Digital sound | How sound boards work |
Measuring digital sound | FM synthesis |
Wave table synthesis | MIDI compatability
If any gadget has transformed the desktop computer from a dreary business workhorse into an entertainment center, it's the sound card.
Whether you're using an add-in sound board on an IBM-compatible computer or the Macintosh's built-in sound hardware, your computer can play back and record voice, music and sound effects.
If your system has MIDI capabilities, you can compose your own music and turn your computer into an orchestra or a controller for other electronic instruments. If you have a CD-ROM drive, you can use your sound board to play music CDs through your computer's speakers while you're working on another task.
Most IBM-compatible computers and all Macs currently come equipped with excellent sound hardware. Older PCs without sound can be upgraded with internal sound boards, which are sold separately or packaged with CD-ROM drives in "multimedia kits." If you don't have either a CD-ROM drive or a sound card, it's best to buy a kit and get the benefits of both worlds.
Macintosh computers have had basic sound capabilities ever since the machine was invented, and they've become favorites with composers and recording professionals. But upgrading older Macs to modern standards is difficult because many lack internal slots for expansion boards such as sound cards.
Sound cards have two basic functions: They record and play back digital sound, turning your computer into an electronic tape deck and mixing studio. Most also can play back MIDI files, generating their own music in simulations of multiple instruments.
In the 1970s and 1980s, scientists developed a new way to record music using computer-based equipment. They turned the sound waves into electronic impulses and "sampled" them, taking thousands of little snapshots every second. These samples could be converted into patterns of binary ones and zeros and stored on computer disks and tape. This digital record would never change with age, and it could always be reproduced precisely.
Over time, digital recording and playback became far more accurate than older, analog methods. Compact disc players brought digital sound to the mass market, and in the course of a few years they virtually wiped out the vinyl record industry. At the same time, computer makers began putting digital sound capabilities into their machines, through both built-in circuitry and sound boards.
Although they're not as sophisticated as professional equipment, sound cards can record, or "sample," sounds from a microphone, CD player, tape deck or other source. On most systems, you can also record directly from a music disc in the computer's CD-ROM drive.
After you've sampled a sound, you can record it digitally in a file on your disk. These files are known as "waveform," or simply "wave" files because they're digital reproductions of sound waves. Games and other multimedia programs often use waveform files to produce background music, voice and sound effects.
All multimedia computers come with simple software that allows you to record and play back waveform files. You can also buy sophisticated programs to edit and mix waveforms. Mixer software allows you to combine files from several sources, just like professional recording studios use mixers to get the right combination of voice and instruments.
The disadvantage of waveform files is that they take up a lot of space -- up to 10 megabytes per minute for high-quality reproduction.
A sound board's overall quality depends on its maximum sampling rate and the number of bits it uses to record information about each sample.
The sampling rate is the number of times per second a board can take a snapshot of the sound it's recording. For CD-quality sound, the sampling rate is 44,000 Hz, or 44,000 samples per second. Lower sampling rates cut off the higher and lower frequencies, but you can get acceptable voice recordings with sampling rates as low as 8,000 Hz. If you're recording, you don't have to use the maximum sampling rate -- in fact, it's often a waste of disk space.
The bit depth is the number of digital ones and zeros used to record information about each sample. The more information you record, the more accurately you can reproduce the sound. Older sound equipment used eight-bit sampling, but most newer sound cards use 16-bit samples. If you have a choice, get a 16-bit card.
MIDI synthesizers add another dimension to desktop computer sound -- the ability to generate music by imitating the sound of various instruments. If you've ever heard an electronic organ or keyboard, you've heard the results of a synthesizer. PC sound cards include synthesizers that can play through your speakers or headphones. But the quality and fidelity of the music they produce varies greatly, depending on how the synthesizer generates its sound.
Used by less expensive sound boards, FM synthesizers use electronics to produce combinations of waveforms that approximate the sounds of different instruments. Approximate is a key word here, because FM synthesizers often sound "tinny," like the house organ at the local skating rink. Most pure FM cards are also limited to playing a few instruments at a time.
Wave table synthesizers can produce remarkably lifelike-sounding instruments on your computer. They work by sampling and digitizing the sound from real instruments and storing it in read-only memory banks on the sound card. When your software tells the sound card to play a tuba, it's actually using the sound of a tuba.
Wave table synthesizers are built into more expensive sound cards. You'll pay more for a sound card that includes extra instruments and better quality samples. Good wave table cards can play up to 32 instruments at a time (enough for the whole orchestra), and some allow you to sample your own instruments and add them to your repertoire.
The Musical Instrument Digital Interface (MIDI) is a hardware and software standard developed during the 1980s to allow electronic instruments -- such as keyboards and drum pads -- to communicate with one another. One MIDI device can tell another device which sound and which notes to play. Over the years, these instruments have been defined as part of the General MIDI specification.
Many PC sound cards include MIDI hardware ports that allow your computer to communicate with other MIDI instruments. Most Macintosh computers use external MIDI interface boxes that connect to the computer's serial port.
Most sound-equipped computers can record and play MIDI "sequences," a function that's more important for average users. Sequences are sets of instructions for the electronic orchestra in your sound card. They're a lot like the printed score that a live orchestra uses.
Unlike digital sound files, MIDI sequences don't take up much space on your hard disk. For example, the MIDI sequence for Bach's entire Brandenburg Concerto No. 3 takes up less than 200 kilobytes of disk space. That's less than a couple of seconds of high-quality waveform sound.
MIDI sequences are also written according to a specific standard. While waveform sound files are stored in a variety of incompatible formats, you can play the same MIDI file on any computer and get something close to the same results.
One of the problems with MIDI notation is that it doesn't resemble standard music at all. It is more complex to write, even with good MIDI software. But if you're a musician, you can find programs that will let you compose regular musical scores on the screen and translate them into MIDI sequences. |