FAQ: Frequently Asked Questions about Praxis

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Installation

General

Hardware Compatibility and Soundcards

Operation and Troubleshooting

About the AudPod

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Installation

How do I get started with installing and using Praxis (in either Full or Free Mode)?  There's a lot of stuff there!

Download the  document "Getting Started with PRAXIS" (pdf, 400KB),  which will guide you through the installation process and toward the included documentation, scripts, and Measurement Guides.  

Note that if you are only using PRAXIS in free mode for postprocessing of data files (and not for acquiring signals) such as when using it for Room Sound Convolution  you do not need to do the "Getting Started With PRAXIS" process nor do any hardware installations -- just install the software and go!

General

What does Praxis mean?? According to Webster's Ninth New Collegiate Dictionary:  

"Exercise or practice of an art, science, or skill".
 

How much does Praxis cost?  
The Praxis software, usable in free/demo mode is...free! (download the software free).  The Audpod interface/key is, however, needed for full access to all of Praxis' functions. For local pricing information, call your dealer - see: ordering information.

If I just download the software but don't buy the Audpod, will Praxis still be of any use to me?
   When Praxis does not sense that the Audpod is connected, it runs in "Free (or demo) Mode".    But Praxis' "Free Mode" does more than just show demos.  

These "Free Mode" capabilities are provided at no-cost for use by amateur or start-up speaker designers.  

Will I be able to run praxis on more than one computer?
    Yes, you can install it on as many computers as you like.  However, all the features will not be available unless the AudPod is attached to the computer being used at the time.  But since the AudPod connects via USB, this makes Praxis extremely portable between such computers..

Can Praxis do anything that Liberty Audiosuite couldn't?
    Yes, quite a bit.  Praxis will accomodate much larger data sizes, additional stimulus types (including high-performance "resynchronized chirp"), wider bandwidths, higher resolution, greater post-processing capabilities, shaped tonebursts, room acoustics calculations, "flex" windowing, and weighting files to name a few. It can work with laptop systems, too.   For more details, download the Praxis manual.

Can Praxis do scripts like LAUD did?

No.  It will do much more sophisticated scripts than LAUD did.  You can program Praxis scripts using using graphical "RAD" (Rapid Application Development) techniques with DelphiScript (an Object Pascal derivative) or with VBScript (a Visual Basic derivative).  Praxis scripts can control the Praxis measurement system, of course, but it can also access the Windows API or even start and control other programs outside of Praxis.  The Praxis download includes the Liberty Script Designer, a Delphi-like programming environment for conveniently developing your own scripts.

How do I update my copy of Praxis to the most recent version?

Download the latest version from the Praxis Download Page.  Then to install, merely run the PraxisInstall.exe file -- the installation program will replace any old versions of files from your previous installation (and will leave your configuration and data files intact).  

There is normally no reason to uninstall the previous version. However, if you should wish to start with completely fresh settings, first uninstall Praxis (using the Windows Control Panel "Add/Remove Programs" function) and then delete the entire "C:\Program Files\Praxis" directory (where "C:\: is the drive on which you have Windows installed).

Hardware Compatibility and Soundcards

What compatibility issues are known about using PRAXIS with various hardware?  
   See: Compatibility Issues

Will Praxis work with all soundcards?
    Praxis is intended to work with all Windows compatible soundcards that meet some minimum standards:

I already have a measurement microphone -- can I use it with PRAXIS?    
  Yes, provided that the microphone:

How can I make an attenuator so I can use a high output voltage microphone with PRAXIS?

For high SPLs, the microphone sensitivity needs to be low.  Even with 1mV/Pa, a 166dBSPL would be 4V output, too much for even most line inputs.  This could be handled with an attenuator.  The tradeoff is of course signal to noise.  Ideally you would use different mics for different purposes, or at least different attenuators. 

