29.97 and 23.98 SMPTE timecode -- The definitive explanation

[charles maynes]

Originally Posted by huub

Please explain?

Thanks,
huub

it is not uncommon for audio to be shot at a pulled down digital rate of 47.752..... especially for commercials....

[huub]

Okay..
I was confused this weekend, because a job was changed last moment to 24p (well 24isf to be exact)
And I was not completely sure if I should record the multitracks at 48khz..
I suppose different samplerates are only used if a producer/audio post person specifically asks for it?

thank again,
huub

[minister]

Originally Posted by charles maynes

it is not uncommon for audio to be shot at a pulled down digital rate of 47.752..... especially for commercials....

48048 is more common as it is pulled down when played at 48000.

[charles maynes]

I would defer to Tom's comments, but I have seen pulled down rates used as well- usually if film is going to intercut with video. If this is an HD project I would expect the SR to be 48 or 96 though.

[jayfrigo]

Originally Posted by starcrash13

Going from 23.976 to NTSC requires a 3:2 pulldown which is not a speed change, but rather an addition of repeated video fields to go from 24 frames to 60 interlaced video fields.

Sorry to be so nit-picky but, after all, this is "THE definitive explanation of 29.97 and 23.98" frame rates, right?

While you're being picky, it may be worth noting that 3:2 is no longer used; currently a 2:3 pulldown is used, i.e. AA BBB CC DDD, not AAA BB CCC DD.

[starcrash13]

Originally Posted by jayfrigo

While you're being picky, it may be worth noting that 3:2 is no longer used; currently a 2:3 pulldown is used, i.e. AA BBB CC DDD, not AAA BB CCC DD.

Right. Most people still call it 3:2, though. It's one of those "cool" industry terms where you say one thing, but maybe mean another like 24p, pullup, or conform.

[Bob Olhsson]

The problem is getting sound and picture editing gear to work together.

I once had a fascinating conversation with an old Ampex engineer. It seems the whole thing was never really necessary. What happened was that NTSC color was rolled out running on a 60 Hz. clock exactly like B&W. At the last minute some people at Ampex who were researching video recording technology noticed bars running through the B&W picture of a color transmission. The FCC went into a panic and quickly figured out that pulling the crystals down .1% got rid of the problem. A few weeks later it was discovered only to be a problem with crystal-controlled lab monitors and not ordinary TV sets.

But it was too late...

[jayfrigo]

Originally Posted by starcrash13

Right. Most people still call it 3:2, though. It's one of those "cool" industry terms where you say one thing, but maybe mean another like 24p, pullup, or conform.

Lots of things like that. Plenty of people still call anything film to video telecine, regardless of whether its film scanning or actual telecine. Heck, I still sometimes say "to tape" (as in "print FX to tape") for anything that records, and I'm certainly not alone. But in a "definitive" thread, I try to be accurate... well, I try...

[hociman]

Originally Posted by Bob Olhsson

I once had a fascinating conversation with an old Ampex engineer. It seems the whole thing was never really necessary. What happened was that NTSC color was rolled out running on a 60 Hz. clock exactly like B&W. At the last minute some people at Ampex who were researching video recording technology noticed bars running through the B&W picture of a color transmission. The FCC went into a panic and quickly figured out that pulling the crystals down .1% got rid of the problem. A few weeks later it was discovered only to be a problem with crystal-controlled lab monitors and not ordinary TV sets.

But it was too late...

I am not going to say that the Ampex engineer lied to you, but I read a conflicting account of this. If you go to my website and read the Timecode PDF, you will find this.

The argument when color was developed was that the frequency of
the color subcarrier would create beating with the sound subcarrier that
would be visible on some black and white television sets. The sound
carrier, however, is frequency modulated. Therefore, beating would have
only occurred at a specific frequency. A GE engineer determined that if
the frame rate was dropped by .1% (from 30 to 29.97), that the beating
would be reduced, and compatibility would be maintained (Lehrman 220).
As a result of this change, 60Hz AC cannot leak into a video signal, or “bars
appear to roll through the picture every 17 seconds” (Schubin 29).

