Blaster series and their compatibles, had only one or two channels of digital sound. Early games and MOD-players needing more channels than a card could support had to resort to mixing multiple channels in software. Even today, the tendency is still to mix multiple sound streams in software, except in products specifically intended for gamers or professional musicians. Outputs The number of physical sound channels has also increased. The first sound card solutions were mono. Stereo sound was introduced in the early 1980s, and quadraphonic sound came in 1989. This was shortly followed by 5.1 channel audio. The latest sound cards support up to 8 audio channels for the 7.1 speaker setup. A few early sound cards had sufficient power to drive unpowered speakers directlyfor example, two watts per channel. With the popularity of amplified speakers, sound cards no longer have a power stage, though in many cases they can adequately drive headphones. Professional sound cards Professional sound cards are sound cards optimized for high-fidelity, low-latency multichannel sound recording and playback. Their drivers usually follow the Audio Stream Input/Output protocol for use with professional sound engineering and music software. Professional sound cards are usually described as audio interfaces, and sometimes have the form of external rack-mountable units using USB, FireWire, or an optical interface, to offer sufficient data rates. The emphasis in these products is, in general, on multiple input and output connectors, direct hardware support for multiple input and output sound channels, as well as higher sampling rates and fidelity as compared to the usual consumer sound card. On the other hand, certain features of consumer sound cards such as support for 3D audio, hardware acceleration in video games, or real-time ambiance effects are secondary, nonexistent or even undesirable in professional audio interfaces, and as such audio interfaces are not recommended for the typical home user. The typical consumer-grade sound card is intended for generic home, office, and entertainment purposes with an emphasis on playback and casual use, rather than catering to the needs of audio professionals. In general, consumer-grade sound cards impose several restrictions and inconveniences that would be unacceptable to an audio professional. Consumer sound cards are also limited in the effective sampling rates and bit depths they can actually manage and have lower numbers of less flexible input channels. Professional studio recording use typically requires more than the two channels that consumer sound cards provide, and