kichigai at comcast.net
Mon Mar 20 16:04:11 UTC 2006
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On Mar 20, 2006, at 01.37, Jonas Lihnell wrote:
> I assume my last mail was too broad and covered an issue too big so
> I'll split it down to a few more specific questions. (old mail:
> When the backend sends the tv signal to the frontend, do the
> backend or
> the frontend decode it? (I assume frontend)
You assume correctly. However, if you have a machine that combines
both front and back ends, then the point is kind of moot. However, if
this is a big worry for you, there are some nVidia cards which can
decode MPEG-2 in hardware, and the Hauppauge PVR-350 has an on-board
MPEG-2 decoder complete with TV-Out, so you can get away with barely
using the CPU for playback.
> When myth takes an incoming signal, does it reencode it before it
> the buffer to disk, or do it first write the raw stream to disk, then
> reencode it to my livetv profile? (I hope it reencodes it before
> writing to disk)
This all depends on your card. For the most part, I don't believe any
real buffering is done, except in RAM, and even then, only if you
have a "dumb" (BT848 style card, or another card with no on-board
video encoder) capture card. If you get a PVR-150, 250, 350, or 500,
all the encoding is done by the card, and the computer pretty much
just exists to write the stream to disk, and play it back for you.
It's highly recommended you get one of those PVR cards, unless, that
is, you hope to do High Def with the machine.
> On another note I'm interested in performance measure myth, so if
> anyone knows a good application for counting cpu-cycles, memory
> requirements, disk usage etc for another application or application
> thread, that would be nice :)
That's pretty much impossible. There are so many variables that go
into Myth's performance, that it is tough to get an accurate
benchmark. This could change if you're using a separate front-end, or
if this is a slave back-end. Or if you're using a PVR-150, or an ATI
TV-Wonder VE. Or if you have 256 MB of RAM, or 512. If you use a
simple IDE hard disk, SATA, RAID 0, RAID 1, NAS, or just another
computer with a big hard disk in it (And then: Samba vs. NFS).
Pretty much, though, you devote an entire computer to Myth. Multi-
tasking isn't what it is best known for.
As far as disk usage, this all depends on your encoding preferences.
At the default settings, a PVR-150 will encode video at about 4
Mibibits/sec (at 720x480), which uses around 2.2 GibiBytes per hour.
However, if you're willing to sacrifice resolution, you can record
video at 2 Mb/s (at 480x480 with the front-end doing deinterlacing)
and disk usage will be closer to 940 MB/h.
Good luck with your system.
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