Again, I'm not saying the Ampex engineer doesn't know what he's talking about, but that's how I understood the issue. If this is going to be a sticky, perhaps we should work on figuring out if the truth is somewhere in-between or more towards one or the other.

[Bob Olhsson]

Perhaps the "some B&W sets" were in the Ampex Research lab and the guy at GE is who igured out how to fix the problem.

By far the very best explanation of pull-down I've ever read was posted by John Klett during in the late '90s. Maybe somebody else remembers where.

[ddavis1086]

Having a project done in 23.98 and wanted it to be delivered in PAL and NTSC.
How do you go about keeping the Total Running Time and Tempo as it relates
to the the audio correct. Are the perfect pitch folks just going to have to grit their teeth?

Is it possible to change the TRT of the show and still have the
tempo of the music the same for PAL and NTSC without hearing the difference?

Is the Total Running Time in 23.98 the same as it would be at 29.97?

Thank you for any info can can provide.

[jacobfarron]

Is the Total Running Time in 23.98 the same as it would be at 29.97?

Yes, there is no speed change.

[ddavis1086]

Thank you for the information.

How about 25 fps to 29.97? Is there a way to keep the running time the same?
Will the audio tempo change also?

thanks

[jacobfarron]

25 fps to 29.97

That one I'll have to let someone familiar with PAL answer.

[musikwerks]

If you are shooting/posting at 23.98 and wanting to go to PAL (25fps) you will need to speed up your audio by 4.1%. You'll also need to pitch shift it to compensate for the speed change. If you're in a 23.98 project in AVID and want to go to 25, AVID will do the video speed up for you but you'll need to do a separate audio re-stripe.

When mastering some feature films a while back (Rabbit Proof Fence, The Quiet American) we mastered to HD-D5 in 23.98 and struck PAL and NTSC digi beta masters from there. The NTSC version needed no adjustment to the audio utilising the D5's internal down converter. The PAL version was re-striped with the compensated soundtrack.

[Sound Sorcerer]

I'd recommend some literature from Mr Wolf, an authority in this subject:
Wolf Seeberg Literature

[Inglesworth]

A couple of issues to clear up in here. I know that timecode is an EXTREMELY confusing subject.
First, on the difference between DF and NDF, it is important to note that of the 108 frames that are "lost", we don't actually lose any frames of video, we are only losing numbers in the timecode window. There are still the same number of video frame changes in one hour of real time either way.

The reason why 23.98 and 29.97 seem to match up every second is that one second is equal to one second. Plain and simple, we can't change the amount of time that one second takes up.

The way I see it, it is similar to sample rates. Humans can hear up to about 20k. We would need a sample rate of 40k to achieve this (nyquist theory is a completely different subject...). Because of this, is there really a reason for a 192k sample rate?
-note, I understand the need for higher sample rates, I am trying to make a theoretical point here

The human eye stops seeing individual pictures and fluid motion at around 20 picture changes a second. Therefore, they originally set up film for 24 fps. As stated earlier, this causes issues due to electrical currents and such. Do we really NEED to have video at 59.94?

Probably not, but we also have 192k audio.

Because it is available, someone will use it. We just need to hope that whoever changes the speed of the video, also changes the corresponding audio at the same time, then we don't need to know anything other than the framerate written on their spec sheet.

Of course, asking for somebody else to know exactly what they are doing is becoming more and more unrealistic these days...

[Audiop]

Originally Posted by Inglesworth

The human eye stops seeing individual pictures and fluid motion at around 20 picture changes a second. Therefore, they originally set up film for 24 fps. As stated earlier, this causes issues due to electrical currents and such. Do we really NEED to have video at 59.94?

I think we stop see individual pictures earlier than 20fps(?) but OTOH for some material up to 80fps is needed for natural motion of fast pans or movements according to tests I've read about.


/Peter