A sound card glossary - What you need to know

This week we look at all the jargon and important details about your sound card that you always wanted to know, but were too polite to ask…


The term latency refers to the time difference between an event occurring in a game or audio software, and the actual playback of the sound. This delay is usually measured in milliseconds, with 12ms being considered acceptable for real time audio applications. Low latency therefore is desirable because it provides an almost real time feel to software.


There are four common standards available in Windows for sound cards: DirectX, ASIO, MME, and WDM. Some cards may or may not have any or all of these available. So when you buy a sound card, be sure to check the driver requirements for any audio software you plan to use.


DirectX is a Windows technology created by Microsoft that provides a fast interface between software and multimedia hardware. Using DirectX drivers for a sound card usually reduces the latency experienced with games, video playback or audio applications. DirectX is supported in Windows 9x, Me, NT, 2000 and XP.

[ Figure 1 - DirectX Driver providing 46ms latency ]


ASIO stands for Audio Stream In/Out and is a technology created by Steinberg, the company responsible for the Cubase recording software product. ASIO, like DirectX, was designed to reduce latency by providing direct access to hardware - specifically, sound cards. ASIO 2.0 is the latest version, which provides some additional features such as hardware sharing between applications and enhanced audio/MIDI synchronisation. Generally speaking, ASIO is really only utilised in professional audio and music applications, although some MP3 DJ software supports it due to the associated low latency. ASIO is supported in Windows 9x, Me, NT, 2000 and XP and can provide latency as low as 2-10ms.

[ Figure 2 - ASIO driver providing 8ms latency ]


MME stands for Multimedia Extensions and was introduced as part of Windows 3.0. This is an old technology, available only on Windows 3.x, Me and 9x. Although recent versions of MME drivers can compete with DirectX in terms of latency, MME drivers should only be used where DirectX drivers are not available. Usually, MME drivers provide about 300-1000ms latency.


These Windows Driver Model drivers are a more recent addition to Windows 98, Me, 2000 and XP. They are often ‘translated’ from DirectX or MME drivers. Although this often introduces 30ms or more extra latency, some WDM drivers can provide latency under 10ms, making them a good alternative to ASIO, for example.

Signal to Noise Ratio (SNR)

This is calculated by comparing the amplitude of the desired sound to the noise signals generated by the sound card. This is usually defined in terms of decibels (dB). Noise can be a result of low quality components, poor insulation, and interference from other hardware such as spinning hard drives. A good SNR is usually around the 100dB range.


Digital Signal Processors provide real time audio shaping such as equalisation, reverb and other effects. Some DSPs are purely software (such as Winamp plug-ins), though some require specialised hardware for CPU intensive processing.

[ Figure 3 - DSP plug-in for Winamp ]


An abbreviation for Sony/Phillips Digital Interface, S/PDIF provides digital connectivity using either an RCA (coaxial) or TOSLINK (optical) cable.


Musical Instrument Digital Interface is an almost 20-year-old standard for synthesisers and other electronic musical devices to communicate. Using a specialised cable connected to the joystick port of your sound card, MIDI notes and messages can be sent to MIDI instruments. Alternatively, most sound cards contain an on-board MIDI synthesiser, which is used when MIDI files are played back from your PC.


Duplex operation is the name given to simultaneous recording and playback of audio with a single sound card. This is a requirement for most multi-track audio software, which simulates the function of multi-track tape recorders.

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Daniel Potts

PC World
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