Nuts & Volts - September 2004

Let’s Get Technical

dot (Its tone lasts longer when played), let us use one bit for a dot and two bits for a dash. So, a dot will be a 1 and a dash will be a 11.

MPEG Audio Layer

Encoder Complexity

Compression

Typical Bit Rate

1

Low

Low (4:1)

384 Kbps

2

Medium

Medium (8:1)

192 Kbps

3

2. Assign binary patterns to the symbol, letter, and word spacing. The symbol spacing is the quiet time needed between one dot/dash and another.

For example, in the code for letter A, we have a dot followed by a dash. If we let the symbol spacing be a single bit (equal to 0), we can represent the letter A by the binary pattern 1011. The letter spacing is the quiet time needed between the letters that make up a word or abbreviation. This spacing needs to be longer than the symbol spacing.

Let us use two 0s for the letter spacing. Now the letters OK will encode as 11011011001101011. Last, the quiet time between words must be longer than the quiet time between letters. For this, let us use three 0s. Then, the words HI JIM will encode as 1010101001010001011011011001010011011000.

High

High (12:1)

112 Kbps

Table 1. MPEG audio layer differences.

3. Count the number of bits required to encode a message in the chosen format. For our example, the binary string encoding HI JIM contains 40 bits.

information compresses compared to a CD quality, uncompressed bit stream of 1,411,200 bits/second (two 16-bit channels, sampled 44, 100 times each second). As indicated in the table, the more processing power you can throw at the compression algorithm, the better your data will compress (to a certain limit).

All three MPEG layers use the same basic techniques for encoding audio and compressing the data. These techniques — called perceptual audio coding and psycho-acoustic compression — utilize knowledge of how humans hear and process sounds to eliminate information that is duplicated or masked out by other sounds.

By eliminating the redundant or unnecessary information, the MP3 file requires less storage space than an uncompressed WAV file for the same audio information. For example, a 30 second WAV file requiring

4. Repeat steps 1 through 3, making one or more changes to the binary patterns assigned. Encode the same message as before.

5. Compare the number of bits required for each encoding. If one method is better than another, can you determine why?

You may also change the hardware requirements and see what effect they have on the format and compression of the encoded binary string.

Another logical type of compression is performed while encoding an MP3 audio file. MP3 files get their name from the MPEG Audio Layer 3 specification. MPEG (Moving Picture Experts Group) audio and video are popular encoding methods for creating high quality, low-bit rate multimedia files.

There are three audio layers defined in the MPEG standard. Table 1 lists some of their features. As indicated, Layer 2 is superior to Layer 1, and Layer 3 is superior to Layer 2. Layer 3 requires the most processing power to implement the decompression algorithm; Layer 1 requires the least. Layer 3 encoding did not become popular until the speed of the PC was able to support its calculations. Before the MP3 audio file, the Windows WAV file provided digitized audio for the PC. The WAV file simply stores the sampled audio data without compression.

The Compression column refers to how well the audio

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