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Introduction
• What are we going to learn?
• Course outline.
• Some details.
• Assessment.
• Introduction to media technology and revision.
Course outline
1. Introduction and revision.2. Text and e-mail.3. Audio.4. MIDI.5. Video.6. Graphics. 7. Image manipulation. 8. Compression techniques.9. Video compression 110. Video compression 211. Audio compression.12. Revision.13. Time constrained assignment.
Who, Where and When?
• Who am I ? – Dr. Malcolm Wilson.
• Where am I ? – Rm. MR15, but not all the time.
• Email ? - [email protected]
• Course notes – eng.nene.ac.uk/~malc.
Who, Where and When?
• New topic every week.• Assignment 1 – Issued week 9-
10, hand in week 18 (After Easter).
• Assignment 2 – Time constrained assignment in the final class.
Media Technology
• Primarily concerned with the following digital media:– Text– Graphics– Animation– Synthesised Sound (Headphones)– Digitised Sound (Headphones)– Digitised Images– Digitised Moving images.
• Multimedia is the integration of the above.
• This is NOT a multimedia course.
Media Technology
• Some of the above are computer generated.
• Others are digitised representation of real-world data.
• The computer data which represents these categories may be also subdivided into:– Static (images)– Continuous (sound, movies)
Media Technology
Continuous
Static
real-world synthesized
animation
graphics
sound moving images
still images text
Data Files
• All of the media data have specific file types.
• The extension identifies the file type.
• Examples:– Mydrawing.gif, “.gif” identifies
a graphics file. “gif” stands for “graphics interchange format”
– Mynoise.wav, “.wav” identifies a sound. “wav” is short for (sound) wave.
Data Files
• Most media data files contain and start with “headers”.
• “Headers” contain information about the file such as:– How long it is. – How it should be played back.– How it is coded.
• Media files are often specially coded forms of the original data.
Text
• Plain text and formatted text.
• Plain text is usually coded in “ASCII” (American Standard Code for Information Interchange).
• A 7 bit code which allows 128 characters.
• Computers usually deal with 8 bits so ASCII appears to “waste” one bit.
Text
• “ASCII” coded text was originally designed to connect terminals (keyboard and text monitors) to remote computers.
• Errors could occur in the connection.
• Bit 8 used for parity checks.
ASCII
• Full list of ASCII codes will appear on my website and will be given as a handout.
• But common letters and numbers are easy to remember.
• Upper case letters– Add 64 (decimal) (40 (hex)) to
position in alphabet.• Eg Code for B is 64 + 2 = 66 • Or 40 + 2 = 42 in hexadecimal.
ASCII
• Lower case letters– Add 96 (decimal) (60 (hex)) to
position in alphabet.• Eg Code for a is 96 + 1 = 97 • Or 60 + 1 = 61 in hexadecimal.
• Numbers– Add 48 (decimal) (30 (hex)) to
number.• Eg Code for 5 is 48 + 5 = 53 • Or 30 + 5 = 35 in hexadecimal.
• Working in hex may be easier.
Parity
• Since we mentioned it.
• Error checking mechanism.
• Odd or even, (but we decide first).
• In 7 bit code (like ASCII) we use the 8th (MSB) for parity.
• We set the bit to one or zero to make the total number of 1’s odd (for odd parity) or even (for even parity).
Odd Parity
• Example 1– Say our seven bit number is
011101. There are 4 ones.– We add an 8th bit of value 1 to
make the total number of ones odd, giving (1)011101.
• Example 2– Say our seven bit number is
010101. There are 3 ones.– We add an 8th bit of value 0 to
keep the total number of ones odd, giving (0)010101.
Even Parity
• Example 1– Say our seven bit number is
001101. There are 3 ones.– We add an 8th bit of value 1 to
make the total number of ones even, giving (1)001101.
• Example 2– Say our seven bit number is
110101. There are 4 ones.– We add an 8th bit of value 0 to
keep the total number of ones even, giving (0)110101.
Parity
• Checked by receiving computer to see if there is an error.
• Can you see a problem with this?
• Clue - 2 errors.
• Midi code (for sound synthesiser communication) very similar to ASCII, but no parity.
