Sound Cards,Digital-to-Analog Conversion, Compressing Data

Digital-to-Analog Conversion

Sound cards use two methods to convert digitally stored sound into real analog sound:

FM (frequency modulation) synthesis creates a sound by artificially creating a wave that is close to the sound wave produced by the instrument. For example, the sound of a trumpet would be produced by imitating the sound wave produced by the trumpet through a series of mathematical calculations. Wavetable synthesis produces the sound by using a sample recording of the real instrument. Wavetable synthesis produces better sound than does FM synthesis, but is also more expensive.

Once the sound has been converted back into an analog signal, you need speakers to play back the sound using a sound card. Unlike speakers used for other sound equipment, speakers made for computers have built- in amplifiers and extra shielding to protect the monitor from the magnetic fields around regular speakers.

Storing Sound Files

Sound cards store sound in files in two ways: MIDI and WAV files. MIDI (musical instrument digital interface, pronounced “middy”) technology, a standard for digitizing sound, dictates a specific number of sound samples and the quality of those samples. MIDI files have a .mid file extension. Nearly all sound cards support MIDI.

Most synthesizers support the MIDI standard, so sounds created on one synthesizer can be played by another. Computers with a MIDI interface can receive sound created by a MIDI synthesizer and then manipulate the data in MIDI files to produce new sounds. The MIDI standards include storing sound data, such as a note’s pitch, length, and volume, and can include attack and delay times. Data compression is used because sound files can be quite large. Sampled files, which Microsoft calls WAV files (pronounced, and stands for, “wave”) have a .wav file extension. When Windows records sound using a sound card, the sound is stored in a WAV file. Most game music is stored in MIDI files, but most multimedia sound is stored in WAV files.

Sound files are often large. For example, CD-quality sound is recorded using a 16-bit sample size and 44.1 kHz sampling rate with stereo. The calculations of data size are:

o 16 bits X 44,100 samples/sec X 2 = 1,411,200 bits/sec or 176,400 bytes/sec This yields more than 30 MB of disk space for a 3- minute song. Because of these large file sizes, methods of compressing data have evolved.

Compressing Data

You can compress data using several standards. Some apply to just audio and others to audio and video. To see the standards currently installed under Windows, double-click the Multimedia icon in Control Panel, and then click the Devices tab. See the figure below. Click the plus sign next to Audio Compression Codecs.

Compressing and later decompressing data is called CODEC (Compressor/Decompressor). A CODEC method that does not drop any data is called lossless compression, and a method that works by dropping unnecessary data is called lossy compression. The term CODEC can also refer to hardware that converts audio or video signals from analog to digital or from digital to analog. When the term is used this way, it stands for coder/decoder. One of the better-known data compression standards is MPEG, an international standard for data compression for motion pictures. Developed by the Moving Pictures Experts Group (MPEG), it tracks movement from one frame to the next, and only stores what changes, rather than compressing individual frames. MPEG is a type of lossy compression. MPEG compression can yield a compression ratio of 100:1 for full- motion video (30 frames per second, or 30 fps).