[TangerineSDR] PSWS Time Stamping Concept/ SBC clock set idea

Engelke, Bill bill.engelke at ua.edu
Mon Oct 21 16:26:35 EDT 2019

Scotty - I thought of something else.

If we put the DE into GPSDO mode (so that it "arms" and then gets the time from the GPSDO PPS pulse), should we have the DE send this highly accurate time to the SBC, and set the SBC's clock accordingly?  This might reduce confusion that could arise if DE and SBC have slightly different time due to the GPSDO being more accurate.   What do you think?

-73- Bill

From: TangerineSDR <tangerinesdr-bounces at lists.tapr.org> On Behalf Of Scotty Cowling via TangerineSDR
Sent: Friday, October 18, 2019 3:08 PM
To: tangerinesdr at lists.tapr.org
Cc: Scotty Cowling <scotty at tonks.com>
Subject: Re: [TangerineSDR] PSWS Time Stamping Concept

Hi Bill,

Just to be clear, if the number of channels does not divide evenly into 1024, then a packet might not start with channel 0 I/Q samples.

Is there a requirement that the time stamp immediately precede channel 0 I/Q data? For example, a packet could look like this:

CH0_I(0), CH0_Q(0), CH1_I(0), CH1_Q(0), CH2_I(0), CH2_Q(0), CH0_I(1), CH0_Q(1), CH1_1(1), CH1_Q(1), CH2_I(1), CH2_Q(1), CH0_I(2)...
...CH2_I(339), CH2_Q(339), CH0_I(340), CH0_Q(340)

So you would start the next packet like this:
CH1_I(340), CH1_Q(340), CH2_I(340), CH2_Q(340), CH0_I(341), CH0_Q(341)...

If I put the time stamp at the beginning:
<sync><time stamp>CH1_I(340), CH1_Q(340), CH2_I(340), CH2_Q(340), CH0_I(341), CH0_Q(341)...

Then the time stamp would apply to the first and second I/Q pairs (CH1 and CH2) as well as to the last I/Q pair of the previous packet (CH0).

If I always put the time stamp before CH0, then the time stamp would apply to the last I/Q pair of one packet and also to the first two I/Q pairs of the next packet.

So are the time stamps always before CH0, or can they be anywhere?  I think for proper synchronization, they will have to be before CH0 only.

Also, while it is clear that time stamps are sent periodically, that period is not specified anywhere. I think we need to specify that, don't we? Maximum count between timestamps? Maximum number of packets?

Did you want to expand on the two commands (or methods) used by the SBC to set the two times (GPSDO and "best effort")?  We talked about an "arm" command that causes the time to be set on the next 1 PPS transition and an "immediate" command that sets the time immediately upon reception of the command.

Scotty WA2DFI
On 2019-10-17 17:37, Engelke, Bill via TangerineSDR wrote:
To all:
Attached is our proposed concept for Time Stamping for PSWS data - for your review and comment.
Note that this is primarily for the case where raw I/Q data is being stored in Digital RF format.
Data recording will be a bit different in the low-bandwidth case where the I/Q data is to be processed by GNURadio running on the SBC, and FFT (waterfall) results are uploaded to the database.

Dave: please post to TangerineSDR.com

TNX ES 73 - W. D. Engelke (Bill), AB4EJ
Center for Advanced Public Safety
Cyber Hall
The University of Alabama
Tuscaloosa, AL 35487
Desk: (205) 348-7244
Mobile: (205) 764-3099

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