Graphics - Vector Images
• Image composed and stored as a sequence of preset shapes or objects.
• Lines, rectangles, ellipses, text etc.
• Described in terms of size, position, drawing colour, fill colour.
• Each object’s characteristics can be edited independently while in this graphical form.
Graphics – Vector Images
• Differs from a bitmap image which we will see later.
• Often called vector graphics.
• Common drawing packages allow the creation of this form of image.
• Once converted into bitmap or (raster form) we can no longer edit individual shapes.
Graphics – Vector Images
• Example of a graphic vector image created using “Autoshapes”.
My text in red
• Other popular vector graphic tools are Paint shop pro and Photoshop.
Bitmaps - Raster Images
• Does not use individual shapes.
• Whole image contains many pixel elements (pixels).
• Pixels are generally defined by colour alone.
Bitmaps - Raster Images
• We cannot edit or change any shape drawn without changing all of the pixels concerned.
• Microsoft Paint produces Bitmap images.
• Once a vector graphic image has been converted to a bitmap it cannot be converted back.
Bitmaps - Raster Images
• If we “paste” from a vector graphics image into Paint the pasting process converts the vector graphic to a bitmap.
• We can no longer edit the pasted image.
• Try it.
• Digitisation of real-life images produces bitmap images.
Moving images and animations
• Images may be given the illusion of motion.
• We display a succession of changing “frames” to give this illusion.
• Moving raster images are usually called “movies” in computer media jargon.
• Moving graphics (vector images) are called animations.
Sound
• Just like images we can have two forms in the computer.
• One form remembers the pitch, duration and loudness and individual sound of the notes.
• This is stored as MIDI (musical instrument digital interface) form.
• Like vector graphics the sound can be edited by changing the individual characteristics of the notes.
Sound
• Other form relies on digitisation of real life sounds.
• Sampled sound.
• A common example of this are “wav” sound wave sounds.
• Like bitmap images we cannot edit individual notes without changing all of the samples which the note is comprised of.
Digitisation
• Real-life images and sounds need to be digitised for computer representation.
• Turning an analogue or continuous signal into a digital signal.
• There are 3 stages to digitisation.– Sampling– Quantisation.– Coding.
Sample rates and Bandwidth.
• The bandwidth of audio and video signals can be considered to be the highest frequency carried by the signal.
• In sound “crispness”.
• In vision “sharpness”.
Sample rates and Bandwidth.
• Sample rates must be (at least) twice the bandwidth
• High quality audio requires a bandwidth of 20 KHz.
• A sample rate of 44.1 kHz or 48 kHz is chosen.
Data rates and file sizes.
• So an 16 bit audio signal sampled at 44.1 kHz produces 16 x 44100 = 705600 bits per second.
• Double this for stereo– 1411200 or 1.4112 Mbps.
• High quality video uses a 270Mbps data stream to allow for a 10bit 625 line television picture.
Data rates and file sizes.
• CD ROM holds about 700 MBytes.
• How much audio?
• How much video?
Data rates and file sizes.
• DVD holds about 15 GByte max.
• How much audio?
• How much video?
Data rates and file sizes.
• Original CD ROM could only deliver data at 1.2 Mbps.
• 40 x is therefore 48 Mbps.
• DVD data rate (single speed) 11 Mbps.
• 16 x now exist giving 176 Mbps.
• Still can’t do telly?
Compression
• Digitised sound and video produces a lot of data.
• In particular digitised television quality pictures produce data at 270 Mbits/second which is faster than most hard disks, CD roms and networks devices can accommodate.
• We need to compress data for use on computers.
Compression
• We have two types of compression.
• Lossy compression and lossless compression.
• As the names suggest lossy compression loses some of the original signal, while lossless does not.
• Lossless techniques such as run-length encoding and Huffman coding achieve compression by creating shorter codes. This is not always possible.
Compression
• Lossy techniques rely on throwing away some information which the viewer or listener will not notice too much.
• Involves changing the data to some other form. (Transform)
• Most lossy techniques are noticeable.
• The more lossy compression that is applied, the more the compression effect will be noticeable.