An attenuator for a microphone could be as simple as a series resistor. The AudPod's mic input has an impedance of 3.45K ohms.  A series resistor R (used between the mic preamp's output and the AudPod's mic input circuit) will scale the output by factor F=3450/(R+3450).  The AudPod mic input can deal with levels to about 700mV peak, so leave some margin and assume 500mV.  If you need to measure to sinewave levels of S [dBSPL] with a mic that has sensivitiy of M [mV/Pa], then calculate:

Max microphone output voltage Vmax = 10^((S-94)/20) * M/1000  [V]

Look up your mic's output impedance Z = (usually 150 ohms)

Scale factor F = 0.5/Vmax

Use Resistor value R = 3450/F - 3450 -Z  [ohms].  Make an adaptor that simply has this series resistor included.

The new effective sensitivity of the mic is then F*M, so edit the microphone's "dat" file so that the value in the first line (where it reads "mV/Pa") is changed to the new sensitivity.  Save that files and load it using PRAXIS' "Config -> Hardware Specific ->Mic" menu.

Example:  Say you have a mic that has 17.8mV/Pa sensitivity and that it is usable to 155dB SPL (and you will be using it to that level).  Vmax is then 19.97 [Volts].  The needed scale factor is 0.025.  The series resistor to use is 134.4k ohms (use the closest 1% value, 133k ohms).  The actual scale factor is then 0.025284, so the new sensitivity of the mic, when the attenuator is used, is 0.450 [mV/Pa]. Edit the file accordingly, and make your attenuator adaptor as shown below:

For use with ACO microphones and supplies, Liberty Instruments has attenuators like these available.
(note that the output of a microphone having 150 ohms output impedance will be reduced from its "open circuit value" by 0.37dB when loaded by the Audpod's microphone input without an attenuator resistor.

What kind of computer do I need?  Does computer speed matter with Praxis?
    Liberty Audiosuite (DSP based) was known to be quite immune to limitations of computer speed.  Praxis, however, relies on the horsepower of your main processor (which is also busy running the Windows OS), so faster is definitely better!  The following are minimum and recommended characteristics of a computer for Praxis:

Which soundcards, motherboards, and operating systems are safest (least likely to cause surprises) with Praxis?
  The safest motherboard choices for Praxis are those based on Intel chipsets and Celeron processors (PIII or P4 are good too, but more expensive).  Intel manufactured motherboards are also good choices. Of course most other types (AMD, VIA, etc) will normally work fine, but are more likely to have "surprises".  This is only because manufacturers of cards, USB devices, etc. will usually make SURE that they work as expected with the standard Intel stuff, but may check less thoroughly with the other brands).

We no longer suggest the "Soundblaster 128" soundcards, as some boards sold under this name use a different chipset ("Vibra") and would not calibrate in at least one system.  
But for a reasonable expenditure you can instead go all the way to 24bit/192kHz performance with the Waveterminal 192X., or for less money, the Waveterminal 192L.

We would NOT recommend the Soundblaster Audigy at this time, as it needs special attention to get it working with Praxis (it defaults to monitoring!), and has no apparent special performance with Praxis that would justify its higher price. (Despite its marketing literature, it is not in any way a 24/96 card!).  We haven't tested Audigy 2 yet, but would recommend that if you are planning to buy a premium card, you'd be better off  getting the real thing and going with the ESI Waveterminal 192X, at comparable cost.

If you need a USB-based soundcard, the most trouble-free ones we've seen are the M-Audio Transit (highly recommended), the ESI U2A or U24, or the AudioTrak Maya 5.1 USB.  

For best overall performance (24bit, 192kHz, fast operation), try the Lynx L22.   The LynxL22  is rather expensive however, and its setup is more involved (but there are detailed instructions for it on our web pages).  A less expensive, choice approaching the same performance (and offering very simple setup) is the ESI Waveterminal 192X.  PLEASE NOTE THAT 192kHz sampling is supported only by Windows XP SP1 and above! (operating system limitation)

For more detailed info: also please see the Soundcard Calibration Pages.

The best operating system to use for Praxis is Windows 2000.  XP is a good second choice (but is still newer and so has more bugs in general).  WindowsME or 98SE can work, but have limitations in the number of Praxis Plots (and other Windows forms shown simultaneously ) that can be handled simultaneously.
  ME is rather a pain in general (plan on crashing at least once a day with it, no matter what programs you run on it!).

