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Video Compression / Codec Suggestion

Monday, 08/20/2007 -- by Alan Jackson. Subject: Video Compression / Codec Suggestion

(I posted this by accident on the end of someone else's thread... meant to post this as a separate thread... sorry...)

Hi, everyone. I am putting together a 3-projector display for a client. We are using IC5 Designer & Player. We have a 7 minute video that spans all three projectors (3 720x480 projectors for a combined image of 2160x480).

Can anyone suggestion the best codec we should use for this video? I have compressed it with MPEG-2 and the playback is OK but not great (some stuttering during quick motion shots). I may still need to tweak the settings to make it look better - or perhaps I need to work with the video card more. But I am curious if anyone can suggest a better format or process for me to follow?

I use Final Cut Pro to edit, export as a full movie, then use VisualHub software to export to MPEG-2, highest quality, 2160x480 pixels. The end result file is about 1.5 GB, and like I said plays back OK but not great.

Specs on our Player PC: OptiPlex 745, Core 2 Duo, 2.4Ghz, 2GB 800Mhz DDR2 RAm, ATI Radeon X1300 Pro video card (256 RAM), using a Matrox TripleHead2Go Digital Edition.

Any help from anyone would be great. Thanks in advance!

Alan

Tuesday, 08/21/2007 -- by John Schilling. Subject: This video stream is a bit rich for the task at hand...

2160x480 is not a valid MPEG-2 file. [outside of "profile"-- No testing would ever be done on such a file]

I'm surprised that the CODEC will even play back the file at all. [1920 is the maximum number of acceptable horizontal pixels for MPEG-2, 2048 for HD-WMV9]

O.K.--let's do the math: A 1.5 GB, 7 minute MPEG-2 file: (1.5GB*1024MB/GB)/(7min*60sec/min) == 3.6MB/sec.

So we are talking abut a 36Mbits per second MPEG-2 file.

This is not a "Playback Quality File"--this is an Archival/NLE-working copy file. No general purpose playback device is expected to display an MPEG-2 files at almost 40Mbps!

Typical playable data rates for 1920x1080 content would be on the order of 20Mbps.

Next--the X1300PRO does pretty well for what it is--but really its not appropriate for displays over ~2720x768, (2x1360x768), or so. Since you are going for 2160x480--3x720x480--you should be O.K. as the total pixel count is still well within the basic performance envelope of this low/low-mid-range card.

As for the encoding of your video file--I would try a custom HD-WMV9 at 1080x480 (bi-cubic scaling and correcting for aspect ratio). I would guess that 6-8, maybe 10Mbps, Mbps, 2-pass CBR, should be enough for the video. If there is an audio track--I've encode it at 64Kbps VBR or 128Kbps CBR. [WMA 9.2--keep the sample freqency the same as the source!] Now here is a _critical_ point--what is the refresh rate of the display devices? IF 60Hz, then encode the video at either 30 or 60 fps. IF 72 Hz, then 36 fps; if 75 Hz, the 25 fps. In any case: Keep the video frame rate an integer multiple of the intended display frame rate. [...and in all cases under 60 FPS--Its best if your transcoder has good "frame rate interpolation" filters]


My current preference for medium-duty cycle encoding would be the Canopus ProCoder3.


--JSS

Wednesday, 08/22/2007 -- by Bryan. Subject: Encoding

Interesting..... Why is the FPS relevant to the refresh rate of the monitor? So does that throw 29.97 out the window as Frame rate to encode at for display purposes. Why is 29.97 so prominent.In the ICplayer 5 FAQ encoder doc I thought I had read 60 fps was optimal at 50hz...Maybe misunderstood.
Excuse me... I enjoy encoding optimization and had never heard that take on it. That's my factoid of the day.

Thanks,

Bryan

Wednesday, 08/22/2007 -- by John Schilling. Subject: Its all about TV...

29.97/59.94 Hz interlace is "NTSC"--as in U.S., (and Japan), Standard Analog Television.

Historically, most Scala Players displayed their content via some form of a "television-related" device--TV rulez--then.

Then along came these new-fangled gizmos--LCD's, Plasmas, Projectors--and they did not necessarily limit themselves to NTSC conventions.

So--what happens when the frame rate of a video clip does not match the display rate?

Well--the Computer+Software _has_ to display data at the frame rate of the display device--no option unless you wanted periodic "blank screens". So--to keep "filling the gaps"--video frames have to be _duplicated_. When the same frame gets duplicated too many times in a row--you the observer notice this effect. What should be smooth motion appears uneven. The technical term, in video circles, for this effect is "Judder".

Q: So, Batman--how do they display film, 24 FPS, on TV, 29.97 FPS???

A: Grasshopper--Go read the Wikipedia article on "3:2 Pulldown".


--JSS


Wednesday, 08/22/2007 -- by Alan Jackson. Subject: Going to give it a shot

John - Thanks for the advice. I am in the process of encoding the video in WMV9 format with the specs you mentioned and will let you know the outcome.

Two more questions, while we are rolling with the topic:

1) You mentioned in your post to encode the video at 1080x480... the horizontal resolution being half of what I am needing (2160). Is that where the bi-cubic scaling comes into effect?

2) Regarding audio... does Scala support 5.1 surround audio, and if so does that change anything I need to do on the encoding side?

Many thanks.

Alan

Wednesday, 08/22/2007 -- by John Schilling. Subject: The filter is for maintaining image quality on the resize

By using a high quality resize operation--and then setting the aspect ratio of the video clip to 2:1 you will get a pretty good result.

We in effect "pass through" 5.1 audio on systems with the appropriate HW--the only control is "global volume". For the most part I avoid 5.1 audio--there are very few _practical_ ways to deploy it in most "Digital Signage" venues. [basically: Movie Theaters]


--JSS

Wednesday, 08/22/2007 -- by Alan Jackson. Subject: Audio

So, as long as the audio track embedded in the video file is 5.1 capable and I run it through a sound card with digital output to an existing 5.1 amp/sound system we should be OK?

Thanks for everything... I ran a test of the WMV exported file and it looks much better.

Friday, 08/24/2007 -- by Stephane Bastien. Subject: If I may add my 2 cents.

We produced video for a 3 screen layout at 1920x480 using (3x 640x480) WMV9 at 8Mbps and it's looking great (all filmed in HD).

NTSC at 720x480 as a pixel aspect ratio of 0.9, most LCD and projector have pixel aspect ratio of 1:1 (square pixels).

You should convert your 2160x480 to 1920x480 square pixels (1:1) this way you won't distort logo and people.

Stéphane


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