust be way over to the right. Clever sound engineers (oraverage sound engineers with clever software) can make thesound pan between the speakers so it seems to be movingfrom side to side.

Audio I


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Audio

Audio II

Surround sound takes your left andright, and adds front and back - making fourspeakers at opposite corners. This way wecan position the sound quite accurately tomake us think it's coming from a very specificlocation. Cinemas always use surround sound,and more and more front rooms are beingequipped with surround sound systems.Fig: Surround sound set up.

When exporting video you will have to decide if it is appropriate to compressyour audio or not (to ensure a smaller file size). Here are two common audioformats:

MP3: If a high bitrate is set, the file size will be small and the audio qualityvery good.


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File formats I

When your computer has a limited storage space and you need to share your videos on theInternet, having a small file size is vital. In addition to this, having a set of standard videoformats that can be read on TVs, computers, Internet websites and handheld devices isessentials.

This is why videos are compressed and exported in a format that most devices will be ableto view.The main ideais that if you

make the filesize smaller,you lose videoand audio

quality.

Fig: Realise the power of your video.


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Main file formats II

Section 1 - Low File SizeFLV (Flash Video) is generally used for streaming video - videosyou watch online but don't usually download. The file size is verysmall and the quality can be remarkably good for such small files.

YouTube uses FLV files for the standard quality videos. A typical 1hour FLV video might be 250MB in size (so you could fit 3 on a CD).

Section 2 - FlexibleWMV (Windows Media Video), AVI (Audio VideoInterleave), MOV (Quicktime MOVie) and MPEG (MovingPicture Experts Group) also known as MP4 files are quitesimilar.

They can all be very low quality, small files and they can all be veryhigh quality, large files - or pretty much anywhere in between.


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WMV (Windows Media Video) is the standard filetype used byWindows - so projects you create with Windows Movie Maker or MicrosoftPhoto Story will generally end up as a WMV file. WMV files aren't thatpopular with experts because they don't work well on other systems (e.g.Macs or Linux) and aren't always the best quality.

AVI (Audio Video Interleave) files are very popular. They can bemade using different codecs(you might have heard of DivX and XviD,

which are popular codecs). A codec is a set of instructions for packagingthe video.

Main file formats III

MOV (Quicktime) format is owned by Apple, the people who makeMacs. The files are generally more popular than WMV files, though, as theystill work well on other systems such as Windows and Linux. People who

edit in iMovie usually end up with a MOV file at the end.

MPEG (or MPG) files are similar to AVI files, because there are severaldifferent versions. MPEG1 was used for the original VCDs (Video CDs) whichcame out before DVDs. MPEG2was (and still is) used for DVDs and MPEG4isused for Bluray and also for iPod video files (they are often called MP4files aswell). All of the file types mentioned so far use MPEG technology.


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Main file formats IV

Section 3 Large File SizesDV (Digital Video)files are what you get from mostcamcorders that record to tape. DV footage is very large insize, but is a good format to edit with because you don't losetoo much quality. One hour of DV footage takes up around13GB (13000MB). For this reason, it is not a good format totransfer across the Internet. iMovie uses DV format bydefault, and that's why even short video clips need largeproject files. You can change your settings to work withmuch smaller files if you want to, but you will lose some

quality and the finished video wouldn't be up to DVDstandard.

File sizevary massively, but here are some typical examples:An hour of video for an iPod might be 400MBA 1 hour video downloaded for your computer might be 800MB

A 1 hour DVD file might be 4GB (4000MB)


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A codec compresses a video file to a smaller size. The video is then contained in asingle file.The codec then decompresses the video file for normal viewing on a PC or handhelddevice.

Compressing video content helps ensure transfer and storage of video files happensquickly and cheaply. It is vital for example that a video file is small so not to generate anexpensive internet or phone bill for people who have limited bandwidth.

The recipient of the video must have a device that is able to read (decompress) the file,otherwise they might not be able to view the video. This is why it is important to consider

which codecs are most widely used and most likely to be installed on the devices ofclients and costumers. This consideration will guide you to decide which file format and

A codec does two things to the pictures and audio that make up a videofile:

compressor decompressor

Codecs I


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H.264 is a popular codec used for delivering online video and content tomobile devices. Its popularity ensures its widespread support. The quality ofaudio and video are very good for the small file sizes it can usually create.

MPEG-2 is used on DVDs and in most digital video broadcasting andcable distribution systems. When used on a standard DVD, it offers goodpicture quality and supports widescreen.

MPEG-4 standard that can be used for Internet, broadcast, and on

storage media, MPEG-4 offers improved quality relative to MPEG-2. MPEG-4 has recently been adopted into a number of company products, includingfor example the PlayStation Portable, iPod, the Nero Digital product suite,Mac OS X v10.4, as well as HD DVD and Blu-ray discs.

Codecs II(Compression Formats)

04 Video - Technical Aspects

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