[MEncoder-users] Dolby downmix

[Rolf Ernst]

On 12/27/2010 11:37 PM, Andrew Berg wrote:
> On 12/27/2010 9:06 PM, Rolf Ernst wrote:
>> The HD PVR actually digitizes off a high quality
>> component analog signal and one of the quirks is that every once in a
>> while it misses a beat (very rarely, but maybe 3 times in an hour
>> unnoticeably so) and only the time stamps keep it in sync. Since
>> mencoder knows how to thread the combined video and audio together none
>> of these glitches matter, which is also why ffmpeg is useless for
>> processing these a/v streams.
> That's odd. I never really encountered sync issues with my HD-PVR caps,
> even though I process audio and video separately. My commands were
> something like this (I may be forgetting something since I use AviSynth
> with x264 now instead of MEncoder):
>
> mencoder input.ts -demuxer lavf -oac copy -ovc x264 -x264opts [x264
> options] -vf yadif,hqdn3d=0:0:4:4,harddup -o temp.avi
> ffmpeg -i temp.avi -f wav - 2>nul | neroaacenc [nero aac options] -of
> temp.m4a
> mp4box -aviraw temp.avi
> tsMuxeR temp.meta (remuxed the raw avc stream - see below)
> mp4box -add temp_video.264:fps=29.97 -add temp.m4a -new out.mp4
>
> As long as I used -demuxer lavf and -oac copy (instead of -oac null or
> -rawvideo), everything was fine. Then again, I always remuxed the video
> (as part of the editing process when cutting out ads) and added
> timestamp info to the new raw AVC stream with tsMuxeR. Also, I don't
> think I've ever recorded 24p ->  30i telecined material with the HD-PVR.
The capturing errors may be related to the STB (Motorola, it initially 
had a lot of A/V sync errors until either the STB was upgraded or a much 
later software update arrived). When I use eac3to to convert to aac it 
explicitly tells me when it finds such a rough spot - sometimes there 
aren't any at all in which case I can do the separate processing but 
more likely than not there are a couple. I don't quite understand 
eac3to/neroaacenc since the messages indicate that the code is patching 
those sync errors up but it really doesn't.

Anyhow, as for your comment on the telecined material: The box always 
captures at 59,94 fps (or 29.97 when 1080i). So you never really get 
23.976 and what people generally do is downmix that to 29.97 interlaced 
(which is how you used to watch film on TV). However, on a flatscreen 
this is deinterlaced and in the end the results are both inefficient in 
encoding and inferior in quality. If instead you rebuild the actual film 
rate (most any broadcast HD is progressive and just about any movie will 
be) you get excellent quality in small space. I get 50 minutes at 
720x480 24fps crf 23 on 250Mb. (Ergo the upscaling via tinterlace=4 and 
then the softskip and filmdint afterwards which winds you up at the 
original film frames). You probably want to use deblock with x264 
because there are already minor blocking artifacts in the source 
material coming from Verizon for me.

Anyhow, if Nero never complains to you about a gap then your capture is 
just better. I believe this is all related to the optical in from the 
Motorola STBs. This symptom does not occur when capturing audio via the 
RCA jacks. Well, I got whatever box I got and I made it work very well - 
just this little quirk is bothersome. I am sure I could recode to AC3 at 
some high bitrate via mencoder and then take that output (no longer 
containing physical gaps and pump it through an encoder but this is all 
a roundabout way. Normally I preserve the 5.1 signal anyhow ...
-- 
/re

"My friends, watch out for the little fellow with an idea"

Tommy Douglass