Chapter 02 audio recording - part i

Date: 08 November 2010

Chapter 02: Audio RecordingPrepared by: Nur Muizz Mohamed Salleh

Talk Layout

✤ Overview of Analogue and Digital Audio

✤ Characteristic of Waveform

✤ Digital Audio Fundamental

✤ Physical Components of Audio

Overview of Analog and Digital Audio

✤ Sound comprises the spoken word, voice, music and even noise.

✤ It is loud and quiet, it is deep and shrill.

✤ Sound is produced by a source that creates vibrations in the air.

✤ Analogue sound consists of continuous waves.


✤ Acoustics and analog audio technology are mainly concerned with continuous mathematical functions, but digital audio is a study of discrete values.

✤ Specifically, a waveform's amplitude is represented as a series of numbers. That is an important first principle, because numbers allow us to manage audio information very efficiently


✤ Using digital techniques, the capability to process information is greatly enhanced.

✤ The design nature of audio recording, signal processing, and reproducing hardware has followed the advance of digital technology; the introduction of software programming into the practical audio environment has been simply revolutionary.


✤ Function of Sound:

✤ Provide cognitive information:

✤ Related to mental processes of knowledge, reasoning, memory, judgment and perception

✤ Outer orientation – space, time, situation

✤ Provide affective information:

✤ Related to emotion, feeling and mood

✤ Inner orientation – energy, structure


✤ Function of Sound: - cont

✤ Sound has undeniable power:

✤ To direct our attention to

✤ Shape our interpretation of an image

✤ To create expectations

✤ To underscore an idea or an emotion

Characteristic of Waveform

✤ A waveform is essentially the graphic representation of a sound-pressure level or voltage level as it moves through a medium over time.

By nature, the sound that we hear is analogue sound.


✤ In short, a waveform lets us see and explain the actual phenomenon of wave propagation in our physical environment and will generally have the following fundamental characteristics:

✤ Amplitude✤ Frequency✤ Velocity✤ Wavelength✤ Phase


✤ These characteristics allow one waveform to be distinguished from another.

✤ The most fundamental of these are amplitude and frequency.

✤ The following sections describe each of these characteristics.

Amplitude and frequency ranges of human hearing.


✤ Amplitude

✤ The distance above or below the centerline of a waveform represents the amplitude level of that signal.

✤ The greater the distance or displacement from that centerline, the more intense the pressure variation, electrical signal level, or physical displacement will be within a medium.

✤ Waveform amplitudes can be measured in several ways (Figure). For example, the measurement of either the maximum positive or negative signal level of a wave is called its peak amplitude value (or peak value).

Graph of a sine wave showing the various ways to measure amplitude.


✤ Amplitude - cont

✤ Power or intensity of the sound.

✤ The size of a sound wave.

✤ The louder the sound, the higher the amplitude.

✤ Measures the strength of a sound.

✤ Unit of measurement = decibel / dB


✤ Frequency

✤ The rate at which an acoustic generator, electrical signal, or vibrating mass repeats within a cycle of positive and negative amplitude is known as the frequency of that signal.

✤ One completed excursion of a wave (which is plotted over the 360∘ axis of a circle) is known as a cycle (Figure)

Cycle divided into the 360∘ of a circle.


✤ Frequency - cont

✤ It is measured in cycles per second.

✤ Standard measurement = Hertz / Hz

✤ Humans are capable of hearing frequencies from 20Hz to 20,000Hz.

✤ Frequency also perceive as pitch : the relative tonal highness or lowness of a sound.


✤ Velocity

✤ The velocity of a sound wave as it travels through air at 68∘F (20∘C) is approximately 1130 feet per second (ft/sec) or 344 meters per second (m/sec).

✤ This speed is temperature dependent and increases at a rate of 1.1 ft/sec for each Fahrenheit degree increase in temperature (2 ft/sec per Centigrade degree).

✤ Changes significantly depends on high or low temperatures or the level of humidity in wetter or drier air.


✤ Wavelength

✤ Determined by the distance a sound wave travels to complete one cycle of compression (push) and rarefaction (pull)

✤ Frequency and wavelength change inversely with respect to each other.

✤ The lower a sound frequency, the longer its wavelength; the higher a sound’s frequency, the shorter its wavelength.

Wavelengths decrease in length as frequency increases (and vice versa).


✤ Phase

✤ Time relationship between two or more sound wave at a given point in their cycles.

