MTD2 class 9

In class

• Review and discuss rough movie edits.
• Digital Theory 2
• Listen to Beethoven examples (Sample Rate and Bit Depth)
• Discuss Digital Errors
• Look at sampling circuit
• Quiz Three Next

Three types of digital errors

• Quantization Error - Difference between actual analog value and quantized digital value
• Aliasing - Undersampling
• Jitter - Time errors

Quantization

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 quantization 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

1. The more bits you have the more dynamic range you have. Each bit is equal to approximately 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 also 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

Aliasing

Aliasing = error caused by under sampling -- cars wheels in movies

Sampling Frequency/Aliasing

Get smapling gif sampling1.gif (39564 bytes)

Nyquist Theorem -- The sampling frequency determines the high frequency limit.

Fh = Fs/2

[itex]f_s > 2 f_H\,[/itex]   (Nyquist rate)

Fh if the highest frequency you want to record and Fs is the Nyquist Frequency 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 input of the sampling circuit with a cut off frequency of 1/2 the sampling 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)

Premiere Audio Demo

Remember that you can make premier open sound files for editing in Sound Forge by changing the default application for all the extensions.

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
1. review for MTD2 Quiz 3