Stuffing the stuff

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August 2012
September 2012

2012-08-19 »

The Low-Latency Analog Measurement Adapter (LLAMA):

I want a device that will be able to measure, with sub-millisecond accuracy, the amount of latency in a complete audio network.  Assuming an audiovisual signal has proper lipsync, audio latency is a good proxy for video latency, which is what I actually care about, for things like video games and videoconferencing.

Here's how I think you could design a system for measuring the audio latency of a TV, for example, given a plain RCA-style or headphone-style input:

  • Create a device with two synchronized ADCs. For each sample, you always sample both ADCs at the same time.

  • Hook ADC1 to the audio signal with a simple three-way splitter (we don't care about analog signal degradation here).  The other splitter port goes onward to the TV.

  • Hook ADC2 to a microphone near the TV's (or home theatre system's, or whatever's) speaker.

  • Generate the audio signal from a PC.  Use a really simple signal that's amenable to a really clear FFT, such as alternating between two frequencies or sweeping up and down, or pulsing on and off.

  • You can't trust that a PC's audio output has low latency.  (Audio drivers are often crap.)  This is why we needed two ADCs.  Take both ADC inputs back in and FFT them.  The lag on ADC1, compared to your expected output, is how slow your PC sound card is.  The lag on ADC2 compared to ADC1 is how slow your sound/video delivery system is.

Does anyone know if a system like this already exists?  (I realize you could do it with an oscilloscope, but I'd rather have something that requires zero expertise to operate so it could be used to compare lots of different equipment objectively.)

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