0001034: A/V drifts out of sync in long HD 1080p 23.98Hz projects

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Date Submitted

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0001034

Kdenlive

Rendering

public

2009-07-10 05:41

2010-09-22 17:53

Reporter

madkins

Assigned To

ddennedy

Priority

normal

Severity

major

Reproducibility

always

Status

closed

Resolution

fixed

Platform

OS Version

Product Version

Recent git

Target Version

0.7.6

Fixed in Version

0.7.6

Summary

0001034: A/V drifts out of sync in long HD 1080p 23.98Hz projects

Description

I've been experiencing a problem where the audio and video drift out of sync in the rendered file where the frame rate is 23.98 Hz. Here's a way to reproduce it:

(1) Create a new project with the HD 1080p 23.98Hz profile.

(2) Setup the timeline like this:

00:00:00:00 - 00:01:00:00 Video Track: Color clip (black)
00:01:00:00 - 00:01:01:00 Video Track: Color clip (white), Audio Track: 1 sec tone
00:01:01:00 - 01:00:00:00 Video Track: Color clip (black)
01:00:00:00 - 01:00:01:00 Video Track: Color clip (white), Audio Track: 1 sec tone
01:00:01:00 - 01:01:00:00 Video Track: Color clip (black)

This has the effect of creating a movie that's about 1 hour long. One minute into the video, the screen turns white for 1 second and at the same time a tone plays. The same white screen/tone combination plays one hour into the movie.

Here's the file I used for a tone: http://www.mikeadkins.net/kdenlive/Tone.wav [^]

(3) Render the project using MPEG-2.

(4) Play the rendered file in an external player. The tone and white screen are well synchronized at the 1 minute mark, but the tone precedes the white screen by almost two seconds at the 1 hour mark.

Alternatively, you could import the rendered file into a new kdenlive project. Exploring the file with the project monitor show that the white screens appear on exactly the correct frames, but the audio thumbnail clearly shows the audio is too early at the 1 hour mark.

My math tells me that the audio is being rendered at a speed about 0.05% too fast for some reason.

If I perform the same experiment using the HD 1080p 24Hz profile the sync is perfect throughout the file.

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Build/Install Method

Manual build from SVN

Attached Files

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related to

0001033

closed

Timecodes indicate 23 fps in HD 1080p 23.98Hz project

Notes
(0003733) madkins (reporter) 2009-07-26 18:17	I now think this is an MLT bug. The function mlt_sample_calculator seems to be trying to compute the number of audio samples in a frame. With this strange frame rate, there is a combination of finite precision floating point arithmetic and integer truncation that causes this calculation to be off by one sample per frame. This error accumulates when rendering a long duration project and causes a noticeable A/V sync issue. I've posted more details and a proposed patch that seems to work for me on the mlt-devel mailing list. We'll see what happens!

(0003777) ddennedy (developer) 2009-08-08 05:08	Your more comprehensive patch was applied to MLT git commit 33098e.

Issue History
Date Modified	Username	Field	Change
2009-07-10 05:41	madkins	New Issue
2009-07-10 05:41	madkins	Build/Install Method	=> Manual build from SVN
2009-07-19 23:38	j-b-m	Relationship added	related to 0001033
2009-07-26 18:17	madkins	Note Added: 0003733
2009-08-08 05:08	ddennedy	Note Added: 0003777
2009-08-08 05:08	ddennedy	Status	new => resolved
2009-08-08 05:08	ddennedy	Fixed in Version	=> Recent git
2009-08-08 05:08	ddennedy	Resolution	open => fixed
2009-08-08 05:08	ddennedy	Assigned To	=> ddennedy
2009-09-10 01:46	xzhayon	Target Version	=> future version
2009-10-08 20:20	j-b-m	Fixed in Version	Recent git => 0.7.6
2010-02-02 10:35	j-b-m	Fixed in Version	0.7.6 => 0.7.7
2010-02-02 10:37	j-b-m	Fixed in Version	0.7.7 => 0.7.6
2010-02-02 10:38	j-b-m	Status	resolved => closed
2010-09-22 17:53	Granjow	Target Version	future version => 0.7.6