Will any benefit(s) be gained from the utilization of high-end audio cards? Any suggestions?

With 24bit cards (such as Waveterminal 192X, CardDeluxe, M-Audio 410, 1010 or Audiophile 2496, or Lynx L22) you could measure to lower distortion residual level for electronics. But for loudspeaker work there is no real advantage to 24bit operation (you'd never find an environment or microphone with enough dynamic range or low enough distortion to make use of the additional bit resolution).

If you want to measure above the normal audio band, a 96kHz sampling card will allow you to get to about 40kHz. A 192kHz card (such as the
Lynx L22 or Waveterminal 192X) will allow measurement to 80kHz, though the higher frequencies will be subject to increased crosstalk between channels from our AudPod (which was mainly designed for operation below 50kHz). 
PLEASE NOTE THAT 192kHz sampling is supported only by Windows XP SP1 and above! (operating system limitation)

Will Praxis work with digital inputs/outputs on soundcards?
    Yes, it can work.  However, calibration (to true voltage or sound pressure levels) will not be possible except when these ports are used with additional converters that provide or accept analog signals.  ("Bits" are not Volts, so they can't be calibrated!).

I'm thinking of getting a new soundcard -- can you recommend any for use with Praxis?
 For a reasonable expenditure you can go all the way to 24bit/192kHz performance with the Waveterminal 192X. or to 24/96 operation with the Waveterminal 192L.   Also see: recommended hardware.   In general, the following are some pointers about good cards for Praxis use:

I've heard there are restrictions with certain cards.  What's the story on this?
    Many soundcards internally use only one hardware sample rate (usually 48kHz), and provide support for other sample rates via a technique called "rate conversion".  Some implementations of rate conversion do not maintain a steady, repeatable mapping between the "native" rate and the desired rate, and thus cannot be used for synchronous measurements (measurements in which data from one burst is used to timestamp or average with a data set from a different  time -- an example is synchronous MLS).  For example, a "Soundblaster Live" will only work with Praxis's synchronous measurements at the rates of 48kHz and 24kHz. For non-synchronous uses (such as RTA measurements), however, there are no such restrictions.  As a practical matter, the restriction causes little or no problem, it only requires that you use the synchronous rates for synchronous measurements -- so, just use 48kHz (which is what you usually want anyway!) and there is no problem.

   Some lower grade cards (or on-board audio chips) have shown occasional problems with varying latency, which can sometimes cause errors with averaging processes. 

   The performance of sound hardware built into some motherboards or laptops can be limited by noise floor or high distortion.

Will Praxis work with laptop computers?
    Yes! 

     --provided that the sound hardware meets the requirements specified above.   But most built-in sound hardware in laptop computers is of medium to poor quality -- for portable use, the best option is probably to use a quality USB soundcard like the TransitESI U24 or Maya 5.1 USB.  

    Most laptop computers are equipped with some means to slow down the processor during periods of low usage.  THIS POWER SAVING FEATURE SHOULD BE DISABLED WHEN USING PRAXIS, as its initiation can interfere with audio data streams and cause problems with measured distortion, synchronization, and spectrum noise floors.  You can find the controls to disable this in your Windows "Control Panel".

Operation and Troubleshooting

I downloaded the Praxis file(s), but it won't install.

Sometimes file corruption can occur during download, or your browser software may incorrectly report that all of a file has been downloaded when it has aborted because of an interruption.  If you are having troubles downloading, try to download the individual 1.4MB files rather than the big all-in-one file, and check that all files but the last are approximately 1400kB in size.  If any (other than the last one) are smaller, download that one again before trying to install.

I get relatively high distortion when using the Maya EX (or Maya 5.1USB)

The Maya EX is USB powered, so it has some trouble achieving high output levels with low (<0.1%) distortion at the line outputs.  One solution is to use PRAXIS with the Output Levels (in the "Levels" form) at -5dB.

