Digital Audio Concepts and Signal Representation

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Questions and Answers

What sampling rate is used in G.711 audio compression for ISDN phone connections?

48 kHz

44.1 kHz

16 kHz

8 kHz (correct)

What is the main advantage of using non-linear PCM over linear PCM?

It has a lower bit rate.

It allows for higher sample rates.

It requires more storage space.

It reduces the quantization error for quieter sounds. (correct)

Which of the following is NOT a typical use case for uncompressed audio formats?

Recording audio in a studio

Editing audio for film

Processing audio for live sound

Streaming audio over the internet (correct)

How would you calculate the size of a stereo audio file that is 10 minutes long at a sample rate of 44.1 kHz and 16 bits per sample?

10 minutes * 44.1 kHz * 2 * 16 bits Signup and view all the answers

Which audio file format is NOT considered openly lossy compression?

FLAC Signup and view all the answers

What is a primary difference between analog and digital audio signals?

Analog signals represent continuous quantities, while digital signals represent discrete values. Signup and view all the answers

What is the purpose of sampling in digital audio?

To convert a continuous signal into a series of numerical values. Signup and view all the answers

Which of the following best describes aliasing in audio processing?

When higher frequency signals are incorrectly represented as lower frequencies. Signup and view all the answers

Which sampling frequency is generally considered adequate for high-quality audio?

44.1 kHz Signup and view all the answers

How is an analog signal converted into a digital format?

Through an analog-to-digital converter. Signup and view all the answers

What does the sampling interval refer to in audio signal processing?

The amount of time between each audio sample. Signup and view all the answers

What is the benefit of using float sample formats over int sample formats in digital audio?

Float formats provide a greater dynamic range for audio signals. Signup and view all the answers

What does an electric signal represent in audio processing?

A continuous variation in voltage. Signup and view all the answers

What is the primary purpose of a digital-to-analog converter (DAC)?

Convert digital signals to analog signals Signup and view all the answers

According to the Nyquist-Shannon sampling theorem, what is the minimum sampling frequency required for a signal with a maximum frequency of 500 Hz?

1000 Hz Signup and view all the answers

What phenomenon occurs when frequencies in a signal exceed the Nyquist frequency?

Aliasing Signup and view all the answers

Which of the following sampling frequencies is commonly used for CDs?

44,100 Hz Signup and view all the answers

Why is it incorrect to assume that higher sampling rates always lead to better audio quality?

There is a limit to human hearing frequency perception Signup and view all the answers

What is typically the effect of quantization in digital audio processing?

Results in loss of information Signup and view all the answers

Which of the following statements about digital signals is accurate?

Digital signals are defined only at sample points Signup and view all the answers

What is the main disadvantage of analog signals compared to digital signals?

Analog signals exhibit noise and distortion Signup and view all the answers

Study Notes

Digital Audio

Sound Design - Games and Multimedia, presentation by Miguel Negrão
Concepts: Digital audio, signal, analog signal, electric signal, audio signal, analog audio signals, digital audio
Signal: Any time-varying quantity.
Analog Signal: A continuous signal representing physical quantities (e.g., pressure).
Electric Signal: A variation of voltage, a flow of electric charge (usually electrons).
Audio Signal: A signal with frequencies between 20Hz and 20,000Hz (audible to humans).
Analog Audio Signals: Signal at microphone output, signal from electric guitar, signal to loudspeaker.
Electroacoustics: Includes microphones and loudspeakers; further details to be discussed later.

Signal Representation & Conversion

Digital Audio: Store and manipulate audio signals as numbers.
Sampling: Converting a continuous signal to a sequence of numbers, representing instantaneous signal values.
Sampling Interval (or Period, T): The time between samples (unit: seconds).
Sampling Frequency (f): Number of samples per second (unit: Hertz).
Formula: f = 1/T
Analog-to-Digital Converter (ADC): Device converting analog to digital.
Digital-to-Analog Converter (DAC): Device converting digital to analog.
Digital-to-Analog Conversion: Digital signal to staircase signal to output sound card (with filter).

Sampling Theorem

Nyquist-Shannon Sampling Theorem: For an exact conversion and reproduction, the sampling rate needs to be at least twice the highest frequency in the signal
Bandwidth: Difference between highest and lowest frequencies of the signal. Typical audio for human hearing has lowest freqeuncy as 0.
Aliasing: Signal reconstruction error occurs when the sampling frequency is not high enough. Frequencies above half the sampling frequency will fold or overlap incorrectly. It produces an undesirable reproduction.
Higher Sampling Rates: Provide more headroom for analog filters in DAC; not necessary for modern methods

Digital Representation & Sample Quantization

Pulse-Code Modulation (PCM): Sampling method where samples are quantized to a grid.
PCM Format (Standard): Standard for audio in computers, CDs, and digital telephony.
Linear PCM: Quantization levels are linearly uniform. Problematic for softer sounds.
Non-linear PCM: Different quantization methods for better resolution in quieter parts of the signal (e.g., G.711 used in ISDN).
Number of Bits: Higher number of bits increases the precision in recording samples.
Quantization Error/Noise: The difference between the original sample value and the quantized representation; higher number of bits lead to less quantization noise.
Dynamic Range: Difference between smallest and largest value; quantization noise limits dynamic range of digital audio; approximately 6dB per bit.

Important Values & Advice

Dynamic Range in Audio: Loudest sound relative to noise floor (Signal-to-Noise Ratio - SNR).
Audio File Formats:
- Uncompressed: WAV, AIFF, CAF
- Lossless Compression: FLAC
- Lossy Compression: MP3, AAC, OGG
Dithering: Technique introducing noise to push quantization noise into the frequency regions of less sensitivity, thus increasing the perceived dynamic range.
Recommendation/Best Practice:
- Recording: Use 24-bit samples for recording, 32-bit floating-point for audio processing, 44.1kHz or 48kHz.
- Exporting: Appropriate sampling rate (44.1kHz, 48kHz, 22.050Hz)
- Video: Export and record at 48kHz
32-bit Floating Point: Used internally by DAWS, larger dynamic range ~1528 dB.
Decibels Relative to Full Scale (dBFS): Used for amplitude ratios in digital audio; 0 dBFS = 1.0 represents the maximum value; other values are negative.

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Description

Explore the fundamentals of digital audio and signal processing in multimedia and game design. This quiz covers key concepts including analog and digital signals, sound design, and fundamental audio principles. Test your knowledge on sampling and the conversion of sound into digital formats.