AP - The Recognized Standard In Audio Test

Digital protocol analysis is essential for any designer of modern digital audio equipment. Compared to earlier protocols such as S/PDIF, metadata streams now can carry hundreds of pieces of essential information. The failure of a device to either send or receive the proper data can lead to serious failures (such as audible glitches) and interoperability problems.

HDMI, for example, carries several types of metadata. Status and user bits carry information such as sample frequency, bit depth, and channel number. The Audio InfoFrame holds data about channel allocations, encoding formats, and level adjustments for multi-channel surround sound. And EDID, while not technically metadata, lets one hardware device tell another what it is capable of (number of channels, sample rates, decoders, etc).

The easiest way to ensure a device handles protocol metadata correctly (or has appropriate exception handling when incorrect metadata is received) is to use a metadata recorder. The metadata recorder plots changing states of multiple metadata fields in real time and displays them together, for example recording behaviour after a hot plug.

The APx metadata recorder has a user interface similar to a logic analyzer to easily identify relationships and correlations between the different metadata streams.

Common Challenges

• Verifying that A/V Mute, audio transmission, and HDCP negotiation all occur in the sequence required by HDMI and HDCP specifications and in manner that avoids the possibility of glitches.
• Verifying glitch free transitions between audio modes. A modern consumer A/V device may switch between linear/PCM data at various sample rates and Dolby and DTS audio quite frequently.  Does the device perform the transitions smoothly?  Do the audio mode and sample rate fields in the channel status blocks update correctly and at the right time?

Tips for Optimum Testing

• Define a sequence of tests that runs through every combination of metadata signals your device might ever see out in the wild to test its error handling.
• Check how your device handles "pause" commands. Does it mute all channels? Does the sample rate settle in a timely manner?