What GOP to use ???

[LoRd_MuldeR]

Hi.

I capture Video with my TV-Card (MPEG-2 Hardware-Encoder).
Since I want to recompress the video afterwards (x264) I need high quality captures. Filesize/bitrate is not important, as long as my HDD can handle it.

So set the MPEG-2 encoder to VBR mode with avarge bitrate of 12 MBit/s and peak bitrate of 16 MBit/s. Think this should be enough...

But what about the GOP setting?
I can choose between "15" and "6"

AFAIK a lower GOP will produce more I-Frames. But will this help the image quality or will it even hurt the image-quality ???

Thanks for reply :)

[Mug Funky]

unless you've got scenechange detection as well, there'll be negligible difference in GOP lengths. go 15 frames, open if you can. also, try set only 1 b-frame - at that bitrate it'll be a help.

[LoRd_MuldeR]

unless you've got scenechange detection as well, there'll be negligible difference in GOP lengths. go 15 frames, open if you can. also, try set only 1 b-frame - at that bitrate it'll be a help.

Don't know if there's "scenechange detection" enabled, at least there *no* option for that aviable. Furthermore there is *no* option for b-frames. All I can set is Resolution, Bitrate and GOP:

http://img430.imageshack.us/img430/1841/clipboard014fc.gif


So you think a GOP of "15" is better than "6" ?!?!?
Someone told me the opposite :confused:

[OEG]

When encoding to MPEG with the purpose in mind of some editing with Cuttermaran later, I always use IPPPP GOP's. (4 P frames for each I frame)

That way, I have 5 editing points for every second, which should be sufficient for accurate cutting and editing.

[LoRd_MuldeR]

When encoding to MPEG with the purpose in mind of some editing with Cuttermaran later, I always use IPPPP GOP's. (4 P frames for each I frame)

That way, I have 5 editing points for every second, which should be sufficient for accurate cutting and editing.

My Hardware-Encoder only lets me choose between a GOP of 6 and 15
Furthermore, cutting is not important, since I want to re-encode the files.
What does matter for me, is image quality...

So does a GOP of 6 or 15 give better quality? I' still not sure...

[Mug Funky]

there's the problem of not knowing what the encoder is doing... if the GOPs are closed then GOP 6 might actually be worse quality by a very small amount. if the GOPs are open, then it may be better.

but at 12-15mbps, you're unlikely to see a difference. and if you're re-encoding the video later, most of the loss is going to happen there. the more important quality settings in your case (seeing as there's so little control over the encoder), involve making sure everything is as good as possible on the analog side of things.

[edit]

btw, if you have the space, try just capture CBR at the highest bitrate you can manage. hardware encoders (generally) do CBR better because there's less to think about. if you can spare the disk space, then the sky's the limit :) (hint - DV is 25mbps, and I-frame only, so mpeg-2 will be quite a bit better at this bitrate).

[LoRd_MuldeR]

there's the problem of not knowing what the encoder is doing... if the GOPs are closed then GOP 6 might actually be worse quality by a very small amount. if the GOPs are open, then it may be better.

but at 12-15mbps, you're unlikely to see a difference. and if you're re-encoding the video later, most of the loss is going to happen there. the more important quality settings in your case (seeing as there's so little control over the encoder), involve making sure everything is as good as possible on the analog side of things.

[edit]

btw, if you have the space, try just capture CBR at the highest bitrate you can manage. hardware encoders (generally) do CBR better because there's less to think about. if you can spare the disk space, then the sky's the limit :) (hint - DV is 25mbps, and I-frame only, so mpeg-2 will be quite a bit better at this bitrate).


What is the difference between "open" and "closed" GOPs?
Do you really think CBR is better on HW Encoders ????

[guada 2]

Hello LoRd_MuldeR, :)

I had never heard spoken about GOP 6.
Normally, your choice would be the 15 (like Mug says you).
But, it is strange that you havent got DC.

Have you have do other researchs?

[Boulder]

Do you really think CBR is better on HW Encoders ????
Logically thinking, it is better if you set the bitrate to the max.

With your card you should be able to set open GOPs if you use WinTVCap to capture.

[Mug Funky]

the CBR comment is based on my experiences with the encoder cards at my workplace. they handle CBR brilliantly, even if the max bitrate is lower than it would be on a VBR encode (within limits of course). it makes sense from a ratecontrol point of view - there's less to think about when all you're doing is giving the most bits you can to every frame. also, if you have plenty of hard disk, it does make sense to just max out everything and capture. it's much better to have a large, good quality file than a smaller file with blocks.

as for open GOP, same deal - the encoders it work suck at closed GOPs (they'll give bad scenechanges if a new GOP by coincidence starts on a scenechange, because it uses a stupid GOP structure for closed GOPs). of course, other hardware will probably handle these situations better.

closed GOPs are only really useful if you're cutting on I-frames, or if you're making a multi-angle encode for DVD. considering you're already capping at about twice the max allowed bitrate for multiangle, i guess you're not worried about that :)

[LoRd_MuldeR]

With your card you should be able to set open GOPs if you use WinTVCap to capture.

Nope, WinTVCap doesn't work with my PVR-150. There is BGRecord aviable, but it doesn't work too. Seems to be some issue with all newer drivers. But I need the latest drivers since they solve the audio-distortion problem as well as the stuttering problem in WinTV2k. So all I have ATM for captureing is WinTV2k. And for now, it works fine. Of course a few more settings would be fine...

[Boulder]

Have you tried checking the forums at shspvr.com? There might be some registry tweaks that could help you.

[LoRd_MuldeR]

Have you tried checking the forums at shspvr.com? There might be some registry tweaks that could help you.

Thanks.
I know about that web-site. Got the only drivers that work properly form this site :)

[DrP]

If it is a PVR150 and assuming the recording program doesn't overwrite the registry, you can control the GOP length and number of B frames from the registry.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E96C-E325-11CE-BFC1-08002BE10318}\xxxx\DriverData" ClosedGOP (REG_DWORD) = 1 close, 0 = open
GOPNumberOfBFrames (REG_DWORD) = number of B frames + 1 (ie 1 = 0, 1 = 2)
GOPNumberOfPictures (REG_DWORD) = number of frames in GOP.

xxxx is different on different computers, so just have a fish around, you'll find the right one by looking for 'AudioCRC'.

Don't take the bitrate over about 16Mbit/sec. The code in the CX seems to have issues past that point and can just stop outputting MPEG data silently.

[LoRd_MuldeR]

If it is a PVR150 and assuming the recording program doesn't overwrite the registry, you can control the GOP length and number of B frames from the registry.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E96C-E325-11CE-BFC1-08002BE10318}\xxxx\DriverData" ClosedGOP (REG_DWORD) = 1 close, 0 = open
GOPNumberOfBFrames (REG_DWORD) = number of B frames + 1 (ie 1 = 0, 1 = 2)
GOPNumberOfPictures (REG_DWORD) = number of frames in GOP.

xxxx is different on different computers, so just have a fish around, you'll find the right one by looking for 'AudioCRC'.

Don't take the bitrate over about 16Mbit/sec. The code in the CX seems to have issues past that point and can just stop outputting MPEG data silently.

Thanks for Info