Created on 2013-12-17 00:10:00
Dolby Confidence Tests with APx
On June 10 2013, 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. These solutions are provided at no charge, but do require the appropriate AP audio analyzer to run.
Configuring your test setup
Many different broadcast 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:
- Metadata recorder
Metadata recorder decodes and displays metadata values as they are received from the device under test. By comparing the metadata being read against the actual audio signal being measured by APx, the device’s ability to transmit metadata correctly can be verified systematically and tests like comparing A/V mute time to sample rate switching can be confirmed. The APx Metadata Recorder captures metadata changes over time, displaying up to 15 different metadata fields for periods up to 168 hours.
- Measurement recorder
Measurement recorder captures level, distortion, frequency, phase and more continuously for up to 168 hours. We use this measurement quite often in the sequence, but it’s also very useful on its own.
- Status Bits
Under normal test conditions, APx's own metadata matches the actual audio being streamed. However, in the real world, devices are likely to encounter incorrect metadata at some point.
To test how a device handles incorrect metadata, the Auto values can be overridden and set to any value, even invalid ones. The device's reaction can then be verified by APx. For example, glitches can be caught with the measurement recorder or the protocol monitors can read what metadata the device passes through: the bad original or a corrected value.
- Bit compare
This feature captures encoded bit streams such as DTS and Dolby Digital and compares the captured stream to a reference file on a byte by byte basis. If differences are found the measurement indicates where the differences are located and will display the erroneous values for analysis. The feature works with HDCP encrypted streams as well as unencrypted streams.
Fig 2 Bit compare window highlighting an incorrect frame versus the original file
- EDID Editor
Reading and editing the EDID allows us to check if the data sent by a device correctly describes its capabilities, and if the data received by it is being handled properly. For example, if a device supports a 192 kHz sample rate on its input, we need to check that the EDID data it sends to the audio source reflects that. Conversely, we need to verify that the audio source correctly interprets incoming EDID data and that it can handle invalid EDID values gracefully.