[mythtv-users] Transcoding HD-PVR Recordings

Doug Vaughan r.d.vaughan at rogers.com
Tue Sep 21 13:43:43 UTC 2010


Bob,

A difference in our approaches is that you transcode before you edit out 
commercials. That may give you some extra flexibility with Avidemux than 
my approach.

It was also pointed out that "MKV File Creator" can convert an avi to 
mkv and that is true but I find ffmpeg is faster.

Someone else suggested that MythTV figures out commercial breaks better 
now. I have not tried MythTVs automatic commercial break detection for 
almost 2 years so things have likely improved.

My MythTV HDPVR recording profiles all have their settings maxed to:
Average bit rates of: 13,500
Maximum bit rates of: 20,200

You mention about file sizes of your 1 hr HDPVR recordings at 1280x720p. 
I found out that different STB's send different bit rates to the HDPVR. 
For example when I use an MythTV HDPVR profile as descibed above I get:
STB(SA3250HD) -1 Hr recording 2.1 to 2.2Gb
STB(SA8300HD) -1 Hr recording 2.3 to 2.4Gb

I assume that the SA3250HD has an older chip set which accounts for the 
lower bit rate. I have also heard that the cable or satilite providers 
transmit specific channels at different bit rates so to me file sizes 
vary according to these factors.
a) The original content and how easily it can be compressed. Darker 
programming like the TV series Supernatural often has smaller file sizes.
b) The provider's choice in what bit rate they transmit a specific channel.
c) Your STB's chip set.
d) The HDPVR recording profile settings you use in MythTV
Please note that I am by no means knowledgable about what the providers 
do with their signal. So much of what I just wrote is based on my 
observations and what I have read.

Here is an example of bit rates I get with my SA3250HD STB and the 
recording profile described above. This data comes from the utility 
MediaInfo and last night's recording of "The Event":
The Event - S01E01 - Pilot.mpg
Format                           : MPEG-TS
File size                        : 2.42 GiB
Duration                         : 59mn 49s
Overall bit rate                 : 5 796 Kbps

Video
ID                               : 4113 (0x1011)
Menu ID                          : 1 (0x1)
Format                           : AVC
Format/Info                      : Advanced Video Codec
Format profile                   : Main at L4.0
Format settings, CABAC           : Yes
Format settings, ReFrames        : 4 frames
Duration                         : 59mn 45s
Bit rate                         : 5 182 Kbps
Width                            : 1 280 pixels
Height                           : 720 pixels
Display aspect ratio             : 16/9
Frame rate                       : 59.940 fps
Resolution                       : 24 bits
Colorimetry                      : 4:2:0
Scan type                        : Progressive

Audio
ID                               : 4352 (0x1100)
Menu ID                          : 1 (0x1)
Format                           : AC-3
Format/Info                      : Audio Coding 3
Duration                         : 59mn 49s
Bit rate mode                    : Constant
Bit rate                         : 384 Kbps
Channel(s)                       : 6 channels
Channel positions                : Front: L C R, Rear: L R, LFE
Sampling rate                    : 48.0 KHz
Video delay                      : -5ms

The ffmpeg command line I use to convert an avi container to a mkv 
container is very straight forward. $avifile" and "$@/$basename.mkv" are 
just the bash script variables.
ffmpeg -v 1 -i "$avifile" -vcodec copy -acodec copy -y "$@/$basename.mkv"

Doug


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