Download Digital Audio Workstations and Analog to Digital Conversion

Digital Audio Workstations and Analog to Digital Conversion
Adam White
Department of Computer Science (CIS)
University Of Wisconsin-Platteville
[email protected]
Abstract
The conversion of real, analog signals that can be heard by the human ear, into digital
audio signals that can be understood by a computing machine is crucial for many
Americans to sustain the high quality of life we live in. Whether this is a person talking
into a telephone or a microphone, the same principals apply. In both cases, an analog
signal must be converted into digital format to be sent before being converted back to an
audible analog signal. Without this conversion, the world of telephones and music
recording would not be the same. To complete this process, a digital to analog converter
and an analog to digital converter are required to translate the signals to their proper
form. This paper will take a look into the process of changing analog signals into digital
format as it relates to audio recording. It will also examine Digital Audio Workstations
(DAW) which is a combination of microphones, cables, analog to digital converts, and
some sort of high powered computer.
Basics of Signal Conversion
To get a better understanding on how DAW works, it is important to first understand the
basic components of data conversion. This concept can apply to more then just audio
signals. In all data conversion, there must be a sender. This could be a host computer or
an analog audio signal for example. This will go through some sort of communication
controller to do some conversions. It will be in charge of serialization. Serialization is
converting bytes into streams of bits. It will need to be converted into bits in order to be
sent over a medium. In computer recording, an Analog to Digital Converter (ADC) must
be used to digitize the audio signal. After it has been put into digital form, it is possible to
manipulate the data. Before the receiver, such as a terminal, can interpret data, it must be
deserialized, or convert the bits back into bytes. In terms of audio signals, a Digital to
Analog Converter (DAC) must be used so that a human can interpret the data. DACs can
be found in CD players, mp3 players, and in sound cards [1].
Sender
Analog
to
Digital
Data
Manipulation
Digital
to
Analog
Receiver
Figure 1: Flow of data
Background on DAW
Digital Audio Workstations (DAW) is usually used to describe the combination of a Mac
or PC computer, highly advanced software, and hardware known as an audio interface.
The audio interface is used to take an analog signal and turn it into a digital signal that
can be processed by the computer. This audio interface is known as an Analog to Digital
Converter (ADC). The ADC acts like a common sound card but it offers higher sound
quality along with more functionality then a common sound card. It is usually found as a
external to the computer often in rack mount form. Most DAWs include computers with
high-end sound cards, fast CPUs, large amounts of RAM, and plenty of hard disk space.
There are two different kinds of DAWs: computer based DAWs and integrated DAWs.
With the computer based DAWs, the audio interface, software, and computer are all
separate stand-alone devices. While Computer based DAWs are more prevalent today, it
is important to examine integrated Digital Audio Workstations. An integrated DAW
combines all of the necessary components such as computing power, software, analog to
digital conversion, and a graphical user interface all in the same unit. The integrated
style was more popular in the 1980’s through the 1990’s but has become less prevalent
with the computing power that is now affordably offered in personal computers.
Using a DAW allows the software to act like a mixer. It will allow for the volumes to be
adjusted for each track, panning to be added, tone to be adjusted, and effects to be added.
DAWs offer the ability to edit, copy, paste, undo, and other features that are usually
associated with most computers. This is a something recordings could not easily offer
before the coming of DAWs. By using DAWs, it is possible to “forecast” what the track
would sound like. A DAW also has the ability to splice segments of audio together into a
single audio track. Using DAWs has allowed for multi-track recording. This makes it
possible to record multiple instruments at the same times. These features, combined with
numerous others, have greatly improved recordings from the time or analog recordings.
Before DAWs, musicians would need to get the tracks exactly as they want the on the
first take [2].
History of DAWs
In 1970, Bob Ingebretsen and Jim Youngber used a custom software package called
“Digital Audio Processor” to do digital editing and adding effects. They used an
oscilloscope that was connected to a separate computer and then made edits by typing a
three-letter command into the computer. The first DAW system available to consumers
was introduced in 1987 by a company called DigiDesign came out with a program called
Sound Tools. The program was a software and hardware package available for
Macintosh users that could do multi-track recording and editing. It would later be
replaced by Pro Tools, which is still used today and considered by many to be the
“industry standard”. It was not until 1992 that Microsoft Windows saw its first DAW
system with a company named Soundscape Digital Technology [2]. Pro Tools and other
Digital Audio Workstations will be talked about later, but first, it is important to take a
glance at how the audio interface works.
Converting Analog to Digital
Real world analog signals must be converted into digital signals to be interpreted and
manipulated by digital equipment. Digital signals are not just used for conversion for
computers to understand. For example, audio signals that are to be sent through a
telephone need to be converted into a digital signal to be sent and then converted back
into an analog signal so the person on the other end of the connection can hear the sound.
This is the same process that is done when recording audio signals with DAWs. An
audio interface is used to convert the signal from analog to digital and when the song is
played back, and digital to analog conversion will take place.
Figure 2: Conversion of audio
One benefit associated with digital signals is noise reduction. An analog signal can
assume any value and therefore, other noises can be introduced into the signal. Since
digital signal are put into binary code there is less unwanted noise entered into the signal.
Another benefit of digital signal is data compression.
