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📉 PCR-PTS Drift

Some encoders or remultiplexers let the PCR clock and the video timing timeline drift apart even when both values remain individually monotonic.

What this test checks​

This rule measures true PCR-PTS drift against an interpolated PCR timeline.

For each sampled video frame:

  • the validator selects the video timing timestamp, preferring DTS when enough decode timestamps are present and otherwise falling back to PTS
  • finds the PCR sample before the frame byte offset
  • finds the PCR sample after the frame byte offset
  • interpolates PCR at that frame byte offset
  • computes PCR-PTS drift = video time - interpolated PCR time
  • subtracts a single per-stream baseline, using the median raw drift value, so the chart is centered for readability

This means the rule is checking rate mismatch, not harmless startup offset.

Why drift matters​

Slow PCR-PTS drift usually points to an encoder or remultiplexer clock problem. Typical symptoms can include:

  • gradual A/V timing instability
  • decoder buffer stress
  • video that appears to run slightly early or late relative to the transport clock
  • streams that start cleanly but become less stable over longer durations

✅ Pass Criteria​

The test passes when the absolute interpolated PCR-PTS drift never exceeds the configured threshold.

The default threshold is 100 ms.

Application result message:

No PCR-PTS drift above 100.000ms was detected.

❌ Fail Criteria​

The test fails when the absolute interpolated PCR-PTS drift exceeds the configured threshold at any sampled video point.

In practical terms:

  • positive drift means the video timeline is ahead of the interpolated PCR timeline
  • negative drift means the video timeline is behind the interpolated PCR timeline
  • the rule compares the absolute magnitude of that drift against the configured threshold

When drift exceeds the threshold, the validator raises ClockDriftDetected.

Chart Modes​

The chart now exposes three modes.

Distance​

Distance shows Frame-to-PCR distance.

This is the distance between each video frame PTS and the most recent PCR anchor. It is not the true PCR-PTS drift.

Use this mode when you want to inspect:

  • PCR cadence
  • spacing between PCR insertions
  • missing or irregular PCR behavior

How to read it:

  • a repeating sawtooth or spike pattern is expected
  • spike height roughly follows the gap from the last PCR anchor
  • this view is useful for transport timing structure, not for pass/fail drift interpretation

Tooltip fields:

  • Frame offset
  • Video PTS
  • Last PCR
  • Frame-to-PCR distance

Drift​

Drift shows the true PCR-PTS drift.

It uses byte-offset interpolation between the surrounding PCR samples and then subtracts the per-stream median raw drift baseline.

Use this mode when you want to answer:

  • does the stream pass or fail the drift rule
  • how large is the true drift in milliseconds
  • where the drift crosses the configured threshold

How to read it:

  • a line staying near 0 ms means PCR and the video timeline remain rate-aligned
  • a steady upward or downward slope means the two clocks are drifting apart
  • a clean stream should stay approximately flat around zero with only small jitter

Tooltip fields:

  • Frame offset
  • Video PTS
  • Previous PCR
  • Next PCR
  • Interpolated PCR
  • PCR-PTS drift

Reference​

Reference keeps the same interpolated drift data but pins PCR at 0 ms so the video drift is easier to read visually.

Threshold guide lines remain at +/- threshold.

Use this mode when you want a more intuitive reference presentation of the same drift behavior.

Missing PCR anchors​

The true drift metric is only computed when a frame has the required surrounding PCR information.

  • if there is no previous PCR anchor, the point is omitted
  • if there is no next PCR anchor, the point is omitted from the true drift series
  • the validator does not fall back to last-PCR stepwise comparison for the real drift metric

The Distance mode can still show points that only have a previous PCR anchor.

Example​

These charts show the same rule in its available presentations.

PTS drift chart

Distance should keep the familiar spike pattern because it follows the last PCR anchor.

Drift should be approximately flat around zero on a clean stream and show a long-term slope only when the PCR and video clocks are actually drifting apart.

PTS drift reference chart

Reference uses the same real drift data as Drift, but keeps PCR fixed at 0 ms so it is easier to judge direction and threshold margin at a glance.

Test file​

  • mpegts-pcr-pts-drift.ts