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02/02/2009

World's First Bit-For-Bit Dolby/DTS Audio Accuracy Test From AP

Audio Precision, the recognised standard in audio test and measurement, has introduced an update for its APx Series audio analysers that allows them to measure the digital error rate of a device handling lossy encoded digital audio in formats such as Dolby and dts - a world first.
Every device that passes digital audio strives to be bit-accurate - that is, to pass the digital audio stream unchanged from its input to its output. However, devices may unintentionally truncate data, introduce dither, scale the signal, or convert the sample rate.
These errors may be hard to detect with a traditional THD+N measurement, but they can cause havoc for devices receiving the corrupted data later in the signal path.
Most audio analysers can already measure in so-called 'bitstream mode' to ascertain that digital audio is passed through a device without corruption. However, until now, there has been no way to test losst encoded audio formats such as Dolby and dts.
Audio Precision's new technique, introduced with the v2.3 update for APx Series audio analysers, allows both lossless and lossy streams to be tested. This is essential in accurately evaluating whether an HDMI source can properly handle all the different formats that may be requested by an HDMI sink.
Tom Kite, Audio Precision's Vice President of Engineering, said: "Encoded audio is a tricky business. We looked at a prototype HDMI converter chip that reported a digital error rate of over 70 per cent. Deeper investigation proved that the device was accidentally killing the last four bits of the 24-bit word. The designer was quite surprised, but at least he found the mistake before his customers did."
APx calculates the digital error by measuring a proprietary waveform with a deterministic relationship between samples. This allows the instrument to detect any errors in the output of a digital device on a sample-by-sample basis. The algorithm used is generator-independent and allows the instrument to test closed-loop digital devices (such as routers and switchers) as well as playback devices including CD and DVD players.
Using specially defined test signals, it is possible to determine the digital error rate of playback devices outputting Dolby- and dts-encoded signals. APx instruments can generate the test signal from any digital output, including SPDIF, Toslink, HDMI and AP's new Digital Serial I/O interface. In addition, the test signal is provided as WAV, AC3 (Dolby 5.1), DTS (dts-encoded 5.1 audio) and DTSHD (DTS-HD Master Audio) files. CD audio and DVD test discs are also available.
The Digital Error measurement can also be used to test bit-for-bit accuracy over time (to be over 10,000 minutes), a useful feature for detecting intermittent errors that would otherwise be hard to reproduce. Available views of the data include Average Error Rate, Instantaneous Error Rate, Total Errors and Cumulative Errors over time.
In addition to digital error rate, APx500 v2.3 adds support for the new Digital Serial interface option as well as the new four-channel APx analysers. New software features include HDMI EDID editing, save-to-disk audio recorder, custom sweeps, signal diagnostics, dBSPL units and easy measurement microphone calibration, support for more coded audio formats in file playback, appended sweeps in a sequence, user-interface improvements and other enhancements.
Further information is available at: www.ap.com.
(KMcA/BMcC)
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