Quantcast Is there an acceptable rate of dropped frames? - digitalFAQ Forum
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06-19-2010, 03:35 AM
maphew maphew is offline
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Hello,

Thanks for putting together this wonderful site and maintaining it for years. It is a relief to find a glade of calm concentrated information amidst this crazy, wildly variable and distributed 'net.

I've read through the guides pertaining to capturing from vhs with a target of dvd using ATI MMC and am putting it to use. I've created a custom capture set of mpeg2-dvd 352x480 ntsc following the guide for set for movies. I'm getting a low number of dropped frames, e.g. 490 dropped of 90,000 captured. Is that acceptable or is the target zero?

The capture system is Windows 2000, 512mb of ram. Task manager shows MMC using 72mb with 292mb of physical memory free, while cpu usage range is 75-95%. The video card is ATI AIW 7200 and the audio SoundBlaster Live. I don't recall the cpu speed at the moment, but I believe it to be ~1GHz. The hard drive is in ultra dma mode.

Changing the preview window size from Large to Tiny dropped the cpu% a few points but no obvious change in number of dropped frames. The computer's only purpose is video capture; I'm not multi tasking on it.
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  #2  
06-19-2010, 05:38 PM
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Excellent question! I originally wrote the "dropped frames" video capturing list that appears on both this site and on videohelp.com (posted by another member) -- and probably appears on a few others sites that plagiarize content -- about 7-8 years ago. And in all those years, I don't think anybody has ever asked this.

The target is always zero. However, that doesn't necessarily mean it's always realistic.

There are several scenario where dropped frames are acceptable:
  1. When the capture first starts, as there can be a delay between the various encoder hardware and hard drive buffers. It's very common to lose several hundred frames at this spot. It's one reason you need to capture signal BEFORE the content you want to keep.
  2. At "snow" or other noise, especially at the beginning/end of VHS tapes, or at other splices or recording on/off segments in the middle of a tape. Same for 8mm, Hi8, S-VHS -- any of the consumer (or low-end pro) analog tape formats. Analog signals are really chaotic, a mess of video engineering that is almost inconceivable that it actually works.
  3. During highly unstable signals.
  4. At the end of a file, as the buffers again inconsistently lose sync with creating the file from the input, as the user stops the capture. (Note that you almost never see these frames listed by ATI MMC's counter.)

And it's not "acceptable" in as much as there's simply no way to prevent dropped frames from happening in those scenarios. Using the suggested hardware -- a good VCR, and a good external standalone time base corrector (TBC) --will reduce the amount of frames lost, however.

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Last edited by lordsmurf; 06-19-2010 at 06:34 PM.
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  #3  
06-19-2010, 06:47 PM
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Quote:
Originally Posted by maphew View Post
Hello,
Thanks for putting together this wonderful site and maintaining it for years. It is a relief to find a glade of calm concentrated information amidst this crazy, wildly variable and distributed 'net.
Thanks!

Quote:
I've read through the guides pertaining to capturing from vhs with a target of dvd using ATI MMC and am putting it to use. I've created a custom capture set of mpeg2-dvd 352x480 ntsc following the guide for set for movies. I'm getting a low number of dropped frames, e.g. 490 dropped of 90,000 captured. Is that acceptable or is the target zero?
Tweak the "motion compensation" preset to about 70, instead of the 95-99 shown on the ATI All In Wonder MPEG capture guide. This can sometimes reduce the CPU use just a bit.

Tweaking the GOP size can help to shave off a few CPU% points, too. Look at using 3 or 4 P frames, instead of just 2.

While 490 our of 90,000 sounds like a relatively tiny amount of lost frames, it really depends on where those frames are! At least with an ATI All In Wonder Radeon card, you don't have to worry about audio sync loss. When the ATI drops a frame, it either repeats a frame to take its spot, or it drops the audio for that frame length, too. Many cheap/crappy cards will only drop the video, and not the audio, and it cumulatively causes sync error that cannot be fixed.

Quote:
The capture system is Windows 2000, 512mb of ram. Task manager shows MMC using 72mb with 292mb of physical memory free, while cpu usage range is 75-95%. The video card is ATI AIW 7200 and the audio SoundBlaster Live. I don't recall the cpu speed at the moment, but I believe it to be ~1GHz. The hard drive is in ultra dma mode.
For a capture system, 1Ghz is really low. If you're only getting 490 dropped frames in a 90,000 frame capture, you're doing amazingly well. In my own tests, 1.5Ghz is really bare bones minimum, and you really don't get to problem-free capturing until you hit the 2.0Ghz range. Some of this depends on the CPU itself, too. A 2.0Ghz Celeron or an AMD, for example, would perform poorly against a 1.8 Ghz Pentium 4.

