SVT Post-processing

[DavidIMurray]

Hi

Not quite the correct place to post this but I thought that some of you might have some experience with this toolkit.

I'm streaming data to a binary file which I would then like to open for analysis on another machine. The data is being collected from a microphone. Now here is the problem. If I scale the data to dB before saving, and then open the file, the values come out correct. Similarly, if I save the values as pressure levels, these values are also correct when I open them. If I wire the pressure levels (having first built a waveform) into one of the sound and vibration toolkit, sound level VI's then I get results out with a massive zero offset (in the negative range) and the 'amplitude' of the level is incorrect - e.g. -50dB to -5dB when the input is 94dB.

Any ideas what I am doing wrong? Basically I want to save the values as pressures, and then during post processing, convert them to dB.

Cheers

Dave

[preston_johnson]

You first want to capture the data as a waveform or similar compatible data type to the S&V scale to engineering units function.  This function stores the scaling features in attributes of the waveform.  In most cases, if you use virtual channels in Max, you will want to indicate your values as voltages with unity scaling.  Let the S&V scale to engineering units do the work on the waveform, then the waveform in the file will have the information for subsequent off-line replay.  You may want to use the waveform read and write VIs for reading and writing the blocks of data to the file. 

[DavidIMurray]

Preston

Thanks for the reply. I have enclosed the two vi's below. If i replace the binary write vi with a waveform I get error 71. The system will be collecting data continually so file size is extremely important, hence the choice of binary files. Is it possible to recreate the correct format to allow the S&V dB scaling to work in the reading VI?

Cheers

Dave

[dsb@NI]

Different pieces of information get lost when the waveform is saved as an array in binary format (t0 which may or may not be important, dt which you have hardcoded in your openbin[1].vi file, and the waveform attributes). The waveform attributes include the channel name and unit description added by the DAQmx driver and they include additional units information (specified via channel info) added by SVL Scale Voltage to EU. When the SVT Exp Avg Sound Level VI sees the reconstructed waveform in the openbin[1] VI, the waveform has no units associated with it. If you again use the SVL Scale Voltage to EU VI in the reader just to set the units and dB reference for the saved data (i.e. set the sensor sensitivity to 1000 mV/EU, units to Pa, and dB reference to 20u), you should see correctly scaled sound level measurements. Let me know if you have any other SVT questions. Doug Sound and Vibration Applications National Instruments

[DavidIMurray]

Hi Doug

Thanks for the reply. Placed the scaling block in with the settings you suggested and it now works perfectly.

Thanks Again

Dave