MM Topic 4 - Video Data 1

Video is a collection of bit-mapped still images (called frames) that are taken one after the other. When the file is played these pictures are shown in quick succession to give the impression of a moving image.

The number of frames is called the frame rate and this is measured in FPS (Frames per Second). The standard rate is 25 FPS.

• Basic Concepts

MM Topic 4 - Video Data 2

The size of code used to represent the colour of each pixel in a frame will also affect the file size and picture quality. As with still images this is called the bit-depth or colour depth.

Resolution is the number of pixels in the area that has been captured. This is usually measured in megapixels. A higher setting for resolution when a shot is being captured will mean better quality but of course, a higher file size.

• Basic Concepts

The bit-rate is the number of bits that are sent in one second when transmitting a video file. If the data cannot be sent quickly enough then the video will not be able to display in real-time.

MM Topic 4 - Video Data 3

1. Digital video camera

3. Video capture card

2. Web cam

A digital video camera(camcorder) functions in the same way as a still digital camera. The user points the camcorder at the scene they wish to capture and a lens focuses the image onto an array of CCDs (Charge Coupled Devices)

Digital Video camera

Each CCD corresponds to one pixel in the image being recorded. The CCD is a sensor which changes the light striking it into an electrical signal.

More expensive cameras have 3 CCDs for each pixel, one each for red, green and blue. This improves quality.

MM Topic 4 - Video Data 4

Unlike a digital camera a camcorder must capture a continuous stream of images. Therefore the ADC needs to work much faster in a camcorder than in a camera.

Another consequence of capturing a constant image stream is that file sizes are very large - indeed video is the largest of any of the media types (text, images, sounds) you look at in Higher, even after it has been compressed.

This analogue signal is then changed into digital using an analogue to digital converter (ADC).

1. Digital video camera

3. Video capture card

2. Web camDigital Video camera

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The camcorder has a piece of hardware in it called a DSP(Digital Signal Processor)

1. Digital video camera

3. Video capture card

2. Web camDigital Video camera

Digital Signal processor

It also compresses and encodes the data stream into MPEG format before saving it onto a storage medium—disk, memory card or tape.

The DSP automatically alters the contrast and brightness of the video for maximum quality.

MM Topic 4 - Video Data 6

Webcam

Webcams are used for real-time chat, including instant messaging (IM), telephone calls over the internet, live broadcasts and video conferencing.

1. Digital video camera

3. Video capture card

2. Web cam

Webcams are a lot less expensive than camcorders because:

● the lower resolution means that the ADC need not be as fast as that fitted on a camcorder.

● there are fewer CCDs.

● they have no backing storage facility

MM Topic 4 - Video Data 7

1. Digital video camera

3. Video capture card

2. Web cam3. Video Capture Card

If a recording was made with an analogue video camcorder then the conversion from analogue and compression will not have taken place.

An analogue camcorder can be connected to a device that can be fitted to computers called a video capture card.

This receives the video data, converts it to digital and compresses it.

A video capture card can also be used to receive data from an analogue video recorder or analogue TV.

MM Topic 4 - Video Data 8

1. Uncompressed AVI

2. MPEG

1. Uncompressed AVIAVI stands for Audio Video Interleave and is a standard developed by Microsoft. It is commonly used with Windows Media Player (also developed by Microsoft).

AVI is so-called because it mixes in (interleaves) sound and video so that they are stored efficiently, rather than as separate entities where video and audio could be separated and played out of sync.

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This makes AVI useful for some files but restricts it to relatively small sections. Microsoft is currently developing a successor to AVI called Windows Media Video which will allow compression.

AVI is uncompressed but it does have methods of keeping file size compact:  

● frame rate is set at a maximum of 30 fps

● resolution cannot exceed 320 * 240,

● final file size cannot exceed 2 Gb.

1. Uncompressed AVI

2. MPEG1. Uncompressed AVI

MM Topic 4 - Video Data 10

1. Uncompressed AVI

2. MPEG2. MPEG

MPEG(Moving Picture Expert Group) is perhaps the most popular standard for video.  

MPEG is a compressed format that uses intraframe and interframe techniques. Sound is stored as a separate ‘layer’ within the file.

