Physics of Music: The Study of Digital Audio Formats. Well it did take me sometime to come up with a project that I thought would be suitable

Transcription

1 Ryan Ochs Physics Dec Physics of Music: The Study of Digital Audio Formats Well it did take me sometime to come up with a project that I thought would be suitable for this course and that could easily be done. Earlier in the semester I was interested in the sound quality of certain audio file types and how they compared to each other and how they sounded to the human ear. At first I wanted to actually look at the music on a signal analyzer and look at a sample of music from a CD and then look at samples of the same music but instead in compressed formats or other alternative uncompressed formats such as MP3, WAV, WMA, etc. I would have then compared the two signals I saw and analyzed where and why there may have been audio loss and loss in sound quality when compared back to the signal generated by the audio played directly from a CD. I thought about this for sometime and after doing some research I found that this has already been done numerous times and that it was fairly well known which audio compression algorithms were the best and kept their music mostly intact when compared back to the uncompressed audio on a CD. So, then I wonder; they say, based on research, that certain compression types should be better than others and that there is a noticeable difference between CD audio and MP3 audio for example or between MP3 and WMA compressed audio. It is then I decided my time would best be spent learning more about how audio compression works and the algorithms they use to go about compressing their audio. I would also research into which audio file types are supposedly better than others and why they are thought to be so. Then to test the claims of scientists, audiophiles, and the people of the internet I would then do a study to see if people can truly tell the difference or find consistently that certain audio file types do seem to have better sound than other audio file types. I will describe later in this paper the method I used for collecting this data and the results of said data. 1

2 So now the question is why is my research important? Well one it is important to me because I have wanted to know for sometime now how audio compression works and then possibly the why behind the differences in audio quality from one audio file type to the next. It is also important to understand if people can truly tell a difference in the different types of audio or if it is just a psychological affect that they think they here differences when they listen to a CD verses when they listen to an MP3. This would be important to know because if the human ear on average can not typically tell the difference between an MP3 and a CD it would be good information to share so that way people do not go out of their way to spend more money on a CD when they can get an MP3 for cheaper and not really be able to tell any difference in quality between the two. Also in this paper I will talk about the audio file types that I have studied, how I tested human listeners to determine the audio quality of these audio file types, and then the results, analysis and conclusions of that study. As with any scientific study I need a hypothesis. As I will be testing the human hear to see if people can truly tell that difference between different audio file types in terms to see if it is detectable under normal circumstances, I will be testing the subjects using a laptop computer with a standard audio card and will be using a fairly standard pair of decent headphones. My hypothesis is that under these normal circumstances the human ear will not be able to tell the true difference between the different audio file types and therefore when people say they can hear the difference between a CD and an MP3, for example, it is a psychological affect and they truly cannot tell the actual difference between the two. However, before we get to the actual experiment and study of the human perception of the different audio types I am going to give some background on the different audio types that I have included in my study, including the information from my research on these audio file types that I did prior to conducting my experiment. 2

3 I started off this project by doing some basic research on the audio file types that I wanted to study. I choose to study the Windows Media Audio (WMA), MPEG-1 Audio Layer 3 (MP3), WAV (Waveform Audio), and Compact Disc Digital Audio (CDDA) audio file types in order to learn more about these audio file types and how they work and then also to look at the acoustical properties in terms of sound quality of these audio file types. I really want to find out if the human ear can truly tell a difference and if there is a certain medium in which we should be listening to digital audio in order to get the best experience or if it is all just a psychological affect and people only think they can tell a difference in the audio quality. I choose to study the audio file types that I choose for a few reasons. First off, the CD (using the CDDA file type) is the main and most pure form of digital audio storage and has an extremely high sample rate and does not compress music at all. I then choose the WAV file even though it is not compressed audio. WAV audio is encoded in much the same way as a CDDA file so I wanted to study it to see if people thought the CDDA and WAV file types did sound the same to them or if they felt that one of the two was still better than the other. I then choose the WMA and MP3 file types as these are compressed audio types. As they are compressed and are lacking certain sets of audio data that have been removed to make the file smaller, they should in theory be of inferior quality when compared to the CDDA and WAV file types. I want to study if this is the case or if, once again, the human ear cannot truly tell the difference. I then want to study the difference between the WMA and the MP3 file types. There is much debate as to which one has superior audio quality. I am hoping to find one of two things in my study of this by testing the human ear. Either, one, there will be no difference and to the human ear the audio played from a WMA and from an MP3 are indistinguishable or that, two, one of the two is perceived to be better than the other according to the human ear. I will now go into more detail over the next few paragraphs as to the research I have done and what I have learned about these four different digital audio file types. 3

