Data AcQuisition And Real-Time AnalysisScope - Spectrum - Spectrogram - Signal Generator
Software for Windows
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Left / Right (Output Enable)
These buttons are simply marked "Left" and "Right", and are located just below the Generator On/Off button in the Generator dialog. They activate or disable the corresponding output channels.
To activate either output, at least one of its component streams must be active. If not, the output will technically be on, but the output level will be zero.
If only one channel is active, the sound card is operated in mono mode, which sends the same signal to both outputs. If you want only one output to be active, use the Solo button.
Note that the Multi-Channel Outputs dialog button is disabled unless both Left and Right outputs are selected. Conversely, when the Multi-Channel Outputs On button is active in that dialog, even if the Generator is off so there is no actual output, these Left and Right output buttons are disabled.
Note that when you change most controls in the Generator, such as wave shape or frequency, Daqarta momentarily stops the sound output, computes the new signal using the changed parameter, and restarts. This gives "immediate" response, but typically results in an audible click which may be unacceptable in some experiments.
The Dual, Solo, and Swap buttons are provided especially for psychoacoustic experiments where you want to change which signal goes to each output, without producing a conspicuous click during switching. Since toggling between mono and stereo modes would typically produce such a click, these buttons allow the card to run in stereo and simply send nulls to an output that is unused. Toggling an output on or off just entails sending different data, not a mode change.
But an abrupt transition to a different data stream can produce a click. Consider the case of a low-frequency sine wave. If you turn it off abruptly while the output waveform happens to be at a peak, there will be an audible transient. To avoid that, it is necessary to fade the sine wave gradually to zero. Daqarta uses a fade length of 1024 samples for fading a signal up or down, which amounts to over 21 milliseconds at a 48 kHz sample rate.
Note, however, that all these changes take place in the data stream. The Generator creates the data stream and sends it to a series of buffers, which Windows sends to the sound card each in its turn. When one buffer runs out, the next one is sent. The empty buffer is then available to receive new Generator data.
This buffer arrangement allows for a multitasking system like Windows to be able to run other concurrent tasks, without needing to spoon-feed the sound card with every sample. When a buffer runs out, Windows can momentarily stop whatever other task is running and switch to another buffer, without actually granting Daqarta any time to create new data. If the other task needs to perform some slow high-priority operation, it might be a while before Daqarta gets control again. So there must be enough data in the buffer system to ride out any interruption.
Thus, simply changing the data stream results in a delay before the changes work their way through the buffer chain to the outputs. You will continue to hear the old setup for a second or so, then there will be a smooth transition to the new one.
Smaller buffers could reduce this delay, but at the expense of making the system more susceptible to drop-outs due to Windows latency. You can adjust this balance with the Wave Block control in the Start Preferences dialog.
GenL=1 turns the Left Generator channel on, GenL=0 turns it off, and GenL=x toggles between the two states. GenR operates the same way for the Right channel.
See also Signal Generator Control Dialog.
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