Chapter 4: Sound - Comprehensive Notes

Chapter 4: Sound

Introduction to Sound

• Vibrations in the air create pressure waves, which are perceived as sound.
• Sound waves vary in:
  • Sound pressure level (amplitude).
  • Frequency or pitch.
• Acoustics: The branch of physics studying sound.
• Sound pressure levels (loudness or volume) are measured in decibels (dB).

Digital Audio

• Digital audio data represents sound stored as samples.
• Samples represent the amplitude (loudness) of sound at a specific time.
• Digital recording quality depends on the sampling rate (frequency), i.e., the number of samples per second.
• Common sampling frequencies in multimedia:
  • CD-quality: 44.1 kHz
  • 22.05 kHz
  • 11.025 kHz
• Sample size: The number of bits used to describe the amplitude of a sound wave when sampled.
• Digital audio is device independent.
• Quantization: Rounding off each sample's value to the nearest integer.
• Crucial aspects of preparing digital audio files:
  • Balancing sound quality with available RAM and hard disk resources.
  • Setting appropriate recording levels for high-quality and clean recording.
• Post-recording editing is almost always needed.
• Basic sound editing operations:
  • Trimming.
  • Splicing and assembly.
  • Volume adjustments.
  • Working on multiple tracks.
• Additional sound editing operations:
  • Format conversion.
  • Resampling or downsampling.
  • Fade-ins and fade-outs.
  • Equalization.
  • Time stretching.
  • Digital signal processing.
  • Reversing sounds.
• Audio resolution determines the accuracy with which sound can be digitized.
• Size of a monophonic digital recording: $sampling \ rate \times (bit \ resolution/8) \times 1$
• Size of stereo recording: $sampling \ rate \times duration \ of \ recording \ in \ seconds \times (bit \ resolution/8) \times 2$

MIDI Audio

• MIDI (Musical Instrument Digital Interface) files are small, so they load and play quickly on web pages.
• The length of a MIDI file can be changed without affecting the pitch or degrading audio quality.
• Working with MIDI requires knowledge of music theory.
• MIDI is a shorthand representation of music stored in numeric form; it is not digitized sound.
• Creating MIDI scores requires sequencer software and a sound synthesizer.
• MIDI is device dependent.

MIDI Versus Digital Audio

• MIDI is analogous to structured/vector graphics, while digital audio is analogous to bitmapped images.
• MIDI is device dependent; digital audio is device independent.
• MIDI files are much smaller than digitized audio files.
• MIDI files sound better than digital audio files when played on a high-quality MIDI device.
• MIDI struggles with spoken dialog playback, while digital audio handles it easily.
• MIDI lacks consistent playback quality, while digital audio provides consistent playback quality.
• MIDI requires music theory knowledge, while digital audio does not.

Recording and Editing Digital Audio

• System sounds are assigned to system events (startup, warnings, etc.).
• Examples:
  • Macintosh: glass, indigo, laugh.
  • Windows: start.wav, chimes.wav, chord.wav.
• Multimedia sound: Digitally recorded audio or MIDI music.

Audio File Formats

• A sound file’s format is a methodology for organizing data bits of digitized sound into a data file.
• Macintosh:
  • Digitized sounds can be stored as data files, resources, or applications (e.g., AIFF or AIFC).
• Windows:
  • Digitized sounds are typically stored as WAV files.
• CD-ROM/XA (Extended Architecture) format enables multiple recording sessions on a single CD-R (recordable) disc.
• Linear Pulse Code Modulation is used for Red Book Audio data files on consumer-grade music CDs.
• MP3 compression saves space.
• MP4 is used for streaming audio and video together.
• ACC (Advanced Audio Coding) is used by Apple’s iTunes store.

Adding Sound to Multimedia Projects

• Must determine:
  • File formats compatible with multimedia authoring software.
  • Delivery mediums.
• Study sound playback capabilities offered by end users’ systems.
• Decide the type of sound (background music, sound effects, spoken dialog).
• Select digital audio or MIDI data based on the location and time of use.
• Steps:
  • Create or purchase source material.
  • Edit sounds to fit the project.
  • Test sounds to ensure proper timing with the project.
• Professional sound:
  • Compression reduces space, but reliability may suffer.
  • Downsampling conserves space by reducing the number of samples per second.
  • File size of digital recording (in bytes) = $sampling\ rate \times duration \ of \ recording \ (in \ secs) \times (bit \ resolution/8) \times number \ of \ tracks$
• Record on inexpensive media instead of directly to disk to prevent hard disk overload.
• Ensure equipment and standards align with project requirements.
• Maintain a high-quality database storing the original sound material.
• Keeping track of sounds with audio CDs.
• Red Book (ISO 10149) standard:
  • Standard for digitally encoding high-quality stereo.
  • Digital audio sample size: 16 bits.
  • Sampling rate: 44.1 KHz.
  • High-quality sound requires substantial disk storage space.
• Sound for mobile and internet usage must be considered.
• Web browsers need instructions for handling downloaded file types.
• Test sound and image synchronization regularly.
• Animation and computer-based video playback speed depends on the user’s CPU.
• Evaluate the sound’s RAM requirements and the user’s playback setup.
• Do not record or use copyrighted material without securing appropriate rights.

Summary

• Vibrations in the air create pressure waves perceived as sound.
• Multimedia system sound is digitally recorded audio or MIDI music.
• Digital audio data represents sound stored as samples.
• MIDI is a shorthand representation of music stored in numeric form.
• Digital audio provides consistent playback quality.
• MIDI files are much smaller than digitized audio files.
• MIDI files sound better than digital audio files when played on a high-quality MIDI device.