Difference between revisions of "MTD2 class 9"
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Nyquist Theorem -- The sampling frequency determines the high frequency limit. | Nyquist Theorem -- The sampling frequency determines the high frequency limit. | ||
| − | Fh = 2 | + | Fh = Fs/2 |
Fh if the highest frequency you want to record and Fs is the Nyquist Freqency or Sample Rate (I can;t get the wiki math functions working yet) | Fh if the highest frequency you want to record and Fs is the Nyquist Freqency or Sample Rate (I can;t get the wiki math functions working yet) | ||
Revision as of 16:00, 28 March 2006
Contents
In class
- Review and discuss rough movie edits.
- Digital Theory 2
- Listen to Beethoven exmaples (Sample Rate and Bit Depth)
- Discuss Digital Errors
- Look at sampling circuit
- Quiz Three Next
- The quiz will cover week digital theory 1Audio Digitization 2 and chapter 3 from Sound Design for Interactive Multimedia
Three types of digital errors
- Quantization Error - Difference between actual analog value and quantized digital value
- Aliasing - Undersampling
- Jitter - Time errors
Quantization
Quantization Signal Processing
Discrete levels based on the number of bits
Analogy
Analog is like a bucket of water while digital is like a bucket of ball bearings. Analog has an infinite resolution while a digital system can only resolve thins to finite values.
"...if sheets of typing paper were stacked 22 feet, a single sheet of paper would represent one quatization level in a 16-bit system. In a 20-bit system, the stack would reach 349 feet. In a 24-bit system, the stack would tower 5592 feet in height - over a mile high." -Principals of Digital Audio, Ken C. Pohlmann McGraw Hill
- The more bits you have the more dynamic range you have. Each bit is equal to aporximately 6 more dB. 16 dB s about 120 dB of dynamic range.
Quantization Error
Quantization error = the difference between the actual analog value and the quantized digital value.
The less bits that you have to quantize a signal the larger the quantization error will be. More Bits = Less Quantization Error
Quantization Error is alos known as Signal
Dither
Dither = noise added to signal
addition of a small amount of white noise to the signal to reduce signal-to-error distortion ...also aid in processing and bit depth conversion... see dither article
READ dither Article
http://www.digido.com/ditheressay.html
Aliasing
Aliasing = error caused by under sampling -- cars wheels in movies
Sampling Frequency/Aliasing
Get smapling gif sampling1.gif (39564 bytes)
http://en.wikipedia.org/wiki/Nyquist-Shannon_sampling_theorem
Nyquist Theorem -- The sampling frequency determines the high frequency limit.
Fh = Fs/2
Fh if the highest frequency you want to record and Fs is the Nyquist Freqency or Sample Rate (I can;t get the wiki math functions working yet)
Sampling frequency = discrete time frequency of the sampler
44.1 kHz = CD audio
square wave @ 20KHz won't make it w/ a 44.1 kHz sampler
Need a brick wall filter at the imput of the samplig circuit with a cut off frequency of 1/2 the smapling rate
Jitter
Error due to clock synchronization...
Signal Flow
low pass filter (ideally a brick wall filter) --> AD Converter --> DA converter --> low pass filter
pcm.gif (23735 bytes)
READ Jitter Article
http://www.digido.com/jitteressay.html
Digitizer
Premiere Audio Demo
How to make Sound forge default editor for Premiere
Flash Tracing Demo
How to trace still images in flash
Homework
- Work on Audio for Storyboard movie
- Post another version of the exported movie
- Quiz Three Next week