4 Well logically it made sense to start with the main form of digital audio storage, which is the CD. The audio format in which audio is digitally stored on the CD is called the Red Book format which is the standard audio CD format for encoding audio digitally onto a compact disc. This is also known as a CDDA file. The term red book comes from the name of an actual book that was bound inside a red cover and was part of the rainbow series of books which were technical books on the standards and technical specifications for all the CD and CD-ROM digital storage formats for both audio and other file types for digital storage (i.e. pictures, etc.). The CD encodes and stores audio digitally but in an uncompressed format meaning that all the audio information that was in the original recording is stored on the CD and that no parts of the audio were left out. For example, all the frequencies for both the right and left channels are present instead of compressing it and cutting out certain parts of each channel or certain sounds at certain frequencies that would be considered miscellaneous by a compression algorithm. This means that the audio from a CD is all the audio that was originally there and has the fullest sound possible. CDDA files also store a vast amount of data which accounts for the high level of digital audio quality. When creating a CDDA the encoder samples the audio it is recording at 44,100 samples per second of audio and each sample is 16 bits in size. In terms of bit rate which people commonly associate with MP3 and other compressed audio file types in terms of the quality of audio from those file types, the CD has a bit rate of 1,411.2 kbits per second. This number of kbits per second is derived from taking the fact that the audio is encoded at 44,100 samples per second and each sample taken is 16 bits in size meaning that 1,411,200 bits are sampled and encoded each second which then when divided by one-thousand give you 1,411.2 kbits per second. Thus meaning that the CD holds a very large amount of data and therefore leaves out 4

5 very little detail when it come to digitally storing music and there for can be consider one of the best forms of digitally stored music and the best way to listen to music digitally. Not only does the CD store vast amounts of audio data but it also stores that data in a fairly simple and clean way. The method used by the red book standard to encode audio information into the CDDA audio file type is using a method called linear pulse-code modulation (LPCM). LPCM encodes music by representing sample amplitudes and frequencies on a linear scale instead of a non-linear scale such as a logarithmic scale. Each value that is stored and encoded by LPCM is directly proportional to the frequency and/or amplitude of the audio that it is encoding. This is where the 44,100 samples at 16 bits per sample come into play when encoding the audio into the CDDA file format. Each of those 44,100 samples per second are given a value as dictated by LPCM. Each of those values are 16 bits in size and the values range from to These are the values that LPCM assigns and again they are directly proportional to the amplitude and/or frequency of the audio it is encoding. So, for example, when the audio encoder samples the music and it gets one single sample of the music at a certain amplitude the encoder will then assign an arbitrary 16 bit value as dictated by LPCM for that particular amplitude and that value is what will become part of the whole audio mixture. When encoding the audio to the CDDA file format this is done 44,100 times for each second of music in order to gather and encode every single piece of audio information that exists in that second of audio. This is a very clear example as to how vast and detailed CDDA files are and how they store very pure forms of digital audio that with LPCM decoders (one of the components of a CD player that takes the LPCM values and reconstructs the sound that was encoded) can easily reconstruct seeing that the LPCM encodes in a very easily convertible format of a linear modulation. Hence this is the reason why CD s were one of the first and main components of digital music and continue to be one of the best and most pure forms of digital audio storage. 5

6 Now that I have described how CD audio is encoded and stored I will now talk briefly as to the processes that CD audio goes through before making the final cut and the start of reproducing the CD for mass production. This process is typically referred to as audio mastering and is done with the audio before encoding onto the final master CD (or other form of digital storage) in preparation for the mass production of CD s with that particular set of audio. One of the first things done in audio mastering is running it through a noise reduction filter that runs the audio through a filter and removes sounds that are not part of the wanted audio. For example noise filtering mainly removes background noise from sources like ventilation and the electrical sound of the lights in the room at the time of the original audio recording. Also in the mastering process the audio engineer may play with the amplitude of different frequencies in order to better balance the music. This will help to give a warmer sound and is meant to balance the audio in a way that makes it more pleasing to the human hear. The audio engineer can also correct small mistakes in the audio and adjust the over volume of the particular song if need. When creating a CD record the audio engineer will typically try and make sure that each song on the record has the same average overall volume in order to keep the music on the CD consistent. The few last steps in the CD mastering process have more to do with how the CD is going to be played. The audio engineer will add in set of data between the songs as identifiers. This allows us to know what track we are on, allows us to navigate to other tracks, and allows us to know the time of each track on the CD. The audio engineer may also do some other basic editing such as fade ins and fade outs of the songs before making the final cut. Once the audio engineer is all done and he or she feels the audio is ready they will encode it into a CDDA file format and onto either a master CD or into another form of digital storage where the music will be stored until it is need to print onto CD s. So, that is basically the main process behind the audio mastering for a CD. 6

