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What is sound?
Sounds are pressure waves
of air. If there wasn't any air, we wouldn't be able to hear sounds.
There's no sound in space.
We hear sounds because our
ears are sensitive to these pressure waves. Perhaps the easiest type
of sound wave to understand is a short, sudden event like a clap.
When you clap your hands, the air that was between your hands is
pushed aside. This increases the air pressure in the space near your
hands, because more air molecules are temporarily compressed into
less space. The high pressure pushes the air molecules outwards in
all directions at the speed of sound, which is about 340 meters per
second. When the pressure wave reaches your ear, it pushes on your
eardrum slightly, causing you to hear the clap.
A hand clap is a short
event that causes a single pressure wave that quickly dies out. The
image above shows the waveform for a typical hand clap. In the
waveform, the horizontal axis represents time, and the vertical axis
is for pressure. The initial high pressure is followed by low
pressure, but the oscillation quickly dies out.
The other common type of
sound wave is a periodic wave. When you ring a bell, after the
initial strike (which is a little like a hand clap), the sound comes
from the vibration of the bell. While the bell is still ringing, it
vibrates at a particular frequency, depending on the size and shape
of the bell, and this causes the nearby air to vibrate with the same
frequency. This causes pressure waves of air to travel outwards from
the bell, again at the speed of sound. Pressure waves from
continuous vibration look more like this:
How is sound recorded?
A microphone consists of a
small membrane that is free to vibrate, along with a mechanism that
translates movements of the membrane into electrical signals. (The
exact electrical mechanism varies depending on the type of
microphone.) So acoustical waves are translated into electrical
waves by the microphone. Typically, higher pressure corresponds to
higher voltage, and vice versa.
A tape recorder translates
the waveform yet again - this time from an electrical signal on a
wire, to a magnetic signal on a tape. When you play a tape, the
process gets performed in reverse, with the magnetic signal
transforming into an electrical signal, and the electrical signal
causing a speaker to vibrate, usually using an electromagnet.
How is sound recorded
digitally?
Recording onto a tape is
an example of analog recording. Audacity deals with digital
recordings - recordings that have been sampled so that they can be
used by a digital computer, like the one you're using now. Digital
recording has a lot of benefits over analog recording. Digital files
can be copied as many times as you want, with no loss in quality,
and they can be burned to an audio CD or shared via the Internet.
Digital audio files can also be edited much more easily than analog
tapes.
The main device used in
digital recording is a Analog-to-Digital Converter (ADC). The ADC
captures a snapshot of the electric voltage on an audio line and
represents it as a digital number that can be sent to a computer. By
capturing the voltage thousands of times per second, you can get a
very good approximation to the original audio signal:
Each dot in the figure
above represents one audio sample. There are two factors that
determine the quality of a digital recording:
Samples:
The
samples contain information telling your computer how
the recorded signal sounded at certain instants in time.
The more samples used to represent the signal, the
better the quality of the recorded sound. For example,
to make a digital audio recording that has the same
quality as audio on a CD, the computer needs to receive
44,100 samples for every second of sound that's
recorded.
1. Sample rate: The rate at
which the samples are captured or played back, measured in Hertz
(Hz), or samples per second. An audio CD has a sample rate of
44,100 Hz, often written as 44 KHz for short. This is also the
default sample rate that Audacity uses, because audio CDs are so
prevalent.
2. Sample format or sample size: Essentially this is the number of digits in the
digital representation of each sample. Think of the sample rate
as the horizontal precision of the digital waveform, and the
sample format as the vertical precision. An audio CD has a
precision of 16 bits, which corresponds to about 5 decimal
digits.
Higher sampling rates
allow a digital recording to accurately record higher frequencies of
sound. The sampling rate should be at least twice the highest
frequency you want to represent. Humans can't hear frequencies above
about 20,000 Hz, so 44,100 Hz was chosen as the rate for audio CDs
to just include all human frequencies. Sample rates of 96 and 192
KHz are starting to become more common, particularly in DVD-Audio,
but many people honestly can't hear the difference.
Higher sample sizes allow
for more dynamic range - louder louds and softer softs. If you are
familiar with the decibel (dB) scale, the dynamic range on an audio
CD is theoretically about 90 dB, but realistically signals that are
-24 dB or more in volume are greatly reduced in quality. Audacity
supports two additional sample sizes: 24-bit, which is commonly used
in digital recording, and 32-bit float, which has almost
infinite dynamic range, and only takes up twice as much storage as
16-bit samples.
