• matth
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Topic: I/O Latencies

I use an RME Fireface 400 (RME driver version 3.124) in a lab as measurement technology (not in a sound studio).
I measured the total latency (input + output) by comparing a playback and a recording signal obtained by connecting an output of the soundcard directly to an input with a balanced jack-jack cable.
SoundMexPro offers the possibility to return the input and output latency of the soundcard specified by RME as properties. Then I added up those two latencies and compare the sum with the total latency measured before.
The two results for the total latency always differ by 3 samples, independent of the buffer size and sampling frequency.  Does anyone know where this difference of 3 samples comes from?

  • Registered: 2019-10-01
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Re: I/O Latencies

Hello!

Just a guess, but AD/DA conversion(the actual conversion taking place inside the chip) + length of cable, maybe?

RME Gear: Digiface USB, HDSP 9632
  • matth
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Re: I/O Latencies

Hello!

thank you for your answer!

For example for a specific buffer size and sampling frequency I activated the loopback option for a hardware output in TotalMix FX. For this software loopback, I measured a delay of 67 samples. By connecting an output of the soundcard directly to an input with a cable I got a delay of 144 samples. The sum of the input and output latency of the soundcard specified by RME are 141 samples.

So I thought the AD/DA conversions are the reason for the difference of the sooftware loopback and the cable (67 vs 144 samples).  Therefore, the difference between the latencies specified by RME and the measured latency with a cable are still unclear to me.

I don't think the length of the cable should introduce a delay but I'll have a look into it.

  • matth
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Re: I/O Latencies

I think I found the response on page 104 of the RME Fireface UCX Manual:

"From a technical view there is no zero. Even the analog pass-through is subject to phase errors, equalling a delay between input and output. However, delays below certain values can subjectively be claimed to be a zero-latency. This applies to analog routing and mixing, and in our opinion also to RME's Zero Latency Monitoring. The term describes the digital path of the audio data from the input of the interface to its output. The digital receiver of the Fireface UCX can't operate un-buffered, and together with TotalMix and the output via the transmitter, it causes a typical delay of 3 samples"

I'm assuming that the digital path from the output to the input is the same as the digital path of the audio data from the input of the interface to its output. Even though i'm not 100% sure this assumption is correct.