7 Next we will take a look at the Waveform (WAV) audio file type and how this file type works and goes about encoding its audio into a digital audio format. The Waveform, which is also known as WAV, audio file format is actually a very interesting file format in the fact that it encodes audio information in much the same way as red book does for the CDDA files for CD s. This means that the WAV file is not compressed audio and thus stores audio in a high quality uncompressed digital format much like a CDDA file does. This is a common misconception among many you believe that the WAV file type is a compression algorithm that compresses the audio. Hence people have the misconception that due to the age of the WAV file type that the WAV file is an old compression algorithm and that the WAV file has an inferior sound quality when compared to audio file types such as the MP3 and the WMA. This however is incorrect as the WAV file is a great way to digitally store high quality audio files and is considered to be better than the WMA and MP3 file types. Since it is also uncompressed audio that is encoded in much the same way as a CDDA file gives a further argument as to why the WAV file stores digital audio in a far superior way than the WMA and MP3 files formats both of which are compressed audio file types. It is however that due with the age of the WAV file format that the type is considered outdated and is no longer used to much though some may use it for audio being played at theatrical performances and other events that require high quality audio. The WAV file is considered outdate because it is uncompressed audio and therefore the file is quite large in size, about 30 megabytes for a typical length song. This is to large for any practical use on mobile audio playing devices such as an ipod or other portable music device. It is also no practical use in terms of CD s as we have already discussed the standard used for encoding onto CD s is the red book standard that encodes audio into a CDDA file format. So, for the most part the WAV file is an outdated audio file format and is no longer greatly used. 7

8 The way in which the Waveform audio standard encodes audio into a digital format as I have already said is almost exactly identical in the way that the red book standard encodes for CDDA files. The WAV file uses the LPCM just as red book does in order to take the sample amplitude and/or frequency and translates it into a 16-bit value that is proportional to the amplitude and/or frequency that it sampled. The WAV file also encodes the audio by sampling the audio at a rate of 44,100 samples per second. This also is also exactly like the red book standard. There is very little known to the differences between the WAV and the red book standard except that it is possible that some of the values that the WAV standard uses for each 16-bit sample are different from the values used by the red book standard. It is also very atypical to ever find audio that has been taken and originally encoded using the WAV standard. Typically a WAV file will come from someone taking a CD and the converting the CD audio into a WAV file format. This is because a WAV file can be stored then on a computer and played back using a program that can play WAV audio files. This allows someone to hold more than one CD worth of audio on a computer while allowing the audio to be of high quality. In my research I ran across some other information about WAV files that I did not quite fully understand and will not really go into great detail anyways as this information is about how the WAV file can store other data other than audio data. Apparently the WAV file was a file structure created by IBM and was meant mainly for use in storing audio. However, due to the way the WAV file encodes data it can be used to store compressed audio and also store other data such as different programs and other data related to computer operations. As I said since this does not have to do too much with audio I do not want to go into great detail nor was I really able to understand as to why and how it is possible for the WAV file to be used to store other forms of data. 8

