block duration in swept sine measurement

[w_lodiman]

Hi All..

I'm still learning to create swept sine measurement using LabVIEW and PCI-4451..

Can anyone explain me about "block duration" in the swept sine measurement? I've tried to search at NI Knowledgebase and some Manuals, but so far they didn't help..

Thank you..


Best Regards,
William

[Ima U]

Hi William,

Where did you get the term "block duration" from.  I do not find it anywhere.  There is much mention of "duration", the time between intervals of averaging. 

I notice you are using NI PCI 4451.  Make sure you have NI-DSA and Traditional DAQ drivers for your PCI-4451.  These drivers should come along with some shipping examples for this device.  There should be a Swept sine example program to assist you.

I am including some helpful links.  The first is a Developer Zone on creating Swept sine in LabVIEW.  The next link is to the PCI-4451 User Manual.

I hope this helps!

[w_lodiman]

Hello Ima U..

Thank you for replying..

I got the term from the Sound and Vibration Toolkit 3.1..

In the manual SVT 3.1 (chapter 13, page 13-6) right at the below, there's a note said that:
"..... A very small block duration can result in a loss of continous processing, causing the swept-sine measurement to stop and return an error."

There's also a special vi in SVT 3.1 which control the block duration, which is "SVY Set Swept Sine Block Duration.vi"
It is located in "Swept Sine" --> "Swept Sine (Traditional DAQ)" --> "Configure Swept Sine" --> "Configure Advanced Settings" --> "SVT Set Swept Sine Block Duration.vi"
(I submit the picture of the vi along with this post)

Well, the block duration default value in the "SVT Initialize Swept Sine (Traditional DAQ)" is 100 ms..

in the "SVT Configure Swept Sine", the value of block duration affect the "AI Block size" and "AO block size"..

It makes me curious and confused too, since I haven't got enough info about block duration..


Best regards,
William

[w_lodiman]

I'm sorry.. I forgot to attach the picture..

here's the picture..

Thank you..


Best Regards,
William

[BrowningG]

Hello William,

Thanks for contacting National Instruments. 

Another good example for creating a swept sine measurement with Tradtional DAQ is called SVXMPL_Swept Sine FRF (Traditional DAQ).  This VI can be found in the NI Example Finder by choosing to browse according to directory substructure and then following the path: Sound and Vibration -> Swept Sine.  This VI sets up the same swept sine test as that shown on page 13-9 of the sound and vibration toolkit user manual. 

The Block Duration VI is used to configure the buffer size of the Analog Input and Analog Output of the DSA card.  If you double-click the SVT Start Swept Sine (Traditional DAQ) VI in the above example, it will show directly how the block duration is used to configure the input and output buffers of the card. 

Page 13-10 of the sound and vibration toolkit user manual indicates that this VI causes a tradeoff between execution time and processor load.  A block duration that is too short will cause an error to occur during the execution of this example. 

Please post back if you have any questions.

[w_lodiman]

Hello Browning G..

Thank you for replying..

---

@BrowningG wrote:
Page 13-10 of the sound and vibration toolkit user manual indicates that this VI causes a tradeoff between execution time and processor load.  A block duration that is too short will cause an error to occur during the execution of this example.

---

I'm still a little bit confused.. It seems that the value that should be set for block duration depends on type of DSA card and personal computer that I use, doesn't it? Then is there any "formula" to find "the right value" to be set for the block duration? Also, how can the default value given is 100ms? (If I don't want to implement the advanced settings configuration in my swept sine measurement, surely I get the block duration is 100ms, right?)


Best Regards,
William

[BrowningG]

Hello William,

Thanks for contacting National Instruments. 

A block duration of 100 ms is specified by default when the SVT Initialize Swept Sine (DAQmx) Task VI is called.  This can be seen by opening the Initialize VI and viewing the block diagram.  In this VI all of the properties needed for a swept sine task are set by default.  So if you do not wish to use the block duration VI, it will be left at 100 ms. 

There is not an official “formula” for setting the block duration.  The block duration controls how large the input and output buffers are.  The smaller the buffer, the faster the VI will execute.  But, if you specify a value that is too small, the VI will have to halt execution while the buffers are updated with new values and this could possibly throw an error.  In most cases it is ok to leave the block duration at the default 100 ms.  Normally trial and error is the easiest method of determining an appropriate block duration size. 

Please post back if you have any questions.