All things related to multicast

[Frank Fulchiero <>]

Tim, actually you made a very good point, that the MPEG4 image looks  
sharper than the MPEG2, which is true.

I believe that's due to the differences between the actual spatial  
resolution of our video and the theoretical upper limit that a format  
can support.
So, the MPEG2 video is 720x480, and theoretically you should be able  
to resolve 720 separate vertical lines (say alternating black and  
white).

(I'm not going to get into the older NTSC standard, which states that  
if, for example, you can see 800 alternating black and white vertical  
lines across the screen, the resolution is described as 3/4 of this,  
or 600 lines of resolution. This often confuses the issue).

However, we started out with miniDV video, which is in the 4:1:1  
color space and compressed 5:1 with the DV codec, this was converted  
to 4:2:0 color space MPEG2 compressed 5:1 from the DV codec, then the  
DVD is playing from a stand-alone player. The digital signal is  
converted to analog here, then goes to the VBrick over S-video, where  
it is converted from analog to digital, and recompressed to 4:2:0  
color space again, to 4 mbps. So, there are too many conversions to  
maintain the original DV quality, which is already consumer level.

I would guess our video you see in VLC is probably closer to an  
actual visual resolution (how many visual elements can be  
distinguished) of 640 wide x 420 high or so. When this image is  
"stretched" to the 720x480 dimensions of MPEG, it's going to look soft.

On the other hand, when the 640x420 actual resolution is shrunk to  
our MPEG4's 320x240, it looks nice and crisp, as no information is  
lost, there is more than enough to fill the frame size.

Not sure I'm explaining it very well, but I think that's why the  
MPEG4 looks "better". We can probably reduce the bit rate on the  
MPEG4 and still get good quality.

Of course, if we were doing things correctly, the MPEG2 should look  
just as good! We kind of hacked the playback system together for  
testing.

Thanks,
Frank Fulchiero
Digital Media Specialist
Connecticut College


On Aug 18, 2006, at 2:44 PM, Tim Chown wrote:

> On Fri, Aug 18, 2006 at 11:14:24AM -0400, Richard Mavrogeanes wrote:
> > MPEG-4 is and should be lower quality than MPEG-2....it is much lower
> > bandwidth, 320 x 240, and is pt 2, SP. Per the standard, the audio  
> > with
> > the MPEG-4 is AAC.  Both are coming in loud and clear here.
>
> Apologies for the brain fart :)   I was confused because I thought the
> MPEG4 version had no sound but that was the higher quality image.
>
> Regardless, both look very good with little evidence visually of loss.
>
> Tim