Chap 3 _data Acquisition Part 2

7/28/2019 Chap 3 _data Acquisition Part 2

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Chapter 3 PART 2Data Acquisition

SITI NORMAZIAH IHSAN


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Representing Digital Images

The result of sampling and quantisation is a matrix of

real numbers as shown in Fig.3.3, Fig.3.4. and Fig 3.5. The values of the coordinates at the origin are (x,y)

= (0,0).

The next coordinate values along the first row are

(x,y) = (0,1). The notation (0,1) is used to signify the 2nd sample

along the 1st row.


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Fig. 3.3. Coordinate convention used to representdigital images


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Fig. 3.4. A digital image of size M x N


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It is advantageous to use a more traditional matrixnotation to denote a digital image and its

elements.

Fig. 3.5 A digital image


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The number of bits required to store a digitizedimage is

b = M x N x k

Where M & N are the number of rows and columns,respectively.

The number of gray levels is an integer power of 2:

L = 2k where k =1,2,24

It is common practice to refer to the image as a k-bit image


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The spatial resolution of an image is the physical size

of a pixel in that image; i.e., the area in the scenethat is represented by a single pixel in that image.

Dense sampling will produce a high resolutionimage in which there are many pixels, each of

which represents of a small part of the scene. Coarse sampling, will produce a low resolution

image in which there are a few pixels, each ofwhich represents of a relatively large part of the

scene.


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Fig. 3.6 Effect of resolution on image interpretation (a) 8x8

image. (b) 32x32 image 256x256 image


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Fig.3.7 Effect of quantisation on image interpretation. (a) 4levels. (b) 16 levels. (c) 256 levels


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Data Capture / Acquisition Device

Analog to Digital Data Conversion

Sampling & Quantization

Image Data Sampling

Audio Data Sampling

Video Data Sampling


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Audio Data Sampling Digital audio uses pulse-code modulation and

digital signals for sound reproduction.

This includes analog-to-digital conversion (ADC),digital-to-analog conversion (DAC), storage, and

transmission. In effect, the system commonly referred to as digital

is in fact a discrete-time, discrete-level analog of aprevious electrical analog.

While modern systems can be quite subtle in theirmethods, the primary usefulness of a digital system isthe ability to store, retrieve and transmit signalswithout any loss of quality.


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Sampling Rate When it is necessary to capture audio covering the

entire 2020,000 Hz range of human hearing, suchas when recording music or many types of acousticevents, audio waveforms are typically sampled at

44.1 kHz (CD), 48 kHz (professional audio), or 96 kHz.The approximately double-rate requirement is aconsequence of the Nyquist theorem.

There has been an industry trend towards sampling

rates well beyond the basic requirements; 96 kHzand even 192 kHz are available.


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This is in contrast with laboratory experiments, whichhave failed to show that ultrasonic frequencies areaudible to human observers; however in some

cases ultrasonic sounds do interact with andmodulate the audible part of the frequencyspectrum (intermodulation distortion).

It is noteworthy that intermodulation distortion is not

present in the live audio and so it represents anartificial coloration to the live sound.

One advantage of higher sampling rates is thatthey can relax the low-pass filter design

requirements for ADCs and DACs, but with modernoversampling sigma-delta converters thisadvantage is less important.


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Bit depth (quantization) Audio is typically recorded at 8-, 16-, and 20-bit depth,

which yield a theoretical maximum signal toquantization noise ratio (SQNR) for a pure sine wave of,approximately, 49.93 dB, 98.09 dB and 122.17 dB.

Eight-bit audio is generally not used due to prominentand inherent quantization noise (low maximum SQNR),although the A-law and u-law 8-bit encodings packmore resolution into 8 bits while increase total harmonicdistortion.

CD quality audio is recorded at 16-bit. In practice, notmany consumer stereos can produce more than about90 dB of dynamic range, although some can exceed100 dB.


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Thermal noise limits the true number of bits that canbe used in quantization.

Few analog systems have signal to noise ratios (SNR)exceeding 120 dB; consequently, few situations will

require more than 20-bit quantization. For playback and not recording purposes, a proper

analysis of typical programme levels throughout anaudio system reveals that the capabilities of well-engineered 16-bit material far exceed those of the

very best hi-fi systems, with the microphone noiseand loudspeaker headroom being the real limitingfactors.


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Data Capture / Acquisition Device

Analog to Digital Data Conversion

Sampling & Quantization

Image Data Sampling

Audio Data Sampling

Video Data Sampling


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Video Data Sampling Standard-definition television (SDTV) uses either 720

by 480 pixels (US NTSC 525-line) or 704 by 576 pixels(UK PAL 625-line) for the visible picture area.

High-definition television (HDTV) is currently movingtowards three standards referred to as 720p(progressive), 1080i (interlaced) and 1080p(progressive, also known as Full-HD) which all 'HD-Ready' sets will be able to display.


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Speech Sampling Signals intended to carry only human speech, can

usually be sampled at a much lower rate.

For most phonemes, almost all of the energy iscontained in the 5Hz-4 kHz range, allowing asampling rate of 8 kHz.

This is the sampling rate used by nearly all telephonysystems, which use the G.711 sampling andquantization specifications.

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Chap 3 _data Acquisition Part 2