issue of latency, unrelated to monitoring latency, is the latency that
occurs in a card's inability to process and digitize audio in, shall
we say, a timely manner. This will result in the actual recorded audio
being a sample or two (possibly more) later than the incoming audio.
This issue will compound itself if the next recorded track is created
while referencing to the first track. In other words, the first track
will be 'x' number of samples late, and the second track will be x +
x samples late.
You won't often hear the question, "What's the input latency?" The question
more often asked is, "Is the card frame accurate?" I don't know why
this is. Maybe it seems more of a critical issue when synchronization
is involved, where digitized audio is immediately referenced to a source.
This issue spills over to any type of capture card, so it's even possible
that it originated in video capture while locking to timecode, more
the norm when capturing video. However you splice it, this is another
important latency issue. There are in fact audio cards on the market
that have zero input latency. Again, check with the manufacturer.
The third version of the latency issue involves a device that uses Windows
drivers, also referred to as MME (multi-media extensions). Windows latency
can be as high as 50 or 75 milliseconds, so that 'tape type' monitoring
is not feasible using only MME drivers. Waiting 50 or 75 ms is a heck
of a lot of time to hear the monitoring signal of something you've just
played. That's similar to the timing you'd use in a slapback delay.
A program like Steinberg's Cubase will have an ASIO MME emulation mode,
as well as ASIO DirectX. These would be the logical choices if using
an ASIO compliant program and an audio card that does not provide ASIO
drivers. If your sound card utilizes DirectX drivers, ASIO DirectX would
be your first choice over ASIO MME.
Latency with DirectX
DirectX is part of the Windows operating system, and basically gives
programmers faster communication between the application and the hardware.
This is the same attempt that ASIO makes, only its less effective. DirectX
is constantly updated, and there are certainly newer versions than the
one that came with Win95 or 98. For the record, there are different
components of DirectX for video and audio, and "Direct Sound" is the
audio portion of DirectX. Many software synths on the PC side use DirectX,
so it is essential that your audio device driver be DirectX compliant.
Audio latency when playing software synthesizers is a side issue, but
involves the "L" word nonetheless.
Getting Around the Latency Issue,
Mac & PC
The way that the digital recordist can get around the issues of latency
while monitoring is to monitor the hardware inputs of the audio card.
Pre-ASIO, this was common practice. Most sound cards have a control
panel that would allow you to select the hardware input as your monitoring
source, or designate at which output you wish to monitor the input signal.
Other sound cards will use the Windows mixer, accessed by double clicking
the speaker icon on the taskbar or by going to Control Panel | Multimedia
| Audio Properties or some variation thereof in Windows 2000). In the
Mac OS, Control Panel | Sounds will allow you to monitor the inputs
by simply checking a checkbox.
In the Mac though, ASIO is almost a necessity since Sound Manager limits
you to only one stereo pair for both input and output (though the input
and output pair may be individually selectable). Most audio programs
on the Mac these days use ASIO or a similar format, though most likely
a proprietary one (such as Emagic's EASI or Motu's MAS). One stereo
pair is often fine for editing, but you didn't buy a multi-port sound
card to be limited to one stereo pair. Windows itself is limited to
one stereo pair, but the programs that use MME override the Windows
settings and enable multiple stereo ports.
By monitoring the hardware inputs while recording, one can quite effectively
use a program that utilizes Windows MME, such as Cakewalk. Even Steinberg
has created a way to get around the ASIO latency with their "Direct
Monitoring" option (ASIO 2 only) in their Audio Setup page, but some
sound cards limit its functionality.
Ultimately, finding a way for programs to monitor the hardware inputs
that disables them while in play mode and enables them in record mode
will be the most effective means by which we perform hard disk recording.
Latency is not really an acceptable commodity, in my opinion, if we're
talking about a pro audio format. The solution may lie in a direct communication
between the software and the sound card via the sound card's control
JD Mars is the producer of DigitalProSound. He has extensive experience
in the professional audio business, including stints as the Chief Engineer
at Duplex Sound in NYC and in technical support at Midiman/M Audio.