MP3 files are good examples of analog to digital conversion. It is simple to take an analog signal-such as your voice-and convert it into a digital file for transmission or playback.

In this activity, we will show you how to record your voice and see how different levels of digital quality affect the sound. First, you need to download a sound editor and MP3 converter. One very good sound editor is Audacity-and it's free. Go to audacity.sourceforge.net and download and install the audacity software. You will also need the plug-in called LAME (an MP3 encoder) which is also free and available at lame.sourceforge.net.

Use Audacity to record music or your voice (you can use a cheap microphone). Audacity records in very high quality, but will produce MP3 files in whatever quality level you choose. Once you have the file recorded, you can edit the Preferences to change the File Format to use in saving the MP3 file. Audacity/LAME offers a wide range of qualities. Try recording at least three different quality levels.

For example, for high quality you could use 320 Kbps, which means the recording uses 320 Kbps of data per second. In other words the number of samples per second times the number of bits per sample produces equals 320 Kbps.

For regular quality, you could use 128 Kbps. For low quality, you could use 16 Kbps. Create each of these files and listen to them to hear the differences in quality produced by the quantizing error. The differences should be most noticeable for music. A recording at 24 Kbps is often adequate for voice, but music will require a better quality encoding.

Deliverable

1. Produce three MP3 files of the same music or voice recording at three different quality levels.

2. List the size of each file.

3. Listen to each file and describe the quality differences you hear (if any).