Sampling and Digitizing the Sound

Optimizing CD-ROM Cache

Recall that for Windows 9x, VCACHE replaced SmartDrive in Window’s 3.x as the disk caching software. For removable drives, VCACHE will cache when reading data but not when writing, VCACHE decides how much memory to use when caching data, based on the speed of the CD-ROM drive and how much memory is installed in the system. You can affect this decision using the Performance tab in System Properties. Click Start, Settings, Control Panel, and select System. From the properties box, click the Performance tab and then click File System. Click CD-ROM on the File System Properties box. By changing the CD-ROM speed in this box, you are changing the amount of memory allotted to the cache. The amount is displayed in the last sentence in this box.

CD-R and CD-RW Drives

A CD-ROM is a read-only medium meaning that CD-ROM drives can only read, not write. Until recently, writing to a CD required expensive equipment and was not practical for personal computer use. Now, CD-Recordable (CD-R) drives cost around $300, and the CD-R disc cost less than $5, making “burning” your own CD a viable option. These CD-R discs can be read by regular CD-ROM drives and are excellent ways to distribute software or large amounts of data. Besides allowing for a lot of data storage space on a relatively inexpensive medium, another advantage of disturbing software and/or data on a CD-R disc is that you can be assured that no one will edit or overwrite what’s written on the disc.

A regular CD-ROM is created by physically etching pits into the surface of the disc, but a CD-R disc is created differently. Heat is applied to special chemicals on the disc and causes these chemicals to reflects less light than the areas that are not burned, thus creating the same CD-R is actually fairly accurate. When you purchase and install a CD-R drive, good software to manage the writing process is an important part of the purchase, because some less robust software can make burning a disc a difficult process. Also, some CD-R drivers are multisession drives and some are not.

Also available at a higher cost is a rewritable CD (CD-RW), which allows you to overwrite old data with new data. The process of creating a CD-RW disc is similar to that used by a CD-R disc. The chemicals on the surface of the CD-RW disc are different, allowing the process of writing a less-reflective spot to the surface of the disc to be reversed so that data can be erased. One drawback to these CD-RW discs is that the medium cannot always be read successfully by older CD-ROM drives.

CD-RW discs are useful in the process of developing CDs for distribution. A developer can create a disc, test for errors, and rewrite the disc without having to waste many discs during the development process. Once the disc is fully tested, then CD-R discs can be burned for distribution. The advantages of distributing on CD-R disc rather than CD-RW discs are that CD-R discs are less expensive and can be read by all CD-ROM drives.

Sound Cards

A sound card is an expansion card that records sound, saves it to a file on your hard drive, and plays it 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, 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 play CD audio by way of a cable connecting the CD to the sound card. For good quality sound, you will definitely need external speakers and perhaps an amplifier.

The three stages that sound goes through when it is computerized are (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).

Each of these stages is discussed next, followed by a discussion of sound card installation.

Sampling and Digitizing the Sound

Remember that converting sound from analog to digital storage is done by first sampling the sound and then digitalizing it. Sampling and digitizing the sound are done by a method called pulse code modulation (PCM) and involves a component called an analog-to-digital converter (A/D or ADC). It follows that the opposite technology which converts digital to analog, is also needed, and that conversion is done by a digital-to-analog converter or DAC. The DAC technology on a sound card converts digital sound files back into analog sound just before output to the speakers.

When recording sound, the analog sound is conserved to an analog voltage by a microphone and is passed to the sound card, where it is digitalized. As explained earlier in the chapter, the critical factor in the performance of a sound card is the accuracy of the samples (determined by the number of bits used to hold each sample value, which can be either 8 or 16 bits). This number of bits is called the sample size. 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. Remember that a low sampling rate provides a less accurate representation of the sound than does a high sampling rate. Our ears detect up to about 22,000 samples per second or hertz. The sampling rate of music CDs is 44,100Hz or 44.1kHz. When recording sound on a PC, the sampling rate is controlled by the software.

As explained above, the 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 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 +127rather 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 +32,767, or a total of 65,536 values.