When streaming LSL data, every fifth timestamp is too quick

[ScottThomasMiller]

Problem

I am pulling LSL samples into my custom iOS app, from the OpenBCI GUI in LSL time series streaming mode. I am using 16 channels, so I expect my sampling rate to be 125Hz (0.008s.) Regardless of whether I pull samples one-at-a-time or in chunks, every fifth timestamp seems to arrive too quickly, <<0.008s. For example:

Expected

All timestamp intervals should reflect the sampling rate of 125Hz = 0.008s

Operating System and Version

macOS Big Sur 11.5.1 on MacBook Pro M1

GUI Version

5.0.5

Running standalone app

standalone

Type of OpenBCI Board

Cyton+Daisy

Are you using a WiFi Shield?

No

Console Log

I can provide a console log upon request.

[ScottThomasMiller]

Chadwick @ LSL provided a detailed explanation of this behavior (he's been absolutely awesome), but I want to hear it from you all as well.

Additionally, when I drop down to 8 channels, the sample rate doubles to 250Hz, and the period of the "quick" samples doubles from every 5 to every 10 samples.

What I need to know is: are the timestamps of the "quick" samples the actual LSL timestamps?

[retiutut]

@ScottThomasMiller Thank you for digging into the LSL streaming of the TimeSeries data type. I've tagged this issue on #971 so as to consider this additional information.

I will likely have more questions soon.

Seems like this could be replicated and investigated using a Python sketch. 🤔

[retiutut]

LSL calculates these timestamps. I'm not sure how it's something we could fix, unless we need to update the LSL Java dependency that the GUI uses.

[ScottThomasMiller]

It's not something OpenBCI can fix. I understand the fundamental issue much better now. Thanks for looking into it. Should we close this issue?

[retiutut]

@ScottThomasMiller Can you explain what's going on with this to me briefly? I'd still like to understand.

The GUI Networking Widget currently collects data in a chunk-compatible array and then sends a chunk of time series data when enough is available.

My key questions are:

• Why is this happening?
• Is it ok?
• Should I try updating the LSL java dependency? Maybe it was fixed some time ago?

[ScottThomasMiller]

The root cause of this particular issue, if I remember correctly, is described by Chadwick here:

https://labstreaminglayer.slack.com/archives/C87GN1S4E/p1628990241050700?thread_ts=1628811549.047400&cid=C87GN1S4E

From that thread:

"The OpenBCI GUI app creates a stream with a nominal rate of 125 Hz. It then pushes a chunk of samples -- 5 at a time. LSL tags the last sample with the current lsl_local_clock time, then decrements the previous sample times in the chunk by 1/nominal_rate (=8 msec)."

--Scott