[mythtv-users] Transcoding profiles

Christopher Meredith chmeredith at gmail.com
Thu May 19 16:01:56 UTC 2011

On Thu, May 19, 2011 at 10:22 AM, Raymond Wagner <raymond at wagnerrp.com> wrote:
> 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
> container.
>> 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.

HandBrake does not appear to support EDL or cutlists. For my use case,
I require the ability to use mythcommflag and mythtranscode and to
achieve frame-accurate cuts. Can mythcommflag and mythtranscode work
with huffman encoding? Also, I choose 3250 kbps because that's the
current convention for archived, HD, commercial-less recordings
(1-hour show = 1.1 GB).

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