I'd say 512MB is minimum, too. RAM isn't as much of an issue. With ~300MB free, it sounds like you're okay on the memory side of things.

The audio card is fine, video card is fine.

At this late date, depending on your motherboard, you may be able to find a processor for $25 on eBay or elsewhere. Those older ATI cards were able to compete with several professional cards, in terms of capturing quality, so it's a great idea to build a system around it.

Because the rest of the computing world has "moved on" (or so they say), new computers are all SATA, PCIe, multi core, etc etc. While you can't walk into a store and buy a new computer to work with the older ATI cards, you can get used parts for a fraction of the cost!

Quote:
Changing the preview window size from Large to Tiny dropped the cpu% a few points but no obvious change in number of dropped frames. The computer's only purpose is video capture; I'm not multi tasking on it.
Not only should you not multitask, you need to not change the size of the preview window. Keep it maybe 66-75% of the screen size, windowed. And then be sure the mouse cursor is off of the preview window. And then don't move the mouse.

You may want to also check the BIOS configuration, and be sure the AGP aperture is set to an optimum size for this card. I'd not set it higher than 32MB. If your card is PCI, instead of AGP, that can account for slightly higher drops on the older systems. The PCI bus is slower than the AGP bus, mostly only noticeable on the older underpowered sub-1.5Ghz systems.

I did many captures on an underpowered Celeron 1.7Ghz system with 512MB RAM and only one hard drive. There were dropped frames sometimes, but it was just for "watch once" TV recordings. For archival use, I used a 2.0Ghz P4 system. Same card on both, the ATI AIW 7200.

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  #4  
06-20-2010, 12:39 AM
maphew maphew is offline
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Thank you for the detailed answers. Dropping the motion compensation to 70 lowered cpu usage to 40-55%. Dropped frames are now 11 per 40K captured, with 7 of those in the first minute.

The cpu is an 1100Ghz Athlon; the ATI is AGP. MMC is v9.08. Buying new/more gear is not an option at this point, but I will keep an eye on the classifieds in case something too golden to pass by shows up.

Quote:
At least with an ATI All In Wonder Radeon card, you don't have to worry about audio sync loss
I have an unknown number of avi's created on this machine which do have audio sync problems. I didn't notice at first because it (seems to) increase to gradually through the file. I had recorded these with ATI's default DVD preset, before I found the guide here. Can these be fixed? and are dropped frames the chief cause of video-audio desynchronization? (should I ask in a new thread?)

Is there a way to determine the number of dropped frames after the fact? MMC's status report disappears when the recording is stopped.
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06-20-2010, 04:41 AM
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Only losing 4 real frames out of 40K is excellent.

Zero is still the goal, but that's honestly close enough to not panic. The source may well be causing those few frames. The TBC you ordered may help reduce it further -- or maybe not. It really depends on the root issue. Some things just can't be fixed.

Yes, some ATI cards, on some drivers, with some ATI MMC's, on some configurations of computers will lose sync on the AVI files. I unfortunately have one of those systems.

The errors are likely for a mix of reasons, including non-dropped audio frames. I don't really know why it happens, just that it doesn't happen often. I've never really been able to troubleshoot this issue, and I'm almost a bit thankful for that. It's a real mess when you come across it.

There is often no way to fix these files. You can always try a "time stretch" of the audio in an audio editor like SoundForge, Audacity or Goldwave, but it may not help.

The solution is to simply capture with a non-DVD high bitrate MPEG-2, maxed out at 15Mbps. It's not the same as uncompressed, but it's often indistinguishable from HuffYUV, and sometimes better than DV due to colorspace being potentially better on the MPEG-2 (4:2:0 instead of 4:1:1 NTSC).

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  #6  
06-20-2010, 09:11 AM
maphew maphew is offline
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Quote:
Originally Posted by maphew View Post
I have an unknown number of avi's created on this machine which do have audio sync problems. I didn't notice at first because it (seems to) increase to gradually through the file.
MMC may not be to blame. I've been trimming some avi's with Avidemux to remove the blank space at the beginning/end of the recordings. There are number of reports in forums here and there that this can cause audio de-sync'ing in certain configurations.

Looks like I get the privilege of starting the project over again.
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