There are basically two types of sound computers can make: digitally sampled sound (WAV) and synthesized sound (MIDI).
Sampled sound is created by recording actual sound waves, running them through an analog to digital converter (which converts the continuous sound levels into discrete digital numbers) and storing them. On your sound card these digital numbers are passed through a digital to analog converter which outputs a voltage level which is sent over the wires to your speakers, making the speakers move and reproducing the sound waves. More or less.
The quality of digital sound depends on two factors: the accuracy of the A/D and D/A converters, (normally stated in bits; the more bits that are used, the more discrete volume levels that can be represented) and the sampling frequency (higher sampling frequencies can capture higher pitched sounds.) Your audio CD contains 16-bit samples at 44,400 samples per second per channel. Typical internet-phone applictions use 8-bit samples at 11,100 samples per second, with one channel (mono).
16-bits x 44400 x 2 = 710,398 bits per second of audio data. A typical modem connection is 28,800 to 36,000 bits per second. Therefore, CD-quality audio data has to be compressed at least 25x to transfer over a modem in real-time. Continuous data transfer rates over the internet are more like 5000 bits per second, which would require compression at 140x. You can't get lossless compression at that high a ratio, so you're simply not going to transfer CD quality audio over the internet with today's bandwidth. Cable modems could handle it, ISDN could handle it, and of course most dedicated lines could handle it. But modems can't.
Synthesized sound (midi) is far more compact, which is why it's already used on lots of websites. Very generally speaking, midi is like printed music; your wavetable synthesis card reads the printed music, and the instruments that are supposed to play each track, and it synthesizes the sound of each instrument playing its part of the music. The quality depends upon both the composer of the music and the quality of your wavetable synthesis. Midi works fine over the internet, but you're not going to find many audio CD's translated into midi. Even the best wavetable synthesis card can't reproduce the sound of a real non-digital instrument. All midi music sounds like it came from a synthesizer keyboard.
Doug. |
