Record Time Waveform and Save its Spectra on CoCo-80X
The CoCo’s Save and Record functions are generally only discussed in the context of separate measurements. Although the Save and Record functions cannot be executed simultaneously, the CoCo is still able to store both the time waveform and FFT from the same set of original data. By exploiting the fact that FFT/block signals are always being buffered in the CoCo’s RAM, we can acquire the FFT signals while simultaneously recording the time waveforms to the SD card.
Figure 1: Recording Time Waveform to SD card while storing FFT signals to RAM
Since the FFT signals are stored in RAM, they can still be accessed after the acquisition is paused. Once the recording is completed, we can save the FFT signals from RAM to the SD card.
Figure 2: Store FFT signals to SD card
By configuring the Save and Record settings appropriately we can ensure that the saved FFT corresponds to the recorded time stream. Here I will outline the process for acquiring time waveform data and FFT data corresponding to the same data set.
How to record the full time history during FFT averaging, and then save the FFT averages as well
1. Press Parameters (F2) and select Analysis Parameters.
a. Set up spectral averaging as usual. In this example we use linear averaging with 16-frame averages.
This ensures that the acquisition will be paused when the frame count reaches 16.
Figure 3: Set up Analysis Parameters
2. On the Spectral Save Setup screen
a. Press Param. (F2) and select Spectral Save Setup
b. Select signals to be saved. In this example we save all signals but leave the checkboxes at the bottom
empty.
c. Press OK (F6)
Figure 4: Configure Spectral Save Settings
3. On the Time Stream Recording Setup screen (found in Param. (F2) menu)
a. Enable ‘Stop record when average reached’
b. Press ‘Apply and REC’ to start the averaging and recording simultaneously.
It is important to initiate the recording through this menu. This triggers the recording to start at the same time it starts counting frames for averaging.
Figure 5: Record Time Waveform and Save FFT Signals
4. The CoCo will automatically stop recording and pause the acquisition when the average number is reached (16 frames in this example). Now that the recording is complete, press the Save button to generate a SIG file with the averaged spectra.
Now if we compare the recording file to the saved FFT signal, we find that the recording duration comes very close to the expected value. In this case the recording file had a duration of 133.12 seconds, which is very close to the time it takes to acquire the FFT signal at 131.07 seconds.
Figure 6: Compare Duration of Time Waveform and FFT
This exercise demonstrates the CoCo’s ability to reliably acquire time domain and frequency domain data from the same set of source data. A special thanks to George Szasz at Stress Engineering Services for bringing this unique functionality to my attention.
A note about how Overlap Ratio affects recording duration
Keep in mind that the recording duration will be affected by overlap ratio. The following expression gives an approximation of the resulting recording length:
