Thanks for asking!
At the introductory level, to understand digitization, its helpful to
backtrack slightly and cover analog recording. Edison's and Berliner's
early phonograph and gramophone recorded analog soundwaves by etching
them into soft metal cylinders and/or records. Playback was
accomplished by drawing a needle through the recording tracks, using a
diaphragm to amplify the sound waves (vibrations) created by the
needle scraping against the track. Scraping metal also produces it's
own scratchy sound, which was also preserved in these early
recordings. "Also, if you play a phonograph repeatedly, eventually it
will wear out -- when the needle passes over the groove it changes it
slightly (and eventually erases it)." [1,A]
Why bother with the analog part of digitization? Because the
technology comes full circle in the production of CDs and DVDs, which
will be covered further on.
"In a CD (and any other digital recording technology), the goal is to
create a recording with very high fidelity (very high similarity
between the original signal and the reproduced signal) and perfect
reproduction (the recording sounds the same every single time you play
it no matter how many times you play it).
To accomplish these two goals, digital recording converts the analog
wave into a stream of numbers and records the numbers instead of the
wave. The conversion is done by a device called an analog-to-digital
converter (ADC). To play back the music, the stream of numbers is
converted back to an analog wave by a digital-to-analog converter
(DAC). The analog wave produced by the DAC is amplified and fed to the
speakers to produce the sound." [1,B]
Graphs and Diagrams of these waveform conversions help clarify the process:
How Stuff Works - Digital Data
http://entertainment.howstuffworks.com/analog-digital3.htm
"The conversion of an analog signal to a digital signal requires two
distinct processes, sampling and quantisation.
Sampling (and oversampling) is concerned with the capture of an
analogue quantity at a certain instant in time." [2,A]
See diagrams and mathmatical formulae that demonstrate the sampling process:
http://httpd.chello.nl/~m.heijligers/DAChtml/Digital%20Theory/Digital%20theory.html#sampling
"Oversampling is used during the analog to digital (A/D) and digital
to analog (D/A) conversion processes in a digital recorder, sampler or
playback device. Essentially, the sampling rate of the converter is
multiplied to a very high rate (i.e. 4x oversampling puts the rate at
176.4 kHz)." [3]
Oversampling reduces phase distortion, and results in a drop in noise.
Jitter is a further component (error) of the sampling process in
analog-to-digital conversion. Jitter is a short term instability of
the amplitude and/or phase of a signal. "Jitter occurs when digital
audio loses sync. Along with the actual audio data, digital audio
signals (either S/PDIF or AES/EBU) carry a synchronization bit that
keeps all the devices attached to that audio signal on the same clock.
However, poor connections, either from just poor circumstances or too
many machines slaved off the main digital output (e.g. - patchers in a
chain at a show), can cause jitter to occur as the sync is lost." [4]
"Quantisation (plus dither and noise-shaping) is concerned with the
representation of this quantity by a digital word of finite length."
[2,A]
"Quantisation of a signal is nothing more complex than rounding it to
the nearest whole number. If the input to a quantiser is between p and
p+0.5, then we round down to p; if the input lies between p +0.5 and
p+1 then we round up to p+1. In general, the output step size (or
quantum) need not be unity, but can be of arbitrary size q; in this
case p is not an integer, but a whole multiple of q." [2,B]
"Quantisation error manifests itself as noise at high signal levels.
However, quantisation errors become quite significant when a low-level
signal approaches the level of the LSB, then the quantizing error
actually becomes the signal, and therefore is an audible component of
the output. Fortunately, in practical systems this adverse effect can
be effectively eliminated through the use of dither.
Dither is the process of adding low-level analog noise d ( n ) to a
signal x ( n ), to randomize the quantizer's small-signal behaviour.
The quantisation error becomes decorrelated from the signal." [2,C]
The noise shaping process is used to reduce the perceived amount of
added noise. Noise shaping algorithms are generally standardized, so
the differences between various techniques are minimized.