9 So why am I studying the WAV audio file type? Well to put it simply, at first I too myself thought that the WAV standard was a form of audio compression algorithm and wanted to go ahead and compare the audio quality of it to the quality of WMA and MP3 audio files. However, once I found out that this was uncompressed audio that was very similar to CD audio I decided to keep it in my study anyways. This way I can have a comparison to a digital audio file that is uncompressed (other than the CD) to compare to the two compressed digital audio file types I am studying, the MP3 and WMA file types. I also wanted to study and compare the sound quality of the WAV file when compared to audio played directly off a CD to see if indeed people did think they sounded the same and could not tell a difference in the audio quality or if there was some sort of noticeable difference to the listener. Now we will move into compressed audio files and what audio compression is starting next with the MP3 audio file standard. MPEG (Moving Picture Experts Group)-One Audio-Layer 3 in a highly known and used audio compression algorithm that is considered to be one of the standard audio file types used in audio compression. MP3 s are highly used for mobile audio devices such as ipod s that can hold several thousands of songs in this format. The MP3 file size of a song is about 1/11 th the size of the same song stored in CD format. So, with this large loss of audio data, just how does the MP3 compression algorithm go about cutting down the size of the file and how does it do so and still sounds pretty decent? The MP3 compression algorithm works in such a way that it can remove mass amounts of the audio data present in the uncompressed (typically CD) audio file, without making it sound too much different and it does so by taking advantage of weaknesses in the human ear that keeps us from hearing a lot of what was removed in the compression process. This weakness is called audio masking in which if there are sounds of similar frequencies only the loudest sound will be heard by the human ear. It is also true that a sound of greater intensity but at a significantly lower 9

10 frequency will also render the higher frequency inaudible by the human ear. The MP3 compression algorithm takes advantage of this by determining where audio masking occurs in the audio it is compressing and then it cuts out or reduces all the other pieces of audio at that timeframe seeing that those other pieces of audio would not be heard by the human ear anyways. This, therefore, makes the file smaller while reserving much of the audio quality because what the human hear would end up hearing anyways is still present in the compressed audio file. The following graphic demonstrates in a basic form how the MP3 goes about choosing the audio data to remove as it compresses an audio stream. Most MP3 files are compressed at a 128 kbits per second rate and this is where the MP3 averages around 1/11 th the size of a CD file. The MP3 can compress audio at many different bit rates, ranging from just 32 kbits to 640 kbits with a rate at 320 kbits being so good that is should be indistinguishable from that of CD audio. This is nice to give the option of bit rate to encode in and is also nice that the MP3 largely has good audio quality and is a small enough of a file to store mass numbers on them on an audio device. However, since the MP3 compressed audio is missing a good deal of audio information that was contained in the original CDDA audio file on a CD, many say that the quality is not as good and that the missing audio information causes the audio that is compressed to lack full sound and is therefore inferior to a CD. Once again I personally cannot tell the difference between CD audio and MP3 audio, however, in my study we will look at this and find 10

11 out if the human ear actually tell the difference between CD and MP3 audio or not. Next we will look at the Windows Media Audio file standard which is also a compressed audio file type. It is highly debated as to which compressed audio type sounds better, the MP3 or the WMA audio file. We will look into this and see what my study has to say. Well we are here now talking about Windows Media Audio (WMA) and well I don t have much to say about it. It is not that I am lazy, I did a lot of searching and could not find much on how the WMA file worked. Why, might you ask? Well because it was created and is owned by Microsoft Corporation and with any large corporations, they like to keep their corporate secrets, top secret. It is however known that the WMA file type is another form of compressed audio using some sort of process and compression algorithm that Microsoft developed. It is thought that maybe the WMA is similar to another compression algorithm called Ogg Vorbis which was also hard to find information on. However, if this is true this means the WMA file has the ability to combine both left and right channels into one stream of data which compresses the file size and has some other supposedly good features that I was not quite able to understand such as quantization and floor mapping and removal. I do, however, want to study this file type for the purposes of its sound quality. Many say that it is in high competition with the MP3 file type as many argue that the WMA file has superior sound than that of a MP3 file. Once again I personally cannot tell the difference myself but we will see later on in this paper what my study and the results of that study have to show us about whether the human ear actually detects a difference between the two. It will be interesting to know because one of the other main arguments is that though the MP3 can sound better than the WMA when the MP3 compresses at higher bit rates, it is argued that the WMA and MP3 at the same bit rates the WMA audio sounds better. Next we will look into wrapping up my comparisons of these different audio types and what should supposedly sound better than others. 11

