[mythtv-users] DVD Burning

[William]

> In my experience of using tcrequant to requantise video, you 
> want to use as small a requantisation factor as possible, as 
> the higher this factor is, the more reduced the video quality is.
> 
> I don't think that the total data size (audio plus video) 
> should be used when determining the requantisation factor. 
> Surely, the factor by which the _video_ data needs to be 
> reduced is the ratio of the size of the original video data 
> divided by the total required data size (4350 in the script) 
> less the audio size:
> 
> RCF = $vsize / (4350 - $asize)
> 
> For example with simple numbers, if the original demuxed 
> files were 9.0GB video, and 200MB audio, and the desired data 
> size was 4.7GB, the factor would be calculated as:
> 
> RCF = 9000 / (4700 - 200) = 2.00
> 
> Here the video size after requantisation would be 4.5GB, 
> which when added to the 200MB audio, gives a total datasize 
> of 4.7GB, which is the capacity of the disk. Obviously there 
> is an amount to be allowed for filesystem/DVD structure 
> overhead, but this should give you a maximally utilised disk.
> 
> Using the 6600MB total data size and assuming 300MB for audio 
> and a disk size of 4350MB, I would use a RQ factor of
> 
> RCF = 6300 / (4350 - 300) = 1.56
> 
> The above scrip would give you a factor of ( (6300 + 300) / 
> 4350 ) ^2 = 2.3. This when applied to the video and then 
> adding back the audio data would give a new DVD size of 
> approx 3.0GB which is broadly in line with your experience.
> 
> The audio is not requantised, so I don't believe the size of 
> the audio data should be combined with the video data size to 
> determine the factor to be used. The amount of space allowed 
> for the video oon the DVD should be determined by subtracting 
> the audio size from the total DVD capacity, and then the 
> ratio of the video size to the remaining space should be used 
> for the requantisation factor.
> 
> I may have missed something very obvious, but this is the 
> method I use to get full DVD disks when requantising DVB 
> recordings that are too large for a DVD.

A couple of things. Your calculations are correct. When the recordings
approach 2X the size of the final result you start to get in trouble. The
requant value can only be in the range of 1.0 to 2.0 with 2.0 approx equal
to a 2x reduction in file size so you are somewhat limited in how far you
can shrink the file. When I wrote the shrinking routine I was working with
files that were in the 4.5-5GB range not requiring much shrinking. I also
put in some fudge factors to allow room for menus and stuff (the X1.05
portion of the calculation)

If you are going to work with files that big you might want to record with a
lower bit rate to shrink the source file.