Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision
Well, 2012 is about to pass into history; and what a year it has been! We just released version 3.2 of our APx500 software—our 3rd release this year to support the expansion and growth of our APx family of audio testing products. Your overwhelming positive response has led to many new business opportunities and measurement solutions. As always, I welcome your feedback in helping us to shape and prioritize our future plans.
2012 ends very strongly for Audio Precision. Notwithstanding the pundits and commentators who report this is a time of financial uncertainty, I believe the audio industry is poised for some very strong organic growth in 2013.
So, from all of us here at Audio Precision, Season’s Greetings and have a Prosperous New Year!
Output: What's New in APx500 v3.2
SUPPORT FOR THE APX511 HEARING AID ANALYZER
The APx511 is a new member of the APx family designed to support R&D and production test of hearing instruments. The APx511 requires APx500 v3.2 or higher.
Firmware Upgrade through the API
Many users have written custom programs that launch the APx500 application but don’t expose the GUI. If APx500 is launched this way, previous versions would halt if the attached instrument required a firmware update. With version 3.2, the firmware update is automatically applied to the instrument by default.
BLUETOOTH OPTION ENHANCEMENTS
Bluetooth mSBC Codec and Controls
Bluetooth functionality is upgraded to support the mSBC codec (also known as wide band speech), extending the Hands Free Profile (HFP) from an 8 kHz sample rate to a 16 kHz sample rate, markedly improving the quality of voice. Note: Use of mSBC wide band speech requires a firmware update to the Bluetooth option. See your AP representative for details.
PDM OPTION ENHANCEMENTS
PDM Decimation Rates from 1x to 32x
The following new decimation ratios are added to the PDM Input Settings: 1, 3.125, 4, 6.25, 8, 8.33, 10.67, 12.5, 16, 16.67, 18.75, 21.33, 24, 25.
PDM Vdd Control Panel
A Vdd Control is added to the PDM Monitor panel that allows you to set the PDM Vdd voltage, and to turn the PDM Vdd Voltage output on and off.
PDM Vdd Ramp measurement
A new measurement called “Vdd Ramp” is available for the PDM option. This measurement allows you to determine the performance of a PDM device during power supply start up. Note: this feature requires a hardware update. Contact your AP Sales Partner for details.
ACOUSTIC INPUT ENHANCEMENTS
Pascal/dBSPL Calibration per input channel
This new feature of APx allows you to set microphone calibration for each channel independently, allowing the simultaneous use of different microphones. Both calibration and results are expressed in dbSPL or Pascals as desired.
Custom Derived Result
Custom Derived Results is a feature that allows you to take external data and combine it with current measurements within a sequence for display directly in APx500, using any arbitrary units.
Reduce minimum regulation level from 1mV to 1μV
The minimum regulation level for Regulated Frequency Sweep and Auto Gen Level in Reference Levels has been changed from 1 mV/1 mFS to 1 μV/1 μFS.
Synchronize recorded wave file in Measurement Recorder with Generator Trigger
The Save Acquisition to File function in Measurement Recorder has been changed such that when the generator is playing a waveform, file capture is triggered precisely at the start of the waveform.
Allow User to Specify .wav Sample Rate in File Analysis Mode
In File Analysis Mode, you may now choose how to specify the sample rate used in measurements. The choices are: Use rates from files, Use rate from first file, and Use fixed rate.
Matlab data export
Matlab is now an additional export choice for all measurement results that currently export in CSV and Excel formats.
Cycles control on scope signal monitor
You may now specify the number of cycles of a waveform that will appear on the Scope Monitor when the x-axis is auto-scaled, with a minimum value of 1 and a maximum of 10.
Support for Windows 8
Release 3.2 is compatible with the Windows 8 32-bit and 64-bit operating systems.
Sound Advice: Equalizing APx Generator .wav Files for PESQ
Our new APx EQ .wav Files Utility facilitates making accurate PESQ measurements by compensating for source speaker errors. Read on below for the details, and check out our just-revised Technote 116 on Smartphone Testing, which uses the utility in some of its measurements.
