Digital Audio
                                                                                   • Representation of audio...
Audio signal as a series of bits                                       Sampling
                                          ...
Low sampling rate Vs High sampling rate
                                                                                 Q...
Digital to Analog Converter                                                                Modern audio hardware

Converti...
Human Hearing Sensitivity                                   Human Frequency Masking
• Experiment: Put a person in a quiet ...
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ITMA09 Introduction To Digital Audio II

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ITMA09 Introduction To Digital Audio II

  1. 1. Digital Audio • Representation of audio signal in digital ITMA 09 form for processing by digital circuits (or Introduction to Audio II computers). Krates Ng Fall 2008 11/17/2008 10:28 AM Krates Ng 2 Binary numbers Can you convert the following from binary to decimal? • In the computer world, there is only “1” and “0” (or “on” and “off”) hence the binary system. • 111 powers of 2 23 22 21 20 • 1011 Equivalent decimal values 8’s 4’s 2’s 1’s Sample 4-bit binary 1 0 1 1 How to solve 8 + 0 + 2 + 1 = 11 (decimal) • 1101 11/17/2008 10:28 AM Krates Ng 3 11/17/2008 10:28 AM Krates Ng 4 Dec. to Bin. by long division Bits and Bytes • e.g. 156(10) to Binary 2) 156 0 • 1 byte = 8 bits Steps: 2) 78 0 1. Write the integer answer (quotient) under the long 2) 39 1 division symbol, and write the remainder (0 or 1) to the right of the dividend. Word size Power of 2 No. of values 2) 19 1 2. Continue downwards, dividing each new quotient 2) 9 1 by two and writing the remainders to the right of 8-bit (1 byte) 28 256 each dividend. Stop when the quotient is 1. 2) 4 0 3. Starting with the bottom 1, read the sequence is 16-bit (2 bytes) 216 65,536 1’s and 0’s upwards to the top. 2) 2 0 24-bit (3 bytes) 224 16,777,216 1 = 10011100(2) 32-bit (4 bytes) 232 4,294,967,296 11/17/2008 10:31 AM Krates Ng 5 11/17/2008 10:28 AM Krates Ng 6 1
  2. 2. Audio signal as a series of bits Sampling • Analog sounds recorded and digitized using an Analog-to-digital Converter (ADC). • A sample of the instantaneous amplitude of the waveform is taken at regular time intervals (called the sampling rate). • The sampling rate is responsible for the frequency response of the digitized sound. 11/17/2008 10:28 AM Krates Ng 7 11/17/2008 10:28 AM Krates Ng 8 Nyquist Theorem • A Swedish researcher. • The theorem states that the sampling rate must be greater than twice the highest frequency (bandwidth) we wish to reproduce. • This frequency is often called the Nyquist frequency. • Fail to do that will result “aliasing”. 11/17/2008 10:28 AM Krates Ng 9 11/17/2008 10:28 AM Krates Ng 10 Aliasing Standard sampling rates Rate Uses 32K Older equipment 44.1K CD, DAT 48K DAT 96K Digital recording equipment 192K Digital recording equipment 11/17/2008 10:28 AM Krates Ng 11 11/17/2008 10:28 AM Krates Ng 12 2
  3. 3. Low sampling rate Vs High sampling rate Quantization • Samples taken are then assigned numeric values that the computer or digital circuit can use or store in a process called quantization. • The number of available values is determined by the number of bits (0's and 1's) used for each sample, also called bit depth or bit resolution . • When a sample is quantized, the instantaneous snapshot of its analog amplitude has to be rounded off to the nearest available digital value. This rounding-off process is called approximation. 