CODEWITHSUNDEEP
ffmpeg
Chienchia Chang
Chienchia Chang

Reputation: 31

the constant time_base in output AVStream

When we want to mux audio and video into OutputContext, we must convert the time_base of AVPacekt. What I confused is why the time_base of output AVStream is (1, 90000)? For some historical reasons or another?

Upvotes: 1

Views: 390

Answers (1)

aergistal
aergistal

Reputation: 31209

Streaming video codecs typically use a 90 Khz (1 / 90000) clock rate, or a higher frequency that is backwards compatible with this one, like the 27 MHz = 300 * 90 Khz clock in MPEG-2.

For MPEG-1 the value was chosen in accordance with a digital video standard recommendation from the 80's called the ITU-R (ex CCIR) 601 which was suitable for use with the 625/50 Hz and 525/60 Hz modes used in PAL/NTSC analogue broadcasts.

Using the sampling frequency for luminance (Y) of 13.5 Mhz from the aforementioned guideline they decided to use a 13.5 Mhz / 150 = 90 Khz value which provided a good enough accuracy for decoding/presentation timestamps.

Source: Fundamentals and Evolution of MPEG-2 Systems: Paving the MPEG Road, Jan Van der Meer

Upvotes: 2

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