计算原理导论. Introduction to Computing Principles 智能与计算学部刘志磊

Transcription

1 计算原理导论 Introduction to Computing Principles 天津大学 智能与计算学部刘志磊

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3 Analog The world is basically analog What does that mean? "Signal" is a varying wave over time e.g. sound as a running example here

4 How Does Sound Work? Sound is vibrations, small waves of pressure in the air (e.g. violin) the wooden violin top flexes in and out, a signal The movement of the wood creates the same signal waves in the air The waves travel through the air, ultimately pushing on the eardrum, recreating the signal From signal goes from one medium to the next, but keeping its basic shape Related to the word analogy

5 Oscilloscope ( 示波器 ) Oscilloscope connects to wires, observes the electrical signal varying over time, and draws that signal on its screen in real time. Voltage is on the vertical axis, and time is the horizontal axis. Oscilloscope takes in something pretty abstract -- signal varying over time -- and makes it visual. Sound Demo

6 Sound: Frequency, Amplitude, Timbre Frequency: how often the wave repeats per second "higher" note = more cycles per second --Middle "A" on the piano cycles --Higher "A" note, one octave up cycles --Even higher "A", another octave up, 1760 cycles Amplitude: how high/low the wave goes, bigger waves = louder Timbre: the shape of the wave sounds different Low C note, sine wave C note, one octave higher (double the cycles per second) Same C note as left, but now a flute timbre instead of sine wave.

7 Sound: Frequency, Amplitude, Timbre Sum behavior of analog: Low C note, sine wave high-c note, 3 octaves above the low-c the combination of low-c and high-c Low C note, sine wave C note one octave higher the combination of low-c and c

8 Sound: Frequency, Amplitude, Timbre Sum Facts Chord -- peaks and valleys match up nicely, sounds naturally good (e.g. double the frequency) Discord -- peaks and valleys don't match up, sounds bad Noise-cancelling headphones -- detect the ambient sound and create an exactly opposite sound, so when added together, it all cancels out to near silence.

9 Analog and Noise -- Hiss Signal does not go through microphones, wires, magnets, etc. perfectly Each step -- the wires, the magnets, etc. can introduce little errors "Noise" -- like little random wiggles around the true signal Noise can sound like "hiss".. e.g. AM radio, cassette tapes Pure signal Signal + noise Why do digital translations of music sound so much better?

10 Digital Technology -- Digitization Horizontal zero line, down the middle "Sample" (look at) the signal +- vs. the zero line Sample very quickly, e.g. 44,000 times per second audio CD Each sample records the height of the signal from 0 as a number CD audio, each sample is in the range -32, ,768 (2 bytes) Reduce signal to a series of "sample" numbers: 1003, 1720, 1939,... Small problem: signal is between 1002 and have to pick one or the other

11 Digital Playback Have a series of digital sample numbers (CD, MP3).. how get sound back? Playback = recreate signal from the numbers Look at each number, plot that height above/below zero line Repeat for next number Connect the dots.. draw the signal between the dots Digital-to-Analog.. chip specialized to do this ("D/A converter")

12 Digital vs. Errors Samples are recorded as 0's/1's on, say, an audio CD What does that signal look like for playback? What does that signal look like for playback with noise? Notice that, even with noise, the 0's/1's are obvious This is why digital playback reliably sounds good compared to analog Read back digital signal, ideal, 0/1 Read back digital signal in reality, with noise

13 Digital -- Compression Compression = apply cleverness to change the data to use fewer bytes Suppose we have this series of samples: 12000, 12002, 12006, 12007, 12010, 12005, The numbers are quite near to each other -- a typical result What if we just record the first number Then record the differences, one sample to the next (aka delta) 12000, +2, +4, +1, +3, -5, +1 The numbers to record are much smaller (fewer bytes to store) This is an example of compression (lossless) Another scheme: Drop every other number. On playback, guess the missing samples as the average of the surrounding numbers. This saves 2x space This is a "lossy" strategy.. sound is not coming back quite perfect MP3 audio compression.. very effective, about 10x less space, but it's lossy

14 Images JPEG Images JPEG = Joint Photographic Expert Group JPEG is a free and open standard for storing digital images JPEG is a "lossy" format, with different quality levels (q1, q2,, q10) Some commonly understood standard format for what is "an image" is needed, and JPEG is the mostly widely used one. Original image, q7, ~48KB Image saved at q5, ~29KB Image saved at q1, ~14KB

15 Images JPEG Images Block artifacts: JPEG divides the image into 8x8 blocks. If the compression is very high, you can see the block boundaries. Edge artifacts/noise: JEPG has a hard time with crisp edges between two colors. In a more-compressed version, little "noise" speckles or distortions can appear to either side of the hard edge. Original image, q7, ~48KB Image saved at q5, ~29KB Image saved at q1, ~14KB

16 Images How Many Bytes? == How many pixels of an image? E.g. : An image with resolution of has 120,000 pixels, each pixel takes 3 bytes (RGB), that's 360,000 bytes, or about 360 KB. If we double the width, so the image is , how many more bytes is that? Why Use Compression? A digital camera typically uses JPEG compression internally as it writes each image to your flash memory card. The higher the compression, the more images will fit in on the card. Web Page Images Image compression speeds up the web page visiting Images only look noticeably bad at very aggressive compression levels -- q2 or q3

17 Images GIF and PNG Images GIF == Graphics Interchange Format PNG == Portable Network Graphics "lossless" image formats, recording every pixel exactly GIF was popular early on with the Internet. However parts of it were patented, but the patents was not enforced. When the patent was enforced more aggressively, the community created the free and open and PNG standard as an alternative, and it also performs a little better than GIF.

18 Audio Formats MP3 MP3 is lossy format MP3 is patented, and legitimately MP3 uses complex and non-obvious techniques to get its excellent 10x compression while still sounding good. If a device or software plays or produces MP3s, a license fee is due to the patent holders, on the order $1-$2 per instance. AAC A more modern form of MP3 used by Apple with their dominant ipod/itunes Ogg Vorbis A free and open standard and royalty free Typically performs better than MP3 Bad name

19 Video Formats MPEG (Motion Picture Experts Group) MEPG standardizes some video formats in the industry, MPEG-LA (Licensing Authority) handles collecting patent royalties MPEG-2 is used in DVDs and some satellite TV systems. MPEG-4 and particular the "h.264" compression system is very good at producing good looking video with the minimum bytes. Open Video Format - WebM A true, free and open video codec to compete with h.264 in support of Internet HTML5 Video Tag Flash Video

20 Thank You! Q&A