hints

来自「Motion JPEG编解码器源代码」· 代码 · 共 170 行

(0)   If you're just starting run, don't walk to the HOWTO.
	  There's a text copy mjpeg_howto.txt where you found this file.
	  Its online at http://sourceforge.net/docman/?group_id=5776.

	  If you're interested in the source-code and what's buried there
	  read the README's.  There's a lot of useful info buried in there...
	  There are specialist README's for mpeg2enc and mplex in the corresponding
	  sub-directories as well as the	general README in the top directory.

	The scripts directory contains shell scripts that provide
	a convenient interface for common tasks.  
	E.g. lav file (editlist, avi, quicktime) -> MPEG for disk
	storage, VCD or SVCD.

	

(1) VCD and SVCDs.
	VCD's have very specific requirements as to the format of the
	multiplexed systems stream. For VCD these are actually at least
	"bendings" of the original MPEG standard and are far from optimal
	as far as maximising possible quality for software decoding goes.

	The simplest way to generate VCD and SVCD streams that can be burnt to CD
	and played using DVD players etc is to use the lav2mpeg script
	with the "-V" and "-S" flags.

	If you want to play around a couple of hints.

	To build a VCD you absolutely *must* used the VCD format
	option for mplex "-f 1".  This turns on a lot of weird stuff that
	otherwise has no place	in a respectable multiplexer!

	Obviously, to play on all players your original MPEG video must be the	
	standard 1151Kbps and audio must be 224Kbps MPEG-1 layer 2 (as produced by
	the "-v" flag of mp2enc.

	The systems streams generated by "-f 1" have been tested and succesfully 
	burned onto CD using vcdimager and vcdburn.


	An SVCD video stream is variable bit-rate MPEG-2
	with a maximum rate of around 2500Kbps and a video buffer size of
	230KB. Currently I recommend encoding -m 2 -F 3 or (for
	progressive material like PAL films) -m 2 -F 0.  -F 1 and -F 2
	will work but are currently handicapped by rather dumb code to
	choose the type of motion compensation.  I have some suspicions
	that the rate control code and multiplexer needs adjusting for
	field sequences (-m	2 -F 1 and -F 2) also. To generate a legal S
	VCD program stream	with mplex use the "-f 3" flag.

(2) For transcoding existing MPEG-2 streams from digital TV cards or
    DVD a still lower data-rate than for broadcast will give good
    results. Standard VCD 1152 Kbps typically works just fine for
    MPEG1.  The difference is in the Signal/Noise ratio of the
    original.  The noise in the analog stuff makes it much 
	harder to compress.

	You will also need to manually adjust the audio delay offset
	relative to video when multiplexing.  Very often around 150ms
	delay seems to do the trick.


(3) If you want to play MPEG1 on hardware decoders don't build variable
	bit-rate video streams.  If you intend to use software players
	variable bit-rate is a much more efficient way of encoding and
	guarantees decent quality.  All in all a *much* better
	bet.  The code for the "-S" (SVCD) flags in the lav2mpeg script shows
	an example for typical usage.

	Buffer sizes.  For hardware players experimentation is the order
	of the day.  My DXR2 seems to use a nice big > 100K buffer for
	VCD as well as DVD streams and be happy to read MPEG-1 streams
	*much* faster than the official VCD 170K/sec.  300K seems fine.
	 
	Generally, you'll probably need a buffer size of around 1/4
	data-rate to avoid buffer underflows (signalled as "TO<framenum>"'s
	by mplex).
	 
(4)  If you want to play MPEG-1 stuff on hardware decoders stick to
     44.1 224Kbps Stereo layer-2 audio.  The 100Kbps or so maximum you'll gain
	using MP3 aren't going to make a visible difference to video anyway.

(5)  If you want to reduce bit-rate without annoying artefacts when
	compressing broadcast material you should try the noise filters.

(6) Storing MPEG's.  If you record the data as XA mode 2 tracks you
	can fit appreciably more on a CD (at the expense of error
	correction/detection).  You can use vcdimager to do this and readvcd
	to extract the resulting files.
 
(6) For MPEG-1 encoding a typical (45 minute running time) show or 90	odd
	minute movie from an analog broadcast I have found a constant bit-rate
	of around 1800 to be ideal.  The resulting files are around 700M for 45
	minutes which fits nicely as a raw XA MODE2 data track on a CD-R.
	(I use cdrdao to write such CD's and a hacked version of vcdread -
	soon to be included to extract them).

	Update: I've now got a digital cable provider.  Digital TV sources
	(even when captured via an analog interface) give *much* better
	results.  There doesn't seem much point going above 1400 or
	1500Kbps.  Often even VCD 1152 works fine. It depends a bit on the
	quality of the original. However, IMHO the VCD rate is silly 'cos
	it means every movie still needs two CD's but the second is mostly
	empty.  Might as well crank the rate (and quality) up until you
	come in at just under 2 CD's.  

	For pure digital sources (DTV or DVD streams and similar) VCD 1152
	works fine.  

	Extrapolating this suggests using 2500bps for MPEG-2 SVCD from
	broadcast sources with 2000bps or less o.k. for pure digital.

(7) Speed.	On modern 400Mhz+ CPU's there is *no point* running with a motion
	compensation setting less than 16.  The speed gains aren't huge as
	other parts of the encoder take a significant fraction of the
	time. It is much better to leave the radius at 16 and push -4 and -2 to 4.
	See README.


(8) Variable bit-rate.  Remember to tell mplex you're encoding VBR as
	well as mpeg2enc (see the example scripts).  It *could*
	auto-detect but I haven't got around to that yet.  You should tell
	mplex a video buffer size at least as large as the one you
	specified to "mpeg2enc".  Sensible numbers for MPEG-1 might be a
	ceiling bit-rate of 2800Kbps, a quality ceiling (quantisation
	floor) of 6 and a buffer size of 400K.



(9) Big files.  Under typical linux 2.2 systems files are limited to
	2G bytes.   This is rarely a problem for MPEG-1 video files but it could
	bite.  If your video threatens to exceed 2G you'll need to do it in
	seperate chunks (for now). 



10)	Interoperability.

	Quicktime files capturing using lavrec can be editted using Broadcast2000.
	mjpeg AVI files captured using the streamer tool from the xawtv package
	can be editted and compressed and played back using software.
	Hardware playback is not possible for such files due to limitations in
	the Zoran hardware currently supported.

	MPEG files produced using the tools are know to play back correctly on:

	dxr2 (hardware decoder card)
	mtv
	ztheater
	xine
	oms
	dvdview
	MS Media player versino 6 (anyone like to test version 7?).

	Obviously, there are some limitations. MPEG-1 players won't play MPEG-2!

11) Optimising encoding.

	Encoder performance is slightly enhanced if you allow it to dynamically
	size the output streams groups-of-pictures to reflect scene changes.
	This is done by setting a maximum GOP (-G flag) size larger than
	the minimum (-g flag).
	
	The default value for both is 12.    For VCD's sensible values might be
	a minimum of 9 and a maximum of 15.  For SVCD 9 and 18 would be good 
	values.

Andrew