By Bruce Hofer, Chairman & Co-Founder, Audio Precision
No big announcements this month: we're full steam ahead on development of APx v2.4.
Read on for a couple of articles about HDMI EDID and a set of Dolby/dts projects we released that allow "hands-free" testing of AV receivers. I'd also to send a special note to all chip designers to ensure you saw the new Digital Serial I/O for APx. With 8 channels, independent clocks etc, it's a unique product for anyone working on multichannel converters or HDMI Tx/Rx chips. The first units shipped in December, and initial reviews are very favorable.
Well, that's enough marketing spin for me. As always, we welcome your questions, and if there's anything we can do, AP will be there to help.
As promised, this month we'll take a look at both the new HDMI EDID read and edit capabilities built into APx500 v2.3, and the Dolby and dts confidence test projects we recently released. For a detailed look at all the new features in the latest version, see What's New in APx500 v2.3.
In version 2.3, we've added EDID read and edit capability for the APx HDMI option. EDID stands for extended display identification data. Originally created for a monitor to communicate its capabilities to a computer, EDID now carries audio format data as well. While audio and video data travel downstream on an HDMI cable to the next device, EDID swims upstream to tell the previous device what to send.
Why is reading and editing EDID useful? So that we can check if the data being sent by a device is correct, and if the data being received by it is being handled properly. For example, if the DUT supports a 192 kHz sample rate, we can check that the EDID data it sends out shows that. We read this EDID data coming into the APx's HDMI source port by clicking on the EDID button in the Signal Path Setup: Output Configuration of the APx500 navigator.
APx500 EDID Viewer
Conversely, on the HDMI sink port, the APx originates the EDID data. You have the options of a default setup, passing EDID through from the HDMI aux out port, or customizing the settings with the EDID editor. With the editor, you can, for example, disable a supported format and see if the DUT falls back properly to another. You get to the EDID editor by clicking on the Settings button in the Signal Path Setup: Input Configuration of the APx500 navigator.
APx500 EDID Editor
APx500 measurement software version 2.3 supports EDID 1.3, and E-EDID, known as Enhanced EDID.
No DVD required: New with APx v2.3, encoded audio can be generated by APx, so AV receiver Dolby/dts compliance testing can be performed "closed loop". Six new APx projects are available with all the required tests and embedded encoded signals.
Every device that bears a Dolby or dts logo is required to go through a compliance test process to insure that it meets the respective technical requirements. The normal test routine involves setting up test parameters, playing individual DVD or Blu-ray tracks from discs supplied by Dolby or dts, and recording the results of each test in a Word or Excel document. It's very time-intensive (as any engineer familiar with cueing up numerous DVD tracks will attest!).
Alongside the v2.3 software release, AP has created a set of six APx project files, some of which allow "hands-free" Dolby/dts testing of multichannel AV receivers.
APx can output a wide array of encoded high-bitrate test streams over HDMI, which makes testing more modern decoder devices such as surround sound receivers far faster and more convenient. With the APx generator stimulating the receiver directly, no DVD is required, so entire sequences of tests can be run without opportunity for human error (such as selecting the wrong track) or the introduction of compatibility issues with the source disc player.
APx project files are available for testing DVD players, Blu-ray players, AV receivers, and new High Definition AV receivers with HDMI, SPDIF, or multichannel analog in and out. Each project includes a battery of tests that mirrors the tests specified by Dolby or dts. Prompts are provided where it is necessary to play a DVD or Blu-ray track (when testing disc players directly) or to change a setting on the device under test. After tests are complete, a rich graphical report is generated automatically.
It's also easy to customize our sample projects for your specific device, and you can jump around in the project if you need to troubleshoot a certain result.
I'm going to be measuring some DACs with my 2700 Series analyzer. Which AES-17 filter should I use?
AES-17 pre-analyzer filters are low-pass filters with a steep brick-wall cut-off just above the measurement range. They conform to AES (Audio Engineering Society) Standard 17 for measuring DACs, digital amplifiers, and other digital devices with strong out-of-band content. The filters remove this out-of-band noise, which gives more accurate measurements and allows the analyzer to properly set input ranges to yield best resolution.
Note that only the 2700 Series analyzers accept optional AES-17 filters. The APx Series and ATS-2 don't need them—their standard low-pass filters are implemented digitally and already have a brick-wall roll-off that meets the AES-17 specifications.
For the 2700 Series, in almost all cases we recommend using the S-AES17 filter. It has excellent performance, with virtually no impact on noise or distortion. The S-AES17 is a two-stage filter, with a gentle roll-off in the first stage before the analyzer’s measurement auto-ranging circuit, and a brick-wall roll-off in the second stage after the circuit.
The gentle roll-off in the first stage protects the auto-ranging circuit from high frequency out-of-band content that might interfere with its operation. It may be invoked independently by selecting 20 kHz SPCL or 40 kHz SPCL in the Bandwidth (BW) low-pass select box on the Analog Analyzer panel.
The brick-wall roll-off in the second stage removes essentially all remaining out-of-band content, so that the measurements accurately reflect only the audio band. It may be invoked independently by selecting 20 kHz (40 kHz) Brick-wall in the Filter (Fltr) select box. The steep brick-wall stage of the S-AES17 filter has been placed after the auto-ranging stage to achieve the best performance. Note that selecting both halves of the filter together—20 kHz (40 kHz) SPCL in the BW select box, and 20 kHz (40 kHz) Brick-wall in the Fltr select box—is equivalent to selecting the entire 20 kHz AES17 filter in the BW select box.
Filter response 10 kHz - 40 kHz
Yellow: 20 kHz SPCL, Magenta: Built-in 22 kHz low-pass, Cyan: 20 kHz SPCL + 20 kHz brick-wall (same as 20 kHz AES17)
The single-stage OPT-2020 filter has exactly the same roll-off curve as the S-AES17, but its THD+N performance is not quite as good. Additionally, the OPT-2020 also has further degraded performance at high input amplitudes, as found in power amplifier testing. Nevertheless, there are some cases where the OPT-2020 may be a better choice. Since its steep brick-wall filter comes before the analyzer’s measurement auto-ranging circuit, it does a better job protecting that stage from overload caused by high amounts of interfering signals just above the cut-off frequency. This can occur in the case of converters using low sample rates, since the sigma delta process causes the noise to start at double the sample rate in use. These low sample rate applications are often in telephony or audiology where very low residual THD+N performance is not of concern. The other advantage of the OPT-2020 is that it does not occupy one of the regular filter slots, so it can be combined with other filters, such as the A-weighting filter.
Switching (Class D) power amplifiers present a special case. Because their speaker outputs often contain high frequency switching noise at much higher voltage levels than line level devices, slew-induced distortion can be caused on the audio analyzer’s input stages. This holds true for all audio analyzer models and brands. To eliminate this problem, passive filtering must be done before the signal enters the analyzer. AP makes the AUX-0025 (2-channel) and APx581 (8-channel) filters for this purpose. As the primary function of the external passive filter is to prevent input overload, the steeper brick-wall S-AES17 filter is still necessary to eliminate all out-of-band noise that might influence measurements.
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