DV capture is 29.96 fps?

[sdm]

I've captured some DV from VHS using a Cannopus advc 100 and Toast 7.
I'm going to export it to AVI (divx/mp3) for use on my Eyehome.

Toast reports the framerate of the DV footage is 29.96 (and would like to encode at that rate too)

FFmpegX and Mpeg Streamclip say the footage is 29.97 fps.

Why would there be a discrepency, and how can I find out the truth?

Any thoughts?

Thanks,
-- sdm

[edDV]

They are both the same thing, just a rounding error.

Interlace NTSC is 29.976 fps.

I don't know why we don't see people saying 29.98 which is more correct than 29.97.

[sdm]

Thanks edDV.

I realise its supposed to be 29.976fps.

Is it possible that the ADVC or the VCR is putting out a very slighly slower framerate? The VCR is pretty low-tech.

Thanks, sdm.

[edDV]

could be, but within spec for capture

[havema-1]

I've always thought that color NTSC is exactly 60000/1001 fields per second or 30000/1001 frames per second = 29.97003 fps.

But obviously 29.976 is the correct value, right? Where does that value come from??

What about 24 fps -- is it really 24000/1001 = 23.97602 fps?? Does this relate to the 29.976 decimals??

(I'm from a PAL country so this is almost purely academic interest).

[Case]

But obviously 29.976 is the correct value, right?

I don't think this value is obvious at all, and the other values are right according to what I could find googling around.

[edDV]

Sorry for my brain fart. 29.97003 is correct. Actual derivation follows.

[edDV]

Original North American monochrome television was 60 fields/sec (30 frames per second) and horizontal scan frequency was 15.750 KHz.

Addition of the NTSC color subcarrier required adjusting horizontal scan frequency down so that the harmonics of the color subcarrier fell in between harmonics of the scan rate (containing the luminance information). The magic subcarrier frequency that made everything line up in frequency space was 3.579545 MHz.

Horizontal scan rate derived from this subcarrier frequency became
(3,579,545 Hz)(2/455) = 15.73426 KHz

Vertical rate became
(3,579,545 Hz)(2/455)(2/525) = 59.9400524 Hz (fields per second)
or 29.9700262 frames per second,

Where
3,579,545 Hz = 3.58 MHz. NTSC color subcarrier frequency
525 = total scan lines/frame (including vertical interval)

2 = allows subcarrier harmonics to fall in-between horizontal scan rate harmonics. This permits color to be separated using a comb filter.
http://members.aol.com/ajaynejr/vidcomb.htm

and 455 was chozen to place the subcarrier frequency and resulting harmonics above 3MHz (adequate luminance resolution for non-comb filter TV sets) and below 4.2 MHz (highest broadcast luminance resolution). If subcarrier harmonics exceeded 4.2 MHz, they would interfere with broadcast TV sound.

In Europe, PAL subcarrier frequency was placed at 4.43MHz. The higher frequency was possible because broadcast bandwidths were 6 MHz* wide rather than 5 MHz* in North America. This gave more choices for subcarrier placement and the choice of 4.43 MHz allowed vertical rate to remain at 50 fields per second.

The wider bandwidth allowed PAL broadcasts to display more horizontal detail. The 625/50 line structure allowed more vertical lines to be displayed as well. The tradeoff was 50Hz (50 fields per second) refresh rate that was 20% slower in motion resolution than NTSC's 59.94 Hz and resulted in mild flicker.


* Each channel included a 1MHz guard band for channel separation so total channel bandwith for NTSC was 6 MHz and 7 MHz for PAL.