Microscopy Image Gallery


Exploration of the Month

November 1996


This month we explore the world of recorded music media.

CD-ROM Recording Tape Listening to music is a very popular activity. Unless you are listening to live musicians you will be using some sort of recorded music. The most common methods to record music use either a CD-ROM, an audio tape, or a record. Other techniques also exist to record music which range from the phonograph to music generated by a computer.

Using the microscope we will explore the way that sound is captured on the recording medium. On the left is a picture of a CD. This one actually contains words and pictures, but it works the same way as a music CD, and on the right is a similar view of the tape from an audio cassette tape.

magnified CD-ROM Magnified Audio tape When we look at these in the optical microscope at very high magnification we can see how they work. On the left is the CD. We can see rows of dots and spaces. The rows actually form a spiral track on the CD. The dots were created when the CD was made and contain all the digital information needed to re-create the music when the disc is played. When you play the CD it is spun round very quickly and a small laser is focused on these dots. The CD player recognizes the dots and spaces and translates that digital information back into sound for you.

On the right we can also see dots in the image from the cassette tape. These are not the same as the digital dots of the CD, but are small magnetic particles stuck onto the plastic tape. By magnetizing the particles in a particular pattern in the recording studio, the musical information is transferred to the tape. The music is played back by detecting the magnetic field produced by the particles as the tape moves past the tape heads. This signal is then amplified by the cassette player, giving us the music we want to hear.

SEM Audio tape Magnified record groove In the Scanning electron microscope the image of the magnetic particles on the left here looks very similar to the high-magnification picture above. The higher magnification is able to see smaller particles, but we still cannot see the magnetic field of the particles. If we could, we could tell whether the tape was recorded digitally in which case it would look a little like the CD with "dots" where all the particles were magnetized in the same direction, and "spaces" where the direction was different.
It is also possible to record music as an analog signal. In this case the direction of magnetization of the particles changes continuously from one place to the next. This is similar to the way that the more old-fashioned record works. In the magnified image on the right we can see a couple of grooves (which are really part of the same spiral track). The ridges along the sides cause the record player needle to vibrate, and it is this vibration which is amplified to play music through the loudspeakers.

Below are three more images of the CD from an area where there was a fingerprint. The first image is focused on the surface of the CD and shows some round patches of finger oil. The dark area at the upper right is some printing on the CD. The middle image is focused on the digital data and (like the laser in the CD player would see it, except that the laser sines in through the other side of the disc and does not see the printing). In the areas where the fingerprint is the dot pattern is distorted, so the music will not play properly here. This is why you should only hold the CDs by their edges.
The third image is focused at a height in between the first two. the oil droplet are acting like little magnifying lenses, giving us an even bigger magnification of the dot pattern.


Finger grease on CD Finger grease on CD Finger grease on CD


Other Explorations


Minnesota Microscopy Society Web Pages maintained by Stuart McKernan
Comments, additions or questions may be addresses to MMS Webmaster
Last Update: 11/25/96