Early Mockingboard models ranged from 3 voices in mono, while some later designs had 6 voices in stereo. Some software supported use of two Mockingboard cards, which allowed 12-voice music and sound. A 12-voice, single-card clone of the Mockingboard called the Phasor was made by Applied Engineering. The ZX Spectrum that initially only had a beeper had some sound cards made for it. Examples include TurboSound Other examples are the Fuller Box, and Zon X-81. The Commodore 64, while having an integrated SID chip, also had sound cards made for it. For example, the Sound Expander, which added on an OPL FM synthesizer. The PC-98 series of computers, like their IBM PC cousins, also do not have integrated sound contrary to popular belief, and their default configuration is a PC speaker driven by a timer. Sound cards were made for the C-Bus expansion slots that these computers had, most of which used Yamaha's FM and PSG chips and made by NEC themselves, although aftermarket clones can also be purchased, and Creative did release a C-Bus version of the SoundBlaster line of sound cards for the platform. External sound devices Devices such as the Covox Speech Thing could be attached to the parallel port of an IBM PC and fed 6- or 8-bit PCM sample data to produce audio. Also, many types of professional sound cards take the form of an external FireWire or USB unit, usually for convenience and improved fidelity. Sound cards using the PC Card interface were available before laptop and notebook computers routinely had onboard sound. Most of these units were designed for mobile DJs, providing separate outputs to allow both playback and monitoring from one system, however, some also target mobile gamers. USB sound cards USB sound cards are external devices that plug into the computer via USB. They are often used in studios and on stage by electronic musicians including live PA performers and DJs. DJs who use DJ software typically use sound cards integrated into DJ controllers or specialized DJ sound cards. DJ sound cards sometimes have inputs with phono preamplifiers to allow turntables to be connected to the computer to control the software's playback of music files with vinyl emulation. The USB specification defines a standard interface, the USB audio device class, allowing a single driver to work with the various USB sound devices and interfaces on the market. Mac OS X, Windows,