Playback of digital audio
uses a Digital-to-Analog Converter (DAC). This takes the sample and
sets a certain voltage on the analog outputs to recreate the signal,
that the Analog-to-Digital Converter originally took to create the
sample. The DAC does this as faithfully as possible and the first CD
players did only that, which didn't sound good at all. Nowadays DACs
use Oversampling to smooth out the audio signal. The quality of the
filters in the DAC also contribute to the quality of the recreated
analog audio signal. The filter is part of a multitude of stages
that make up a DAC.
How does
audio get digitized on your computer?
Your computer has a
soundcard - it could be a separate card, like a SoundBlaster, or it
could be built-in to your computer. Either way, your soundcard comes
with an Analog-to-Digital Converter (ADC) for recording, and a
Digital-to-Analog Converter (DAC) for playing audio. Your operating
system (Windows, Mac OS X, Linux, etc.) talks to the sound card to
actually handle the recording and playback, and Audacity talks to
your operating system so that you can capture sounds to a file, edit
them, and mix multiple tracks while playing.
Waveform
Most people's introduction to audio on a computer
will either be through a video editing program or a specific
audio editing program. In most cases, the audio is represented
in a way similar to below:
This particular waveform shows the two stereo
channels, left on top and right on the bottom. Looking at the
waveform we can see that there is a definite "beat" to the music
here, which is actually a strong bass drum introduction to a
music track. What the waveform is
really showing is the volume of the music at time progresses;
the larger the waveform, the louder the music. From
this we can see that the initial drumbeat in the above example
is louder than the following beats.
This method of displaying audio is a good way of
representing how the audio plays in a visual way and closely
matches how audio is stored in a normal uncompressed WAV file or
on a CD. Uncompressed audio is stored as a sequence of numbers
("samples") that describe how loud each sample is. On a CD there
are 44100 samples every second, so you can see that there are a
lot of numbers to be stored for even short pieces of music.
Standard file
formats for PCM audio
There are two main types
of audio files on a computer:
- PCM stands for Pulse Code
Modulation. This is just a fancy name for the technique
described above, where each number in the digital audio file
represents exactly one sample in the waveform. Common examples
of PCM files are WAV files, AIFF files, and Sound Designer II files. Audacity supports WAV, AIFF, and
many other PCM files.
- The other type is compressed
files. Earlier formats used logarithmic encodings to squeeze
more dynamic range out of fewer bits for each sample, like the
u-law or a-law encoding in the Sun AU format. Modern
compressed audio files use sophisticated psychoacoustics
algorithms to represent the essential frequencies of the audio
signal in far less space. Examples include MP3 (MPEG I,
layer 3), Ogg Vorbis, and WMA (Windows Media
Audio). Audacity supports MP3 and Ogg Vorbis, but not the
proprietary WMA format or the MPEG4 format (AAC) used by Apple's
iTunes.
For details on the audio
formats Audacity can import from and export to, please check out the
File formats section of this documentation. Please remember that MP3
does not store uncompressed PCM audio data. When you create an MP3
file, you are deliberately losing some quality in order to use less
disk space.
File Formats
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Audacity Project format (AUP)
Audacity
projects are stored in an AUP file, which is a format that
has been highly optimized for Audacity so that it can open
and save projects extremely quickly. In order to achieve
this speed, Audacity breaks larger audio files into several
smaller pieces and stores these pieces in a directory with a
similar name as the project. For example, if you name a
project "chanson", then Audacity will create a project file
called chanson.aup
which stores the general information about your project, and
it will store your audio in several files inside a directory
called chanson_data.
The Audacity Project format is not compatible with any other
audio programs, so when you are finished working on a
project and you want to be able to edit the audio in another
program, select
Export.
WAV (Windows Wave format)
This is the
default uncompressed audio format on Windows, and is
supported on almost all computer systems. It can also be
lightly compressed (about 4:1) using the ADPCM codec, but
this is less widely supported on non-windows platforms.
Audacity can read and write this format, including ADPCM on
all platforms.
AIFF (Audio Interchange File Format)
This is the
default uncompressed audio format on the Macintosh, and it
is supported by most computer systems, but it is not quite
as common as the WAV format. Audacity can read and write
this format.
Streaming Audio
There are currently
more than a dozen formats for streaming audio over the Web,
from widely used formats, such as RealNetworks' RealAudio,
streaming MP3, Macromedia's Flash and Director Shockwave,
Microsoft's Windows Media, and Apple's QuickTime, to more
recent entries that synchronize sounds with events on a web
page, such as RealMedia G2 with SMIL and Beatnik's Rich
Music Format (RMF). Also included are a host of downloadable
formats, including Liquid Audio, MP3, MIDI, WAV, and AU.
