On Fri, 21 Jan 2005, kevin thayer wrote:
This is going back a ways, but here's my remeberances regarding GOP:
1) there are settings for gop as you have noticed
That's what I've recently found out.
Nor do I, but I'm trying to learn more about it all. Making a direct-to-DVD-from-8mm with even linear edits was a real bitch to try to keep sync in order. Basically, I "wrapped" the MPEG up in an avi so I could use avidemux for editing... the 8mm capture had sync glitches that tweaked the PTS within the capture. That didn't work with any of the mplexing A/V offset options (since it *changed* throughout the clip!).2) i don't think any of us working on it in the early days had the mpeg-knowhow to tell if they worked or didn't
I saw a "scene-change" option in a press-release for the chip, but I haven't found out how/if it's used. It's not mentioned in http://ivtv.sourceforge.net/firmware-api.html, although the open/closed GOP, GOP format, etc is.3) i do seem to recall a scene-change option, but it's not in our current documentation (it may be an extra parameter to 0x97 or 0xc5.. it may have been in a press-release from conexant or something.
Absolutely... if we're putting together a list of questions to ask. I'd throw in a few votes for:
perhaps add that to the hauppauge question list?
- Variable/adaptive GOP structures. It could produce much better quality at lower bitrates.
- Sync issues on captures resulting in glitches and PTS variations.
- Differences between the streams produced under different types (ES, PS, TS, DVD, etc)
- Different PU formats possible... something more like what's produced by software encoders. If you tcscan a mythtv-produced capture, you get lots of things like:
.....
------------- presentation unit [709] ---------------
stream id [0xbe] 3
stream id [0xc0] 1
stream id [0xe0] 19
23 packetized elementary stream(s) PES packets found
presentation unit PU [709] contains 3 MPEG video sequence(s)
Average Bitrate is 284000. Min Bitrate is 6000, max is 6000 (CBR)
---------------------------------------------------
found ISO/IEC 13818-2 or 11172-2 MPEG video stream [0xe0]
found padding stream [0xbe]
found unknown stream [0xbb]
sequence: 704x480 4:3, 29.97 fps, 6000 kbps, VBV 224 kB , Custom Intra Matrix , Custom Non-Intra Matrix
found ISO/IEC 13818-3 or 11172-3 MPEG audio track 0 [0xc0]
------------- presentation unit [710] ---------------
stream id [0xbb] 1
stream id [0xbe] 3
stream id [0xc0] 1
stream id [0xe0] 31
36 packetized elementary stream(s) PES packets found
presentation unit PU [710] contains 3 MPEG video sequence(s)
Average Bitrate is 286000. Min Bitrate is 6000, max is 6000 (CBR)
---------------------------------------------------
found ISO/IEC 13818-2 or 11172-2 MPEG video stream [0xe0]
found ISO/IEC 13818-3 or 11172-3 MPEG audio track 0 [0xc0]
found padding stream [0xbe]
.....
.... that's only 20-30 PES packets in a PU "chunk" of video. Basically every 1/10th of a second, there's a new PU. That tends to confuse lots of editors and muxers, as well as provide the lots of opportunity to change PTS offsets. Compare that to a stream produced from re-encoding a capture with avidemus (2-pass, denoised, etc):
tcscan -i reencoded.mpgfound first packet header at stream offset 0x0
found unknown stream [0xbb]
found navigation pack [0xbf]
found ISO/IEC 13818-2 or 11172-2 MPEG video stream [0xe0]
sequence: 352x480 4:3, 29.97 fps, 8000 kbps, VBV 224 kB
found ISO/IEC 13818-3 or 11172-3 MPEG audio track 0 [0xc0]
found padding stream [0xbe]
found program end code [0xb9]
------------- presentation unit [0] ---------------
stream id [0xbb] 2118
stream id [0xbe] 2122
stream id [0xbf] 4236
stream id [0xc0] 14700
stream id [0xe0] 172297
195473 packetized elementary stream(s) PES packets found
presentation unit PU [0] contains 2118 MPEG video sequence(s)
Average Bitrate is 1916. Min Bitrate is 8000, max is 8000 (CBR)
---------------------------------------------------
(scan_pes.c) detected a total of 1 presentation unit(s) PU and 2118 sequence(s)
That's *one* PU, with 1/2 hour's worth of video/audio stream.... much easier for editors to parse.
I might try messing around with different formats and see if it actually changes anything (ES, PS, TS, DVD, etc).
i'd appreciate it if those who can test these things did :) i'll help with some sample code if you need.
-Cory
************************************************************************* * Cory Papenfuss * * Electrical Engineering candidate Ph.D. graduate student * * Virginia Polytechnic Institute and State University * *************************************************************************
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