[mythtv-users] HDMI capture card with linux drivers
mythtv-list at dinkum.org.uk
Fri Apr 16 08:56:04 UTC 2010
On 16 Apr 2010, at 09:03, Raymond Wagner wrote:
> On 4/16/2010 02:15, Douglas Peale wrote:
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>> Raymond Wagner wrote:
>>> On 4/16/2010 01:24, Douglas Peale wrote:
>>>> Your computer could do nothing else but write the disk, so there is no
>>>> possibility of encoding while recording.
>>> Why not? Obviously the BlackMagic Windows software already performs
>>> this impossible task of encoding to MJPEG on the fly.
>>> mythtv-users mailing list
>>> mythtv-users at mythtv.org
>> Unless they are using a raid system, they are not writing the
>> uncompressed file directly to disk.
> Correct, their software compressed to MJPEG for storage in real time. Your original claim was that the CPU would be so busy trying to write to the disk, that it would not have the power to compress the feed to something smaller. That is false.
I've used several of Blackmagic's HDSDI cards, this one is much the same but with HDMI instead of HDSDI input, the software happily compresses to a choice of formats from uncompressed through to h264, there are a couple of blackmagic codecs supplied that match up to the capture formats the card can handle (4:4:4, 4:2:2, 4:2:0, 8bit, 10bit etc.), they appear to be some sort of lossless compression, I'm not certain but I've always thought these used hardware compression on the cards. There's no mpeg2 or h264 hardware but these are broadcast TV cards and no-one would use those codecs for broadcast capture.
And yes you usually need a raid array to capture this card's output, three striped middle of the road 7200rpm disks will cover pretty much everything, I have two Samsung 501LJ's in this machine and they manage ok for 1080i25 at 8 bit 4:2:2 can't do 4:4:4 or 10 bit.
The Linux drivers are news to me but these guys are the most innovative and helpful broadcast capture hardware people I've ever worked with, their hardware is not always top notch but they have added features to drivers for me a couple of times quickly and with no fuss. They were the first to have a capture card that worked with Windows & Mac, everyone else made you buy two cards, with the number of dual boot Mac Pro's around in TV production houses they are really useful. If they have added Linux support across their range that's great, that gives me some ideas...
>> Are you sure they are doing this in software? I'm not aware of any
>> software that can handle HD compression in real time on a PC. Unless of
>> course you don't care about the quality.
> 'HD compression' is an absolutely meaningless term if you don't specify the codec in use. Huffyuv is extremely fast, such that the author's 400MHz Celeron was able to compress DVD video (480p30) in real time. Any Core2 or Athlon64 should be able to handle 1080i60 in real time. MJPEG is almost as fast, if a bit lossy and so less suited for video editing.
Actually various MJPEG flavours used to be used a lot in editing, at very high bit rates, 50Mb/s for SD for example, because each frame is compressed separately simple cuts give no quality loss and for any other process only frames affected need to be de-compressed and re-compressed. These days there are a lot of choice of codecs that compress and de-compress extremely quickly, are low loss for editing or transport across studios, many are based on H264 but bear almost no resemblance to final broadcast H264 that comes OTA or OTW. For example Quicktime Prores is a form of H264 at 140Mb/s or 220M/s for HD, this is commonly used in HDTV editing, this can be produced from one these cards in the background on a Mac Pro while editing is going on.
Bottom line is, this card is not aimed at domestic systems.
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