While the high quality of MP3 has sent shockwaves through
the recording industry, streaming formats like RealAudio
remain the dominant audio technology on the Web right now.
Indeed MP3 is being folded into multimedia streaming formats
like QuickTime and Windows Media.
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Sun Au / NeXT
This is the
default audio format on Sun and NeXT computers, and it is
usually u-law compressed so not very high quality. U-law
compression is a very simple, fast but low quality way to
reduce the size of the audio by about 50%. It is widely used
in American telephone systems.
This format was one of the first audio formats supported by
Web browsers, and it is still often used for short sound
effects where quality is not as important. Audacity can read
this format, and write files either in 8-bit u-law
compressed or 16-bit uncompressed variants.
MP3 (MPEG I, layer 3)
This is a
compressed audio format that is a very popular way to store
music. It can compress audio by a factor of 10:1 with little
degradation in quality. Audacity can both import and export
this format. For more information on how to export MP3 files
from within Audacity, see
Exporting MP3 Files.
Ogg Vorbis
This is a
new compressed audio format that was designed to be a free
alternative to MP3 files. Ogg Vorbis files are not as
common, but they are about the same size as MP3 with better
quality and no patent restrictions. Audacity can import and
export this format.
AAC (Advanced
Audio Coding)
AAC
is a standardized, lossy compression and encoding scheme for
digital audio. Designed to be the successor of the MP3
format, AAC generally achieves better sound quality than MP3
at similar bit rates.
AAC
is also the default or standard audio format for:
Apple's iPhone, iPod, iPad, Nintendo DSi, iTunes, DivX Plus
Web Player, Sony's PlayStation 3 and is supported by Sony's
PlayStation Portable, latest generation of Sony Walkman,
phones from Sony Ericsson, the latest S40 and S60 models
from Nokia, Android based phones, Nintendo's Wii (with the
Photo Channel 1.1 update installed for Wii consoles
purchased before late 2007), and the MPEG-4 video standard.
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What is an MP3 file and how does it differ from WAV and AIFF
files?
MP3 (MPEG II, layer 3) is a
popular format for storing music and other audio. A typical
MP3 file is one tenth the size of the original WAV or AIFF
file, but it sounds very similar. MP3 encoders make use of
psychoacoustic models to, in effect, "throw away" the parts
of the sound that are very hard to hear, while leaving the
loudest and most important parts alone.
Unfortunately, no MP3
encoder is perfect, and so an MP3 file will never sound
quite as good as the original. Still, most people find that
the quality of an MP3 file is virtually indistinguishable
from a CD when played on headphones or on small computer
speakers, which is why the format is so popular. |
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What is mp3?
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MP3s (which stands
for Moving Picture Experts Group, Layer 3) are digital
audio files that are compressed to about 1/10 the size of a CD
recording. Your computer compresses files by scanning them for
repeating patterns of digits. It then replaces these patterns with
smaller codes that take up less space. So, you can compress a CD
song that has 50 megabytes of data into an MP3 that has only 4 or 5
megabytes (1 megabyte = 1,000,000 bytes; 1 byte = the size of 1
computer character, like the letter "a"). As a result, MP3s are
compact enough to send over the Internet with little difference in
sound quality. |
A Brief History. . . .
-
The
German
company
Fraunhofer-Gesellshaft
developed
MP3
technology
and now
licenses
the
patent
rights
to the
audio
compression
technology
- United
States
Patent
5,579,430
for a
"digital
encoding
process".
The
inventors
named on
the MP3
patent
are
Bernhard
Grill,
Karl-Heinz
Brandenburg,
Thomas
Sporer,
Bernd
Kurten,
and
Ernst
Eberlein.
-
In 1987,
the
prestigious
Fraunhofer
Institut
Integrierte
Schaltungen
research
center
(part of
Fraunhofer
Gesellschaft)
began
researching
high
quality,
low
bit-rate
audio
coding,
a
project
named
EUREKA
project
EU147,
Digital
Audio
Broadcasting
(DAB).
-
Two
names are mentioned most
frequently in connection
with the development of
MP3. The Fraunhofer
Institut was helped with
their audio coding by
Dieter Seitzer, a
professor at the
University of Erlangen.
Dieter Seitzer had been
working on the quality
transfer of music over a
standard phone line. The
Fraunhofer research was
led by Karlheinz
Brandenburg often called
the "father of MP3".