All the APx500 measurements for frequency response allow you to apply equalization to sine waves produced by the generator. Now, the new APx EQ .wav Files Utility lets you apply equalization to generator source .wav files as well.
Generator equalization is commonly utilized when measuring microphones, in order to compensate for frequency response variations in the stimulus speaker. To do this, a high quality measurement microphone is used to measuring the frequency response, and then the results are inverted. This creates an equalization curve that can be applied to the generator to cancel out response variations. For greater precision, the (typically small) response variations of the measurement microphone can be factored in as well. When utilizing an artificial voice to test portable device mouthpieces, the equalization compensates not only for speaker performance, but for the acoustics of the artificial voice’s mouth and throat cavity.
Fig 1 G.R.A.S. KEMAR manikin Type 45BM with 45EA handset positioning system.
The procedure explained above for creating and applying an inverse EQ curve is illustrated below:
Fig 2 Using a flat sine stimulus to measure speaker deviations and create a compensating inverse EQ curve.
Fig 3 Applying the compensation EQ to the generator, resulting in flat output from speaker.
To measure the frequency response of a device, the DUT is substituted for the measurement microphone.
Fig 4 Measuring the frequency response of the DUT.
When the generator stimulus source is a .wav file instead of a sine wave, the APx EQ .wav Files Utility is used to apply the inverse EQ curve to the .wav file.
Fig 5 Using the APx EQ .wav Files Utility to apply EQ to a .wav file.
Then the equalized .wav file is used as the generator source.
Fig 6 Loading the equalized .wav file into the APx generator.
The ability to equalize .wav files is perhaps most valuable for PESQ subjective audio measurements. In APx500, .wav files containing speech samples are used as the generator stimulus signal for the PESQ measurement. PESQ works by comparing the audio received by the analyzer to the original speech samples. By using equalized versions of the speech samples, we can remove response errors introduced by the stimulus speaker and assure that any measured degradation of audio quality due to frequency response errors is caused by the device under test (DUT).
Fig 7 Loading the equalized .wav file into the APx generator, and using the flat .wav file as the reference.
It is important to note that the reference signal must be the original file and not the equalized file, because the reference is not degraded by the loudspeaker. APx500 gives you the choice to use separate files for the generator and the reference, or to use the same file for both (when not applying equalization).
Fig 8 The APx PESQ measurement settings panel allows the generator waveform and the reference signal to be chosen independently.
The APx .wav Files Equalization Utility can also be used to apply equalization to audio at the analyzer inputs before calculating measurement results (post EQ). This can be useful when utilizing an artificial ear or HATS (head and torso simulator). In some cases, it is necessary to convert the frequency response at the DRP (eardrum reference point), where the measurement microphone is located, to the response that would theoretically be measured in the free field. In this scenario, use the APx500 Measurement Recorder to first capture the acquisition to a .wav file. Then use the utility to create an equalized version, and load the equalized file into APx500 for analysis (Input Connector: File (Analog Units) or File (Digital Units)).
Fig 9 A sine wave or .wav file is used as the generator stimulus, and a new .wav file is recorded with Measurement Recorder.
Fig 10 The new .wav file is EQed with the utility.
Fig 11 File analysis mode (input source File) is used to perform measurements on the equalized acquisition.
The utility includes both a GUI version and a command-line version. The GUI version has a simple interface for selecting the input file, the .csv file with the EQ curve, and an output file. Some sample .csv files are included with the utility as examples to show the proper format. The data export feature in APx will also create a properly formatted .csv files.
Fig 12 APx EQ .wav Files Utility
The command-line (CLI) version can be run automatically within an APx project from a Run External Program step. A sample APx project file is included with the utility to guide you through the steps of creating an EQ file for a loudspeaker and then testing a device with correction applied. The project uses the CLI version and makes use of APx500’s automated data import/export capabilities as well as its Invert Derived Result to create the inverse EQ curve.
Test Results: AP News & Events
© 2012 Audio Precision, Inc.