It comes in many different versions: MPEG 1, MPEG 2, MPEG 3, MPEG 4 and MPEG 7with most DVDs using MPEG 2.

MM Topic 4 - Video Data 11

1. Uncompressed AVI

2. MPEG

2. MPEG

This involves removing data from each frame.

Intraframe compression

MPEG looks for and removes data that either cannot be detected by the human eye or will not affect the overall quality too much.

MM Topic 4 - Video Data 12

1. Uncompressed AVI

2. MPEG

2. MPEG

If an area of a video clip such as a background does not change over a long period then this does not have to be stored for each frame. 

Interframe compression

This look for savings between frames.

MPEG does not store all the frames but instead it stores certain key frames (i-frames) as jpegs.

It then looks for the changes between frames and stores these changes as delta frames. Thus the whole of every frame is not stored—only those areas that might change.

MM Topic 4 - Video Data 13

To calculate the size of a video file in bytes you need to know the following:

● The resolution of a single frame

● The frame rate (no of frames per second)

● The length of the video (time it runs for)

● The colour depth

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a. Calculate the number of pixels in a single frame by multiplying height by width

b. Calculate the storage needed for a single frame by multiplying the number of pixels by the bit depth.

c. Calculate the storage needed for 1 second by multiplying the size of a frame by the frame rate.

d. Calculate total file size by multiplying size of 1 second by the length of the clip in seconds.

e. Convert the answer into a suitable unit.

Carry out the following steps

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Calculate the file size of a one minute uncompressed video clip recorded at 25 fps (frames per second. The resolution was set at 640x480 and each frame is stored in 16 bit colour.

1. Number of pixels = 640 x 480 = 307,200 pixels.

2. Frame size = 307,200 x 16 = 4,915,200 bits.

3. 1 second of clip = 4,915,200 x 25 = 122,800,000 bits.

4. Total file size = 122,800,000 * 60 = 7,372,800,000 bits.

5. Convert into bytes = 7,372,800,00 /8 = 921,600,000 bytes.

6. Convert into Kilobytes = 921,600,00 / 1024 =

900,000 Kilobytes.

7. Convert into Megabytes = 900,000 / 1024 = 878.91 Megabytes.

MM Topic 4 - Video Data 16

256

65,536

16

No of colours

30

25

fps

1024 x 768

800 x 600

640 x 420

Resolution

20

13

12

Time(secs)

24

Calculate the file size for the following:

(Note that the No of colours is given. You have to work out the colour depth. (bit depth)

64.09 MBFile Size

285.64 MB

270 MB

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Once a video clip has been captured and transferred to a computer it can be viewed and, if desired, edited.

To edit video clips you will need to have video editing software such as Adobe Premiere Elements, Pinnacle Studio, ULead Video Studio or Windows Movie Maker.

A user would typically be working with several video clips to create a movie.

Typical activities would include copying clips, cutting frames from clips, adding or removing audio tracks, adjusting audio volumes, adding titles and credits and inserting transitions and effects.

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Three features which are common to all video editing applications are:

1. Timeline

3. Sequencing

2. Transition

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1. Timeline

3. Sequencing

2. Transition1. Timeline

The timeline area of the screen displays the sequence of video clips in order of viewing.

This where you view the various media elements, video, audio, still images, titles in an order and sequence which can then be edited.

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1. Timeline

3. Sequencing

2. Transition2. Transition

Transitions are added between clips and to create an effect when the change is made from one clip to another.

Common transitions are wipe (a line wipes across the old image, taking it away as it goes and replacing it with the new picture), fade out (the old image gradually fades to black), dissolve (the old image fades out while the new one fades in without the screen going black), hard cut (the clip suddenly jumps to a completely new image) and peel or page turn (the old image peels away to reveal the next clip).

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1. Timeline

3. Sequencing

2. Transition3. Sequencing

Sequencing simply refers to the order in which video and audio clips are placed.

Clips can be easily put into any order and then duplicated, inserted or deleted

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The final stage of the process is to display the video on screen and a video card is used for this.

The DAC (digital to analogue converter) changes the binary information into electrical signals that can be sent to monitors.  

The DSP (Digital Signal Processor), also called a GPU (Graphics Processing Unit) carries out all of the work needed to display full quality video clips.