A better approach is to connect the Maya's "headphone" output to the AudPod's "From Sound Line Out" jack (instead of feeding this from the Maya's line outputs).  The headphone output has better output drive and lower distortion at high levels.

I see low level spurious tones in FFTs when I use my USB-connected soundcard

This can be caused by ground loops and inductive pickup when using excessively long cables to connect between the USB soundcard and the AudPod.  The 12-foot cables supplied with the AudPod are not good choices, and are not necessary for this connection to a USB card!  The spurs can also be caused by internal noise in the USB card (from its microcontroller), but in better cards such as the Transit, the spurs will all but vanish when cables of 12inches or less are used.

Measurements made with PRAXIS are changing when I move the cables going to the soundcard, the microphone or the AudPod.

One possibility is that an internal connection in one of the cables is intermittent.  This may be the case if bending the cable (near where it meets the connector molding, but without moving the connector to its jack) causes the measured signal to abruptly change.

Another possibility is that there is an oxidized or dirty contact inside or going to the AudPod.  You may want to get some CAIG "DeoxIT" contact cleaner,  or a silicone-based tuner cleaner from an electronics dealer such as Radio Shack, spray some on a cloth, and wipe it onto all connector contacts -- then work these connectors back and forth a little into the AudPod's audio connectors to clean both the plugs and the Audpod's jacks.  Doing this can do wonders.

During soundcard calibration, in the Playback Mixer adjustment, the blue bar-graph shows at full scale no matter where I set the mixer controls! 

The output slider adjustment process during soundcard calibration can be confusing. 

The brief answer is:  
Make sure you really did have the Windows Playback Mixer set for full scale (or near-full scale) in the step PREVIOUS to the step where you are asked to adjust the mixer for the bar-graph!.

The long answer, with explanation is:
It is very important to note that the blue bar graph during playback mixer adjustment doesn't show absolute level -- it shows relative level. That is, it is showing an indication of the current level, relative to the signal that was detected and determined in the previous step of the calibration. What PRAXIS is trying to do here is to get you to set the output gain slider to a setting where a full-scale output signal is coming out of clipping. The step just before, where previous instructions directed you to have the slider at or near full maximum, is assumed to have been clipping the output during a test that PRAXIS runs when you proceed.. When PRAXIS runs a sinewave in the "bar graph" step, and sees the peak level at something less than the previously measured level (the Red dot goes out), then it knows that you are no longer clipping. BUT, if the level controls were not really set for clipping (or at least for a large signal) in the previous step -- a common error seems to be that people try to start with the level sliders at minimum or with them muted -- then PRAXIS can NEVER detect a signal less than that -- except randomly from noise. This is usually what is happening, should you find that changing the slider shows the bar graph at full scale no matter where you adjust the mixers.   If you didn't get a near-clipping reference, but by chance get past the "bar-graph" step, the calibration will usually fail at a later step, when it tries to distinguish one playback channel from the other or to determine output polarity-- two channels with low gain show insufficient difference in level when one is turned off.   So the moral of that story is:  make sure you start the calibration with the playback level controls cranked. (There is one exception -- some few very high output level cards, such as the CardDeluxe if set for "pro" level, can overdrive the AudPod if full gain is used; for these, you may need to start with the output level at about 75% of full scale if calibration problems should appear).

I can't get the Maya 5.1USB soundcard (or other compatible USB soundcard) to calibrate in PRAXIS

Before doing the calibration (Levels Form, "Setup, Do Calibrate" menu) procedure: 
Make sure that PRAXIS is set for 16 bit operation (in the Levels Form, "Setup, D/A&A/D" menu) and that the "WAVE_EXT" menu option (of the Main Form, "Config, Hardware" Specific menu) is checked.  And that the proper soundcard is selected in the Levels form!

I can't get the soundcard calibration process to succeed  with my soundcard.

