[TangerineSDR] PSWS Time Stamping Concept

Engelke, Bill bill.engelke at ua.edu
Mon Oct 21 15:30:29 EDT 2019

Hello Scotty:

I agree that we should have a convention that CH0 always comes immediately after a time stamp.  We are designing this to be tolerant of missed packets; without this convention, in case of one or more missed packets, we would have a hard time knowing what channel occurs after the time stamp.

On frequency of time stamp, I'm not sure what this needs to be; however, the following does occur to me:

  *   At start of data collection, we can only commence after we get the first time stamp. In the odd case where we miss the first packet, we don't want a long time to elapse before we get the next time stamp so we can start collecting. Would it work if we were to put exactly one time stamp in every packet, right before the first time CH0 is output in that packet?

On setting the time, I can certainly amplify what happens here, even include a process diagram. You can decide on the details of exactly what these commands will look like when designing the ICD for LCC.

I got some additional comments which I will also include as part of a minor revision....     Tnx es 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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