✤ eg: Whenever two waveforms having the same frequency, shape, and peak amplitude are completely in phase (meaning that they have no relative time difference), the newly combined waveform will have the same frequency, phase, and shape...but will be double in amplitude (Figure).

The amplitudes of in-phase waves increase in level when mixed together.


✤ Phase - cont

Waves of equal amplitude cancel completely when mixed 180∘ out of phase.

When partial phase angles are mixed together, the combined signals will add

in certain places and subtract in others.

Digital Audio Fundamental

✤ Conversion Process

✤ To the computer, analogue sound is like a foreign language.

✤ Sound too has to be converted into a digital format. This is where the sound card comes in.

✤ Sound cards = devices that convert analogue sound to digital sound.


✤ Conversion Process - cont

✤ The process of converting from analogue sound to digital sound is performed by the Analogue-to Digital Converter (ADC) and later the re-conversion back to analogue sound from digital sound is performed by the Digital-to-Analogue Converter (DAC).

ADC to DAC for Sound


✤ Digital Audio

✤ Digital audio is made up of binary codes.

✤ Binary codes are represented by 0s and 1s.

✤ To work effectively with digital audio, it is important to understand some basic terminology and properties of digital audio.

digital audio sampling


✤ Digital Audio- cont

✤ Understanding these terms will help in making informed decisions about how to compress digital audio for the web and streaming.

✤ The three basic properties of digital audio are:

✤ channels✤ sample rate ✤ bit-depth


✤ Digital Audio

✤ Channel✤ A stereo audio signal has two discreet channels of audio whereas a mono signal has only one. ✤ A stereo signal can be mixed down to a mono signal. ✤ An uncompressed stereo digital audio file will be

twice the size of a mono file.


✤ Digital Audio - cont

✤ Sample Rate✤ Sample rate is the number of samples of a sound that are taken per second to represent the event digitally. ✤ The more samples taken per second, the more

accurate the digital representation of the sound can be.

✤ For example, the current sample rate for CD-quality audio is 44,100 samples per second.



✤ Sample Rate - cont

✤ Sample rate is the number of samples of a sound that are taken per second to represent the event digitally.

✤ The more samples per second that are taken, the more accurate the digital representation is of the analog waveform.

✤ Each sample is really only a number that contains the amplitude value of a waveform measured over time. This brings us to bit-depth.

analog signal

and resulting sample signal



✤ Bit Depth

✤ Bit depth defines how many bits are used to describe each of the samples taken from the sample rate .

✤ It describes the potential accuracy of a digital audio file.

✤ Higher bit depth audio will sound better than smaller bit-depth audio. 8 and 16-bit audio are currently the most common sample sizes.

✤ 8-bit audio takes up less hard drive space but is inherently noisier than 16 or 24 bit-depth audio. CD quality is 16-bit.


✤ To estimate the size of a digital sound file, a few several points need to be consider:

i. Sampling rates✤ A higher sampling rates means more samples. The more the

samples there are, the more storage will be needed.✤ CD quality = 44100Hz / Broadcasting = 11025Hz

ii. Bit Depth✤ The higher the bit sampling, the larger the file size.


✤ To estimate the size of a digital sound file, a few several points need to be consider:

iii. Channel✤ Mono: Sound will be playing from one channel✤ Stereo: Sound will be playing from two channel

iv. Duration✤ The longer the duration of the sound, the larger the file size.


✤ How to calculate the size of a digital sound file?

✤ Example:

How much storage space is needed if you want to record a 16-bit, 44.1 kHz, stereo music for a duration of 30 seconds?


✤ How to calculate the size of a digital sound file?

✤ Answer:

✤ Step 1:

44 100 x 2 bytes (or 16-bits) = 88 200 bytes

✤ Step 2:

88 200 x 2 (for stereo) = 176 400 bytes

✤ Step 3:

176 400 x 30 seconds = 5 292 000 bytes

So, recording CD quality music (16-bit, 44.1 kHz, stereo) for 30 seconds will take approximately 5.292 Mbs of storage space.

Physical Components of Audio✤ Basic Audio Recording Components:

✤ Source of analog sound - voice or cassette✤ A good computer system✤ A good sound card or integrated audio on your

computer✤ Quality audio software✤ Stereo cables✤ A good quality dynamic microphone with XLR

connector✤ A high quality of speaker or headphone

THANK YOUQ & A Session

Chapter 02 audio recording - part i

Technology

Transcript of Chapter 02 audio recording - part i