[mythtv-users] Transcoding profiles

Raymond Wagner raymond at wagnerrp.com
Thu May 19 15:22:05 UTC 2011

On 5/19/2011 11:04, Christopher Meredith wrote:
> On Thu, May 19, 2011 at 9:47 AM, Raymond Wagner<raymond at wagnerrp.com>  wrote:
>> On 5/19/2011 10:33, Bob wrote:
>>> I have been reading the many threads about transcoding with 6 channel
>>> audio.  I would like to transcode some h264 content with 5.1 from my
>>> HD-PVR.  In the profiles I only see "MP3" and "Uncompressed" as audio
>>> options.  Do most people choose "Uncompressed" when wanting to preserve
>>> the 6 channel audio?
>> Your content is already in h264.  It already is in the most efficient
>> common codec available.  While hardware decoders are not going to be as
>> efficient as something like x264, and you could arguably shave off
>> 20-30% with minimal quality loss, it will be at a huge cost in CPU power
>> (and electricity).  You shouldn't bother transcoding that content unless
>> you are either clipping commercials, or need it in a specific format for
>> some device.  In either case, you really want to use an external
>> utility, rather than MythTV's internal transcoder.
> AFAIK, mythtranscode is still incapable of working with HD-PVR
> recordings anyway.

Mythtranscode can work with HDPVR recordings just fine, however the 
special lossless cutting mechanism only works with MPEG2.  It works by 
performing a GOP cut, followed by filling the frames between the extent 
of the GOP structure and the real cutpoint itself with I-frames.  Any 
HDPVR content will instead be transcoded to MPEG4 in a NuppelVideo 

> If you're looking to cut commercials, I've found
> that recording at max quality, then transcoding to MPEG2, then cutting
> commercials with mythtranscode, then converting back to h264 using
> HandBrake at 3250 kbps produces better-looking results than an
> unconverted HD-PVR recording done at 3250 kbps.

That seems to be an extremely long winded way of doing things.  Surely 
handbrake can be fed a cutlist and do the cutting directly on the first 
pass.  At least choose something lossless and fast as your intermediary 
format, like huffman, rather than MPEG2.  While the 13.5mbps max quality 
recordings out of the HDPVR are overkill by any measure, 3250kbps seems 
awfully low for even 720p60.  I would expect more like 4-6 depending on 
the grain and motion in the content.  At any rate, you shouldn't be 
setting the bitrate anyway.  Save yourself the second pass, set a 
constant quantizer to 20 or so, and let the compressor decide what's best.

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