12 We will also take a look at my thoughts and hypothesis and then the background behind my study/experiment and how I went about carrying out this experiment. So, I have researched the red book audio standard used for encoding CDDA files which are used on audio CD s and then, WAV files which are very similar to CDDA files in terms of how the audio is encoded and is an uncompressed form of audio storage, and then WMA and MP3 files which are both compressed audio forms. Seeing that CD audio and WAV audio are both uncompressed audio forms they should sound better, in theory, than the WMA and MP3 audio file types. This is because the WMA and MP3 compress their audio using algorithms that find and choose pieces of audio information to remove from the audio data stream in order to make the file smaller for easier storage. Since these files are missing portions of the audio many say they do not sound as good. Then there is the argument between which compressed audio type is better than the other, MP3 or WMA? There are arguments either way but, we will see what my results show to see if the human ear under standard conditions (standard sound system) can actually hear any true difference between the two or not. As for my experiment I wanted to test the claims of some people saying things like mp3 sounds better than wma or CD is by far the best form of digital audio etc. To do so, I created a testing procedure that uses test subjects listening to a clip of music in one format and then again but a clip using a different format. I will now further describe how I went about testing my test subjects. I would tell the test subject that they are about to listen to two 30 second clips from a song, each clip is of the same section of the same song, however, each clip maybe of the same or different audio format file types. I asked them to listen carefully so that they could tell me afterwards which clip they felt sounded the better of the two. I then told them I would do this test three times. Each test would be a different song but still each test clip would be of the same section of the same song and would be either the same or different audio file types from one 12

13 another. I then told the subject to put on the headphones that I provided them and to be prepared to start the test. At the end of each test I asked them for their response as to which one they felt sounded better. As I will discuss a little later on, I got a lot of responses stating they could not tell a difference and also that one did not sound better over the other. In order to gather statistics I told them they needed to make a decision of one or the other. I then recorded the results to go back and compare later which audio file type they actual choose as the better sounding one. I then commenced the next test and did this until all three tests were fully completed. As controls for this test I conducted the test as much as possible in the same spot which would be my bedroom at home with the door shut and as many different background noises eliminated as possible. I did also conduct some of my tests at my high school, so a different location, but I conducted them in a room with the door closed with no one else in the room besides the test subject and I; and background noises were eliminated as much as possible. Also for controls I used the same laptop and same set of headphones to test every one of my test subjects. Also not at anytime was the audio file type known to the listener as to keep their response as unbiased as possible. For my test subjects I used my family, a few close friends, and then some students and teachers at my high school. I did all testing over my fall break will I was back home. So after doing all the testing and collecting all my data, I went back and matched everything up to get usable data and to see and analyze if there is any form of a superior digital audio format. So, first I will mention that my curiosity centered around three things, seeing the WAV and CDDA files are both uncompressed forms of audio that essentially work in the same way I wanted to see if this is true and if to the human ear they sound a like. So one of my three tests was always playing a clip from a song in WAV format and then playing it directly off the CD. All 20 test subject came back with the initial response that they could not tell a difference or that they felt they sounded the same and that one did not sound better than the other. When told 13

14 they needed to make a choice, 9 persons choose CD while 11 choose the WAV file. The next thing I wanted to focus on was the argument between which compressed audio format sounds better, mp3 or wma. In both cases I had the wma and mp3 files compressed in a 128 kbits per second format. For this test, I played a clip of music in the mp3 format and then the same clip again but in the wma format. Again, this yielded the same result of the first response being that they could not tell a difference between the two. When told they needed to make a choice, 13 people choose the mp3 file while 7 choose the wma file. I was then curious to see if it is true that uncompressed forms of audio are truly better sounding than compressed forms of audio and if the human ear can actually detect the difference or not. For this test I did a little more random. I either played a CD clip or WAV clip and the either a MP3 clip or a WMA clip of the same section of the same song or I did it the other way around. This was to test to see if humans can hear the difference between compressed and uncompressed audio. For the results of this test I actually had 7 tests subjects say that they could hear a difference with all 7 responding saying that the audio file type that ended up being the uncompressed audio (either CD or WAV) sounded better to them than the other audio clip. The other 13 responded with the usual, they could not tell a difference. When told they needed to make a choice 8 people choose what ended up being the compressed audio types (WMA and MP3) while 5 choose the uncompressed types. So those were my results. Below is a graph of those results that I created in an excel spreadsheet in order to better visual and help to understand the results a little better. Remember that when you are looking at and comparing the responses for the WMA verses MP3 and the CD verses WAV that those were responses made most likely randomly by the test subject as their first responses were that they could not tell a difference. Next I will briefly describe the meaning behind my results. 14

