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MPEG 2 Compression: Compressor 1 vs Compressor 2

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jrmcgee
MPEG 2 Compression: Compressor 1 vs Compressor 2
on Dec 22, 2005 at 4:58:45 pm

Greetings,

Just wondering about "MPEG 2 HD", and what one means by that. Is it just a way of saying HD Video that's been compressed using MPEG 2 codec? Also, is there a difference between the following two MPEG 2 streams:
(1) 1920x1080 MPEG 2 Stream compressed using Compressor 1's "MPEG-2 6min High Quality Encode Widescreen Preset"
(2) 1920x1080 MPEG 2 Stream compressed using a similar, but "HD supportive as advertised" MPEG-2 encoder?

Is there a difference in the resulting MPEG-2 stream as far as structure goes? Or will there JUST be some slight quality difference, per say?
By the way, MPEG-2 streams that come out of Final Cut Pro: are they program streams or transport streams?
Thanks!
v/r
JMcGee, Physics and Astronomy Undergrad,LSU
Visualizations Research



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Charles Simonson
Re: MPEG 2 Compression: Compressor 1 vs Compressor 2
on Dec 22, 2005 at 5:15:49 pm

1: Compressor 1 did not encode to HD resolutions with MPEG2. So that is a major difference between Compressor 1 and 2.

2: Yes, there can be. The MPEG formats have various profiles and levels that are supported to ensure a video will play back correctly on a certain platform/device. Apple's encoder only supports a few of the profiles/levels. The MainConcept encoder can support them all.

There are many variables that can make a stream incompatible with one device or decoder to another. It is best to know and understand the required specifications that you will need to encode to in order to ensure proper playback. The page you linked to in your previous post does a good job explaining these. And the streams that come out of Compressor for MPEG2 are usually elementary streams. This means the audio and video are separate and not muxed. You can easily mux the tracks together into a program or transport stream using some of the freeware tools on the mac. A program stream should have broad compatibilty, whereas a transport stream may not as the specifications for the types supported are more specific.

Feel free to email me if you wish.



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jrmcgee
Re: MPEG 2 Compression: Compressor 1 vs Compressor 2
on Dec 22, 2005 at 5:22:02 pm

Charles,

You see, I'm confused as to why Compressor I will not do a good job with HD Videos when it clearly has an encoding preset to do so:

Example of Compressor 1's (BEST) MPEG-2 encoding syntax preset

Name: MPEG-2 60min High Quality Encode Widescreen
Description: 6.8 Mbps, 2-Pass VBR, 16:9
File Extension: m2v
Video Encoder
Format: M2V
Width and Height: Automatic
Pixel Aspect Ratio: default
Crop: None
Frame Rate: (100% of source )
Aspect Ratio: 16:9
Field Dominance: Auto detect
Average Data Rate: 6.8 (Mbps)
2 Pass VBR enabled
Maximum data rate: 7.5 (Mbps)
High Quality
Best motion estimation
Closed GOP Size: 1/2 second, Structure: IBBP
DVD Studio Pro meta-data enabled




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Charles Simonson
Re: MPEG 2 Compression: Compressor 1 vs Compressor 2
on Dec 22, 2005 at 6:03:32 pm

Right, those settings are for a 16:9 anamorphic 480p encode. Notice how resolution isn't specified there. Its not that Compressor 1 will do a bad job encoding your material, its that it won't encode it to HD to begin with. Also, even if it could encode MPEG-2 at HD resolution, Compressor 1's output quality would be terrible at the average bit rate of 6.8 (this doesn't even take into account the fact that the MPEG-2 encoder from Apple has made huge strides from Compressor 1 to 2) as 6.8Mbps is too low for MPEG-2 HD (1080i or p). Fact is, Apple didn't support HD MPEG-2 encoding until Compressor 2. Compressor 1 and previous versions of the QT MPEG-2 export component only supported a max resolution of 480p and a max bit rate of 9Mbps.

I can't understand why this hard for you to grasp. The easiest thing to do would be to do some short sample encodes and determine this yourself. Simply grab or edit a 10 second source clip and use that to test your MPEG-2 encodes with.



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jrmcgee
Re: MPEG 2 Compression: Compressor 1 vs Compressor 2
on Dec 22, 2005 at 6:20:07 pm

Charles,

The reason why it's difficult to grasp is the terminology here. It's highly ambiguous. For instance, part of the MPEG 2 syntax (aside from what encoding options you chose), includes a HD-supportive Profile/Level: Main Profile/High Level(MP@HL). So, if you give it HD resolutions, it can encode them.(my assumption). Also, " Final Cut Pro supports capture, editing, and output of high definition(HD) video when using an appropriate third-party capture card. 1080i, 1920x1080, 25,29.97(30fps), 59.94(60fps)
1080p ", etc. -Final Cut Pro 4.5 User Manual: I-26
So, even if I don't "capture" but instead generate HD-resolutions, there is some ambiguity.
Finally, in the encoding summary for Compressor 1 I sent you, it says
Aspect Ratio 16:9 ( indicative of at least 720 resolution which is HD ).
However, you are correct that it doesn't specify res. But, like I said before, MPEG 2 was inherintly designed to encode all ATSC-compliant resolutions by law:
ATSC compliant formats include:
1080 x 1920, 60 Hz interlaced, 30 Hz progressive
480 x 704, 60 Hz progressive/interlaced, 30 Hz progressive
720 x 1280, 60 Hz progressive, 30 Hz progressive
480 x 640, 60 Hz progressive/interlaced, 30 Hz progressive

You're also making a good point about the maximum bit rate. Apparently the encoding techniques of compressor 1 are not quite sufficient to drive quality HD since minimum broadcast HD quality is done at 12-20 MBps.




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Charles Simonson
Re: MPEG 2 Compression: Compressor 1 vs Compressor 2
on Dec 22, 2005 at 7:08:38 pm

Sorry, but 16:9 does not mean "indicative of at least 720 resolution which is HD". When you buy a DVD, the PAR is usually anamorphic 16:9, yet it is not 720p, it is only 480p. This is what is meant by the Apple preset in Compressor 1. Compressor 1's MPEG-2 encoder presets are targeted only towards standard DVD output. As I have stated before, the encoder in Compressor 1 and QT6 is only capable of 480p max output, no matter what the input resolution of the source is. Trust me on this and if you don't, please stop writing on here and go and encode for yourself to check this out. As far as FCP, there is no ambiguity, it can certainly output HD, but this refers to output of HD over SDI using an appropriate capture board or to an appropriate export format. MPEG-2 was not an "appropriate" export format for FCP in versions before 5 (at least not directly with Apple's products). What you can do is export your HD project from FCP to a HD source file using a different codec that is more resolution independent.

On the ATSC front, two of the resolutions you noted are for SD (there are actually a few more formats, but no need to worry about those right now). I can't remember right now if QT6 or Compressor 1 supported output to the fixed pixel size 704x480 or 640x480, but they may have. If you have a 16:9 HD source, and are looking to encode to an ATSC format with Compressor 1, then see if 704x480 (sometimes referred to as EDTV) is an option. Also, ATSC as it is today can be broadcasted at right around 8Mbps for 720p. FOX does this for some of their programs, but mostly they broadcast at 9Mbps. At 1080i, I have seen HBO broadcast at 10Mbps, but usually 11Mbps (on the flip side, TNT broadcasts at about 18Mbps). These numbers will continue to get lower and lower as further advances are made to compression with MPEG-2 (although they are very close today to what the maximum can be achieved with MPEG-2 and low bit rate encoding).



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