[LAD] Latency measurements: External round-trip vs. absolute in/out values

Fons Adriaensen fons at linuxaudio.org
Sun Apr 7 12:48:28 CEST 2019


On Sat, Apr 06, 2019 at 12:32:29PM -0400, Tim wrote:

> So the software would be able to deal with sines better,
>  and measure phase differences rather than a pulse edge.

Sure, the pulse will be 'smeared out' a bit, and its actual
position may seem to be ambiguous. But that's no problem.

What you get in the sample stream is the impulse response
of the A/D converter. Assuming that this is a linear and
time-invariant system, the IR contains all information.

If the pulse is smeared out, one way to find the delay is

1. Compute the FFT of the part of the sample stream that
   contains the smeared-out pulse.

2. Compute the phase of each frequency bin. If the delay
   is independent of frequency (it should be if the A/D
   is any good), then the phase will be a linear function
   of frequency. The slope of that function indicates
   the delay (relative to the start of the FFT input).

The only advantage of using sine waves is that your test
signal has much higher energy than a single short pulse,
so you get a better S/N ratio. But for this test, the
S/N will be very good anyway.


Ciao,

-- 
FA









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