This can be tricky with some soundcards and operating systems.  There are a wide range of characteristics, performance parameters, and functions for all the different makes and models of soundcards, and over the various Windows operating systems and drivers.  In some cases the direct procedure of the Praxis Hardware Calibration guide (for AudPod use) will not be able to sort it all out automatically and will need more effort.  You can find some specifics of calibration results, and the settings needed for certain mixers, using a number of soundcards in Windows 2K, ME and XP, including some particularly troublesome cases at our new Sound Card Calibration Pages

How does PRAXIS deal with the soundcard Windows mixers?
    See: PRAXIS and Windows Sound Mixers.

I'm having trouble getting Praxis to operate with my soundcard.  I get an error message "WAV failed at start". What should I do?

Praxis can be configured to use one of two types of soundcard APIs.  Some soundcard drivers (usually not "WDM" types) are not able to use the so-called "WAVEFORMATEXTENSIBLE" structure.  For these, uncheck the menu option "WAVE_EXT", which can be found on the Main Form, under "Config, Hardware Specific". To make this setting your default, close down Praxis afterward (and allow it to save all configurations). 

This message will appear if the soundcard device cannot support the bit resolution or sample rate that is requested.

This can also happen if your soundcard's driver freezes or crashes for some reason.  Usually the only way to fix that is to reboot the system.  (It may be a good idea to see if the soundcard manufacturer has issued a fixed driver, too).

I can't operate my 24bit capable soundcard with 24bit capabilities -- Praxis does not allow me to select 24bit mode.

With soundcards using "WDM" type drivers, 24bit operation can only be obtained by using the so-called "WAVEFORMATEXTENSIBLE" structure.  For these, check the menu option "WAVE_EXT", which can be found on the Main Form, under "Config, Hardware Specific". To make this setting your default, close down Praxis afterward (and allow it to save all configurations). 

The noise floor in my spectrum plots swells or rises periodically, particularly when using larger FFT sizes.

This usually indicates that record or playback samples are being corrupted or dropped for some reason.  When Windows is unable to keep  up with audio streams (because it is too busy, or gets interrupted by high priority operations) it will sometimes just toss out or ignore sections of the audio stream.  Possible causes are:  

When I measure the frequency response sensitivity of my loudspeaker, the values seem to be about 9dB lower than the results from measurements with LAUD or with other systems.  Am I doing anything wrong?

This happens because Praxis reports acoustic to electronic sensitivity in terms of SPL normalized to 1Vrms drive (this is called "dBS" on Praxis plots).  This is different from some measurement systems which report sensitivity in terms of SPL normalized to 2.83V (2.83V represents 1Watt with an 8 ohm resistive load).  

Because what is actually being measured is voltage during such a test, and because loudspeakers do not always have an 8ohm impedance (and are virtually never resistive loads), the normalization to 1Vrms was deemed to be more correct and makes no assumptions about the nature of the load. 

It is easy to convert from "dBS" to "dBSPL at 2.83Vrms applied".  Simply raise the curve by the factor  
20*log(2.83) =  9.03dB.   You can accomplish this by entering this value into the "Offset" control that can be found on the "Weighting" tab of the Plot Forms.

When Praxis measures sensitivity for 2.83V, how is the level set to 2.83?

A better question is "why would you want it to?".  The measurement is not MADE at 1Vrms (usually), but the resulting sensitivity value is referenced to the 1Vrms drive level.  Think of it as being like the speed of a car - you can drive at 60mi/hr without driving for an hour or for 60 miles.  You can go 60 miles/hour for just 10 seconds and the speedometer will still read correctly.  But if you did drive at 60mi/hr for exactly one hour, you'd go....60 miles.  Don't confuse ratiometric measurements with absolute measurements.

 
The sensitivity measure is ratiometric  - sound per volt.  More rational units of measure for speaker sensitivity would be dB(Pa/V),.  But the world is hooked on SPL units which are already in dB.  So the sensitivity is given in terms of "dBS": how many dBSPL of sound you'd get if you did drive the speaker with a volt.  But if you instead drive the same speaker with half a volt, you'd get 6dB less absolute SPL (because 0.5 is -6dB).  If you drive with 2Volts, you'd get 6dB more absolute SPL (2 is +6dB).  Drive with 2.83V, you'd get 9dB more.  But in all these cases,  the ratiometric "dBS" stays the same, just as your car still can go 60mph down the highway for one second or for 4 hours, depending how long you run it at that speed.
 
