Even amidst iTunes and online streaming apps like Spotify, there are a couple of reasons why I still buy albums. For instance, some things only come with a physical album: the artwork of the booklet, hidden/bonus tracks, even an autograph.
My main motivation, however, can be summed up in one word: science. The difference between CD-quality audio and MP3 audio alone is enough for me to refuse to listen to MP3 files online.
CD Quality Audio
What is the science behind CD-quality sound? To answer this question, the first step is to learn how CDs work. Here is a 2-minute video that walks you through the process:
When a person records music, he/she produces sound waves. These sound waves are referred to as analog since they are not digitally generated: the medium producing the sound is "analog". As introduced in the video, recording music is the process of encoding analog sound wave into digital audio signals using a complex technique called Pulse-Code Modulation.
Two key factors determine the sound quality of the digital audio during the process.
The first factor is the number of times per second the analog signal is digitized, or sampled, known as the sample rate. The video describes the sample rate as the number of segments of electric signals per second.
The sample rate is important because it determines the range of analog sound wave frequencies that can be digitally recorded. And the frequencies determine the pitch of the sound. The higher the sample rate, the higher the maximum frequency the digital audio system can record. The standard sample rate of CD-quality audio is 44,100 times per second, or 44.1kHz. With this sample rate, the system can record frequencies of up to 20,000Hz, the highest frequency that human ears can detect.
Two key factors determine the sound quality of the digital audio during the process.
The first factor is the number of times per second the analog signal is digitized, or sampled, known as the sample rate. The video describes the sample rate as the number of segments of electric signals per second.
The sample rate is important because it determines the range of analog sound wave frequencies that can be digitally recorded. And the frequencies determine the pitch of the sound. The higher the sample rate, the higher the maximum frequency the digital audio system can record. The standard sample rate of CD-quality audio is 44,100 times per second, or 44.1kHz. With this sample rate, the system can record frequencies of up to 20,000Hz, the highest frequency that human ears can detect.
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| Left: analog sound wave | Middle: digital audio, low sample rate | Right: digital audio, high sample rate |
The second factor is the quality with which each analog signal is digitized, known as the bit depth. When the digital audio system digitizes an incoming analog signal, it stores the result as a series of 0's and 1's called a binary word. The bit depth refers to the length--or number of digits--of the binary words used to describe each sample. Longer word lengths allow more accurate digitization, since more data can be described in longer words than shorter ones. Hence, the higher the bit depth, the higher quality the digital audio is. The standard bit depth of CD-quality audio is 16 bits; in other words, each sample is represented as a 16-digit-long binary word.
The combination of high sample rate and high bit size makes possible the high quality of audios recorded in a CD. The quality of the digital audio can be quantified as the amount of data required to represent one second of the audio, or the bit rate, since a high bit rate would mean there is more data contained. The unit of bit rate is kilobits per second, or kbps -- the same unit as sampling rate times bit depth. The standard bit rate of CD-quality audio can be calculated as 1411.2kbps. In other words, this is what "CD-quality" means.
MP3 Audio
Now that we know about CD-quality audio, let us compare it with MP3 format audio. MP3 refers to a specific format of audio compression. The compression process is complicated, but in general it uses certain aspects of how humans perceive sound to make permanent changes to an audio file.
For instance, human ears have different sensitivity to different frequencies. MP3 compression removes sounds of frequencies that human ears are less sensitive to.
In addition, human ears tend to prioritize hearing certain sounds over others based on frequency or loudness, a process known as masking. For instance, if two sounds are played at the same time, the louder sound masks the quieter sound. MP3 compression calculates and removes masked sounds.
Because MP3 removes some of the sound in the original, CD-quality audio, MP3 format audio has smaller file size than CD-quality audio. This is why MP3 format is so widely used; a typical song is about 3-4MB in MP3 format, whereas it is about 30-40MB in CD-quality format. It takes up 1/10 of digital space, making storage and transfer of MP3 audio easier.
However, because some of the sound is removed, MP3 audio has significantly lower bit rate, and thus lower sound quality, than CD-quality audio. Typically, the bit rate of MP3 audio is either 128kbps or 192kbps. Even the highest bit rate is 320kbps. Recall that the bit rate of CD-quality audio is 1411.2kbps; clearly MP3 audio is of much lower quality.
Hear the difference yourself.
The combination of high sample rate and high bit size makes possible the high quality of audios recorded in a CD. The quality of the digital audio can be quantified as the amount of data required to represent one second of the audio, or the bit rate, since a high bit rate would mean there is more data contained. The unit of bit rate is kilobits per second, or kbps -- the same unit as sampling rate times bit depth. The standard bit rate of CD-quality audio can be calculated as 1411.2kbps. In other words, this is what "CD-quality" means.
MP3 Audio
Now that we know about CD-quality audio, let us compare it with MP3 format audio. MP3 refers to a specific format of audio compression. The compression process is complicated, but in general it uses certain aspects of how humans perceive sound to make permanent changes to an audio file.
For instance, human ears have different sensitivity to different frequencies. MP3 compression removes sounds of frequencies that human ears are less sensitive to.
In addition, human ears tend to prioritize hearing certain sounds over others based on frequency or loudness, a process known as masking. For instance, if two sounds are played at the same time, the louder sound masks the quieter sound. MP3 compression calculates and removes masked sounds.
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| Masking in MP3 compression |
Because MP3 removes some of the sound in the original, CD-quality audio, MP3 format audio has smaller file size than CD-quality audio. This is why MP3 format is so widely used; a typical song is about 3-4MB in MP3 format, whereas it is about 30-40MB in CD-quality format. It takes up 1/10 of digital space, making storage and transfer of MP3 audio easier.
However, because some of the sound is removed, MP3 audio has significantly lower bit rate, and thus lower sound quality, than CD-quality audio. Typically, the bit rate of MP3 audio is either 128kbps or 192kbps. Even the highest bit rate is 320kbps. Recall that the bit rate of CD-quality audio is 1411.2kbps; clearly MP3 audio is of much lower quality.
Hear the difference yourself.
As this video shows, I lose too much if I choose MP3 over CD-quality. That is why I buy CDs.
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