Data AcQuisition And Real-Time AnalysisScope - Spectrum - Spectrogram - Signal Generator
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Sample Rate Select
The Sample Rate button in the X-Axis Dialog opens a separate Sample Rate dialog with buttons for popular sample rates: 11025, 12000, 22050, 24000, 44100, 48000, 88200, 96000, 176400, and 192000 Hz.
The default is 48000 samples per second (Hz) if your card supports it, otherwise 44100 samples per second.
Selecting one of these buttons closes the Sample Rate dialog and forces the Rate Adjust control (in the X-Axis dialog, just below the Sample Rate button) to the selected value. Alternatively, you can directly enter any arbitrary value into the Rate Adjust control.
Important: Note that although many modern sound cards are capable of samples rates higher than the default 48000 Hz (upper limits of 96000 or even 192000 Hz are becoming common, even on inexpensive laptop chipsets), their drivers may have 48000 set as the upper limit. The problem is that Windows doesn't reject higher rates, it just simulates them using sample rate conversion (SRC). This doesn't really give the rate you request, and may introduce low-level distortion as well, but you won't be aware of that without careful testing. (See Sample Rate Adjust for more info.)
To enable the higher rates, you will need to delve into Windows Control Panel or your card's proprietary utility. Try this first on Vista/7: Right-click the speaker icon in the system tray and click Playback Devices. In the dialog that opens, click on Speakers and then click on Properties at the lower right. The Speakers Properties dialog will open; click the Advanced tab and select the desired sample rate from the list. Repeat all of this for Input Devices.
Daqarta acquires 1024 samples per display. For a waveform, the resolution is the sample period, which is the reciprocal of the sample rate. The resolutions and X-axis extents for the full 0-1023 samples are:
Rate Period X-axis Hz microsec millisec 11025 90.703 92.79 12000 83.333 85.25 22050 45.351 46.39 24000 41.667 42.63 44100 22.676 23.20 48000 20.833 21.31 88200 11.337 11.60 96000 10.416 10.66 176400 5.668 5.799 192000 5.208 5.328
For a spectrum, those 1024 samples become 512 spectral lines ranging from 0 (DC) to just under half the sample rate (known as the Nyquist frequency). The resolution is the sample rate divided by 1024. The resolutions and X-axis extents for lines 0-511 are:
Rate Line X-axis Hz Hz Hz 11025 10.77 5501.73 12000 11.72 5988.28 22050 21.53 11003.47 24000 23.44 11976.56 44100 43.07 22006.93 48000 46.88 23953.31 88200 86.13 44013.87 96000 93.75 47906.25 176400 172.3 88027.73 192000 187.5 95812.50
The astute observer will note that higher sample rates give better time resolution but narrower overall axis time for waveforms, while lower sample rates give better resolution and lower overall frequency range for spectra.
If you change the sample rate, you may need to adjust certain Generator controls. In particular, Burst durations are stored internally as sample counts, based upon the sample rate in effect when they were originally set. For example, if you set a High duration of 0.100 sec with a sample rate of 44100 hertz, it is stored as 4410 samples. Changing the sample rate to 22050 Hz doesn't change the number of samples, so it doubles the effective duration because each sample takes twice as long.
Caution: Changing Sample Rate causes all Generator frequencies (including modulation rates) to be updated using the current Frequency Entry Step Mode. For Step Modes that use quantization (those other than Direct), this may cause unexpected results. Always set Sample Rate before setting Generator frequencies.
However, note that you can use Decimation to view signals at a slower effective sample rate, while maintaining an unchanged high actual Generator output rate.
Note that sample rate controls are disabled during averages.
SmplRate=44100 sets the Sample Rate to 44100 samples per second.
See also X-Axis Control Dialog
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