Created on 2013-12-17 00:33:00
APx Waveform Computes Utility
The APx Waveform Computes Utility extends the capabilities of APx500 by making Fast RMS (root mean square) measurements. In addition to RMS level measurements, it utilizes Fast RMS for a number of useful tasks: audio/video synchronization, dropout detection, and pop/click detection.
Fast RMS analyzes every sample of a waveform to make gapless measurements at speeds up to 1000 readings/sec. This capability is essential when looking at synchronization, dropouts, and pops and clicks, because these transient events may get missed at slow reading rates that have gaps between readings. In addition, the fast reading rate enables accurate determination of duration, allowing precise pass/fail criteria to be set.
The utility runs as a command line interface (CLI) application. It has no user interface—it gets all its settings from command line parameters. This allows it to run in the background and send the results to CSV format data files. In APx500 you can use the Defined Result feature to plot the data. Defined Results lets you can create bar or XY graphs from data contained in a CVS file.
Audio Video Synchronization
The Audio/Video Synchronization function (DoAVSync) calculates audio delay (positive result) or advancement (negative result) relative to corresponding video. Synchronization errors seem more prevalent now than in the analog past, as any digital processing that occurs in the video or audio chain can cause delays that put the picture and sound out of sync. Testing of production, distribution, and playback devices; transport protocols; CODECS; and operating systems all have become necessary.
Fig 1 Fast RMS result from the AV Sync measurement. The audio (blue) lags the video (red) by 10 ms.
To capture the video time reference, it is common to play a video file with regularly spaced periodic flashes. A photocell placed on the receiving device’s screen changes the light pulses into a voltage signal that can be fed directly into one of the audio inputs on an APx500 audio analyzer. Audio Precision sells the LTV-1 light-to-voltage converter especially for this purpose. The audio reference point is a recorded beep on the audio track. The flash/beep pairs can be repeated multiple times to capture variation in the delay over time.
Fig 2 Audio Precision LTV-1 light-to-voltage converter for A/V sync measurement.
The utility is invoked to run in the background by adding a Run External Program step in the project navigator, as shown in Figure 3.
Fig 3 AV Sync Run External Program Step.
A controlled test file with a 10 ms audio delay is used in the following example. Figure 1 shows the Fast RMS result, displaying both the video and audio pulses. Figure 4 shows the Delay result. Because synchronization error isn’t constant on some devices, the mean, maximum, and minimum delay over the duration of the test are calculated.
Fig 4 AV Sync Delay result.
The Statistics result in Figure 5 shows how many flash/beep pairs were received and the standard deviation.
Fig 5 AV Sync statistics result.
Fig 6 Dropout, 6 dB for 21 ms.
The Dropout Detection function (DoDropout) lets you set level and duration command line arguments in a Run External Program Step dialog for defining what constitutes an unacceptable dropout. The graph above shows RMS level vs. time for a waveform prepared with a defined dropout of 6 dB for 21 ms. If this test waveform exceeded the specified limits, the utility would return “FAIL” and the measurement would show a failure icon in the APx500 navigator.
Fig 7 Pop/Click detection.
Pop/click detection (DoPopClick) is similar to dropout detection. Command line arguments determine the level and duration of the transient that constitutes a failure.
The APx Waveform Computes Utility download includes the program, complete instructions with command line parameters, and an APx project to demonstrate usage of all its capabilities.