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High def on a Disk Drive? How can it all fit?
PostPosted: Thu Jun 17, 2004 9:19 pm Reply with quote
Hiro_Protaganist
 
Joined: 17 Jun 2004
Posts: 1




sorry if this has been posted berfore.

My buddy and I are looking into creating a media center computer capable of recording High Def signals. now looking around we found the consesus to be that HD at 1080i is 25mbps and 720p is 19mbps. simple math will tell us that an uncompressed HD signal will require roughly 1.5gb per minute at 1080i. tivo offers a PVR capable of 12hours of HD recording on a 250gb hard drive. It should take a 1.4 terabyte hd to hold this much signal. so i guess what i'm wondering is what kind of compression codec (i think thats the right word) is used and at what cost to the HD signal. any links to sites explaining how the PC is capable of doing this would be greatly appreciated.

thanks,
Hiro
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PostPosted: Fri Jun 18, 2004 10:31 am Reply with quote
Scott Larson
 
Joined: 15 Oct 2003
Posts: 713
Location: Portland, OR




It uses standard MPEG-2 compression, the same system used for DVD's and in digital video cameras and in countless other applications. There are MPEG compressor chips that require very very little CPU power. The CPU in the original Tivo runs at some laughingly slow speed and lets the MPEG hardware do all the hard work.

MPEG compression doesn't affect the picture that much. DVD's look great. NTSC rarely looks very good to begin with and it doesn't look much worse after a Tivo has compressed it.

HDTV is different in that it's broadcast as a MPEG-compressed stream since there's no practical way to broadcast uncompressed HDTV. That means you need no elaborate MPEG hardware to to record HDTV -- simply save the bytes to a file as they come in. A raw ATSC stream will use about 2 Mb of disk space a second which adds up to 7 or 8 Gb per hour. A 250Gb disk is more than enough space to save 12 hours of raw ATSC streams. You can use even less space by extracting only the video and audio streams you want to save from the ATSC stream (that's what dtvstream does).
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PostPosted: Mon Jun 21, 2004 2:13 pm Reply with quote
Spinoza
 
Joined: 08 Jan 2004
Posts: 27




Quote:

HD at 1080i is 25mbps and 720p is 19mbps. simple math will tell us that an uncompressed HD signal will require roughly 1.5gb per minute at 1080i.


Scott larson is correct about the Tivo and how HDTV is transmitted. I think he just missed the problem in your train of thought here.

You got your simple math wrong Wink 1080i isn't 25mbps it's closer to 20mbps.. but we'll use your numbers. 25 mpbs is a transmission rate, read as 25 Mega*bits* per second. So simple math gives us 1500 Mega*bits* per minute. Dividing that by 8 bits per storage byte gives us 188 Mega*Bytes* per minute, or about 11 gigs an hour or so. Since it's closer to 20mbps, it's closer to 7-8 gigs an hour.

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-vito
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PostPosted: Wed Jun 23, 2004 10:24 am Reply with quote
Scott Larson
 
Joined: 15 Oct 2003
Posts: 713
Location: Portland, OR




At least my final number was correct! Smile

Once anyone starts talking about storage space instead of transmission speeds I immedately assume they're talking bytes and I usually don't notice if it's a big B or a little b. Also when they say "uncompressed HD", I assume they're talking about raw video coming from an HD camera, not recompressed MPEG which I think Hiro was talking about now that I read it that way.
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High def on a Disk Drive? How can it all fit?
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