Biomedical Engineering Reference
In-Depth Information
first and last channels, a situation that is intolerable for
many applications. The way of eliminating time skew between channels is to sample-and-
hold each channel individually at the same time. Digitization can then occur at the A/D's
leisure. Most often, however, the impact of channel skew is minimized by a technique
called
burst sampling
. Here, all EMG channels would be sampled at the maximum rate of
the converter once every 1/10,000 of a second. Assuming the use of a 1-MHz A/D, the
maximum time skew between the
time skew of 3.1 ms between the
fi
s.
Channel skewing has become a very critical consideration, since multichannel record-
ings enjoy wide popularity in research and diagnostic uses of electrophysiological activity.
For example, 32-, 64-, and even 128-channel systems are commonly used in the acquisi-
tion of EEG and evoked potentials; and 64-channel systems are now commercially avail-
able for body potential mapping (BPM) ECG.
fi
first and last channels drops to barely 31
µ
FREE DATA ACQUISITION CARD IN YOUR COMPUTER
If you need to measure one or two audio-frequency signals that do not include a dc compo-
nent, don't overlook the data-acquisition card that your PC probably has already—your PC's
sound card. Audio-range signals can be applied to a sound card via stereo
8
-in. mini jacks.
Inside the sound card, these signals are ac-coupled, with one pair of inputs being sent through
preampli
1
ers for boosting low-level signals (intended to amplify microphone signals), and
another pair feeding signals within a
fi
0.5 V
p-p
range directly into the card's analog multi-
plexer/mixer. The input impedance of a sound card is typically above 10 k
, and channels
typically respond within the frequency band 20 Hz to 20 kHz. Signals at the output of the
mixer/multiplexer are digitized by a delta-sigma A/D that is part of the sound card's codec IC.
Consumer-grade sound cards o
Ω
er 11.025-kHz (“telephone quality”), 22.05-kHz (“music
quality”), and 44.1-kHz (“CD quality”) sampling rates. Low-cost sound cards often use a sin-
gle codec chip that is multiplexed between the two input channels, which means that these
sound cards would sample both inputs only at 22.05 kHz or lower. Better cards feature two
A/D converters or at least two sample-and-hold devices so they can sample both inputs simul-
taneously at rates up to 44 kHz. There are some “professional-quality” sound cards that fea-
ture more input channels, higher resolution, and dc input response, but they require proprietary
software to operate, and their prices are similar to those of a full-function data acquisition card.
Of course, the most common use for a sound card is to generate the various annoying
sounds of explosions, phasers, and kicks that make video games so realistic. The output
jack of a typical sound card carries an ampli
ff
Ω
speakers directly with some 2 W of power. Sounds are generated by using an FM synthe-
sizer within the sound card to combine harmonically related sine waves or by selecting
data from a wave table that contains digitized samples from various musical instruments.
A third option available on higher-end sound cards is to use an onboard musical instrument
digital (MIDI) synthesizer to produce an even wider variety of sounds.
There is quite a bit of free software available on the Web to turn a PC equipped with a
sound card into all sorts of audio-range virtual instruments, such as two-channel oscillo-
scopes, spectrum analyzers, signal generators, frequency counters, and noise generators.
The following are some of the freeware packages that deserve special attention.
fi
ed ac-coupled signal capable of driving 8-
Signal Generator
• BIP Electronics Labs Sine Wave Generator v3.0 (sine30.zip, freeware for Windows
3.1, but works well in most cases under Windows 9x) by Marcel Veldhuijzen
• Sweep Sine Wave Generator v2.0 (swpgen20.zip, freeware for Windows 9x) by
David Taylor
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