Biomedical Engineering Reference
In-Depth Information
complex waveform can be created within WaveWorks Pro by looping and seamless link-
ing of previously created waveforms.
As you can imagine, the introduction of cheaper, faster, high-resolution DACs, wider
RAMs, and higher-performance microprocessors is making it possible for digital wave-
form generators to replace analog sources rapidly in many applications. High-performance
integrated DDS generators have taken over in the spread-spectrum communications
eld
and are the key elements that enable low-cost high-speed data links to be integrated into a
wide variety of patient monitors and wirelessly networked medical instruments.
Arbs are also becoming very popular with design and test engineers. Arbs are more ver-
satile sources than their analog counterparts. In fact, even when generating “standard”
waveforms, arbs can compete with analog generators. For example, arbs can output ramps
and triangle waves with higher linearity and sharper corners than an analog generator.
Similarly, an arb-generated sine can have far better THD and frequency accuracy than the
one generated by an analog circuit.
Of course, the neat control and waveform-design screens of commercial arbs, their
powerful DSP microprocessors, and their exotic high-frequency mixed-mode circuitry
makes them costly pieces of equipment. Most commercial arbs are priced in the range
$3000 to $7000. On the other hand, an analog signal generator with similar bandwidth
costs just a few hundred dollars. So don't feel that your reliable analog waveform genera-
tor no longer deserves its space on the workbench, but keep the arb in mind when your
application demands ultimate
fi
fl
flexibility without a compromise on performance.
PC Sound Card as an Arb
The PC sound card is a true audio-range arb (useful in the range of 20 Hz to 20 kHz). It
takes the waveform de
nition stored in the computer's memory and plays it back as an
analog signal. The simplest way of generating an arbitrary wave through the PC sound card
is to store it as a .wav
fi
fi
file and play it back using Window's Media Player utility. A .wav
fi
file is just a series of samples, preceded by a header that tells the player program impor-
tant things such as the sampling rate and the number of bits in the sample. The player pro-
gram reads the header, sets up the sound card, and then feeds the samples to the card's
digital-to-analog converter.
PC multimedia data are often encoded in the RIFF
file format. RIFF is based on chunks
and subchunks. Each chunk has a type, represented by a four-character tag. This chunk type
comes
fi
file, followed by the size of the chunk, then the contents of the chunk. The
.wav format is a subset of RIFF used for storing digital audio and requires two types of
chunks: (1) the format (fmt) chunk, which describes the sample rate, sample width, and so
on, and (2) the data chunk, which contains the actual samples. .wav can also contain any
other chunk type allowed by RIFF, including LIST chunks, which are used to contain
optional kinds of data, such as the copyright date and author's name. Chunks can appear in
any order. In its simplest form, the .wav format starts with the RIFF header of Table 6.3.
fi
first in the
fi
TABLE 6.3
Contents of the RIFF Header of a .wav File
Offset
Length (bytes)
Contents
0
4
RIFF
4
4
file length - 8
where the “8” is the length of the first two entries
(i.e., the second entry is the number of bytes
that follow in the file)
8
4
WAVE
Search WWH ::
Custom Search