What sets the "1V" in Praxis is its measure of how much signal it sees from the microphone (sound) compared to how much voltage it sees at the same time on the speaker terminals (voltage, from the probe).  So it comes from Praxis' calibration and your mic's calibration data.  

The volume level you measure with doesn't matter (provided you don't overdrive anything by being too loud or get buried in noise by being too low), as long as PRAXIS can determine the ratio of sound pressure to drive voltage.  The best check for overdrive  is to reduce the volume control on your amp and see if you get the same ratiometric answer as at the higher level.  If so, you can trust it.

If the applied level DOES matter, then you are in the realm of distortion measurements, not frequency response measurements!

Why can't I select the frequency range when I measure frequency response using Chirp stimuli?

In order to let you window out reflections, PRAXIS needs to get the frequency response into a time domain impulse response (IR) representation so it can include or exclude portions of the IR.  But you can't mathematically get an Impulse Response using only part of a spectrum, it must include all representable frequencies -- it's all or nothing! 

 

So any Chirp measurement which needs to pass through an Impulse Response  format needs to sweep all of from DC to Nyquist frequency (half the sample rate).  For Chirp measurements, these are all acquisitions except maybe Impedance or FFT (since those don't need to deal with time domain and you can therefore select what frequency range you want to chirp over).

 

I think my Calibration files or my Praxis Configuration files may have become corrupted or damaged.  How can I start again with new Configuration or Calibrations?

 

If you find that PRAXIS no longer seems to start up or work as before (when it had been working correctly), but recalibration does not seem to help, it is possible that the problem could be an error or data corruption in either a calibration file or the PRAXIS configuration files.

 

One way to avoid propagating damaged configuration or calibration values is to uninstall PRAXIS, delete the entire "C:\Program Files\Praxis" folder, and then re-install PRAXIS.  But if you do that, you could also lose data files you may have generated.  To keep the installation and data files, but start with fresh Calibration Files and/or Configuration files, close PRAXIS, then do the following:

For Configuration Files:  Browse (in Windows Explorer or "My Computer") to your "C:\Program Files\Praxis" directory and find the files named "Praxis.ini" and "Praxis.hdi". Rename these to something else (such as PraxisOld.ini and PraxisOld.hdi". Also, rename or delete the file "PraxisSaveData.pbu".  These steps will force PRAXIS to start with default settings and to make a brand new configuration file. Your data files will still be available, but the settings for scaling, formatting, etc. will go to the "new installation" defaults. You will again need to select the sound card to use, select the microphone calibration files, and make any other settings that are needed with a fresh PRAXIS installation.

For Calibration Files:  Browse (in Windows Explorer or "My Computer") to your "C:\Program Files\Praxis\AudPodMixer" subdirectory. Rename that subdirectory to something else (such as "AudPodMixerOLD"). This will force PRAXIS to create a new subdirectory with all new calibration files and device-mixer matching records.  Then start PRAXIS and try to recalibrate. You will need to first check and "OK" the device-mixer matching settings, and select your desired soundcard, rate, resolution, etc.  

About the AudPod

So, what does the AudPod do?

The AudPod is powered by the USB port (no batteries or wallwarts to fuss with), is direct coupled (frequency response down to DC), and contains an internal microcontroller and symmetrical power supplies.  It is housed in an attractive, rugged, and handy 3"x4"x1.3" extruded aluminum enclosure. 

What are the AudPod electrical interfaces?

Microphone Input (2 channels on one 3.5mm stereo jack):  

Probe Inputs (2 channels, each on a separate 3.5mm stereo jack):

Aux Inputs (2 channels on one 3.5mm stereo jack):

Stimulus Output Characteristics (2 channels on one 3.5mm stereo jack):

Interface to Sound Line Out (2 channels on one 3.5mm stereo jack):

Interface to Sound Line In  (2 channels on one 3.5mm stereo jack):

USB connector:

 


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