Data Acquisition Overview
SIGVIEW supports real-time data acquisition from the following device types:
• Windows compatible sound cards (up to 16-bit resolution, up to 2 channels, old-style drivers)
• DirectSound compatible sound cards (maximal supported sound card resolution, typically 24-bit for professional sound cards, typically all available channels)
• ASIO sound card drivers (by Steinberg®) for low-latency and high fidelity sound card recording
• Data acquisition simulation by reading data from a specially prepared signal file
SIGVIEW offers a generic user interface and settings for data acquisition from all supported devices. Device-specific settings (if any) are available through additional dialogs (by clicking on the "Default-specific settings" button). To understand the settings available in these custom dialogs, please consult the user manual for your specific data acquisition device.
To start data acquisition, select the “Data acquisition/Open data acquisition” menu option or the toolbar button. A dialog appears which allows you to choose the data acquisition device and to change the data acquisition settings:
Device type: Device type to be used. This corresponds to the above list of device classes supported by SIGVIEW. If you have no devices of specific type installed in your system, you will not see the corresponding entry in this combo-box.
Device: Actual device to perform data acquisition. Only available devices from the selected device type will be shown in the combo-box.
Device-specific settings button: Some devices allow special settings to be created, for example, hardware triggering, input range, gain, etc. By pressing this button, a new device type specific dialog will open, allowing you to change device specific settings. For more information about these settings, please consult your hardware manual for the data acquisition device. If there are no specific settings for currently chosen devices, this button will be disabled.
Calibration file: See calibrationtopic for details. Alternatively, some professional DAQ devices offer an “Automatic calibration” option. This means that SIGVIEW will try to obtain data in physical units (usually Volts) from the DAQ device.
Channels: You can choose one of the following:
• A number of channels to use for the acquisition (first [1...NumberOfChannels] channels of the device)
• Or the exact list of channels to use for the data acquisition (for example "1,4,7")
By pressing the "List" button, an edit box will appear, instead of the combo-box for the selection of the channels number. In the edit box, you will be able to enter a comma-separated list of channel indexes (for example "1, 4, 7") which should be used for the data acquisition. Please note that not all devices support this exact channel selection.
By pressing the "Count" button, you will switch back to the channel count selection mode.
Sample rate (samples/sec): This is the sampling rate for one channel. The maximum value is determined by your DAQ device capabilities. The combo box will show some standard types supported by most cards. If you want to use some other custom sampling rate, press the “Custom” button and enter the sample rate manually. Please note that some sound cards will simply report that they support any sample rate, even if they don’t support it in hardware. Those will simply try to simulate it on a driver level. If you want to avoid this, please consult the card's manual to determine which sample rates are really supported. Most devices do not allow arbitrary sample rate settings. SIGVIEW will try to find the next possible sample rate settings applicable to your device and apply it automatically. You will see the actual sampling rate in the status bar of the data acquisition window after you close this dialog.
Block length (samples): SIGVIEW retrieves signal from your DAQ device in blocks of a constant length. After one block is retrieved, it is displayed in the DAQ window and processed with any attached analysis functions. This option enables you to define the block size, i.e. how many samples will be retrieved from device and shown in a window during acquisition.
Recording length: This option enables you to define the overall length of the data acquisition. The following options are available:
• Absolute recording length (seconds): After the defined recording length in seconds has been reached, acquisition will stop. During acquisition, you will always see the last "Block length" samples in your acquisition window but you will see all samples after zooming-out. Please note that SIGVIEW stores all data in physical memory (RAM). Therefore, saving more than several minutes of data by using this method is not recommended. You can use the Logging data to file feature for that purpose
• One block: Only one block of data (as defined by "Block length") will be retrieved
• Continuous monitoring: If you want only to monitor the signal without any saving, use this option. Data acquisition will run until explicitly stopped. You will always see only the last "Block length" samples in your data acquisition window. No other samples will be stored
Decimation ratio: To reduce the amount of data which has to be processed during analysis, you can use the decimation feature, i.e. convert multiple samples from the DAQ device to one sample in SIGVIEW. You can define a decimation ratio by determining how many samples from the DAQ device will be converted to one sample in SIGVIEW. If you choose a decimation ratio greater then 1, SIGVIEW will calculate the actual sample rate and show it as a read-only information under this field. For example, if the sample rate on a sound card is 44100Hz, and the decimation ratio is 100, then the actual sample rate in SIGVIEW will be 441Hz.
Trigger recordings in windows with external trigger: Select this option if you want that the start of this recording (manual or triggered) also starts all other recordings which are waiting on "external trigger" (see below for more details about the "external trigger" type).
