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Audio.TST May 2009 AES Munich, APx Metadata Recorder, DIM 30/100

Created 03 Jun 2009

Audio.TST May 2009

Notes from the Test Bench
Output
Sound Advice

Notes from the Test Bench

By Bruce Hofer, Chairman & Co-Founder, Audio Precision

I’ve just returned from AES in Munich. While the number of exhibitors was down sharply from previous years, attendance did not seem to have experienced the same relative decrease. Thus it turned out to be a great show from the viewpoint of having interesting conversations in our exhibit booth and meeting new prospects. The technical program was also very interesting, and I managed to sit in on a number of good papers. Unfortunately I was not able to attend the evening organ recital given by Graham Blyth this year.

Our big splash at the show was to preview the new digital audio metadata features of APx (due to be released in APx v2.4). The new metadata recorder looks a lot like a logic analyzer, which makes sense considering how fast channel status bits can change. This will doubtless be an extremely useful tool in pinpointing the cause of glitches and other errors in new HDMI hardware. You can read the official announcement below.

In an interesting experiment, we asked people who stopped by the stand to “fill in the blank” on a photo of an APx585 (see below). The overwhelming vote was for adding SDI, though MADI and USB were also contenders. This kind of input is invaluable as we continue to optimize our APx product roadmap, though the front panel layout and artwork silkscreen might need some additional engineering work before final production...

APx585 photo with suggestions written over the lower right front panel blank.

Don’t worry if you missed this chance to make your voice heard—we’ll be conducting a “Future of Audio Test” survey next month to get your opinion too.

Bruce

Output: Tech Tips and New Applications from AP

Here’s our latest press release straight from the AES convention floor in Germany.

Audio Precision Introduces "Logic Analyzer" View for Digital Audio

126th AES Convention, Munich, May 8th, 2009: Audio Precision, the recognized standard in audio test and measurement, has announced a number of new features for their APx Series audio analyzers relating to the display, analysis, and control of metadata in HDMI and IEC 60598/AES3 digital audio streams.

The additional features are available in the first beta release of APx500 v2.4, which Audio Precision staff are demonstrating in their booth at the AES show in Munich, from May 8-10, 2009. The full release of APx500 v2.4 is due to be available later this year.

“Errors in metadata can cause real problems in HDMI that we just don't see in S/PDIF,” explains Dan Knighten, Director of Products at Audio Precision. “A logic analyzer view shows exactly what's going on over time, so you can identify what's causing glitches or other issues. There's no better way to troubleshoot these kinds of problems.”

The new APx500 software release has three new features relating to metadata analysis and management:

Two new Metadata Monitors. The first of these allows the decoding and display of the entire HDMI audio InfoFrame, including current N and CTS values, HDCP state, audio layout and A/V mute condition, while the second displays the channel status and user bits information embedded in IEC 60958/AES3 format audio (SPDIF, Toslink, AES/EBU). Both the HDMI and IEC 60598 audio monitors also display digital audio stream type and sample rate, along with Dolby and DTS parameters including ACMOD/AMODE value, dialog normalization level, and bit rate.
Two new Metadata Editor Panels, which allow users and test engineers to override default metadata settings and set channel status information and HDMI Audio InfoFrame data to any value. These panels can be used to verify the response of a device under test to both valid and invalid settings.
The Metadata Recorder, an easy-to-read colored display capable of plotting the changing states of multiple metadata fields in real time, like a logic analyzer. Up to 15 fields can be displayed simultaneously, allowing engineers to monitor in detail what happens to metadata information when an HDMI device is hot-plugged, or exactly when and where data glitches occur in a corrupted audio stream. Discrepancies between the indicated metadata and that being received are also automatically identified and flagged (for example, if the input sample rate is 48 kHz, but the channel status indicates 44.1 kHz). The results of such measurements are invaluable in verifying that a device under test is behaving in compliance with relevant specifications.

Related Downloads

APx500 v2.4 beta

Sound Advice: AP Knowledge Base

This month’s Sound Advice column is contributed by AP’s Director of Technical Support, Joe Begin.

DIM 30 and DIM 100 Measurements per IEC 60268-3 with AP2700

DIM stands for Dynamic Intermodulation distortion. It is a technique used to measure the nonlinearity of a device, and it’s designed to be particularly sensitive to distortions produced during transient conditions typical of audio program material. In DIM measurements, a square wave at a frequency of 3.15 kHz is low-pass filtered and then linearly combined with a sine wave at a frequency of 15 kHz. DIM 30 and DIM 100 use single-pole low pass filters with cutoff frequencies of 30 kHz and 100 kHz, respectively.

Fig 1 DIM 30 waveform.

If nonlinearities are present, the DIM signal induces intermodulation distortion products at 9 difference frequencies ranging from 0.75 kHz to 13.35 kHz. DIM is then calculated as the ratio of the root mean square (RMS) sum of the levels of the 9 intermodulation components to the level of the 15 kHz sine wave. It is typically expressed as a percentage or in dB.

Fig 2 Nine DIM intermodulation components.

An analog domain Audio Precision 2700 series analyzer equipped with the IMD option has the ability to generate DIM 30 and DIM 100 waveforms. But the DIM measurement provided by the system’s Analog Analyzer is just an approximation of the DIM level specified by IEC 60268-3, because it only considers two of the nine intermodulation components specified in the standard. A DIM measurement that conforms exactly to IEC 60268-3 can be conducted using the DIM generator in conjunction with the FFT analyzer available in DSP equipped systems (SYS-2712 and 2722). We have developed an AP2700 DIM measurement macro to accomplish this easily.

To use the macro, set up the AP2700 software just as you would to do an analog DIM measurement: In the Analog Generator panel, set the Wfm: control to IMD - DIM 30 or DIM 100, in the Analog Analyzer panel, set the Function Reading meter to DIM, and in the Sweep Panel, set Data 1 to Anlr.DIM and Source 1 to Gen.Ampl A (Figure 3). Now, start the macro to make the measurements and plot the graph. You can also make a single point measurement by checking “Single Point” on the Sweep panel.

Figure 3 AP2700 panel setup for macro.

The macro works as follows: First, it checks to see if the panels are set up as outlined above. If they are not, the macro displays an error message and then exits. Next, the macro reads the sweep parameters from the sweep panel, conducts an FFT-based DIM measurement for each step in the sweep, calculates the DIM levels according to IEC 60268-3, inserts the DIM results into the Data Editor panel (Figure 4) and plots them in the Graph panel (Figure 5). Finally, the macro reverts the Sweep Panel to the setup it had before the macro was run.