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
Typical Bit Rate
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
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
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
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
The Compression column refers to how well the audio
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