[FFmpeg-devel] [PATCH] Fix loss of precision for silencedetect on large files

[Michael Niedermayer]

On Tue, Mar 26, 2019 at 10:07:10PM +0000, Allan Cady via ffmpeg-devel wrote:
> When the silencedetect filter is run against very large files, the
> output timestamps gradually lose precision as the scan proceeds
> further into the file. This is because the output is formatted (in
> libavutil/timestamp.h) as "%.6g", which limits the total field length.
> Eventually, for offsets greater than 100000 seconds (about 28 hours),
> fractions of a second disappear altogether, and the timestamps
> are logged as whole integers. This is insufficient precision for
> my purposes.
> 
> I propose changing the format to "%.3f", which will give millisecond
> precision for all timestamps regardless of offset.
> 
> Patch file is attached.
> 
> EXAMPLE
> 
> For a sample scan on a file that's about 35 hours (126000 seconds) length:
> 
> $ ffmpeg -i input.mp3 -filter_complex silencedetect=n=-30dB:d=3,ametadata=mode=print:file=silence-out.txt -f null -
> 
> The output looks like this near the beginning:
> 
> frame:83085 pts:47856431 pts_time:2170.36
> lavfi.silence_start=2166.86
> frame:83139 pts:47887535 pts_time:2171.77
> lavfi.silence_end=2171.77
> lavfi.silence_duration=4.91061
> 
> Further on we get this:
> 
> frame:2450348 pts:1411399919 pts_time:64009.1
> lavfi.silence_start=64005.6
> frame:2450371 pts:1411413167 pts_time:64009.7
> lavfi.silence_end=64009.7
> lavfi.silence_duration=4.10082
> 
> Eventually after it passes 100000 seconds:
> 
> frame:4738029 pts:2729104175 pts_time:123769
> lavfi.silence_start=123765
> frame:4738055 pts:2729119151 pts_time:123770
> lavfi.silence_end=123770
> lavfi.silence_duration=4.17918
> 
> The start and end times are now in whole integers (seconds).
> 
> After making this patch and building the code myself, I now get this
> output near the end:
> 
> frame:4738029 pts:2729104175 pts_time:123768.897
> lavfi.silence_start=123765.411
> frame:4738055 pts:2729119151 pts_time:123769.576
> lavfi.silence_end=123769.584
> lavfi.silence_duration=4.173
> 
> This gives me the output I want.
> 
> Thank you,
> 
> Allan Cady
> Seattle WA

>  timestamp.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> a997e2f7d02d14a3761cf389b8096e55f3670fb5  0001-Fix-loss-of-precision-for-silencedetect-on-large-fil.patch
> From 59b82d49516926173ab03944a73fd7fc9e5d7bcc Mon Sep 17 00:00:00 2001
> From: Allan Cady <allancady at yahoo.com>
> Date: Tue, 26 Mar 2019 14:11:03 -0700
> Subject: [PATCH] Fix loss of precision for silencedetect on large files

this breaks make fate

also if fate is updated it should be ensured it still checks enough precission
and that it does produce the same results (fate passes) on all relevant
platforms. This change may bring non significant differences into significance

thanks

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The smallest minority on earth is the individual. Those who deny 
individual rights cannot claim to be defenders of minorities. - Ayn Rand
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