11/17/2008 10:28 AM Krates Ng 13 11/17/2008 10:28 AM Krates Ng 14 Quantization Error Dynamic range • The smaller the number of bits used per sample, the greater the distances the analog values need to be • The greater the magnitude of approximation errors, the greater the level of digital or quantizing noise rounded off to. The difference between the analog produced. The solution to reducing digital noise is value and the digital value is called the to use larger sample sizes (greater bit depth). approximation or quantizing error as shown in the • rule of thumb is an added 6 dB of dynamic range illustration below: for every additional bit used per sample. • The original CD standard proposed by Sony was for a 14-bit sample size, with a dynamic range of only 84 dB, but was changed to 16 bits before inception. 11/17/2008 10:28 AM Krates Ng 15 11/17/2008 10:28 AM Krates Ng 16 Low and High bits per sample • The CD/DAT standard 16-bit samples, with their impressive 65,536 values for quantizing, provide the theoretical playback system optimum of a 96 dB dynamic range. 11/17/2008 10:28 AM Krates Ng 17 11/17/2008 10:28 AM Krates Ng 18 3
  4. 4. Digital to Analog Converter Modern audio hardware Converting digital bits to analog sound signals. • Creative latest X-Fi sound card •24-bit ADC at 96KHz sample rate. •24-bit DAC at 96KHz to analog 7.1 speaker output. •24-bit DAC of stereo digital sources at 192kHz to stereo output •16-bit to 24-bit recording sampling rates: 8, 11.025, 16, 22.05, 24, 32, 44.1, 48 and 96kHz 11/17/2008 10:28 AM Krates Ng 19 11/17/2008 10:28 AM Krates Ng 20 Digital Audio Tape Deck Common audio file formats • Tascam DA-45HR • .aif or AIFF (audio interchange file format) • .ra or .ram (Real Audio) •20 bit A/D, 20 bit D/A. • .wav or Microsoft WAVE •Recording resolution: SP 16-bit, HR 24-bit. • WMA (Windows Media Audio) •Recording sample rate: 44.1KHz, 48KHz. • AAC (Advanced Audio Coding) •Fading out. Giving ways to computer-based equipment. • MPEG (Motion Pictures Expert Group) – .mp3 (MPEG I-Audio layer 3 compression) – And watch out for MPEG-4… 11/17/2008 10:28 AM Krates Ng 21 11/17/2008 10:28 AM Krates Ng 22 Audio compression basics Psychoacoustics • Two main categories • Principles of the human perception of sound. – Lossless, e.g. .zip • MPEG compression algorithm uses model of human – Lossy, e.g. JPEG hearing to remove data (perceptual coding algorithm). • Traditional lossless compression methods usually • Frequency range is about 20Hz to 20kHz, most don’t work well on audio (and image) compression. sensitive to 2 to 4 kHz. • Thour characteristics: • Dynamic range (quietest to loudest) is about 96db. – Compression ratio, e.g. 12:1 • Normal voice range is about 500Hz to 2 kHz. – Bit rate (average no. of bits that 1 sec. of audio will • Low frequencies: vowels (元音), bass. consume, usually in kbps) – Speed (compressing, uncompressing) • High frequencies: consonants (子音). – Quality 11/17/2008 10:28 AM Krates Ng 23 11/17/2008 10:28 AM Krates Ng 24 4
  5. 5. Human Hearing Sensitivity Human Frequency Masking • Experiment: Put a person in a quiet room. Raise • Experiment: Play 1 kHz tone (masking tone) at fixed level (60db). Play test tone at a different level (e.g. 1.1 kHz), and raise level until just level of 1 kHz tone until just barely audible. Vary distinguishable. the frequency and plot: • Vary the frequency of the test tone and plot the threshold when it becomes audible. 11/17/2008 10:28 AM Krates Ng 25 11/17/2008 10:28 AM Krates Ng 26 Frequency Masking Temporal Masking Repeat for various frequencies of masking tones: • If we hear a loud sound, then it stops, it takes a little while until we can hear a soft tone nearby. • Experiment: Play 1 kHz masking tone at 60 dB, plus a test tone at 1.1 kHz at 40 dB. Test tone cannot be heard (it is masked). Stop masking tone, then stop test tone after a short delay. • Adjust delay time to the shortest time when test tone can be heard (e.g. 5ms). • Repeat with different level of the test tone and plot: Frequency masking on critical band scale: 11/17/2008 10:28 AM Krates Ng 27 11/17/2008 10:28 AM Krates Ng 28 Combination Reference • Total effect of both frequency and temporal • MPEG’s web site masking: – http://www.mpeg.org 11/17/2008 10:28 AM Krates Ng 29 11/17/2008 10:28 AM Krates Ng 30 5

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