Start recording after trigger: If you check this option, recording will not start immediately after pressing the START button – SIGVIEW will wait for a specific trigger (i.e. signal level) value in the signal and then begin recording automatically. Trigger behavior and the definition of trigger value is determined by using the following options:
Trigger on channel: Determines which of the data acquisition channels SIGVIEW will monitor for a trigger value. It is possible to receive data on one channel and use the other one only for triggering.
Threshold (% full scale): This option determines which signal level is exactly regarded as a trigger value in the signal. Each sample with the amplitude higher than the threshold, by default 50%, from the maximum possible signal amplitude will be regarded as a trigger. For example, if your trigger is very strong signal impulse, you can set the threshold to 90%.
Trigger type: This option enables you to choose the type of trigger. The following types are available:
• Signal level: If signal level is above threshold (in % of the full scale), start acquisition
• Absolute signal level: If absolute signal level is above threshold (in % of the full scale), start acquisition
• Rising edge: If absolute signal level is above threshold (in % of the full scale), and the signal is rising, start acquisition
• Falling edge: If absolute signal level is above threshold (in % of the full scale), and the signal is falling, start acquisition
• External trigger: If external trigger type has been set, this recording will start after another window with the "Trigger recordings in windows with external trigger" option starts its recording. Please note that the start of this recording and trigger recording will not be perfectly synchronised as if using hardware-based triggering. You can expect to see a time difference of up to 1 block in length between recordings
Start recording X samples before/after the trigger: This option determines on which position, relative to the trigger appearance, SIGVIEW will start recording. The position is defined in number of samples. A negative value will cause SIGVIEW to start recording before the trigger and a positive value after the trigger. The maximal number of samples you can set for the pre-trigger option is equal to the "Block length" setting (see above).
Re-Arm trigger after each recording: After data acquisition is started after trigger and required signal length has been recorded, data acquisition would normally stop. By turning this option on, you can instruct SIGVIEW to automatically open a new data acquisition window after the triggered data acquisition has been completed. A new data acquisition window will use exactly the same settings as the previous one, i.e. will wait on trigger, record, and then start new data acquisition window. This way, you can let SIGVIEW automatically record multiple triggered signal events. Each recording will be stored in a separate window. To limit the number of new windows which can be created during this operation, you can set the "Max Re-Arm Count" option. Please note that trigger Re-Arm options make sense only if you define some specific recording length. If you use the value of zero (monitoring only), trigger will never be re-armed because recording will never stop.
Reuse the same window after Re-Arming: This option works similar to the above "Re-Arm trigger after each recording" option, but does not create a new window after each re-arming. Instead, each new acquisition is started in the same window, deleting all previously recorded samples. This can be used only in combination with the setting, Block length = "One block". If you attach an averager window to your acquisition window, you will be able to perform Time synchronous averaging. To limit numbers of retrieved data blocks, you can set the "Max Re-Arm Count" option.
For further information about the triggering, please see the How-To: Using data acquisition triggering options.
Starting/stopping data acquisition
After you set all these data acquisition parameters and press the OK button, one or more signal windows will appear (depending on the number of channels). Now you can press the button on the toolbar or select the “Data acquisition/Start” menu option to start data acquisition. If you choose "Continuous monitoring" as block length, you can stop it by pressing the button on the toolbar or selecting the “Data acquisition/Stop” menu option. Otherwise, data acquisition will stop automatically after the desired acquisition length is reached. While data acquisition is running, you can work with the “signal window” as you would with any window containing static signal: for example, you can perform FFT of a running signal, track its changes as a 3D graphic, and as a result, you will have a real 3D spectrum analyser.
After the data acquisition has finished, the signals in the data acquisition signal windows are zoomed in to show only the last recorded block. You will have to select the “Zoom-out” option to view the whole recorded signal.
You can also use the Control Window to start or stop multiple data acquisitions at once (context-menu option).
Resetting Data acquisition
After you have performed data acquisition and recorded some signal, you will not be able to start data acquisition in the same window again because it already contains recorded data.
If you would like to repeat the acquisition in the same window (for example, because the result of the first one was not satisfactory), you can reset the acquisition window by using "Data acquisition/Reset acquisition window" from the main menu. This function will bring the acquisition window to its initial state, enabling you to re-run data acquisition.
Using "File" driver
If you choose the ‘File’ driver type you will be able to simulate data acquisition by reading data from file in a 16-bit signed binary format (You can store any data in that format by using the Binary file export feature). After clicking on the “Device specific options” button, you will be able to choose the file name. SIGVIEW will try to keep the desired sample rate if there is sufficient data in the file.