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Old July 19th, 2004, 04:39 AM   #1
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Bitrate drops when copying MPEG edits?

I'm using KDDI's MPES PRO which was bundled with my JVC GR-PD1 camera.

I've tried other NLEs and even though they offer more in terms of options and flexibility - none are as stable.

I'm quite happy with MPES PRO as it suits my needs.

I have just one question!

I have learnt that it's better to keep timeline short, render, then dump rendered portion into new timeline - it's a way to avoid crashing and doesn't bother me as it's the way I work naturally anyway.

My question is: does this rendering - and potential re-rendering depending on how a given editing project develops - mean I am tampering with the picture quality?

I mean, hypothetically, if I were to render and re-render a scene say 6 times, in the course of editing the whole project, as I am moving stuff around and whatnot... is there a risk to the MPEG 2 quality?

As a test, I rendered a project and placed it back in the timeline, changed it slightly and re-rendered - I did this 6 times. After I created my 6th render I examined the quality.

I didn't notice any degradation on my 21" Ultrascan - I'm also using a 128 MB Radeon card and generally get nice picture...

However when I checked the 'property' of the file that had been rendered 6 times it's bitrate was different than the bitrate of normal, uncut shots in bin.

UNCUT, NORMAL SHOTS IN BIN:
* Stream Type : MPEG-2 TS
Bitrate : 19750973 bps


PORTION RENDERED 6 TIMES
* Stream Type : MPEG-2 TS
Bitrate : 19081265 bps


I'm not technical and for all I know this bitrate difference may be irrelevant to pic quality.

I know this might sound a bit finicky - but I am simply trying to establish whether rendering and rendering and re-rendering in the context of this NATIVE MPEG2 ONLINE NLE is a quality consideration or not a quality consideration at all...

Any help would be appreciated.

I hope it's okay that I am also posting this in HDV editing solution section - as I guess the users of the HD1 and HD10 may be most familiar with MPES PRO. Just thought it should go here too?
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Old July 20th, 2004, 10:44 AM   #2
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Graham,

It doesn't surprise me that file sizes are a little different when you re-render. With the six generations of rendering, it sounds like you noticed very little degrading of picture - which is good. Generally, it's best to avoid re-rendering, but obviously has to be done to add titles or effects. Try to minimize.

Due to error correction, an m2t file might play perfectly and still contain errors. These errors typically occur on tape, but not on disk. I've thought about adding a long, empty title to force rendering to remove errors from the original tape. These errors have caused some glitches around edits, which I have corrected by rendering small segments.
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Old July 20th, 2004, 11:13 AM   #3
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Thanks Dave. I plan to render and re-render as little as possible. I was doing it six times only to test whether quality was/wasn't an issue to consider when rendering. I am still unclear whether it is...

If I shoot something, capture it to disk and then export it back to tape without any changes, I am confident of virtually 100% quality being maintained.

If I do a lot of processing at the middle stage (in my NLE) I know it can take away from the quality - but that just straight cuts don't take away as much.

But what of rendering? What IS rendering? Am I copying in a manner that tampers with my digital info - or not?

Much appreciated
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Old July 20th, 2004, 12:44 PM   #4
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Again (just in case anybody is wondering why I must render and re-render) it's a way to make MPES PRO stop crashing and hey, if there's no impact on pic quality, it's not a problem as I work in pieces naturally anyway.

Would LOVE to hear opinions on this. Though I have come to the conclusion that the bitrate difference is irrelevant I would love to know what people think of the MPEG-TS condition through re-renders...

Hypothetically I guess the quality shouldn't be suffering, right?

Much appreciated
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Old July 20th, 2004, 02:43 PM   #5
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Graham,

With standard DV, since each frame is an entity, the data for that frame is added to the timeline intact. When the frame is altered, say for transition, or title key, or color correction, then the frame must be decompressed, modified, then recompressed.

Since HD1-10 has 6 frame group-of-pictures (GOP) MPG, then nearby frames must also be modified around an edit. While the normal sequence requires an Independent frame (I-frame) followed by five more frames built upon that frame, a shorter GOP is OK. As you can see, there will most likely be some short GOPs around edit points and effects. You can see the sequence by clicking (double-click?) on the blank frames below the preview window. White dot in the corner is I-frame.

Say you take a standard dv file and compress it to mpg, then an I-frame will be created at each cut in the program - even if an I-frame had just occured.

When you said you made minor changes in the clips, what exactly did you do?
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Old July 20th, 2004, 03:04 PM   #6
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Just straight cutting - no dissolves, effects etc.

I see why there is still a little processing in a few frames around such 'straight cutting' when editing Mpeg2 due to GOP.

It's the effect of rendering I'm curious about - curious that I am reliant on rendering due to MPES PRO's inability to hold much in timeline and the neccessity to 'render and dump' in timeline every, say, 5 mins.

Does that rendering (and potential re-rendering) mean quality loss? That's all.

Thanks.
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Old July 20th, 2004, 03:05 PM   #7
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Typo:

'It's the effect of rendering I'm curious about - curious AS I am reliant on rendering due to MPES PRO's inability to hold much in timeline and the neccessity to 'render and dump' in timeline every, say, 5 mins.'

Thanks.
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Old July 20th, 2004, 03:06 PM   #8
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That's every 5 screen minutes I mean, by the way!
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Old July 20th, 2004, 03:36 PM   #9
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Gaham,

Meant to say something about this earlier. What I have done is make one part of pgm and set it aside. Then make another, and so on. As a final edit, I put all the pieces together. My experience is that a large number of clips cause problems - not length.

Good luck!
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Old July 20th, 2004, 04:04 PM   #10
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I'll keep respective projects, so that if I need to make changes I can revert to project stage - thus meaning I will only re-render the once.

Thanks again - and let me know if you have any more ideas about mpes pro!

Graham
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Old July 26th, 2004, 04:46 AM   #11
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more on this... I just started doing converts, and am now watching results in Windows Media Player with full screen. I re-tested it, and couldn't see any image difference between an output straight from bin to render and one that was re-rendered eight times... guess I got the answer I wanted!
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