Dithering and Noise Shaping are discussed in greater technical detail
in a 1994 article written by Christopher Hicks:
Digital Theory
The Application of Dithering and Noise-Shaping to Digital Audio
http://httpd.chello.nl/~m.heijligers/DAChtml/Digital%20Theory/n_shape.txt
ADC Devices/Methods
-------------------
All Analog-to-Digital-Conversion devices have transistors. Various
methods are outlined below:
Flash ADC system (Direct Conversion)
"The 8-bit flash ADC starts sampling the summed signals at WNG1, and
continues until ca 400 ns after the leading edge of WNG2. The time
after WNG2 is needed to get the complete shape for the last minibunch.
The data is stored in a FIFO (figure 3). The sampling rate is 20 Mhz,
i.e. one sample each 50 ns. If the maximum sampling time is 750 ns
(Bunchtrain length) + 400 ns = 1150 ns, there is in total 24 32 bit
words for each VSAT event." [5]
Figure 3
http://www.quark.lu.se/~jum/minibunches/node29.html#FADCreadoutlogichard
Ramp-Compare ADC
"A ramp-compare ADC produces a saw-tooth signal that ramps up, then
quickly falls to zero. When the ramp starts, a timer starts counting.
When the ramp voltage matches the input, a comparator fires, and the
timer's value is recorded. Timed ramp converters require the least
number of transistors. The ramp time is sensitive to temperature
because the circuit generating the ramp is often just some simple
oscillator. There are two solutions: use a clocked counter driving a
DAC and then use the comparator to preserve the counter's value, or
calibrate the timed ramp. A special advantage of the ramp-compare
system is that comparing a second signal just requires another
comparator, and another register to store the voltage value." [6]
Successive-Approximation ADC
A successive-approximation ADC addresses the digital ramp ADC's
shortcomings using a very special counter circult known as a
successive-approximation register. "Instead of counting up in binary
sequence, this register counts by trying all values of bits starting
with the most-significant bit and finishing at the least-significant
bit." [7] These are one of the more complex ADC designs.
"A delta-encoded ADC has an up-down counter that feeds a digital to
analog converter (DAC). The input signal and the DAC both go to a
comparator. The comparator controls the counter. The circuit uses
negative feedback from the comparator to adjust the counter until the
DAC's output is close enough to the input signal. The number is read
from the counter. Delta converters have very wide ranges, and high
resolution, but the conversion time is dependent on the input signal
level, though it will always have a guaranteed worst-case. Delta
converters are often very good choices to read real-world signals.
Most signals from physical systems do not change abruptly. Some
converters combine the delta and successive approximation approaches;
this works especially well when high frequencies are known to be small
in magnitude." [8]
"A pipeline ADC (also called subranging quantizer) uses two or more
steps of subranging. First, a coarse conversion is done. In a second
step, the difference to the input signal is determined with a digital
to analog converter (DAC). This difference is then converted finer,
and the results are combined in a last step. This type of ADC is fast,
has a high resolution and only requires a small die size." [9,A]
Commercial Analog to Digital converters
"These are usually integrated circuits.
Most converters sample with 6 to 24 bits of resolution, and produce
fewer than 1 megasample per second. Mega- and gigasample converters
are available, though (Feb 2002); megasample converters are required
for digital video editing. Commercial converters usually have ±0.5 to
±1.5 LSB error in their output.
The most expensive part of an integrated circuit is the pins, because
that makes the package larger, and each pin has to be connected to the
integrated circuit's silicon. To save pins, it's common for ADCs to
send their data one bit at a time over a serial interface to the
computer, with the next bit coming out when a clock signal changes
state, say from zero to 5V. This saves quite a few pins on the ADC
package, and in many cases, does not make the overall design any more
complex. (A notable exception is in connecting the converters to
microprocessors which use memory-mapped IO.)
Commercial ADCs often have several inputs that feed the same
converter, usually through an analog multiplexer. Different models of
ADC may include sample-and-hold circuits, instrumentation amplifiers
or differential inputs, where the quantity measured is the difference
between two voltages." [9,B]
If you'd like to see an example (including transistors) of an ADC
construct your own simple ADC, this tutorial provides descriptions,
instruction, circuit diagrams and photographs of the completed
circuit.