Sampling
The process of going from continuous time to discrete time is known as sampling [5].
The job of the ADC is to use samples from the audio signal and convert it into machine
code by utilizing the voltage levels from the analog input. The sample rate of the signal
is the frequency at which the sampling takes place. The sample rate is measured in hertz
(Hz). If the sample rate is 25,200 Hz, it means that 25,200 points will be converted per
second. The higher the sample rate, the higher the quality of the conversion from analog
to digital, but a higher sample rate also means that it will take up more data space.
So the question becomes what is the best sampling rate? Nyquist’s Theorem suggests
that one uses at least two times the highest frequency you want to sample. In the music
industry, this is 20 KHz, so a sampling rate of at least 40,000 Hz should be used. In a
phone system, the sampling rate is only 8,000 Hz. This is why if you try to transmit
music over a phone it is of poor quality [3].
ADC designs
Flash
There are a few different kinds of ADCs to look at. One type is called Parallel ADC or
Flash ADC. This method will compare the input voltage to a reference voltage of the
maximum value that the input signal can be. “For example, if the reference voltage is of
5 volts, this means that the peak of the analog signal would be 5 volts. On an 8-bit ADC
when the input signal reached 5 volts we would find a 255 (11111111) value on the ADC
output, i.e. the maximum value possible.” [3] Flash ADC is the most efficient and fastest
of all the methods. A disadvantage of using flash is that it uses 2 n -1. This means that a
large amount of comparatives must be used. For example, an 8 bit comparator requires
28 – 1 which is 255 [4].
Figure 3: Flash ADC
Ramp Counter
Another type of ADC design is a ramp counter. A ramp counter uses a counter by
counting up until it reaches a value that matches the analog input signal coming in.
When it finds the digital value for the voltage signal, the value is recorded. The major
problem with the ramp counter is that it is very slow and inefficient because it needs to
compare numerous values until it finds the correct one [3].
Figure 4: Ramp Counter ADC
Successive approximation
A successive-approximation technique uses a comparator to reject ranges of voltage until
it settles on the correct voltage range. This method is based on the binary search
method. Successive-approximation will use a buffer allowing the digital data to be
available while the next sample is being processed. This will use a successive
Approximation Register which has commands to start a conversion and end a conversion.
It will consist of four main parts [4].
1. A sample and hold circuit to acquire the input voltage(Vin).
2. An analog voltage comparator that compares Vin to the output of the
internal DAC and outputs the result of the comparison to the successive
approximation register (SAR).
3. A successive approximation register sub-circuit designed to supply an
approximate digital code of Vin to the internal DAC.
4. An internal reference DAC that supplies the comparator with an analog
voltage equivalent of the digital code output of the SAR for comparison
with Vin.
Figure 5: Successive approximation ADC
Software Options
There are many different options when it comes to picking out DAW software. There are
many different solutions depending on what people are looking for. There are free
software and open source software available. There is also mid-level software around the
$100 range. Some of the more expensive software can be upwards of $600.
For an individual who is interested in get into recording on a small budget, they might
want to look at downloading open source DAW software options. One of the most
popular free DAW software available is Audacity. This program is available on Mac,
PC, and Linux forms. It has the ability to import and export WAV, MP3, AIFF, and
other file types. In this program there is the ability to record, playback, and edit tracks.
This software will support multi-track recording. One draw back of Audacity is it does
not have dynamic equalizer controls or real time effects.
For the more serious recording engineer, they will be interested in a higher performing
DAW. There are numerous DAW software options available to choose from but we will
examine two of the more popular options available. Pro Tools is a Digital Audio
Workstation that is used in the majority of large studios across the nation. A company
called DigiDesign produces pro Tools. When the first version of Pro Tools hit the market,
it sold for $6,000. Pro Tools, like Audacity, will record, playback, and edit audio signals.
It uses a Digital Signal Processing chips to create effects and virtual instrument samples.
One downside of using Pro Tools is that you must buy a DigiDesign ADC to run Pro
Tools. Pro Tools recently introduced a special line called Pro Tools M-Powered, which
is used with M-audio interfaces.
Conclusion
When looking at the music recording, Digital Audio Workstations have had a enormous
impact on the industry. They provide a way to convert real, analog signals into digital
signals that can be interpreted by a computer. The concept of converting physical audio
to digital using Analog to Digital converters has had a great influence on things that
affect many individuals’ normal lives including talking on a phone or listening to music.
With new technology always being developed, we should hear higher quality audio
available at our finger tips without breaking the bank.
References
[1] Strawmyer, M. (2003). Serialization/Deserialization in .NET. Retrieved from
http://www.developer.com/net/csharp/article.php/3110371/Serialization Deserializationin-NET.htm
[2] Digital Audio Workstation. Retrieved from
http://en.allexperts.com/e/d/di/digital_audio_workstation.htm
[3] Torres, R. (2006). How Analog to Digital Conversion (ADC) Works. Retrieved from
http://www.hardwaresecrets.com/article/317/1
[4] Hollos, R. (2007). Analog to Digital Converters. Retrieved from
http://www.exstrom.com/journal/adc/flashadc.html
[5] Cristi, R. (2004). Modern Digital Signal Processing. Pacific Grove, CA:
Brooks/Cole-Thomson Learning.