[FFmpeg-user] Meaning of ffprobe output

[Moritz Barsnick]

Moin Ulf,

On Tue, Jan 29, 2019 at 20:50:25 +0100, Ulf Zibis wrote:

> My understanding is, that a VHS cassette player always provides a fully
> interlaced analogue stream (50 half-frames per sec. for PAL).

Originally coming from the analog world, I can confirm that this is
true.

> With this a DVD recorder has two options, (1) a interlaced vob stream
> or (2) a deinterlaced progressive vob stream. Why should it create a
> mixed stream?

I agree.

> My explanation is, that the original VHS was marked with copy
> protection, so the DVD recorder by legal reasons has to sustain the
> copy protection by creating an intentionally corrupted DVD file
> system (which is not readable from a computer) and additionally a
> corrupted vob stream, which is detected and forbidden to copy by
> legal DVD copy software.

This seems far fetched ("weit hergeholt"), at least in terms of
creating mixed progressive/interlaced content. Broken file systems and
streams: Yes, often.

> It seems to me, a 25 fps interlaced stream has 25 frames per sec., each
> with 2 fields, top and bottom (50 fields per sec. in total) and a 25 fps
> progressive stream has 25 frames per sec., each with 1 field.
> So my video with "TFF: 36759 BFF: 19705 Progressive: 58193
> Undetermined:    27" has 36759*2+19705*2+58193+27 = 171148 fields in
> total, partly half, partly full ones. But idet says: "Repeated Fields:
> Neither:114683 Top:     1 Bottom:     0"
> 
> Can you please help me to understand this?

(I hate the concept of interlaced digital video, BTW.)

AFAIU, idet does an *interpretation* of the input, somewhat like the
visual inspection. In certain cases, it will see no obvious signs of
interlacing, and count the frames as progressive. So, in summary, you
have a statistical analysis. Unless your DVD recorder is really doing
funky magic, your recording should be interlaced.

> >> Also I do not understand, that after the transcoding to mp4 the numbers
> >> are different. I interpret this, that the transcoding process does some
> >> deinterlacing, but you say, the encoder does not. The vob with 114684
> >> frames at 25 fps results in 1:16:27.36 length, but the resulting mp4 with
> >> 114502 frames is 1:16:20.08.

BTW, this difference is marginal, and probably due to the lossy nature
of re-encoding biasing idet's results.

> I need concentration on the subject and additionally time to
> translate my thoughts from german to english.

For a few of us, you could reiterate in German. That will not help the
community at all, though.

> What is vfr and cfr?

Variable vs. constant frame rate. (Smart phone cameras *tend* to record
VFR, as do some webcams which adapt frame rates to lighting conditions.
Other examples exist.)

Cheers and Gruß,
Moritz