ITEC2110, Digital Media Chapter 1 Background & Fundamentals GGC -- ITEC2110 -- Digital Media

Content Analog vs. Digital Digitization (Sampling & Quantization) Bits basic concepts How bits represent information

Types of Information Analog information Digital information

Analog Information (continuous information) Examples: time weight temperature Brightness of a light Loudness of your voice …

Accuracy of Analog information: difficult to achieve How long is the black bar?

Digital Information (discrete information) Examples: The number of students in this class The number of cars in the parking lot The number of characters in your name …

Phenomena in the Real world: discrete vs continuous Things in the real world can be discrete These things can be counted Examples: The number of cars GM produced each year The number of beans in a jar

Phenomena in the Real world: discrete vs continuous Things in the real world can be continuous Continuous can’t be counted, it must be measured Examples: Air pressure Height of an ocean wave Frequency of a sound wave

characteristics of Analog vs. Digital Analog information Continuous Cannot be counted, must be measured made up of infinite number of data points Difficult to be accurate Digital information Discrete Countable Can be Accurate

computers can only deal with digital information Discrete data is easy for a computer count it and store it as a number Continuous data How?

Question... If computers only store 0s and 1s... How does all this continuous stuff end up in a computer so that we can save it and play it back?

Answer Continuous data must be converted to discrete data

Sampling: from continuous to discrete Converting continuous information to digital data: -You must SAMPLE to convert it to discrete

Sampling rate the number of samples per time period is called the sample rate

Sample and Hold: from digital to continuous Digital data back to continuous information:

Digitization Digitization is the process of converting analog information into digital data that computers can handle 2-step process: sampling quantization

Wave height Wave height

single sample

single sample

single sample (sample and hold)

two samples

two samples

two samples (sample and hold)

three samples

three samples

three samples (sample and hold)

four samples

four samples

four samples (sample and hold)

five samples

five samples

five samples (sample and hold)

Taking it a lot farther…

Taking it a lot farther…

How frequently should I sample? too few small file size (good) not a faithful representation when replayed too many large file size (bad) excellent representation when replayed The Nyquist rate twice as many samples as the frequency being captured Results in an ok file size Results in faithful representation when replayed

CD quality is 44,100 samples per second Why? Human hearing response is in the range of 20 to 22,000 cycles per second Nyquist sample rate = highest frequency to be captured = 22,050 CPS 2 x 22,050 = 44,100 samples per second

Further reading http://en.wikipedia.org/wiki/Nyquist_rate http://en.wikipedia.org/wiki/Sampling_%28signal_processing%29 http://en.wikipedia.org/wiki/Mp3

Quantization Quantization is the process of representing the sample data using only given number of values. The given number of values determines the bit depth of the quantization. In other words, bit depth refers to the number of allowable levels you map (or round) the sample data to.

Example: 10 levels For 10 discrete levels, you may have the 10 allowable values as 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 0, 5, 10, 15, 20, 25, 30, 35, 40, 45 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 0, 100, 200, 300, 400, 500,600,700,800, 900 ... and so forth

Suppose you choose 1,2,3,4,5,6,7,8,9,10 For 1,2,3,4,5,6,7,8,9,10 Any value below 1.0 will be recorded as 1.0. Any value higher than 10 will be capped at 10. Any value will be rounded to closest decimal value It works well if all data fall in this range. But it may not be always the case.

Increase Number of Allowable Levels Will achieve better accuracy when more levels are used for quantization. However, for digitization, increasing the number of allowable levels (i.e. increasing bit depth) will increase the file size.

The side effects of sampling: sampling artifacts Sampling Artifacts are the negative side effects caused by having to sample continuous data

Sampling Artifacts Under-sampling: not enough samples being taken of continuous data can produce undesired artifacts

Sampling Artifacts (cont.) Not enough quantization levels when sampling continuous data can produce undesired artifacts Examples might be: too few grey levels: gradients become steps too few brightness levels: posterization

Sampling Artifacts Retrograde Motion 2 samples/cycle, 2 cycles

Sampling Artifacts

Sampling rate is related to: Bit depth is related to: Overview of how sampling rate and bit depth affect digital media file quality Sampling rate is related to: Bit depth is related to: digital images image resolution, or number of pixels number of allowable colors in an image digital video number of pixels in the video, frame rate number of allowable colors digital audio sampling rate of the audio (it limits how high the pitch of the audio can be captured) number of allowable levels of amplitude Details will be covered in chapters for each media type.

Now you should be able to answer the following questions Computers store and transmit information using digital data. What exactly is digital data? Is there anything not digital? Why do we bother to learn about anything not digital in a digital media course? What does digitizing mean?

Review Questions

Review Question Digital data is _____ and analog information is _____. A. continuous; discrete B. discrete; continuous B

Review Question Digitization means converting _____ into _____. analog information, digital data

Review Question Converting from analog to digital involves a two-step process: _____ and _____. sampling, quantization

Review Question When analog information is converted to digital data, two properties affect the exactness of the digital representation, one from sampling and one from quantizing. Which of the following is a result of quantization? A. sampling rate B. bit depth B

Review Question Which of the following refers to the number of allowable levels of digitized data? A. sampling rate B. bit depth B

Review Question Which of the following can reduce file size of digital media? (More than one choice) A. decrease sampling rate B. increase sampling rate C. decrease bit depth D. increase big depth A and C