Week 10: Lecture (moving image and video)

The Moving Image

John Logie Baird is often credited with inventing the television, but in fact despite designing and making a working mechanical TV system, his system was not adopted due to it being so unreliable. 

Persistence of Vision

'Persistence of Vision' is an old theory that now is not believed to be true. It is the phenomenon of the human eye were an after image was thought to persist for approximately one twenty-fifth of a second on the retina. This is now regarded as the 'myth of persistence' and it is no longer accepted that human perception of motion (in the brain) is the result of persistence of vision (in the eyes). A more modern and more plausible theory to explain motion perception are two distinct perceptual illusions : phi phenomenon and beta movement. 

Bitrate

The bitrate is the number of bits that are processed/conveyed per unit of time. It describes the rate at which bits are transferred from one location to another (i.e. it measures how much data is transmitted in a given amount of time).

Interlaced and Progressive video

Interlaced video is a technique of doubling the perceived frame rate without consuming extra bandwidth. The TV tricks your eyes by first drawing the odd number lines on the screen 25 times per second. Then the even lines of the next frame and so on. Progressive video does not interlace and appears sharper.

Display resolution

The display resolution of a digital television, computer monitor or display device is the number of distinct pixels in each dimension that can be displayed. Modern HD televisions can display 1080i or 1080p - the 'i' and 'p' stand for interlaced or progressive video. 

Video file formats

I decided to research more about video file formats and how they are used. 

Platform:   PC

File formats used:  

WMV (Windows Media Video): Microsoft’s family of proprietary video codecs, including WMV 7, WMV 8 and WMV 9. This format uses the VC-1 codec.

MPEG-4: a standard developed by the MPEG (Moving Picture Experts Group)

MPEG-2:  an older standard developed by the MPEG (Moving Picture Experts Group)

AVI (Audio Video Interleave): a multimedia container format introduced by Microsoft in 1992.

FLV (Flash Video): Flash video files usually contain material encoded with codecs following the Sorenson Spark or VP6 video compression formats. It is developed by Adobe and used on the popular video streaming site YouTube.

MOV (Apple QuickTime Movie): common multimedia format often used for saving movies- compatible with both Macintosh and Windows. QuickTime format uses the H.264 codec.

PC: WMV (Windows Media Video)

Windows Media Video 9 (WMV9) is Microsoft’s implementation of the VC-1 codec.

WMV9 supports three profiles: Simple, Main and Advanced. The Simple and Main profiles support a wide range of bit-rates. These include bit-rates appropriate for high-definition content as well as video on the internet at dial-up connection speeds. The Advanced profile supports higher bit-rates.

VC-1 can be used to compress lots of different streaming and downloadable video content, from podcasts to HD movies on demand. It is designed to achieve state-of-the-art compressed video quality at bit rates that can vary from very low to very high. It can easily cope with 1920 x 1080 pixel video at 6-30 Mbps for high-definition video. It is capable of handling up to a maximum bit rate of 135 Mbps. VC-1 works using a block-based motion compensation and spatial transform scheme similar to that used in other video compression standards such as H.264 and MPEG-4.

Platform: Apple Macintosh

File formats used:

MOV (Apple QuickTime Movie): the standard format that QuickTime uses, along with MPEG-4.

MPEG-4: a standard developed by the MPEG (Moving Picture Experts Group)

MPEG-2:  an older standard developed by the MPEG (Moving Picture Experts Group). Only supported by the most recent version of Apple’s operating system, OS X Lion.

AVI (Audio Video Interleave): a multimedia container format introduced by Microsoft in 1992.

Apple Macintosh: MOV (QuickTime Movie)

The MOV video file format is a QuickTime movie. It is used for saving movies and other video files and uses a proprietary compression algorithm developed by Apple, which is compatible with both Macs and PCs. The format is a multimedia container file that contains one or more ‘tracks’- which each store a particular type of data including audio, video, effects or text (e.g. for subtitles). Each of the tracks either contains a digitally-encoded media stream or a data reference to the media stream located in another file.

Both the MOV and MP4 formats can use the same MPEG-4 codec (also known as H.264), so they are mostly interchangeable in a QuickTime-only environment.   The codec uses lossy compression, which means that an algorithm is used to remove any image data that is unlikely to be noticed by the viewer. Lossy compression does not retain the original data meaning that the data that is removed is lost. The amount of data lost depends on the degree of the compression.

MP4/MPEG-4 - based on MOV (QuickTime Movie)

MP4 is a video file format that uses MPEG-4 compression, which is a standard developed by the Moving Picture Experts Group (MPEG). It uses separate compression for both the audio and video tracks. The video is compressed using AAC compression, which is the type of compression used in .aac files.

MPEG-4 uses lossy compression, which uses an algorithm to remove image information that is unlikely to be noticed by the viewer. Lossy compression does not retain the original data meaning that the data that is removed is lost. The amount of data lost depends on the degree of the compression.

The Apple iTunes store uses the M4V file extension for their MP4 video files, including TV episodes, movies and music videos. These files use MPEG-4 compression but also may be copy-protected using Apple’s FairPlay digital rights management (DRM) copy protection. This means to play the file the computer must be authorised to access the account it was purchased with.

Comparison of file formats/codecs

MPEG-4/H.264 and VC-1 are both industry standards. However, just because a platform or mobile device supports one of these “standards,” it doesn’t guarantee compatibility, because H.264 and VC-1 consist of many different “Profiles” and “Levels.”

A Profile outlines a specific set of features that are required in order to support a certain delivery platform. A Level gives the limits of the Profile, such as the maximum resolution or bit-rate. H.264 Profiles that are suitable for web video applications are: Baseline Profile (BP) for limited computing power and Main Profile (MP) for typical user’s computers. VC-1 uses different options- these are Low and Main Profiles, both at low, medium or high levels depending on the user’s target platform.

At high bit-rates, the differences between MPEG-4/H.264 and VC-1 are small: 

• Under high bandwidth stress, H.264 tends to degrade to softness instead of blockiness, while VC-1 tends to retain more detail, but can show more artefacts due to its stronger in-loop deblocking filter. At web/download bit-rates, both codecs offer great quality and there is no difference between them.

• The MPEG-4/H.264 codec tends to take more CPU power to decode at maximum resolution. Comparing VC-1 Advanced Profile and H.264 High Profile, each with all of its options turned on, VC-1 can decode about twice as many pixels per MIPS (million instructions per second). This is more important when considering HD-resolution content than lower resolution content and doesn’t matter at all when there’s a hardware decoder. When constrained by software decoder performance, VC-1 can handle Advanced Profile, while H.264 is limited to Baseline, where VC-1 outperforms H.264 in quality/efficiency. This has become an important consideration in recent years, because consumer broadband access has outpaced the hardware upgrade cycle of many households and businesses.

• While there do not seem to be many differences between the VC-1 and MPEG-4/H.264 in terms of desktop computing (except for HD quality video), the Windows Media Video format has considerably less support on the Mac platform so H.264 has the advantage there. There is not the same difference in support on the Windows PC platform, however since the Windows Media Video format (and therefore VC-1 codec) is proprietary to that platform it would make sense to say it has a slight advantage.