Karlheinz Brandenburg
was a specialist in
mathematics and
electronics and had been
researching methods of
compressing music since
1977. In an interview
with Intel, Karlheinz
Brandenburg described
how MP3 took several
years to fully develop
and almost failed.
Brandenburg stated "In
1991, the project almost
died. During
modification tests, the
encoding simply did not
want to work properly.
Two days before
submission of the first
version of the MP3
codec, we found the
compiler error."
-
In
1997, 18-year-old Justin
Frankel of Sedona,
Arizona, created the
first popular MP3
player. Called Winamp,
the player is a computer
program that converts
MP3 audio (sound) files
from computer numbers or
digits into sound waves
you hear through your
computer's speakers.
Today, listening to MP3s
is so widespread, the
most common word used on
all Internet search
engines (sites that find
Web pages to match words
you type in) is "MP3."
And Frankel, a mere 20
years old, is a
multimillionaire!
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How MP3 Works |
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MP3 Myths
There seems to be a great deal of
confusion about what is, or is not, legal regarding music these
days. People don’t seem to know where the line is between enjoying
music from an artist or band that they like, or violating the
copyright protection of that same music. Below is a list of common
myths associated with buying, sharing and listening to digital music
and what the realities are.
Recording Industry Association of
America
Myth No. 1: The RIAA will sue you for
downloading music.
As I mentioned in
a previous column, the RIAA is currently suing only users who
share more than 1,000 songs. This doesn't mean that it is legal to
download copyrighted music without permission of the copyright
owner. It's not (see below). At this time, however, it appears that
the RIAA is not targeting people who download copyrighted music.
They are going after uploaders, not downloaders, which means that as
long as you aren't sharing a significant number of files, the
ongoing purge will pass you by.
Myth No. 2:
Downloading songs for free from the Internet is fine.
Unfortunately, with very few exceptions,
this is untrue. The songs are copyright protected and the owner of
the copyright is owed compensation for the song. If you find music
on the Internet for free, the individual or business sharing the
music is most likely violating the law and if you download the song
without paying for it you will be stealing.
Myth No. 3: Downloading copyrighted material
without permission is legal.
In the interest of fairness, I must point out that downloading
copyrighted material without permission is 100 percent illegal. When
you download an MP3, you make a copy of that file from a remote
location onto your hard drive. Copying the song is the exclusive
right of the copyright holder, and doing so without permission is an
act of copyright infringement.
Myth No. 4:
I can share my
music with friends because I own the CD
The fact that you
purchased a CD entitles you to listen to the music
all you want, but not to share that privilege with
others. You can make a copy of the CD for yourself
in case you damage or lose the original. You can rip
the music from the CD onto your computer or laptop
and convert the music to MP3 or WMA or other formats
and listen to it on portable MP3 players or other
devices. Your purchase of the music entitles you to
listen to it pretty much any way you want, but you
can’t give copies of it to friends or family. I am
not suggesting that you can't *play* the music when
other people are around, but that you can't give
them a copy of the music, in any format, to take
with them when they leave.
Myth No.
5: Its OK though, because I gave my friend the
original CD
You can
sell or give away the original CD,
but only as long as you no longer
have any copies of the music in any
format (unless of course you have
another copy that has been
legitimately paid for). You can not
copy the CD onto your computer and
load MP3’s of it onto your portable
MP3 player, and then give the
original CD to your best friend
because you don’t need it any more.
Think of
it like you bought a couch. You can
use the couch in your living room if
you want. You can move it to a
bedroom if it works better for you
there. You can remove the throw
pillows and use them in a different
room than the couch. But, when you
give the couch to your friend, the
couch is gone. You can’t *both* give
the couch away *and* keep the couch
at the same time, and the music that
you buy should be treated the same
way.
Myth No
6:
It isn’t “stealing”
because I wasn’t going to pay for it
anyway
Some people feel
that because they
would never actually
spend the money to
buy the CD,
illegally copying or
downloading it from
somewhere else
really isn’t costing
the artist or the
industry any money.
Along these same
lines, some people
may copy or download
music to try and
decide if they like
it enough to buy it,
and just never get
around to buying it.
However, sites like
Amazon.com now have
clips or samples
available to listen
to of virtually
every song on every
CD available. Rather
than crossing the
ethical line, you
should just visit a
site like this and
play the clips to
help you make your
purchasing decision.
In the end, you may
very well find that
you would rather buy
just one or two
songs for $1 each
rather than spending
$15 for a CD filled
mostly with music
you don’t care for.
Case Studies /
Lawsuits
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