[mythtv-users] HD-PVR thoughts

Yeechang Lee ylee at pobox.com
Thu Apr 15 07:03:29 UTC 2010


In the 4 1/3 years I've been using MythTV I've never had to deal with
an "encoder card," or /dev/video0 nodes, or bitrate settings, of any
kind. All my recordings have always been from two types of
sources--FireWire from cable boxes or ATSC from over-the-air--both
100% reliably producing identical digital MPEG-2 1080i or 720p
recordings, straight from the source and undiluted by any trace of
analog. This changed last week with a HD-PVR, now connected via
component and S/PDIF cables to one of my two cable boxes.

Even two years after its release in April 2008, it's hard to believe
that this tiny, unassuming black-an-gray box offers the Holy Grail of
cable/satellite MythTV users (well, other than myself): Real-time
encoding and compression of high-definition video, a feat that was so
long believed to be impossible to accomplish on a consumer budget. But
it does.

For the first time ever I've had to deal with video4linux--the bane of
so, so many mythtv-users posts I've always been glad to be able to
blithely ignore--but thanks to ATrpms there isn't anything more needed
for CentOS 5 users like me than 'yum install video4linux'. I did add
the three recommended HD-PVR patches to my customized MythTV
0.22-fixes packages
(<URL:http://www.gossamer-threads.com/lists/atrpms/users/13627#13627>). I
use the preexisting FireWire connection between my frontend/backend
and the HD-PVR's cable box to handle channel changing.[!] Also, I did
have to set the output on the HD-PVR's cable box to 720p; playing
recorded 1080i output from the HD-PVR results in mild, but consistent
audio stuttering with mythfrontend complaining of "VidOutVDPAU: Frame
g is in use by avlib and so is being held for later discarding" and
the like. (I don't know whether this is because of the 0.23 audio code
backport I am using, or the non-latest firmware on the HD-PVR.[*])

I didn't expect having the cable box turn all output into 720p to make
a material difference to video quality and it hasn't. Yes, I can see a
difference between the pure FireWire digital signal and the softer
720p analog encodings, but the difference is very, very mild and I
suspect that, if I didn't know what to look for, I'd be hard pressed
to distinguish the two.

As implied above I don't actually need to use the HD-PVR to record
from my cable boxes. Beyond the novelty value, there is a meaningful
reason to use it instead of FireWire: Disk space. A high-definition
MPEG-2 FireWire/ATSC recording of ~7.5GB/hour turns into a h.264
recording of 1.6-1.7GB. Admittedly it isn't quite an apples-to-apples
comparison since the former is 1080i and the latter is 720p, but I
wouldn't expect a 1080i HD-PVR recording to need more than ~2.5GB or
so.[#] I certainly like the idea of saving 60-75% of the space
formerly required for each high-definition recording, as well as
future-proofing my setup should FireWire become non-viable one of
these days. For that much savings I can live without closed captioning
and lossless transcoding via mythtranscode.

In conclusion, we MythTV users live in fortunate times. For $400--$200
for a Revo and $200 for a HD-PVR--anyone can assemble a compact,
attractive, nearly self-contained frontend/backend system that can
record and play back excellent-quality high-definition video from any
cable or satellite system. How fantastic is that?

[!] A script that simply calls '6200ch'. I formerly had a 1.5-second
sleep after 6200ch, but with the aforementioned patches I don't think
I need the delay and could probably just call 6200ch directly from
within mythtv-setup.

[*] I still don't have a Windows box to upgrade the firmware with and
unfortunately VirtualBox's USB support is insufficient, at least with
a Windows 7 RC2 VM.

[#] I'm still unsure as to whether the default HD-PVR bitrate settings
are optimal, and would appreciate thoughts on my question at
<URL:http://www.gossamer-threads.com/lists/mythtv/users/431028#431028>.

-- 
Yeechang Lee <ylee at pobox.com> | San Francisco CA US


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