Building An Analog/Digital Converter Circuit (ADC)
http://www.pages.drexel.edu/~dwk24/ADC.htm
SEE ALSO:
Design Of A High-Performance Analog-To-Digital Converter
Ground-up design of a Pipeline Analog-To-Digital Converter, including
discussion and diagrams of circuit design.
http://www.commsdesign.com/main/9810/9810feat2.htm
Desktop Media - Digital Audio
Details equipment, hardware and software required for analog to
digital audio conversion.
http://www.jtmedia.com/dtmp/audio.html
Dynamic Specification for Sampling A/D Converters
http://www.national.com/an/AN/AN-769.pdf
Circuit Diagrams - ADC
http://www.electronic-circuits-diagrams.com/computersimages/computersckt2.shtml
http://arsenal.media.mit.edu/media/gsr-adc.jpg
http://turquoise.wpi.edu/ISCAS99_2/node2.html
Circuit Symbols of Electronic Components
http://www.kpsec.freeuk.com/symbol.htm
Digital Recording Media
----------------------------------------------------------------------
Remember the tinfoil?
"A CD is a fairly simple piece of plastic, about four one-hundredths
(4/100) of an inch (1.2 mm) thick. Most of a CD consists of an
injection-molded piece of clear polycarbonate plastic. During
manufacturing, this plastic is impressed with microscopic bumps
arranged as a single, continuous, extremely long spiral track of data.
We'll return to the bumps in a moment. Once the clear piece of
polycarbonate is formed, a *** THIN, REFLECTIVE ALUMINUM LAYER *** is
sputtered onto the disc, covering the bumps. Then a thin acrylic layer
is sprayed over the aluminum to protect it. The label is then printed
onto the acrylic." [10,A]
See the CD cross-section diagram at:
http://entertainment.howstuffworks.com/cd1.htm
A CD has a single, spiral track of data, circling from the inside of
the disc to the outside. The track is approximately 0.5 microns wide,
with separation of 1.6 microns between tracks. The elongated bumps
that make up the track are each 0.5 microns wide, a minimum of 0.83
microns long and 125 nanometers high.
Data Storage in CDs
"Data storage in CD format is not a simple thing. Typically, a user
pictures the "1s" and "0s" in the memory of the computer as being
directly transferred to "pits" and "bumps" on the CD disk.
Unfortunately, it is far from that easy.
To begin with the incoming data is subjected to a series of coding
operations. These coding operations add a number of additional parity
bits to the data for error detection and correction purposes. The data
is also subject to an interleaving process (which means that adjacent
data on the disk is not adjacent data from the incoming file).
Additionally, the physical form of the data is changed (EFM coding) to
eliminate the possibility of adjacent "1s". (This is done because it
is the edges of the pit -- not the pit itself -- that represent l's in
the data stream.)" [11]
Storing Data on a DVD
"DVDs are of the same diameter and thickness as CDs, and they are made
using some of the same materials and manufacturing methods. Like a CD,
the data on a DVD is encoded in the form of small pits and bumps in
the track of the disc. A DVD is composed of several layers of plastic,
totaling about 1.2 millimeters thick. Each layer is created by
injection molding polycarbonate plastic. This process forms a disc
that has microscopic bumps arranged as a single, continuous and
extremely long spiral track of data...
Once the clear pieces of polycarbonate are formed, a thin reflective
layer is sputtered onto the disc, covering the bumps. Aluminum is used
behind the inner layers, but a semi-reflective gold layer is used for
the outer layers, allowing the laser to focus through the outer and
onto the inner layers. After all of the layers are made, each one is
coated with lacquer, squeezed together and cured under infrared light.
For single-sided discs, the label is silk-screened onto the
nonreadable side. Double-sided discs are printed only on the
nonreadable area near the hole in the middle." [12,A]
Data Storage CD vs. DVD
DVDs can store more data than CDs because they store digital data in a
higher density format, because there is less overhead than needed for
the older CD "instructions" area, and because DVDs are capable of
multi-layer data storage.
See the complete set of articles at How Stuff Works:
Introduction to How DVDs Work
The Basics
DVD Advantages
Storing Data on a DVD
It's Truly Tiny
Data Storage: DVD vs. CD
Multi-Layer Storage
The DVD Video Format
Frame This!
DVD Audio
Sound Check
The DVD Player
Reading a DVD
Features
Other Features
Video Outputs
Audio Outputs
Models
Connecting the DVD Player
Video
DVDs and Laser Discs
FAQ
How Stuff Works - How DVDs Work, by Karim Nice
http://entertainment.howstuffworks.com/dvd.htm
For a step-by-step tutorial on the Digital Video Process, I recommend
this comprehensive guide from the Video Development Initiative (ViDe).
