Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision
We have two big announcements to start off the New Year. First, APx500 v2.9 is now available for download. Version 2.9 has a host of new measurements and features, described in the article below and in last month’s Audio.TST. Second, I am very happy to announce our newest digital interface for the APx Series: PDM.
PDM (pulse density modulation) is a one-bit digital format that’s popular on digital microphones used in devices like smartphones and tablets. The audio signal is converted to a digital stream right at the microphone, reducing susceptibility to crosstalk and noise, and lowering costs (compared to PCM) due to the simpler conversion process. Read the article below to learn more about the new PDM module.
2012 looks promising for audio—innovation is coming from every corner of the industry, and the mood is very positive out in the field as we visit both existing and prospective new customers. We’re looking forward to being part of your success.
PS: I hope you will join me in a hearty congratulations to Bob Moses, who was named Executive Director of the AES (Audio Engineering Society) in December. Bob’s industry experience will serve the society well.
Output: New APx PDM Option
AP will release a new PDM interface module for APx Series audio analyzers early in the second quarter of 2012. PDM joins the wide array of APx connectivity options, which includes balanced and unbalanced analog, optical / unbalanced / balanced AES digital, serial digital, HDMI, and Bluetooth.
AP’s new PDM interface module enables full audio testing of MEMS microphones (with PDM outputs) and PDM microphone inputs. MEMS (microelectromechanical systems) microphones are commonly found in smartphones, tablets, laptops, cameras, and other compact consumer devices.
Fig 1 The new APx PDM module.
The new APx PDM interface module features both PDM inputs and outputs, configurable clock direction (in or out), and a variable power supply.
Some of the features that distinguish AP’s new PDM module:
Comprehensive audio testing plus PSR. The full range of APx audio measurements may be performed on PDM signals. In addition, single frequency or swept frequency Power Supply Rejection measurements can be made.
Digital domain audio analysis. By keeping the PDM signal in the digital domain, the signal integrity is preserved. Outboard adapters that convert PDM into an analog signal introduce D/A conversion errors than can affect measurement accuracy.
Full programmatic control over physical interface parameters. User settable logic/Vdd voltage, sample rate, and clock edge.
PDM signal generation. Full, comprehensive audio testing of the PDM inputs on microphone preamplifiers, smartphone processors, and many other chip-level and finished devices.
Independently set input and output formats. PDM can be set for both input and output, or PDM input or output can be paired with a variety of analog and digital interface formats. This enables analysis to be performed at virtually any point in a device or circuit (may require optional modules), and allows PDM to be included as part of a more comprehensive device test.
What is PDM?
PDM stands for pulse density modulation. However, it is really better summarized as “Oversampled 1-bit audio.” It is nothing more than a high sampling rate, 1-bit digital system. If you increased the sample rate of CDs by a factor of 64 (referred to as “upsampling”), and reduced the wordlength from 16 bits to 1 in a reasonable way, you’d have the basis of a PDM system.
Fig 2 Typical outline of a MEMS (microelectromechanical systems) microphone with PDM output. The sound port may be located on the top or the bottom.
An undithered 16-bit system has a theoretical signal-to-noise ratio of around 98 dB. An undithered 1-bit system has an signal-to-noise ratio of about 8 dB. However, by use of noise shaping, most of the noise is pushed above the audible band (above 20 kHz) and performance can be quite high.
Download Understanding PDM Digital Audio for a more in-depth discussion about PDM.
Price and availability
Pricing for the PDM option is $4500 in the US. Upgrade price for existing instruments is $4500 until December 2012, when it moves to $5500. Price may vary in other countries. All AP Sales Partners are now accepting orders, and the first units will ship early in the second quarter of 2012. Please contact your local AP Sales Partner for demos or more information.
Downloads / Resources
Sound Advice: APx500 v2.9 New Features
APx500 version 2.9 includes many new features. Last month we highlighted the Appended Sweep Colors feature (see Audio.TST December 2011). Selected additional new features are described below.
The team at Audio Precision has been working hard to make sure that engineers working with audio have the best and most capable tools possible. Sporting a host of requested features and improvements, APx500 v2.9 is designed to improve productivity, increase flexibility, and streamline workflows both in the lab and on the factory floor.
Save Graph Data: All graph data is now saved with a project by default (may be disabled).
Import and Export Data in a Sequence: Import Result Data and Export Result Data steps have been added to the Sequence Step Properties dialog.
Data Output Appends a Row at a Time to CSV File: For automated production lines, the Data Output feature sends meter and pass/fail results of the sequence to a CSV file as tabular data. Each time the sequence is run, the new data is appended to the file on its own line ready for trend analysis.
User Defined Filters: Low-pass and high-pass filters are now user-definable by entering a corner frequency in the filter field, and the high-pass filters are now separate from the weighting filters.
Fig 1 Filter options. Users may now type the desired low and high pass cut-off frequencies in the boxes.
Man in the Middle: Controls have been added to the Bluetooth configuration dialog to enable “Man in the Middle” (MITM) pairing modes. MITM requires human interaction during pairing.
New Sweeps and Other Measurements:
High Accuracy Crosstalk Options in Continuous Sweep: The Continuous Sweep Crosstalk Type selection feature allows you to use the original High Speed method or to choose one of two High-Accuracy modes (One Channel Driven and One Channel Undriven) that do not use time-offset sweep signals. The High Accuracy crosstalk modes require longer testing times, but provide more accurate results across a wider spectrum.
Fig 2 Crosstalk Sweep, One Channel Driven (view entire APx500 application window).
Crosstalk vs. Frequency Stepped Sweeps:
Bandpass Stepped Sweeps:
Bandpass Level vs. Time: Added to Measurement Recorder
IMD vs. Frequency Sweep: For any IMD type (SMPTE, DFD, CCIF or MOD), the frequency of one of the two stimulus tones may be swept while holding the other tone at a fixed frequency.
Fig 3 Signal Analyzer, Power Spectral Density result (view entire APx500 window).
Spectral Density Results:. Amplitude Spectral Density (V/√(Hz)) and Power Spectral Density (V2/Hz) results are now available in the Signal Analyzer.
Noise Recorder: Shows RMS Noise Level vs. time, similar to the Measurement Recorder but without the generator. Weighting filters are available.
Q-peak Noise: Measurement is in accordance with the CCIR/ITU-R BS 468-4 weighting curve.
Fig 4 THD+N measurement now gives all these results: THD+N Ratio, THD+N Level, THD Ratio, THD Level, Noise Ratio, Noise Level, Distortion Product Ratio, and Distortion Product Level.
THD, N Results in THD+N: The THD+N measurement now includes "N-only" and "THD-only" results. Added measurements are THD Ratio, THD Level, Noise Ratio, and Noise Level.
RMS Derived Result: The "Min/Max Statistics" family of Derived Results now includes an RMS result. When applied to a Measurement Recorder Level measurement, this allows RMS calculation for periods of up to 7 days.
Downloads / Resources
Test Results: AP News & Events
© 2012 Audio Precision, Inc.