[TangerineSDR] PSWS Time Stamping Concept: maybe we should use the 128-bit time

Scotty Cowling scotty at tonks.com
Mon Oct 21 19:04:02 EDT 2019

Hi Bill,

128-bit format should be no problem. Do you know what format VITA-49 uses?

I don't like rollovers either. And I use lots more memory than 1MB.

Scotty WA2DFI

On 2019-10-21 12:56, Engelke, Bill via TangerineSDR wrote:
> Tom – thanks for bringing this up, I definitely should have included 
> it.  Referring to https://tools.ietf.org/html/rfc5905#section-6 .
> After thinking about this, I’m wondering: *should we consider using 
> the full 128-bit NTP date format* (instead of the NTP Short format of 
> 64 bits), with the idea that we then *don’t have to cope with the year 
> 2036 rollover*?  The Era value will take care of it.  Since our time 
> stamps don’t occur all that often, it doesn’t seem to me like a big 
> cost. Our experience has been that these software projects take on a 
> life of their own and last much longer than anyone ever expects; 
> thinking of it in that way, 2037 is right around the corner.
> Would anyone else like to weigh in on this matter?
> 73- Bill, AB4EJ
> *From:* TangerineSDR <tangerinesdr-bounces at lists.tapr.org> *On Behalf 
> Of *Tom McDermott via TangerineSDR
> *Sent:* Friday, October 18, 2019 4:29 PM
> *To:* TAPR TangerineSDR Modular Software Defined Radio 
> <tangerinesdr at lists.tapr.org>
> *Cc:* Tom McDermott <tom.n5eg at gmail.com>
> *Subject:* Re: [TangerineSDR] PSWS Time Stamping Concept
> HI Bill - thanks for documenting this.   The format of the 64-bit time 
> stamp
> should be referenced.  My recommendation is NTP format.  We cannot use 
> all of
> the precision that affords, and thus may want to consider setting the 
> LSBs to zero below
> some accuracy limit.
> The format is documented in: IETF RFC 5905 Section 6, "NTP Timestamp 
> format", figure 3.
> page 13.
> https://tools.ietf.org/html/rfc5905
> There are standard library tools to manipulate it.  Whilst UNIX time 
> will rollover in 2037,
> my view is that standard methods, procedures, and standard code will 
> emerge to consistently handle it.
> If we adopt a proprietary format, then custom code would otherwise 
> need to be crafted.
> The fractional portion of one second is encoded as a 32-bit integer. 
> The LSB represents 0.233 nanoseconds.
> We could consider setting the 5 LSBs to zero for the 50 nanosecond-ish 
> accuracy.  Then use those LSBs
> later if some GPS receiver in the future had better accuracy.
> -- Tom, N5EG
> On Fri, Oct 18, 2019 at 1:09 PM Scotty Cowling via TangerineSDR 
> <tangerinesdr at lists.tapr.org <mailto:tangerinesdr at lists.tapr.org>> wrote:
>     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.
>     73,
>     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
>     -- 
>     TangerineSDR mailing list
>     TangerineSDR at lists.tapr.org <mailto:TangerineSDR at lists.tapr.org>
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