brad+myth at templetons
Jan 14, 2007, 1:55 AM
Post #11 of 12
(899 views)
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On Wed, Jan 10, 2007 at 07:15:49PM -0800, Tom Greer wrote:
> Here is a device that takes HDMI in and provides HD video streams:
> http://www.blackmagic-design.com/products/intensity/
>
> Fits in a PCI Express slot.
> Cost: $249
>
> Downsides:
> 1. Output is uncompressed, so there is heavy duty work to compress.
> 2. Supports MS-Windows and Mac OS-X only.
> 3. Does not support HDCP (but then neither does XBox or iTV).
As noted, not going to do you any good on HDCP output, which is what
most set top boxes put out or plan to put out. But I don't know if
anybody has done a survey one what they actually put out.
However, uncompresed HDTV -- that's pretty heavy duty stuff.
1080i at 24 bits/pixel is 1.5 GIGABITS per second uncompressed,
or 187 megabytes/second. No hard disk I know of can write that
fast, in fact you would probably have to get at least 4 in a stripe
array, perhaps even 5, to keep up with that, and an hour of the stuff
is 670 megabytes, so they had better be pretty large disks.
That's because your CPU is not going to be compressing this stuff
in real time. However, one presumes you will have a thread trying
to compress it, and depending on how fast you can get that to go,
then burning a lot of CPU you could get it down to nicd mp4 sizes
before too long, so you only need perhaps just a terabyte of buffer
space. "just a terabyte." Sheesh.
There are things you can do to improve this. You can probably do
_some_ compression in real time. I would venture you could probably
do 4 to 1 or better, in which case you're now down to something you can
write to 2 stripes. Get it down to 8 to 1 and you might be able to use
just a single drive, and have far less buffering worries. Then finish
the job in non-real time.
Or, better news is there will soon be cheap encoding chips on the
market, able to transcode HD in real time and costing $10. Get one
of those and you are in great shape (If you get it unencrypted.)
What you really want is a card with some hardware to do the first
line compression to really work this. And then you will need A2D because
the analog outputs are not encrypted.
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