AP - The Recognized Standard In Audio Test


HDMI is rapidly becoming the standard for consumer audio/video interconnectivity. Most new televisions, A/V receivers, and disc players rely on HDMI to communicate with each other. Other devices such as cell phones, computers and cameras are also adopting HDMI.
HDMI has extremely high bandwidth that allows super high audio and video resolution. As a transport standard, it's also interesting in that HDMI allows two-way communication between the source (output) and sink (input) devices. HDMI's high bandwidth has enabled new high definition formats as well as.
Testing HDMI devices raises some unique challenges for an audio engineer:

  1. Physical interface
    Testing HDMI requires an HDMI connection! Many engineers will want to test an HDMI transmitter or receiver chip. This requires an analyzer with HDMI on one end and I2S, analog or S/PDIF on the other. Ideally the other end will have eight channels to match HDMI’s capability.
  2. Support for new high definition audio formats
    HDMI introduces a whole alphabet of formats ranging from older 5.1 Dolby Digital and dts Digital Surround to lossless formats like dts-Master Audio and Dolby TrueHD to linear PCM audio with 8 channels, 24 bits, and a 192 kHz sample rate.
  3. Metadata
    HDMI is a complex two-way protocol. EDIDs let a device tell other devices what formats it can handle. Failure to read or write these properly can lead to problems. Likewise, the HDMI Infoframe informs the receiving device about the characteristics of the audio and video.  Mistakes with the metadata can produce glitches or complete failures of the link.
  4. Video support
    HDMI is primarily a video transport. Any HDMI signal must include basic video data as well as audio content.

The HDMI option is available on the APx525, and APx585. The APx525, combined with the HDMI and Bluetooth options, is ideal for testing smartphones and tablets.

The Audio Precision APx585 (with HDMI option) is the world’s only Blu-ray and HDMI-capable audio analyzer. The instrument can directly interface to all the audio interfaces on modern consumer devices. In addition to HDMI source (output) and sink (input) the instruments provides eight channels of analog I/O, S/PDIF, Toslink, and optionally an 8 channel I2S/TDM serial data interface.

Common Challenges

  • Verifying EDID behavior:  Is the source device correctly parsing the EDID of a sink device and sending the correct audio data.
  • Are all the bits there: Some HDMI devices truncate the audio data rather than passing it bit transparently.  Turning 24 bit audio into 20 bit audio or 16 bit into 12 bit.
  • Does the metatdata match the audio: Many HDMI receivers require that the channel status sample rate, word length, and sample word length be set correctly for the signal actually being transmitted.

Tips for Optimum Testing

  • Use a protocol analyzer to systematically test EDIDs and Infoframes match actual output.
  • Test for bit-for-bit accuracy with a Digital Error Rate test. Be sure to confirm that encoded audio streams are handled correctly using an encoded audio test.
  • Use a metadata recorder to systematically test invalid metadata error handling.

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