[mythtv-users] MPEG-2 to MPEG-4 Transcoding comparisons

[James L. Paul]

Clif Deanhardt wrote:
> Actually, I'm just trying to make it as small as possible (while,
> unfortunately, knowing as little as possible).  Back when I had the CVS
> running (early september) I had it transcoding 3.7gb files into 700Mb
> MPEG-4s with no noticable loss in picture quality (to my eyes).  I can't
> seem to recapture that, though.
> 
> I assume I'd just drop my MPEG-2/DEFAULT bitrate to 2200 or so and check the
> quality... is that right?  Should I expect to get any space savings from
> converting a 2200 MPEG2 into a 2200 MPEG4?  I have no reason to use MPEG4
> other than trying to save space.

It's not the type of codec that saves space, it's the bitrate. (The 
number of bits per second.) 2200kbps is just that, 2200kbps. No matter 
what codec you use (excepting for VBR variations) a 2200kbps 1 hour file 
is going to be approximately 2200kbits x 60 seconds x 60 minutes = 
7920000kbits = 7734Mbits = 7.734Gbits = 0.967GB. Call it about 1GB per 
hour then.

The quality issue is about which codec gives the best quality at that 
bitrate. MPEG4 is going to be the winner there, but only if it's the 
original codec. You aren't going to transcode from one codec to another 
and get higher quality video, you lose quality each time you transcode. 
(And at low bitrates, the loss can be substantial.)

If it's only size you are after, I wouldn't transcode from MPEG2 to 
MPEG4. Any size savings is likely to be offset by quality loss and heavy 
CPU time. Better to simply encode at a lower bitrate in the first place.

I have about 150GB for video, and at 1.8GB/hour, that's enough for me. I 
have a couple extra profiles set up for higher and lower 
quality/bitrates, but my default is currently 3500k/5000k. Oh, and I 
gain quality by reducing resolution. I use 1/2D1 DVD resolution, which 
is 352x480. This is technically equivalent to SVHS resolution, and most 
CRT televisions are incapable of representing higher resolutions than 
this. When I make a DVD from my 1/2D1 3500kbps MPEG2-PS streams output 
from my PVR-250, it looks absolutely great on a standalone DVD player. 
It's only about VHS quality output from the Myth box, due to my 
GeForceII GTS TV-out not being the greatest.

Set your capture resolution to 352x480 and the bitrate to 2500 or 3500, 
or as low as you decide you like the resulting quality. Play with the 
Live TV profile first, it's easy to just check and see the results.

For comparison, the SVCD standard calls for 480x480 at 2524kbps max. And 
that's overkill. 352x480 looks even better because the extra pixel depth 
significantly increases image quality much more than the extra 
resolution. Try it and see. :)

Anybody interested in the benefits of 1/2D1 or CVD resolution should 
read this:

http://www.dvdrhelp.com/forum/userguides/98177.php

> Clif
> 
> -----Original Message-----
> From: James L. Paul [mailto:james at mauibay.net]
> Sent: Wednesday, October 29, 2003 1:39 AM
> To: Discussion about mythtv
> Subject: Re: [mythtv-users] MPEG-2 to MPEG-4 Transcoding comparisons
> 
> 
> 
> 
> Clif Deanhardt wrote:
> 
>>Would someone who's using .12 to transcode PVR-250 MPEG-2s into MPEG-4s
> 
> like
> 
>>to compare notes on compression rates?  I'm trying to get a handle on
>>whether I've got mine set up optimally and I'm a bit of a noob at setting
>>this thing up.
>>
>>With the exception of a apparition when I first started using it, every
> 
> one
> 
>>of my 1hr shows records at 3.7gb and after transcoding it's 2.2gb,
>>regardless of the type of show or number of colors or quantity of
> 
> movement.
> 
> I wonder... Is the quality of a 3.7G MPEG2 file transcoded to 2.2GB
> MPEG4 better or worse than just encoding the original MPEG2 at 2.2G in
> the first place? I have trouble believing it's better quality and worth
> the effort, unless for some reason you really want the results as MPEG4.
> 
> 
>>Likewise, every one of my half hour shows records at 1.8gb and transcodes
> 
> to
> 
>>1.1gb.
> 
> 
> My PVR-250 bitrate is set to result in 1 hour for 1.8G and looks
> absolutely great, better than VHS to my eye. I just keep the MPEG2 and
> prefer not to transcode at all. Then again, I have little use for MPEG4.
> 
> 
>>Is anyone else similarly set up?  What numbers are you getting?
>>
>>Thanx!
>>
>>Clif
>>
>>
>>My settings:
>>
>>----------------------------
>>MPEG-2 ENCODERS / DEFAULT
>>----------------------------
>>   Width/Height: 480x480
>>
>>   Codec: MPEG-2 HARDWARE ENCODER
>>   Stream: MPEG-2 PS
>>   Aspect: 4:3
>>   Bitrate: 4500
>>   Max Bitrate: 6000
>>
>>   Codec: MPEG-2 HARDWARE ENCODER
>>   SampleRate: 32000
>>   Type: Layer II
>>   Bitrate: 384kbps
>>
>>----------------------------
>>TRANSCODERS / MPEG2
>>----------------------------
>>
>>   Width/Height: 480x480
>>
>>   Codec: MPEG-4
>>   Bitrate: 2200
>>   Max Quality: 2
>>   Min Quality: 15
>>   Max Quality Difference Between Frames: 3
>>   Scale Bitrate for Frame Size: Selected
>>   Enable High Quality encoding: Not Selected
>>   Enable 4MV: Not Selected
>>
>>   Codec: MP3
>>   SampleRate: 32000
>>   Quality 7
>>
>>----------------------------
>>GENERAL
>>----------------------------
>>It's configured to automatically detect commercials, but not to skip them.
>>And the resulting, compressed files still have the commercials in them.
>>
>>
>>
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>>
>>
> 
> 
> 
> 
> 
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