Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision
Recently, AP celebrated the 15th anniversary of our custom designed building. Photo albums were displayed showing the construction and the move into our facility back in 1998. (My, how some people have changed in appearance!) Among my many memories of that time, I was reminded of a particularly nasty problem we experienced as we restarted operations in our new production environment.
Almost immediately after moving we began to experience failures of certain bench tests that are performed by our technicians during the course of product assembly and adjustment. After some research, we discovered our new building had some extremely large magnetic fields in the production area, almost as if it was haunted. These fields coupled high levels of hum into our products that were causing the test failures. AP products are designed to reject reasonable levels of stray magnetic fields that would typically be encountered in a lab or production environment. However the magnitude of the fields we faced were at least 20 dB worse.
We ultimately discovered that several of our AC outlets had been wired incorrectly, having their neutral and ground connections swapped. This is a big no-no from the safety viewpoint, but it also caused all of the neutral currents in a particular circuit (outside of our production area) to return through the safety ground connection and ultimately through plumbing and drainage pipes. Some of these plumbing pipes were located in the space directly above our production area while the main drainage pipe was buried directly below; thus our production area was effectively inside of a huge coil. Our electrical contractor was embarrassed but confirmed our diagnosis, and the problem was quickly fixed.
Sometimes one has to think “outside of the box” to correctly perceive or understand a given problem. In this case, our new building plans provided the necessary insight to recognize the inadvertent source of our unwanted magnetic fields (plumbing that formed a coil around our production area). Perhaps you might want to check the neutral and safety ground connections of the AC outlets in your own work space—there could be some ghosts present!
Enjoy this month’s edition of our newsletter…
Output: Dolby Confidence Tests
On June 10, Dolby Laboratories posted the first APx Dolby Compliance Test developed in tandem with Audio Precision and Dolby engineers. The APx solution is part of the Dolby Digital Plus Decoder for Broadcast Products System Development Kit and is available to all licensees from the Dolby Deliverables on Demand website.
AP has provided support for Dolby “confidence tests” for many years, and the 2700 Series is still Dolby’s reference instrument in many of their older, consumer-oriented SDKs. However, this release marks the first official cooperation between Dolby and AP using the APx platform. APx is the natural fit for Dolby test because of its HDMI interface, multiple analog channels, extensive automation options and rich reporting capability.
AP’s application runs through all the tests specified by Dolby in the SDK and then automatically populates Dolby’s report template ready for submission to Dolby.
The SDK compliance test includes well over 1200 individual measurements. Performed manually, these tests could easily take over a week. The APx solution reduces that time to hours. More important, because APx generates the test sequence with all appropriate limits, there is no time wasted on recreating a test methodology or deciphering test requirements.
Dolby test solutions for Audio Precision analyzers are available exclusively from Dolby as part of the deliverables included in Dolby System Development Kits (SDK) for specific Dolby technologies. The solutions are provided at no charge, but do require the appropriate AP audio analyzer to run.
Configuring your test setup
Many different devices might use the DD+ Decoder SDK, so the APx solution optimizes itself for your device’s inputs and outputs, A/V sync delays, and desired transport streams.
Fig 1 The Dolby Project Controller
AP’s Dolby solution generates several reports. An official Dolby report is generated for each Dolby signal path, populated with data as requested by Dolby. A supplemental report is generated for the DUT providing all measurements, supplemental graphs and data points that we think might be useful for later reference or troubleshooting.
Dolby R&D vs. compliance testing
Of course, most of the time implementing Dolby technologies is spent on actual implementation, not compliance certification. To make troubleshooting easier, users can run any of the measurements in the sequence independently.
Outside of running the compliance tests, APx has several tools for further analysis. Some of the tools we found particularly useful while we were developing the Dolby app include:
Fig 2 Bit compare window highlighting an incorrect frame versus the original file
Fig 3 The EDID editor
Sound Advice: Testing MADI systems with APx
Multichannel Audio Digital Interface, MADI or AES10 is an Audio Engineering Society (AES) standard electronic communications protocol. MADI allows up to 64 channels of uncompressed audio to be transmitted over a single optical cable over a distance of up to 2km, and is used extensively in live sound and recording environments as a replacement for analog "snakes" used to handle multi-channel audio in the past.
In order to test the functionality of a MADI system, one must have a reliable means of accessing the individual channels of digital audio over the range of operating sample rates and channel counts. Although APx does not have a native MADI I/O, we can use the APx ASIO driver to interface between a MADI device and an APx audio analyzer.
The SSL MadiXtreme is a high quality, purely digital interface for PCIe slots in workstation PCs. It is available in both 64 and 128 channel versions, with 1 or 2 optical MADI connectors respectively, and supports all combinations of sample rate and channel count. We have selected and tested this sound card as an example; many professional-grade digital interfaces can be used to connect to APx500, and like any intermediary must be accounted for in any final measurement.
Some test examples
With a qualified MADI interface, we’re ready to add a Device Under Test into the signal path.
Fig 1 Analog to MADI DUT test signal path
The DUT is connected via MADI to the SSL MadiXtreme installed in a PC running APx500. An APx analyzer provides stimulus to the analog inputs of the DUT, while the output of the MADI interface is measured directly in the PC via the ASIO connection to APx500.
The functionality of a DUT with a MADI input can be tested by reversing the connections used in the previous example. The DUT is connected via MADI to the SSL MadiXtreme installed in a PC running APx500, as shown in Figure 2, while the analog output of the DUT is fed to the analog input of an APx analyzer. APx500 will send any desired stimulus to the DUT via its ASIO outputs, while measurements are taken from the analog signals as usual.
Fig 2 MADI to Analog DUT test signal path
Up to 16 channels of stimulus can be received or transmitted simultaneously with an APx525 or APx585 family analyzer; with an APx515, up to 2 channels of ASIO audio can be transmitted. A mapping dialog in APx maps available MADI channels to available APx generator and analysis channels.
Testing other digital interfaces with ASIO
Taking advantage of ASIO for greater connectivity extends beyond MADI. Other formats and protocols also share the common features of bit-exact streams and ASIO drivers. The key is to verify that the interface device is configured correctly in bit-exact mode before inserting any other device into the signal chain.
If it has an ASIO driver, APx can talk to it.
Examples of some other digital audio transports for which ASIO interfaces are available:
Test Results: AP News & Events
Dolby training seminars in Tokyo, Seoul, Shenzhen and Shanghai
In cooperation with the local Dolby offices in each country, Audio Precision is planning a series of half-day seminars to train our customers how to run the new projects. In addition, AP Senior Application Engineer Bill Rich will share some of the day-to-day test techniques and tricks he’s developed after working with Dolby technologies nearly every day for over a year.
These seminars should be attended by all engineers working with Dolby technologies.
© 2013 Audio Precision, Inc.