Copyright restrictions prevent display of the Guide here, however, you
can print a personal copy from links provided in the Web version. Use
the drop-down Table of Contents menu to navigate.
Digital Video for the Next Millennium - Digital Video on Demand
http://www.vide.net/resources/whitepapers/video/1.shtml
See Also:
Digital Video Tutorial
http://www.monet.k12.ca.us/Challenge/VStudio_Class/digital_video_tutorial.htm
Digital Video Glossary of Terms
http://www.marcpeters.co.uk/glossary.html
Digital Video Production Process (Illustrated)
Handbook of Digital Publishing
http://www.printerport.com/kdp/hbdp/new/A-D.sec.pdf
Wikipedia - Digital Signal Processing
http://en.wikipedia.org/wiki/Digital_signal_processing
The Scientist and Engineer's Guide to Digital Signal Processing
Downloadable Textbook (Free)
http://www.dspguide.com/
References
----------------------------------------------------------------------
[1] How Analog and Digital Recording Works
How Stuff Works, Marshall Brain
http://entertainment.howstuffworks.com/analog-digital.htm
A - Analog Wave
http://entertainment.howstuffworks.com/analog-digital2.htm
B - Digital Data
http://entertainment.howstuffworks.com/analog-digital3.htm
[2] Introduction to Digital Audio
Marc Heijligers and the DAC group
http://httpd.chello.nl/~m.heijligers/DAChtml/Digital%20Theory/Digital%20theory.html
A - Digital Signals vs. Analog Signals
http://httpd.chello.nl/~m.heijligers/DAChtml/Digital%20Theory/Digital%20theory.html#digan
B - Quantisation
http://httpd.chello.nl/~m.heijligers/DAChtml/Digital%20Theory/Digital%20theory.html#quantisation
C - Dither
http://httpd.chello.nl/~m.heijligers/DAChtml/Digital%20Theory/Digital%20theory.html#dither
D - Noise Shaping
http://httpd.chello.nl/~m.heijligers/DAChtml/Digital%20Theory/Digital%20theory.html#noiseshaping
[3] The Tech-T Glossary - Oversampling
http://www.technical-t.com/Tglossm-s.html
[4] Kaiyen's Trading Glossary
http://www.kaiyen.com/trading/pages/other_pages/glossary.html
[5] Ulf Mjoernmark, University of Lund, 1995
The VSAT bunch train scheme - Flash ADC system
http://www.quark.lu.se/~jum/minibunches/minibunches.html
[6] Encyclopedia4U Analog To Digital Converter
Ramp-Compare Converter
http://www.encyclopedia4u.com/a/analog-to-digital-converter.html
[7] All About Circuits - Chapter 13
Digital Analog Conversion - Successive-Approximation ADC
http://www.allaboutcircuits.com/vol_4/chpt_13/6.html
[8] Science Daily Encyclopedia - Analog to Digital Converters
http://www.sciencedaily.com/encyclopedia/analog_to_digital_converter
[9] Wikipedia - Analog/Digital Converters
http://en.wikipedia.org/wiki/Analog_to_digital_converter
A - ADC Structures
http://en.wikipedia.org/wiki/Analog_to_digital_converter#ADC_structures
B - Commercial Analogy To Digital Converters
http://en.wikipedia.org/wiki/Analog_to_digital_converter#Commercial_Analog_to_Digital_converters
[10] How Stuff Works - How CDs Work, by Marshall Brain
http://entertainment.howstuffworks.com/cd.htm
A - Understanding the CD Material
http://entertainment.howstuffworks.com/cd1.htm
[11] Audio Compact Disk - Writing and Reading the data, Kelin J. Kuhn
http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio2/95x7.htm
[12] How Stuff Works - How DVDs Work, by Marshall Brain
http://entertainment.howstuffworks.com/dvd.htm
A - Storing Data on a DVD
http://entertainment.howstuffworks.com/dvd3.htm
Search Strategy
----------------------------------------------------------------------
Google Search Terms:
digital audio process
digital video process
ADC circuits
audio digital converter
I hope you find the information and links useful.
Best regards,
---larre |
