SMPTE 2059-1 - NextAlignmentPoint

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zerodefect
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Joined: Tue Nov 13, 2018 5:39 pm

SMPTE 2059-1 - NextAlignmentPoint

Post by zerodefect » Tue Nov 13, 2018 6:10 pm

I'm looking into SMPTE ST 2059-1:2015 - specifically I'm interested in understanding how to sync broadcast video against PTP (PTP is defined in iEEE 1588).

Section 7 of ST 2059, dubbed "Video Signal Generation", describes how to find the next `alignment point` at time `t` for a given broadcast video signal. Subsection 7.2, dubbed "Digital Standard Definition Television", has a table of entries with one of the rows containing the following data:

Table 2 – Signal Generation – Digital Standard Definition Television

Code: Select all

+--------------------------+---------+----------------+-----------------------------------+---------------------------+------------------------------+--------------------------------+-----------------+------------------------+
|    Television System     | Fr Rate | SMPTE Standard | Horizontal Alignment (Y Sample) P | Vertical Alignment Line L | Y samples per total line (H) | Y Samples per active line (HA) | Total Lines (V) | Sampling Clk Freq (SR) |
+--------------------------+---------+----------------+-----------------------------------+---------------------------+------------------------------+--------------------------------+-----------------+------------------------+
| 625 line Interl 13.5 MHz |      25 | SMPTE ST 125   |                               736 |                         1 |                          864 |                            720 |             625 |  13.5 x 10^6           |
+--------------------------+---------+----------------+-----------------------------------+---------------------------+------------------------------+--------------------------------+-----------------+------------------------+

Following the table are some formulae:


- T = 1 / SR
- NextAlignmentPoint = int(t / (H x V x T) + 1) x (H x V x T)

where:

- `t` is elapsed continuous time from SMPTE Epoch in seconds. Note: This is the same as PTP time for PTP-based implementations.
- `T` is the period of the sampling clock in seconds.
- `H` is the duration of an entire video line in sample clock periods.
- `V` is the duration of a complete frame in lines.
- `R` is Sample Clock Frequency

The formula for `NextAlignmentPoint` is the one of interest.

So for 625-line interlaced (and using the table above), I believe that:
  • T = 1 / 13.5 x 10^6
  • V = 625
  • H = (no clue - does it use 864 here?)
Any assistance would be greatly appreciated. It's situations like this where I wish I had a better understanding of analog video!

Kelly
Posts: 509
Joined: Thu Aug 21, 2008 5:32 pm
Location: Washington D.C. Area

Re: SMPTE 2059-1 - NextAlignmentPoint

Post by Kelly » Fri Dec 14, 2018 12:45 pm

Skype:kellyalford Twitter: @KellyAlford

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