principles of digital audio

Download Principles of Digital Audio

Post on 15-Feb-2016

55 views

Category:

Documents

0 download

Embed Size (px)

DESCRIPTION

Principles of Digital Audio. Analog Audio. 3 Characteristics of analog audio signals: Continuous signal single repetitive waveform Infinite sound propagates as long as oscillator is activated Measured in voltage . Digital Audio. 3 Characteristics of digital audio: - PowerPoint PPT Presentation

TRANSCRIPT

Principles of Digital Audio

Principles of Digital AudioSAnalog Audio3 Characteristics of analog audio signals:Continuous signal single repetitive waveformInfinite sound propagates as long as oscillator is activatedMeasured in voltage Digital Audio3 Characteristics of digital audio:Discrete made up of numerous samples taken from a continuous analog signalFinite has a defined start and end point in timeMeasured in binary digits (0 and 1)Binary SystemBinary code is a coding system using messages made up of strings of the digits 0 and 1 1 binary digit (or a single instance of 0 or 1) is referred to as 1 bit (or binary digit)8 bits = 1 byteDigital ConversionAnalog signals can be converted to digital signals using an analog to digital converter (ADC)Takes numerous measurements of a signal at regular time pointsIn order to hear a digital signal it must be converted back into an analog signal, using an analog to digital converter (DAC)Take the sampled digital audio and converts it into a continuous signal to be output through loudspeakers, headphones, etc.Digital ConversionWhat is the benefit of converting analog signals into digital signals?Eliminates the need for physical oscillators and large cumbersome electronic equipmentAllows for more accurate timing in processingAllows for more refined processing methods without the need for external hardware (such as a physical delay module, flanger, phaser, pitch shifter, etc.)SamplingThe digital conversion process makes use of the process of sampling, or taking measurements of a signal at regular points in time (see analog to digital converter on previous slide)In the sampling process, a continuous signal is divided into equal segments. Segments are then reassembled inside various digital platforms (computer programs, audio recorders, sampling units, etc.) and create a recorded representation of the original signal.SamplingINSERT PITCURE OF SAMPLED AUDIO SIGNALCharacteristics of Digital Audio2 measurements characterize digital audio signals:Sampling rate/sample frequencyQuantization (also known as bit depth)

Sampling rate how often an audio signal is sampledMeasured in samples per second, the more samples per second, the accurate the representation of the original signal.Bit rate/Quantization number of bits per sample Refers to the overall resolution and dynamic range of a signal. Higher bit depth yields wider dynamic range, lower bit depth results in limited dyanmic range (1 bit audio results in an on or off signal)Nyquist TheoremTo digitally represent a signal containing frequency components up to X Hz, the sampling rate must be at least 2X HzMaximum frequency perspective refers to sampling frequency in reference to the highest frequency in the signal, for example:The maximum frequency sampled at SR is SR/2 Hertz. The 2x frequency is also called the Nyquist frequency

Types of SamplingUndersamplingUndersampling contains frequency content that is beyond the sampling rateIn undersampling there are actually frequencies in the original signal that are not captured and represented in the sampled digitized version of the signalIn other words, undersampling is bad and results in digital distortion and aliasingUndersamplingINSERT IMAGE OF UNDERSAMPLING AND DIGITAL PHOTOGRAPHY ALIASINGCritical SamplingCritical sampling is when the sample rate and the highest frequency in the original signal are the same valueThis may capture the original signal with no problem, but could result in distortion, aliasing and foldoverBecause the original signal hits 0 points (when the sample rate and signal are at the same frequency)Foldover when the signal moves more quickly or as quickly as the sampling rateCritical SamplingINSERT IMAGE OF CRITICAL SAMPLING HEREOversamplingWhen the sampling rate is higher than the highest frequency in the original signalOversampling captures all audible frequencies in the original signal and some that are outside the audible frequency band.Oversampling is the most desirable method of samplingSample rates: 44.1 kHz, 48 kHz, 96 kHzCD audio sampling rate is 44.1 kHzHistory of Sampling RateEarly digital master recordings were stored on magnetic video tapeBits were stored as black and white pixels on the tape The sampling rate was determined with the following figures:525 lines per frame with 35 blank lines = 490 lines per frame3 samples are stored on each lineTape is taken at 60 fields per second, at 245 lines per frame (490/2)So, 3 x 245 x 60 = 44100 (in other words, 44.1 kHz)QuantizationResolution with each sample is recordedDependent of how many bits are available to represent the signal dataDetermines the amount of original signal to amount of unwanted noiseThe unwanted noise is referred to as quantization error, and is unavoidable in the digital conversion processQuantization error is also referred to as the signal-to-noise ratioSignal-to-Noise RatioRatio of original signal to amount of quantization error

Dependent on the nature of the audio contentQuantization error is less noticeable in high-level signalsIt is more obvious in low-level signals. Why? Because low-level signals dont use all available bits in the conversion process?Less bits means the signal-to-error level is greater and quantization error becomes audible Problems with quantization error result in digital distortion and low-level noise (well talk about how to remove this from recordings later) DitherDither low-level noise added to signal before it is sampledWhat? Adding noise to the signal?Adds random error to the signal. Transforms quantization error into added noise, and makes noise becomes a constant factor.This noise can be removed to the point that it is not audible, but still removes quantization error from low-level recordings, helping them to sound cleaner with less system noise