DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

ARB BASICS 271complex waveform can be created within WaveWorks Pro by looping and seamless linkingof previously created waveforms.As you can imagine, the introduction of cheaper, faster, high-resolution DACs, widerRAMs, and higher-performance microprocessors is making it possible for digital waveformgenerators to replace analog sources rapidly in many applications. High-performanceintegrated DDS generators have taken over in the spread-spectrum communications fieldand are the key elements that enable low-cost high-speed data links to be integrated into awide variety of patient monitors and wirelessly networked medical instruments.Arbs are also becoming very popular with design and test engineers. Arbs are more versatilesources than their analog counterparts. In fact, even when generating “standard”waveforms, arbs can compete with analog generators. For example, arbs can output rampsand 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 theone generated by an analog circuit.Of course, the neat control and waveform-design screens of commercial arbs, theirpowerful DSP microprocessors, and their exotic high-frequency mixed-mode circuitrymakes 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 bandwidthcosts just a few hundred dollars. So don’t feel that your reliable analog waveform generatorno longer deserves its space on the workbench, but keep the arb in mind when yourapplication demands ultimate flexibility without a compromise on performance.PC Sound Card as an ArbThe PC sound card is a true audio-range arb (useful in the range of 20 Hz to 20 kHz). Ittakes the waveform definition stored in the computer’s memory and plays it back as ananalog signal. The simplest way of generating an arbitrary wave through the PC sound cardis to store it as a .wav file and play it back using Window’s Media Player utility. A .wavfile is just a series of samples, preceded by a header that tells the player program importantthings such as the sampling rate and the number of bits in the sample. The player programreads the header, sets up the sound card, and then feeds the samples to the card’sdigital-to-analog converter.PC multimedia data are often encoded in the RIFF file format. RIFF is based on chunksand subchunks. Each chunk has a type, represented by a four-character tag. This chunk typecomes first in the 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 ofchunks: (1) the format (fmt) chunk, which describes the sample rate, sample width, and soon, and (2) the data chunk, which contains the actual samples. .wav can also contain anyother chunk type allowed by RIFF, including LIST chunks, which are used to containoptional kinds of data, such as the copyright date and author’s name. Chunks can appear inany order. In its simplest form, the .wav format starts with the RIFF header of Table 6.3.TABLE 6.3Contents of the RIFF Header of a .wav FileOffset Length (bytes) Contents0 4 RIFF4 4 file length - 8where the “8” is the length of the first two entries(i.e., the second entry is the number of bytesthat follow in the file)8 4 WAVE