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Author Topic: VNAs in audio applications.  (Read 3100 times)

Lyle Williams

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VNAs in audio applications.
« on: December 26, 2013, 03:04:04 am »


Does anyone use VNAs ( Vector Network Analysers ) for audio system testing?

This would be the standard tool for measuring phase response at RF frequencies.

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Lyle Williams

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Re: VNAs in audio applications.
« Reply #1 on: December 27, 2013, 07:34:38 pm »

In the absence of an answer, I gave it a go.

All the bits of the signal path that are meant to be linear test out just fine with a VNA.  From the mic cable through to speaker terminals (which obviously need quite a bit of attenuation when driven by an amp).

Once I go out an "air interface" into a room and mic the results are horrible.  It takes a huge number of sweeps to start to see what is going on. 
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Jason Lavoie

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Re: VNAs in audio applications.
« Reply #2 on: January 08, 2014, 01:25:29 pm »

In the absence of an answer, I gave it a go.

All the bits of the signal path that are meant to be linear test out just fine with a VNA.  From the mic cable through to speaker terminals (which obviously need quite a bit of attenuation when driven by an amp).

Once I go out an "air interface" into a room and mic the results are horrible.  It takes a huge number of sweeps to start to see what is going on.

I'll give it a go..
they are both essentially doing the same thing: applying a stimulus to the input of a system and watching the response at the output, at multiple frequencies

the biggest difference is that your typical lab VNA will measure a single frequency at a time, and if you like it will do a sweep and log the results. These results are accurate, but noisy so they are useful for seeing trends or letting you know if your circuit is unstable.
Although most dual channel FFT software packages can be made to work exactly like a VNA nobody does because as you've found out it's a PITA
Smaart and others have options to use averaging (from small averages right on up to many seconds) and smoothing. as well as many other little tricks like time windows to avoid unwanted signals from getting into the measurement.

FWIW your results would be the same with Smaart.. cables, electronics etc all measure great with nice clean responses (as they should) and speakers/rooms measure comparatively terrible.
If you were in an anechoic chamber the differences would be more subtle.

Jason
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Lyle Williams

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Re: VNAs in audio applications.
« Reply #3 on: January 08, 2014, 03:49:43 pm »

Another issue with swept measurements is that the VNA is expecting speed-of-light type latency through the circuit rather than speed-of-sound.  The sweep rate needs to be turned right down to let the VNA find it's input signal in the output.
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Jason Lavoie

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Re: VNAs in audio applications.
« Reply #4 on: January 09, 2014, 09:51:46 am »

Another issue with swept measurements is that the VNA is expecting speed-of-light type latency through the circuit rather than speed-of-sound.  The sweep rate needs to be turned right down to let the VNA find it's input signal in the output.
Right. other than measuring sound through air a VNA will always have the response come back way faster. this may make your VNA useless
if you have a DSP handy or a digital delay try setting a delay of 20, 50, 100ms and see if the VNA gets a proper measurement.

Jason
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Lyle Williams

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Re: VNAs in audio applications.
« Reply #5 on: January 09, 2014, 03:03:31 pm »

I need the time machine variant that lets me set negative delays.  :-)
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Tim McCulloch

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Re: VNAs in audio applications.
« Reply #6 on: January 10, 2014, 11:07:45 am »

I need the time machine variant that lets me set negative delays.  :-)

Mr. Peabody's Wayback Machine is available... :)
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

Lyle Williams

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Re: VNAs in audio applications.
« Reply #7 on: January 10, 2014, 02:13:00 pm »

Sadly I am old enough to have watched a lot of the original on TV...
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Re: VNAs in audio applications.
« Reply #7 on: January 10, 2014, 02:13:00 pm »


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