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Old 02-02-2013, 09:46 AM   #31
Rob Helmerichs
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Quote:
Originally Posted by HerronScott View Post
The 720p programs take up less space on my TiVo's versus the 1080i. why do you think they would be the same size?

Scott
Mathematically, a 720p image should be 11% smaller than a 1080i. I don't know how much of an impact various compression schemes can have, but I wouldn't be surprised if that were a bigger factor than 720p vs 1080i in final file size...

The reason those two formats were chosen for HD broadcast standards is because they're pretty much the same bandwidth...with different quality trade-offs to get there.
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Old 02-03-2013, 07:32 AM   #32
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Quote:
Originally Posted by Rob Helmerichs View Post
Mathematically, a 720p image should be 11% smaller than a 1080i. I don't know how much of an impact various compression schemes can have, but I wouldn't be surprised if that were a bigger factor than 720p vs 1080i in final file size...

The reason those two formats were chosen for HD broadcast standards is because they're pretty much the same bandwidth...with different quality trade-offs to get there.
Hi Rob,

How are you calculating 11% smaller? A single frame of 1080i (1920x1080) has 2,073,600 pixels while a single frame of 720p (1280x720) has 921,600 pixel per frame. That's over twice as large uncompressed.

Of course cable feeds are compressed (and more so than OTA). On my local Comcast channels, Fox and ABC 1 hour shows at 720p are 2.5-3.5GB on the TiVo while 1 hour shows on the other 1080i stations are 4.5-5.5GB depending on the channel.

I had not looked at file sizes in some time and I see that Comcast is doing even more compression than they use to as I seem to recall the 1080i stations being over 6GB in size for an hour shown in the past.

Scott
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Old 02-03-2013, 07:38 AM   #33
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Originally Posted by HerronScott View Post
How are you calculating 11% smaller? A single frame of 1080i (1920x1080) has 2,073,600 pixels while a single frame of 720p (1280x720) has 921,600 pixel per frame. That's over twice as large uncompressed.
No, a single frame of 1080i only has half of the image (that's where the "i" comes in...interlacing alternate frames to get the whole image).
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Old 02-03-2013, 08:04 AM   #34
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No, a single frame of 1080i only has half of the image (that's where the "i" comes in...interlacing alternate frames to get the whole image).
My understanding is that for interlaced standards, the field rate and frame rate are different. In order to achieve the same same final 30fps *frame* rate with an interlaced standard, the field rate is double so 60 fields per second. Each field as you mentioned is half the full frame (540 lines) for interlaced. For progressive standards, the field rate and frame rate are the same.

Even given the varying compression the cable companies apply, you can still see this by the 1.5 to 2 times file sizes of 1080i content versus 720p content.

Scott
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Old 02-03-2013, 11:00 AM   #35
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I would like to thank you all for the education. I felt, using TiVo Desktop, that resolution and content were important considerations for file size. The OP wanted to save space and, like my cable feed, has the option of the same network in SD and HD. I don't use my TiVo for storage and offload anything I want to save. I appreciate your math and logic that convinces me that all storage is not the same. I also saw this with the CM-7400, TViX and my Sony DHG. Have a nice day.
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Old 02-03-2013, 01:47 PM   #36
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Originally Posted by HerronScott View Post
My understanding is that for interlaced standards, the field rate and frame rate are different. In order to achieve the same same final 30fps *frame* rate with an interlaced standard, the field rate is double so 60 fields per second. Each field as you mentioned is half the full frame (540 lines) for interlaced. For progressive standards, the field rate and frame rate are the same.

Even given the varying compression the cable companies apply, you can still see this by the 1.5 to 2 times file sizes of 1080i content versus 720p content.

Scott
I can't contribute any expert knowledge to this discussion, but my understanding has been the same as Rob's is (apparently), which is that for progressive scan the frame rate is the full 60 frames per second. If that isn't the case, surely 1080p60 could be transmitted over-the-air and a TiVo could record it. Neither is possible because 1080p60 would require twice as much bandwidth as 1080i60. I've read that, and it makes sense. If H.264/AVC is used then 1080p60 broadcasting could be done because H.264 compresses better than MPEG-2 by about 2 to 1, but most TV receivers in the field today can't handle H.264. Am I wrong about all of this? (Stranger things have happened.)
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Old 02-03-2013, 01:51 PM   #37
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I'm no expert, so if my understanding of 720p vs. 1080i is faulty or incomplete, I'd love to be educated.
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Old 02-03-2013, 02:04 PM   #38
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Progressive scan and frames per second are not tied to each other. You can have 1080p24 1080p50, or 1080p60.

The ATSC standard actually allows for H.264 1080p50 and 1080p60, but nobody uses it because receivers only work with MPEG-2. Thus you get the chicken and the egg problem, since no broadcaster will upgrade to H.264 when nobody can receive it, and nobody will make a receiver that can receive H.26 when nobody is broadcasting it.

You're stuck with 720p60 or 1080i60 on the majority of broadcasts in the US, at least until the next round of updates, probably to 2160p.
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