AP - The Recognized Standard In Audio Test

Testing Digital InterfacesHOME/SOLUTIONS/DIGITAL INTERFACES

The electrical quality of a digital audio signal traveling from one piece of equipment to another isn’t often thought about until a failure occurs. While digital signals are generally reliable, some situations call for the ability to analyze the signal, not only to troubleshoot problems, but also to preempt issues and ensure reliability as part of a regular maintenance routine. Digital interface failures may occur due to limitations of the digital audio transmitters and receivers themselves, or due to the quality or length of the cable connecting them. Equipment manufacturers, installers, broadcasters, and many others need to have a way to analyze the electrical qualities of the digital signal.

The Audio Precision 2722 and the ATS-2 both have this ability. These instruments provide analysis of numerous signal quality parameters such as jitter (unsteadiness in the timing of a digital signal), voltage (voltage level of the signal,should not be too high or too low), delay (time delay of the signal traveling in and out of a device), and error flags (an error indicator added by the receiver to the audio stream when a decoding errors occur).

The eye-pattern display gives a valuable visual indication of electrical signal quality. An oscilloscope may easily be connected to the 2722 or ATS-2, making it possible to watch for signal reflections and other abnormalities.

To test a device’s ability to accept a sub-par signal, jitter should be added to the incoming digital audio stream until the device can no longer lock or receive the signal properly. The 2722 and the ATS-2 both are able to add various types of jitter. The results can then be compared against established standards. These analyzers also can add noise, add a common mode signal, limit rise/fall time, and vary the output voltage. All these tests stress a receiver and demonstrate its abilities under adverse conditions.

For multichannel ICs such as HDMI receiver/ transmitter chips or multichanel converters, the multichannel APx Digital Serial I/O is very useful, with 8 channels of simultaneous audio data on TDM or multiple data lines; dedicated, buffered oscilloscope monitors; built-in support for left or right justified, I2S and DSP formats; automatic rate calculators and active timing diagrams; and independent input and output clocks.

Tips for Optimum Testing

  • Make sure to use cables specified for digital audio. The impedance must be correct (75 Ohms for coax, 110 Ohms for balanced) and connectors must be installed properly and in good condition to avoid signal reflections.

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