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Re: MTC and Time Code Types

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Re: MTC and Time Code Types

Subject: Re: MTC and Time Code Types
From: Herbie Robinson <email@hidden>
Date: Thu, 27 Jan 2005 05:03:31 -0500

At 9:10 PM -0600 1/26/05, Cianflone, Chris wrote:

I have read through most of the MIDI Time Code chapter from "The Complete MIDI 1.0 Detailed Specification" (and used MTC devices in the past as a user, perhaps not enough) and I thought I understood the various formats outlined there. In particular for a quarter frame message the spec refers to "Time Code Type" 0 = 24 Frames/Second 1 = 25 Frames/Second 2 = 30 Frames/Second (Drop Frame) 3 = 30 Frames/Second (Non-Drop Frame)

However, from CoreAudioTypes.h we see the following: // constants describing SMPTE types (taken from the MTC spec) enum { kSMPTETimeType24 = 0, kSMPTETimeType25 = 1, kSMPTETimeType30Drop = 2, kSMPTETimeType30 = 3, kSMPTETimeType2997 = 4, kSMPTETimeType2997Drop = 5 };

So, what is kSMPTETimeType2997 and kSMPTETimeType2997Drop? I have been searching the web all day and not really finding a sufficient answer to this (although a few things I found made reference to these extra formats). Is there a more up-to-date MTC spec I should be reading?

30 Frames per second color video is really recorded at 29.97 frames per second (for analog video quality reasons that are too complicated to explain here). Black and white video used to be at 30 frames per second, but probably isn't any more.

Time code for 30 frames per second can either be straight or it can be what is called drop frame. Straight, non-drop, 30 frames per second time code counts through 9000 frames every 5 minutes -- the frame count goes from 0 to 29 every second. Drop frame time code counts through 8991 frames every 5 minutes -- the frame count goes from 0 to 29 most of the time, but 9 times every 5 minutes, the frame count goes from 0 to 28. On other words, frames are occasionally dropped from the count. Note that the frame rate is constant, it's the counting that is a little jerky.

To make matters a little more interesting, early devices didn't really care what kind of counting one did with a particular frame rate. it was up the user to use the right counting method. This was also before everybody had computers to do math for them and doing time code math by hand for drop frame time code is really ugly. For this reason, a lot of production at the 29.97 frame rate was (and still is) done with straight non-drop time code.

In the early days of MIDI, the way time code was counted mattered, but it really didn't matter whether the actual frame rate was 29.97 or 30. When digital audio came along, all of a sudden one has to know the actual frame rate to lock the digital audio to video. For example, when the sampling rate is 48K, there will be 240,000 samples every 9000 frames at 30 frames per second and 240,000 samples every 8991 frames at 29.97 frames per second. -- -***************************************** ** http://www.curbside-recording.com/ ** ****************************************** _______________________________________________ Do not post admin requests to the list. They will be ignored. Coreaudio-api mailing list (email@hidden) Help/Unsubscribe/Update your Subscription: This email sent to email@hidden Prev by Date: MTC and Time Code Types Next by Date: Re: MTC and Time Code Types Previous by thread: Re: MTC and Time Code Types Next by thread: Mailing list archive Index(es): Date Thread