602 Chapter 20
audio fi le’s original fi delity. Layer 1 takes the least amount of time to compress, but layer
3 yields higher compression ratios for comparable quality fi les.
20.2.5 VOC
Creative Voice (.voc) is the proprietary sound fi le format that is recorded with Creative
Lab’s Sound Blaster and Sound Blaster Pro audio cards. This format supports only 8-bit
mono audio fi les up to sampling rates of 44.1 kHz and stereo fi les up to 22 kHz.
20.2.6 Raw PCM Data
Raw Pulse Code Modulated (PCM) data are sometimes identifi ed with the .pcm, but it
sometimes has no extension at all. Since no header information is provided in the fi le,
you must specify the waveform’s sample rate, resolution, and number of channels to the
application to which it is loaded.
20.3 Sound Cards ..........................................................................................................
Available in a bewildering array of different guises, for serious audio work, 16-bit cards
only are suitable, and even then beware of very poor noise levels. Computers differ
widely in their suitability as devices for high-quality audio. The Creative Technology
Ltd. Sound Blaster card family has some of the most widespread sound cards used in the
PC world. Supplied standard with a four operator FM sound generator chip for sound
synthesis, Creative Labs offer a wavetable-based upgrade. Sound Blaster ships with sound
fi le editing software. The card comes with a utility program for controlling the analogue
mixer on the card where the various sound sources are combined and routed. This utility
is called Creative Mixer and it’s illustrated in Figure 20.5. Note that fader style controls
are implemented in software so as to provide a familiar user interface. Control over
CD, MIDI synthesizer, and WAV fi le replay, as well as line and microphone inputs, are
provided. Global (all sources) equalization is also provided.
20.4 PCI Bus Versus ISA Bus .......................................................................................
Most PCs, until the arrival of the Pentium, were provided with a PC/AT bus (or ISA bus)
for connecting peripherals (such as sound cards, frame grabbers, and so on). The ISA bus
operates with 16-bit data bus and a 16-bit address bus and operates with a divided clock.
The ISA bus limits real-world transfer rates to around 1–2 Mbytes/s, which is just enough