Sound Cards, 8-Bit And 16-Bit Sound Cards

Sound Cards

A sound card is an expansion card that records sounds, saves them to a file on your hard drive, and plays them back. Some cards give you the ability to mix and edit sound, and even to edit the sound using standard music score notation. Sound cards have ports for external stereo speakers and microphone input.

Also, sound cards may be Sound Blaster compatible (as shown in the figure below), which means that they can understand the commands sent to them that have been written for a Sound Blaster card, which is generally considered the standard for PC sound cards.

Some sound cards play CD audio by way of a cable connecting the CD to the sound card. For good quality sound, you definitely need external speakers and perhaps an amplifier.

Sound passes through three stages when it is computerized: (1) digitize or input the sound, that is, convert it from analog to digital, (2) store the digital data in a compressed data file, and later (3) reproduce or synthesize the sound (digital to analog).

Sample Rate And Size

Sound is analog, but computers are digital. Computer systems need a way to convert from one to the other. The critical factor in the performance of a sound card is the accuracy of the samples as determined by the number of bits used to hold each sample value. This number of bits is called the sample size .

A sound card uses an analog-to-digital converter (A/D or ADC) to convert sound into digital values that can be stored on hard drives. This process, sometimes called sampling, is done by the PCM (pulse code modulation) method.

The analog sound is converted to analog voltage by a microphone and is passed to the sound card where it is digitized. The sampling rate of a sound card (the number of samples taken of the analog signal over a period of time) is usually expressed as samples (cycles) per second or hertz. One thousand hertz (one kilohertz) is written as kHz.

Sample size is the amount of space used to store a single sample measurement. The larger the sample size, the more accurate the sampling will be. The sampling rate of music CDs is 44,100 Hz, or 44.1 kHz. When you record sound on a PC, the sampling rate is controlled by the software.

The number of values used to measure sound is determined by the number of bits allocated to hold each number. If 8 bits are used to hold one number, then the sample range can be from –128 to 128. This is because 1111 1111 in binary equals 255 in decimal, which, together with zero, equals 256 values. Samples of sound are considered to be both positive and negative numbers, so the range is –128 to 128 rather than 0 to 255. However, if 16 bits are used to hold the range of numbers, then the sample range increases dramatically because 1111 1111 1111 1111 in binary is 65,535 in decimal, meaning that the sample size can be –32,768 to 32,768, or a total of 65,536 values.

8-Bit And 16-Bit Sound Cards

An 8-bit sound card uses 8 bits to store a sample value, or uses a 256-sample size range. A 16-bit sound card has a sample size of 65,536.

Sound cards typically use 8- or 16-bit sample sizes with a sampling rate from 4,000 to 44,000 samples per second. For quality sound, use a 16-bit sound card. Samples may also be recorded on a single channel (mono) or on two channels (stereo).

Don’t confuse the sample size of 8 bits or 16 bits with the ISA bus size that the sound card uses to attach to the systemboard. A sound card may use an 8-bit sample size but a 16-bit ISA bus. When you hear someone talk about an 8-bit sound card, they are speaking of the sample size, not the bus size.