15 Study of Digital Audio Quality CDDA WAV MP3 WMA Could tell a difference between compressed and uncompressed Could not tell a difference between compressed and uncompressed Compressed Uncompressed My results were really quite interesting. I will first discuss the fact the since no one could really tell the difference between WAV and CD and then also between MP3 and WMA, and also given the fact than when asked to give a response anyways it was nearly 50/50 as to which one they felt was better showing that in general the human hear truly can t tell a difference between the two and most likely any difference a person may think they hear is just most likely a psychological affect. We ll then look at the difference in sound between the two compressed forms of audio that I studied the MP3 and the WMA audio file types. Here once again the first response from my test subjects was that they could not hear a difference between the two different types of audio. When told that they needed to make a choice, the results did not come out quite 50/50 but they did not sway in one major direction or the other. Seeing that there were only 20 test subjects it is safe to assume that my statistics are far from perfect which can be 15

16 considered an error in my experimental data. I feel as I were to test more and more subjects that when asked to make a choice the numbers would tend to approach a 50/50 distribution seeing that the choice in this case is most likely nearly random. This shows that when it is unknown to the listener, using standard audio equipment, the human hear cannot tell the difference in the audio quality between WMA and MP3 which are both compressed forms of audio and are both thought to compress audio using different methods. Lastly, we look at the results when looking at the differences in compressed and uncompressed audio. Here is the only place that the results of my study produced data that pointed to one thing over the other. Here 7 people actually said they heard a difference between the two audio file types. Of those 7 all of them said they thought the audio that ended up being the uncompressed audio sounded better than the compressed audio. When asked why they felt that one was better than the other, they could not really explain it. They said that they felt they heard more and that it didn t feel as empty. One of my subjects even said they felt that the music sounded warmer. This can be contributed to the fact uncompressed audio has many more data points and much of the actual audio is there verses compressed audio that removes certain components of the audio data in order to make the audio file smaller. Meaning that technically there actually is less to listen to in compressed audio. It is just a little confusing given the fact that compressed audio (especially MP3) runs on the fact and basis that the audio data it removes should not be able to be heard by the human ear anyways. However apparently this isn t true and goes to show that even advanced and complex algorithms cannot precisely achieve this feat and that some people can detect the difference. As for the fact why 13 others said they could not tell a difference is most likely due to the fact that they have difference hearing both from genetics and then also from the fact they may have damage to the hear that limits their ability to hear sounds in the high and/or low and/or mid frequency ranges. This may mean that their hearing 16

17 isn t the greatest and that they cannot detect the differences in the sound quality. So also in analysis of my results we should look at some of the errors in my experiment which include very standard equipment which was a basic laptop with a basic audio card and basic CD disk drive using a pair of decent but pretty standard headphones. Using these may mean certain sounds were missed or blurred affecting the listener s ability to hear difference in audio quality. Also another source of statistical errors is the fact that my test subject pool was extremely small. In order to get decent results a test group of subjects would be ideal. In order to have a nearly perfect statistic a study of subjects of different ages, different parts of the world, different life styles, and different race would have been good, however, extremely unrealistic with my current resources. Then one other source of error is any background noise that may have interfered with the listener s ability to hear the differences in sound quality. We also see based on my results and analysis of my study that my original hypothesis was correct and that the human ear for the most part cannot tell the differences in audio formats except when it is between compressed and uncompressed audio file types. In conclusion, I really enjoyed doing this project. Learning about how digital audio is stored and how audio compression works has always interested me and I have just never had the time to sit down and look into it. Seeing that this course gave me an opportunity to do so was enticing and so thus the reason I studied digital audio formats. I have also always heard about the claimed differences in audio quality between the different audio formats which is hence the reason why I did the study that I did. I found my results interesting and I can now say that when ever someone argues with me or I hear an argument I can settle it with the fact that I have study this and have written a paper about it. Overall, I enjoyed this course and the information it provided to me that helped with my project and then also just peaked my natural curiosity about how things in the universe work. In the future I could look into analyzing audio on the scope and 17

18 trying to reproduce my results using a different test group. Even further down the road would be moving towards larger and more diverse test groups and also using even better audio equipment to better refine my results. 18

19 Works Cited These resources were used in my first half of my paper in learning and describing how digital audio works. Bellis, Mary. The History of MP3. accessed 3 Nov Brian, Marshall. How MP3 Files Work. accessed 3 Nov Wilburn, Thomas. The AudioFile: Understanding MP3 compression. compression.ars accessed 3 Nov Unknown Author. Windows Media Audio. accessed 3 Nov Unknown Author. MP3. accessed 3 Nov Unknown Author. WAV. accessed 3 Nov Unknown Author. How Audio Compression Works. accessed 3 Nov

20 Unknown Author. Red Book (CD Standard). accessed 3 Nov