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CD vs MP3 Sound Quality

There is no argument in the audio world about the quality of CDs vs MP3s – CDs win outright. Yet, in spite of this, the MP3 file format has gained in popularity, and is widely used on the Internet. In a side by side listening test however, how do the two formats compare? This is something I wanted to find out for myself.
 
In order to make a fair comparison, all elements in the audio chain should remain constant, in other words, the sound should go through the same amplifier and be heard through the same speakers or headphones. Another important factor to avoid bias is for the listening volume to remain exactly the same as, up to a point, louder is often perceived as better.
 
To achieve all this I loaded two CD tracks into a computer and, using high-end mastering software, copied and converted them into MP3 files. Now, with the CD and MP3 tracks residing side by side in the computer, it would be easy to flip from one to the other to make direct comparisons. The screen shot below shows the waveform of the CD in blue (the PCM 16-bit CD file has been converted into a WAV file, but for the purpose of this discussion I will continue to refer to it as a CD), and the MP3 file in red.

Picture of waveforms on a computer screen.

In order to expose the strengths and weaknesses of the formats I chose one classical piece of music, and one pop. The Classical music, Ravel’s Le Tombeau de Couperin, was a DDD recording, and therefore of the highest quality, and for the pop music I chose a quiet track, Secret Garden, from Quincy Jones’ album Back On The Block. This is an exceptionally high quality recording made by one of America’s best producers. Choosing a loud track could have helped mask any deficiencies in the MP3, and therefore defeated the object of the test.
 
A major factor affecting the quality of the MP3 is the conversion (bit) rate, measured in kilobits per second (kbps). The higher the bit rate, the higher the sound quality. A 320 kbps MP3 would equate to a high-quality MP3, whereas a 128 kbps MP3 would be at the lower end of what is generally regarded as acceptable for listening to music.
 
The results are shown in the table below and are based on what I heard. They are, therefore, totally subjective. However, as a musician I believe I have a very discerning ear.
 

MP3 Bit Rate

CD vs MP3 Comment

320 kbps

At a sample rate of 320 kbps, much to my surprise, I was struggling to hear any difference between the CD and MP3 tracks. I ‘thought’ the CD sounded ever so slightly brighter and clearer, although having said that, if I heard just one of the files, MP3 or CD, I could not, with any certainty, say which I was listening to. Although I didn’t think I would be, I was impressed with the quality of the 320 kbps MP3.

192 kbps

Again, I was impressed with the 192 kbps MP3.  However, this time I believe I really could hear a difference. The CD tracks did sound a little brighter and clearer. Although I wouldn’t choose to do any serious listening to a 192 kbps MP3, I would be quite happy with passive listening.

160 kbps

At 160 kbps degradation in sound started to show, most noticeably with ‘sh’ and ‘s’ vocal sounds, which began to sound slightly metallic. Generally, everything sounded a little harsh and unrefined. However, background noise, on the street or in a motor vehicle for example, would help mask much of the degradation, making the file acceptable for listening to music ‘on the move’.

128 kbps

At 128 kbps certain blemishes started to show through quite clearly, most noticeably on the drum hi hat cymbals. These had a very processed, artificial, sound. Vocals started to sound a bit metallic, and the whole recording sounded harsh.

 
In all fairness, MP3s were never designed to be played on high-end audio equipment costing well into four figures. For this reason, it's probably unfair to make direct comparisons between CDs and MP3s. However, much to my surprise, I found that MP3s, even ones recorded at 128kbps, are fine for casual listening, listening to music on the move, or listening in noisy environments where having CD quality would be of little benefit. MP3 sound quality that fails to make the grade in a living room, when played through a high quality system, may be quite acceptable on a portable MP3 player in a bus, train, or on the street.
 
With regard to file size: the higher the quality the larger the file. There may be occasions when having a smaller file size, and thus access to a greater number of tunes, is more important than high quality audio (older MP3 players with small memories spring to mind).  At the end of the day, it’s a trade-off in which the listener must decide where to draw the line at what is, and what is not, acceptable sound quality.
 
Beyond MP3
 
Although this article is primarily about CD vs MP3 sound quality, it’s worth mentioning the newer AAC (Advanced Audio Coding) file format, also referred to as MP4. By way of comparison, a 128 kbps AAC file is said to be roughly equal to a 160-192 kbps MP3 file, with a 192 kbps MP3 being (as my test subjectively confirms) ‘near’ CD quality.
 
According to the information I have read, AAC files at lower bit rates (such as 128 kbps) do sound noticeably better than MP3s. However, at higher rates (such as 320 kbps), there is little audible difference.
 
With larger memories on portable music players, and Internet speeds increasing, lower audio file size is becoming less of an issue. Consequently, the future is looking bright for sound quality.

 

 

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