Latency is the time a message takes to traverse a system. For music recorded via computer, latency is major concern. A human playing an instrument, for example, needs nearly instantaneous feedback from that instrument in order to play it correctly. While this is generally not a problem with non-digital instruments, audio routed through a computer always has some delay in the signal path. Latencies higher than 100 ms make working with real-time music programs or instruments impossible, and many musicians find much lower latencies objectionable. While virtually every digital process involves some latency (just converting a signal to digital and back to analog takes some small amount of time) there are some systems where it is much more of an issue than others. Historically host based computer recording systems (ones that don’t rely on dedicated audio processing hardware, but use the computer’s CPU for instead) have been the worst offenders. A TDM based Pro Tools DAW, for example, has virtually no latency because the computer is merely acting as a host while most of the audio processing is done on the DSP cards residing in the computer.
Out of the need for low-latency interconnects, Steinberg created ASIO, a protocol designed for low-latency transmission (on the order of a few ms) of digital instrument and other music data. The term ‘Zero Latency Monitoring‘ was introduced in 1998 by RME with the DIGI96 series of audio interfaces and refers to the technique of routing the input signal directly to the output on the audio card. This has become one of the most important features of modern, host based hard disk recording.
Progress is continually being made in lowering the latency of these systems. With ASIO Direct Monitoring (ADM, since ASIO 2.0), Steinberg has not only introduced Zero Latency Monitoring to ASIO, but also extended it substantially. ADM also allows for monitoring the input signal via the hardware in real-time. Over and above that, ADM supports panorama, volume and routing, which requires a mixer (i.e. DSP functionality) in the hardware though. Thus it is possible to copy a routing through a software mixer into the hardware in real-time, so that the sound difference between playback and monitoring is very small. In total, ADM renders a substantial step towards ‘mixer and tape recorder inside the computer’. There are similar advancements being achieved with other brands. On the whole zero latency monitoring is a reality now, but there are still some compromises to be made in terms of workflow to achieve it. The only easy way around this is still to go with more costly solutions until processing speeds allow the power and flexibility of dedicated systems to be truly replicated with host based systems.