Does video latency change due to Decklink scaling?

[Postman]

I had a conversation today with someone who is experiencing big changes in his pro tools video sync when he uses Decklink's scaling options. For instance, he can set PT's offset to align audio and video to be in sync when running a 1280x720p movie to his 720p projector, but then will be out of sync by several frames if he runs a 1920x1080 movie (same codec, just more pixels) and chooses to downscale in realtime to 1280x720. The projector should be seeing exactly the same signal either way, therefore the difference has to be due to downscaling. Or so his theory goes.

Have any of you noticed this? I do not have any type of Blackmagic hardware so I cannot easily help him.

[tom_lowe]

I don't have Blackmagic stuff either, but have discussed using their stuff with PT a fair bit with their guys at expos etc. and they have never mentioned any such delay.

Seems odd that scaling would introduce delay, as the whole purpose of this function of the cards is so you can downscale/upscale when playing out to tape and for monitoring, a delay being added would obviously throw a massive spanner in the works.

On the other hand, the amount of work being done realtime would suggest that a short delay isn't totally unfeasible - look at how much a Dolby E decode has to be compensated by - and that's only audio!

Maybe you should suggest he contacts BM directly and post the answer up here?!

[Postman]

I have the same opinion but I hope someone who owns a Backmagic card with realtime scaling can check on this. Please?

[hummerZ]

Which model Decklink does he have? My Decklink HD Extreme doesn't have the option to down-rez to 720 from 1080. There are options to down-convert HD to SD/anamorphic, or to cross-convert 720 to 1080i, but that's it. This leads me to believe that he is actually feeding his projector 1080, and the projector is introducing a processing delay by adjusting to 720. Or maybe he has another processor in the chain? Like an HD Link or some other SDI -> HDMI converter?

[tom_lowe]

hummerZ, it should be able to cross convert from 720P to 1080i/P and vice-versa. Cross converting 1080 to 720 is really the same as down-scaling.

This is taken from the product page for the DeckLink HD Extreme 3D:
Built-in, high quality hardware cross conversion from 720HD to 1080HD and 1080HD to 720HD on playback.

[hummerZ]

Yes, I understand the terminology quite well... Still, there is no option for it:

http://i1085.photobucket.com/albums/...at110327AM.png

And this is while a 1080p ProRes is loaded and playing in Pro Tools.

[Postman]

A little more info now, he has Decklink Extreme which gives him two SDI outputs, one scaled and one not scaled. He has an SDI to DVI converter between the card and his projector. He says the scaling option is in the preferences pane of the decklink.

As long as the SDI-DVI converter is handling the same SDI signal each way, I don't see how it could be the cause.

It seems unlikely that hardware scaling would be causing this, I have a feeling there is something else (user error perhaps), but a quick answer from the forum would help me rule it out.

Thank you HummerZ and Tom for your help.

[Postman]

HummerZ, I just saw your photo link. Can you not load a 1080i movie and cross convert to 720p? I have a feeling that is what the guy is saying he is doing (but don't really know yet).

Why would your preference give you a choice to convert 720p to 1080i while a 1080p movie is loaded? That is kinda weird, no?

[Postman]

Hey, wait a minute! Is it even possible to have a cross-converted HD output and a non-converted HD output at the same time? I don't see that capability when I read BM's website. All I see is that one can be HD and the other SD.

[hummerZ]

The options for downconversion settings in the BM pref pane are always the same for me, regardless of what format I'm playing out of Pro Tools. There are indeed two SDI outputs on the card, one is the native HD/SD monitor output, the other is capable of downconversion, or at least that's my understanding. Maybe my lack of options is because I'm using one output, as opposed to the other.

I have noticed varying degrees of offset when using the Decklink/HDLink combo, depending on the input resolution. I use one of three video formats, all ProRes: 1080p23.98, 1080i59.94, or NTSC 29.97, each having a slightly different video sync offset setting in PT (16QF, 15QF, and 14QF, respectively). Whether or not this is due to the Decklink, HDLink, or my LCD panel, I'm not quite sure (nor do I care). I routinely use a syncheck to verify these offsets, and I can say with absolute certainty that once you get it set correctly for your codec/resolution, the sync is rock solid.

As you can see, I never use 720p, and I never do any realtime down/cross-conversion, so maybe I have yet to run into a multi-frame offset difference?

Can your friend just measure the offsets and switch them as necessary?