Digital Audio Workstations and Analog to Digital Conversion
Adam White
Department of Computer Science (CIS)
University Of Wisconsin-Platteville
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].
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 ananalog 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 2n -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].
- A sample and hold circuit to acquire the input voltage(Vin).
- 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).
- A successive approximation register sub-circuit designed to supply an approximate digital code of Vin to the internal DAC.
- 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 availableat our finger tips without breaking the bank.
References
[1] Strawmyer, M. (2003). Serialization/Deserialization in .NET. Retrieved from Deserialization-in-NET.htm
[2] Digital Audio Workstation. Retrieved from
[3] Torres, R. (2006). How Analog to Digital Conversion (ADC) Works. Retrieved from
[4] Hollos, R. (2007). Analog to Digital Converters. Retrieved from
[5] Cristi, R. (2004). Modern Digital Signal Processing. Pacific Grove, CA: Brooks/Cole-Thomson Learning.