Scenario: Capturing 29.97 fps content at 30 fps - Possible issues?

[ratereducer]

Hi.

I have the following theoretical situation. There will surely be variables I haven't considered. Please help my understanding.

Say we've got source content that is at 29.97fps. It's playing through a media player on a laptop, which is then plugged into a capture card over HDMI. This is captured using said HDMI capture card which only has an option of "30fps" not 29.97.

I don't really understand what would happen here. I'm guessing the capture will end up with dropped frames due to the rate mismatch? Would that be the case?

Is the capture card actually going to be seeing the content at 29.97 anyway? Wouldn't the laptop's output be presented to the capture card like a normal monitor at 60hz or similar?

If there is an issue that would occur, is it one that can be fixed up later?

[newpball]

Is the capture card actually going to be seeing the content at 29.97 anyway?

Yes!

Question is why on Earth would you want to capture the source, just copy it!

[ratereducer]

Is the capture card actually going to be seeing the content at 29.97 anyway?

Yes!

Question is why on Earth would you want to capture the source, just copy it!

That would be the logical solution, however for this scenario assume that it has to be captured.

[Cornucopia]

Many devices will say "30" for convenience, but what they're really capturing is "29.97" (or more accurately, 30 * 1000/1001). Especially if they're intended for sources/destinations that are meant for TV/CE* consumption vs. PC consumption. It's a rare** item which does actually capt at straight 30.

Scott

*CE=Consumer Electronics
**= ~<10%

[jagabo]

When the capture device tells you it's capturing 30 fps it's probably just shorthand for 29.97 (30000/1001) fps. If it's really capturing 30 fps you will get one duplicate frame every 1001 frames (every ~33 seconds) from a 29.97 fps source. If that duplicate frame appears in the middle of a panning shot you'll see a little jerk on playback. If it appears in a relatively still shot you won't notice it.

[newpball]

Is the capture card actually going to be seeing the content at 29.97 anyway?

Yes!

Question is why on Earth would you want to capture the source, just copy it!

That would be the logical solution, however for this scenario assume that it has to be captured.

So what is the scenario? Why talk hypotheticals?

[ratereducer]

Many devices will say "30" for convenience, but what they're really capturing is "29.97" (or more accurately, 30 * 1000/1001). Especially if they're intended for sources/destinations that are meant for TV/CE* consumption vs. PC consumption. It's a rare** item which does actually capt at straight 30.
[/I]

The cards in question are :

Avermedia Game Broadcaster HD C127

I believe this card supports Directshow, so couldn't you just tell it to capture 29.97 and it will do it? or are the capture speeds also dictated by the hardware itself as well?

(judging from this thread here, Avermedia have said it captures at 30, not 29.97 )

The other card is the Avermedia Live Gamer Portable Lite


No directshow support. Captures at what it claims is "30". I've downloaded some recordings people have made with it and my player does seem to show the rate as 30 (maybe this is only a header label though?)

[ratereducer]

When the capture device tells you it's capturing 30 fps it's probably just shorthand for 29.97 (30000/1001) fps. If it's really capturing 30 fps you will get one duplicate frame every 1001 frames (every ~33 seconds) from a 29.97 fps source. If that duplicate frame appears in the middle of a panning shot you'll see a little jerk on playback. If it appears in a relatively still shot you won't notice it.

Is this something Virtualdub can fix with a built in feature/rate adjustment? or would you have to remove every frame in increments of 1001 to remove the duplicates, via a script or something?

Wouldn't this also send the audio off sync?

[newpball]

99% certain this is a total non-issue.

However what makes me less certain is if the source is actually 29.97fps, why would anyone capture such a source if you can play it on the computer?

Does not make any sense.

[jagabo]

When the capture device tells you it's capturing 30 fps it's probably just shorthand for 29.97 (30000/1001) fps. If it's really capturing 30 fps you will get one duplicate frame every 1001 frames (every ~33 seconds) from a 29.97 fps source. If that duplicate frame appears in the middle of a panning shot you'll see a little jerk on playback. If it appears in a relatively still shot you won't notice it.

Is this something Virtualdub can fix with a built in feature/rate adjustment? or would you have to remove every frame in increments of 1001 to remove the duplicates, via a script or something?

It would be very hard to fix this accurately. VirtualDub would simply remove a random frame out of every 1001. To remove only the duplicate frame you would have to figure out which frame is the duplicate. Then remove every 1001'th frame from there. If there are other duplicate or dropped frames you will bend up discarding the wrong frames later in the video.

Wouldn't this also send the audio off sync?

Not if you play the video back at 30 fps. Or discard every 1001th frame and play at 29.97 fps. Think of it like this: I have 52 playing cards. I'm going to flip one card over every second. How long will it take to flip all 52 cards? 52 seconds. Now I discard half the cards, leaving 26. I flip a card over every 2 seconds. How long will it take to flip the 26 cards? 52 seconds. So the duration of the video is unchanged in this scenario because the amount of time each frame is displayed makes up for the difference in the number of frames.

[ratereducer]

99% certain this is a total non-issue.

However what makes me less certain is if the source is actually 29.97fps, why would anyone capture such a source if you can play it on the computer?

Does not make any sense.

The content is protected, so I'm unable to play it on my portable devices. The only way is to capture and then encode from there.

[jagabo]

So the duration of the video is unchanged in this scenario because the amount of time each frame is displayed makes up for the difference in the number of frames.

Unless the capture device is really capturing 29.97 fps but flagging the video file as 30 fps. In that case you will suffer from A/V sync problems.

[ratereducer]

Okay. I've tried the Blackmagic Decklink Mini Recorder, and also the Avermedia Game Broadcaster HD C127.

The Blackmagic card has no problem running in 29.97, and the Avermedia is able to also when using Uncompressed capture in Avermedia's own software, or when using Directshow/WDM.

On my laptop the option of "29hz" can be selected. On my PC's GPU however, this isn't an option and you can only choose 30hz and capture at what displays as "30".

[Brad]

Okay. I've tried the Blackmagic Decklink Mini Recorder, and also the Avermedia Game Broadcaster HD C127.

The Blackmagic card has no problem running in 29.97, and the Avermedia is able to also when using Uncompressed capture in Avermedia's own software, or when using Directshow/WDM.

The AVerMedia devices improperly capture 1080p59.94 at 30.00/60.00. Of their 1080p devices, I've only used the C127, which as I recall does operate correctly when presented with any 29.97 source (like your laptop) as well as 720p59.94.

http://www.thethrillness.com/2015/07/avermedia-live-gamer-extreme-gc550.html

[pandy]

Say we've got source content that is at 29.97fps. It's playing through a media player on a laptop, which is then plugged into a capture card over HDMI. This is captured using said HDMI capture card which only has an option of "30fps" not 29.97.

I don't really understand what would happen here. I'm guessing the capture will end up with dropped frames due to the rate mismatch? Would that be the case?

Is the capture card actually going to be seeing the content at 29.97 anyway? Wouldn't the laptop's output be presented to the capture card like a normal monitor at 60hz or similar?

If there is an issue that would occur, is it one that can be fixed up later?

Each 1001 frames you should have one of frame duplicated - should be easy to fix - but everything depend from capture device and how capture is performed.

And Scot is right - frequently 30 is used as synonym for 29.97 - even in HDMI you don't have separated video descriptors for 30 and 29.97 (60&59.94 similar) - in fact in color NTSC world there is no 30 fps at all (chroma subcarrier describe in very precise way relation between H and V frequencies).