Who, Where and When? Who am I ? – Dr. Malcolm Wilson. Where am I ? – Rm. MR15, but not all the time. ? - 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)
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 = 66 Or = 42 in hexadecimal.
ASCII Lower case letters –Add 96 (decimal) (60 (hex)) to position in alphabet. Eg Code for a is = 97 Or = 61 in hexadecimal. Numbers –Add 48 (decimal) (30 (hex)) to number. Eg Code for 5 is = 53 Or = 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 8 th (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 There are 4 ones. –We add an 8 th bit of value 1 to make the total number of ones odd, giving (1) Example 2 –Say our seven bit number is There are 3 ones. –We add an 8 th bit of value 0 to keep the total number of ones odd, giving (0)
Even Parity Example 1 –Say our seven bit number is There are 3 ones. –We add an 8 th bit of value 1 to make the total number of ones even, giving (1) Example 2 –Say our seven bit number is There are 4 ones. –We add an 8 th bit of value 0 to keep the total number of ones even, giving (0)
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 = bits per second. Double this for stereo – or 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.