Sound and Digital Audio

1. Sound and Digital Audio From the air to the iPod

2. What is Sound? • Minute disturbances in the air, caused by a vibrating object • Air molecules bunch together, then spread out • Changes in density (air pressure, or sound pressure) • Causes a chain reaction; sound pressure wave propagates

3. Sound Wave Propagation

4. High Sound Pressure Normal Low Time Compression / Rarefaction Time domain plot of a waveform

5. Period: Frequency: How long does one cycle last? How many cycles per second? Periodic Waveforms Amplitude 1 Cycle Expressed in Hertz (Hz) Ex: 440 Hz (the A above middle C) • Period = 1 / Frequency • (for A440: 0.0023 sec.)

6. Frequency and Pitch • Frequency is an acoustic fact.Pitch is a human perception. • Our sense of pitch has a logarithmic relation to frequency — it’s based on ratios. • Our ear is like a microphone. It changes the physical wave (acoustic energy) into an electrical signal. • Range of human hearing • 20Hz to 20,000 Hz (20 kHz), approximately(for young folks; old folks can’t hear as high)

7. The Harmonic Series Arithmetic series of frequencies gives ever-shrinking intervals. (flat) Frequencies in Hz: Double frequency: octave higher

8. Analog Representations of Sound Magnified phonograph grooves, viewed from above: When viewed from the side, channel 1 goes up and down, and channel 2 goes side to side.

9. Analog versus Digital Analog Continuous signal that mimics shape of acoustic sound pressure wave Digital Stream of discrete numbers that represent instantaneous amplitudes of analog signal, measured at equally spaced points in time.

10. Analog to Digital Recording Chain ADC Microphone converts acoustic to electrical energy. It’s a transducer. Continuously varying electrical energy is an analog of the sound pressure wave. ADC (Analog to Digital Converter) converts analog to digital electrical signal. Digital signal transmits binary numbers. DAC (Digital to Analog Converter) converts digital signal in computer to analog for your headphones.

11. Analog to Digital Conversion Instantaneous amplitudes of continuous analog signal, measured at equally spaced points in time. A series of “snapshots”

12. Analog to Digital Overview Sampling Rate How often analog signal is measured [samples per second, Hz] Example: 44,100 Hz Sampling Resolution [a.k.a. “sample word length,” “bit depth”]Precision of numbers used for measurement: the more bits, the higher the resolution. Example: 16 bit

13. Sampling Rate Determines the highest frequency that you can represent with a digital signal. Nyquist Theorem: Sampling rate must be at least twice as high as the highest frequency you want to represent. Capturing just the crest and trough of a sine wave will represent the wave exactly.

14. A high frequency signal sampled at too low a rate looks like … … a lower frequency signal. Aliasing What happens if sampling rate not high enough? That’s called aliasing.

15. Common Sampling Rates Which rates can represent the range of frequencies audible by (fresh) ears? Most software can handle all these rates.

16. 7 6 5 4 3 2 1 0 3-bit Quantization A 3-bit binary (base 2) number has 23 = 8 values. Amplitude Time — measure amp. at each tick of sample clock A rough approximation

17. 14 12 10 8 6 4 2 0 4-bit Quantization A 4-bit binary number has 24 = 16 values. Amplitude Time — measure amp. at each tick of sample clock A better approximation

18. 16-bit Sample Word Length A 16-bit integer can represent 216, or 65,536, values (amplitude points). We typically use signed 16-bit integers, and center the 65,536 values around 0. 32,767 0 -32,768

19. Quantization Noise Round-off error: difference between actual signal and quantization to integer values… Random errors: sounds like low-amplitude noise

20. Common Sampling Resolutions

21. The Digital Audio Stream It’s just a series of sample numbers, to be interpreted as instantaneous amplitudes: one for every tick of the sample clock. Previous example: 11 13 15 13 10 9 6 1 4 9 15 11 13 9 This is what appears in a sound file, along with a header that indicates the sampling rate, bit depth and other things.

22. Audio File Size CD characteristics… - Sampling rate: 44,100 samples per second (44.1 kHz) - Sample word length: 16 bits (i.e., 2 bytes) per sample - Number of channels: 2 (stereo) How big is a 5-minute CD-quality sound file?

23. Audio File Size How big is a 5-minute CD-quality sound file? 44,100 samples * 2 bytes per sample * 2 channels = 176,400 bytes per second 5 minutes * 60 seconds per minute = 300 seconds 300 seconds * 176,400 bytes per second = 52,920,000 bytes = c. 50.5 megabytes (MB)

24. MPEG Compression • MPEG 1-Layer 3 (.mp3) • “Motion Picture Experts Group” • Different levels for different purposes • E.g. MPEG 2 used for DVDs • Takes out parts of the sound signal that humans can’t hear • How does the size change? • Lossy compression

25. MPEG Compression

26. What is an iPod? • It’s a computer! • Look at the von Neumann architecture • I/O • 2-4GB microdrive, click wheel, LCD panel, USB connection, FireWire connection, audio output • Memory • 32MB RAM, 32MB ROM • CPU • Special digital to analog converter chips • Well-designed UI • Stores music in various digital formats • AAC (16 to 320 Kbps), MP3 (32 to 320 Kbps), MP3 VBR, Audible, AIFF, Apple Lossless and WAV

27. iPod Specifications • CPU: ARM7TDMI • Playtime: up to 8 hours • Scroll wheel: solid state touch • Buttons: mechanical • CPU Speed: dual 80 Mhz embedded • Data Path: 32 bit • ROM: 32 MB • Onboard RAM: 32 MB • Screen: 1.67" LCD w/ LED backlight • Maximum Resolution: 1-bit 138x110 • Hard Disk: 1" 4 GB 4200 RPM – holds • 1000 songs. • USB: via Dock Connector • FireWire: via Dock Connector • Audio Out: stereo 16 bit mini • Weight: 0.225 lbs. • Dimensions: 3.6" H x 2.0" W x 0.5" D • OS: iPod OS 1.1 • Introduced: January 2004

28. Let’s Look Inside!