Raw uncompressed bandwidth of 1080i HD?

electrictroy

Here's what I come up with:
1920x1080 pixels
24 bit color per pixel
1/2 frame sent per second(interlaced)
==
24.9 million bits/second

That doesn't sound right. Help please. Where's the error in my math?

__________________


My Free TV streams 19 Mb/s == 6000 GB/month per channel. No cellphone can do that. WHY kill off this excellent service??

NonMcTubber

Even if I can replicate your math, I am not sure all assumptions are correct.

But the main incorrect assumption is the fact that a digital signal can and is heavily compressed. A 2X compression is about what is normally expected knocking bandwidth down to 12.5 Mb/sec, or enough to allow a multicast or 2. in the total 19.4 Mb/sec OTA bandwidth.

And it may also explain why many choose to broadcast in less bandwidth intensive 720i or P.

And I certainly do not claim any expertise in this subject area, I am confident someone on this forum knows far more.

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DTV_Engineer

A couple things: the most important is you forgot to take the frame rate into account. If you use the figure of 29.97 frames per second and drop your "1/2 frame per second" parameter, your number becomes much closer. (If you are considering 720p, the frame rate becomes 59.94 f/s.)

Also, you will find that HD-SDI (high definition - serial digital interface) uses 20 bits per pixel, not 24: half for luminance, half for chrominance. Finally, there is other data riding along in the video that adds into the rate: embedded audio, captioning, automatic format decriptors, and so forth. The bottom line is that uncompressed 1080i and 720p both run at 1,485 megabits per second; 1080p runs at twice that. If you want more explicit technical details, the standard you are looking for is called SMPTE 292M.

-- Jeff

DTV_Engineer

Actually, 720p and 1080i require a fairly similar amount of bandwidth to transmit with comparable subjective quality, if you factor in both resolution and motion performance.

At the station level, we generally encode for over-the-air transmission in the same format our network uses -- cost and avoiding extra processing and conversion steps are the main reasons, though there are others. At the network level, the format decisions were based largely on the type of programming was perceived to be most important. Back when ABC chose 720p, Monday Night Football was their "premier" live HD program, and using progressive scan was a no-brainer... especially when you factor in slo-mo and overlaid field graphics. Other networks picked 1080i, feeling that resolution was more important than motion for their content.

720i isn't an ATSC standard format... and I've never seen broadcast equipment that offered that as a parameter.

-- Jeff

electrictroy

nonmctubber: I was discussing uncompressed video, not compressed. :-) Oh of course. Duh. Corrected:

1920x1080 pixels
24 bit color per pixel
~60 fields per second
1/2 frame per field(interlaced)
==
1493 million bits/second

That's more realistic and matches the numbers I've found on the internet (1.5 Gbit/s). Thanks for the help. And just for the sake of completion, here's the raw bitrate for 720p/30:

720p
1280x720 pixels
24 bit color per pixel
~30 fields per second
1 frame per field(progressive)
==
664 million bits/second Interesting. 1024 shades of gray and 1024 different colors. That seems rather limiting compared to a 24 bit palette with 16 million defined colors.

__________________


My Free TV streams 19 Mb/s == 6000 GB/month per channel. No cellphone can do that. WHY kill off this excellent service??

electrictroy

I found this interesting table - http://broadcastengineering.com/mag/...atsc_standard/ - It's based upon 10 bits of luma and 10 bits of chroma:

- 720 × 480 x 30 interlaced == 207 Mbit/s (270 with overhead)
1280 × 720 x 30 progressive == 553 Mbit/s (742)
1920 × 1080 x 30 interlaced == 1244 Mbit/s (1485)

And those numbers have to be squeezed down to 19 Mbit/s or less.

__________________


My Free TV streams 19 Mb/s == 6000 GB/month per channel. No cellphone can do that. WHY kill off this excellent service??

rbinck

The broadcasters use 720p/60 not 720p/30.

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DTV_Engineer

Although 720p/60 and 1080i/30 are indeed valid options on our encoding equipment, as a rule broadcasters generally use 720p/59.94 or 1080i/29.97 for over-the-air HD, which reflects what our various networks originate.

These slightly odd numbers correspond with NTSC field and frame rates, and allow standard definition content to be upconverted without having periodic stutter frames.

-- Jeff

DTV_Engineer

It's a minor point of terminology, but fields derive from frames, not the other way around. So we refer to an interlaced format as having 30 frames per second (for example), with two fields per frame.

Progressive scan formats don't break frames into fields, so field rate is meaningless in that context.

That isn't as great a limitation as it would seem; as a practical matter, over a million discrete color values still allows for a much greater level of luminance and chrominance precision than camera sensors and display devices are capable of resolving.

-- Jeff

BrianO

A few points.

The actual frame size for 1080i/p is 1920x1088. The extra 8 scan lines are blank and are discarded by the receiver before the images are sent to the display. They are needed because of the requirements of MPEG compression which require the "dimensions" in both directions to be divisible by 16.

The "divisible by 16" requirement applies to each field in interlaced scanning. Consequently the number of scan lines in an interlaced scan frame must be divisible by 32. 1088 is divisible by 32 making the 1920x1088 frame size compatible with both interlaced and progressive scan.

720i does not exist. 720 is divisible by 16 but is not divisible by 32. Consequently the frame size 1280x720 is compatible with progressive scan but incompatible with interlaced scan.

All interlaced formats in the ATSC 30/60 Hz standard are broadcast at 30 frames per second (or 29.97) OTOH, the progressive scan formats can be broadcast at 24 frames/sec (or 23.976), 30 frames/sec (or 29.97) or 60 frames/sec (59.94), subject only to the overall bandwidth restriction.

The 29.97 frame rate and 59.94 field rate are a throwback to the analogue NTSC colour system. The 0.1% reduction from the integral values based on the AC power transmission rate was needed to eliminate visible anomolies caused by interaction between the colour and audio carriers. This 0.1% reduction is not needed in the ATSC/8VSB standard but broadcasters are free to use it, should they so desire. For all intents and puposes, including bandwidth calculations, the exact frame rates of 24, 30 and 60 are all that need to be considered.

The overall bandwidth restriction eliminates 1080p at 60 frames per second from the ATSC standard but 1080p at 24 fps and 30 fps are valid ATSC OTA broadcast formats that have been part of the standard since Day 1. Disregarding the fact that progrssive scan has been found to be a little easier to compress using MPEG-2, 1080p/30 requires the same bandwidth that 1080i requires and 1080p/24 takes less bandwidth than 720p/60. The oft quoted factoid that 1080p requires too much bandwidth for OTA broadcasting is nothing more than a myth that won't die, propagated by people who do not understand that progressive scan involves multiple frame rates.

DTV_Engineer

You have a point here, but there are several practical issues concerning the different variants of 1080p that need to be kept in mind. 1080p/24 especially, and 1080p/30 to a lesser degree, have readily perceived flicker on a number of displays; more to the point, these aren't formats used by any broadcast networks or syndicators, and much of our HD studio equipment does not even support these frame rates. Are they valid under the ATSC standard? Sure... but as a practial matter they are irrelevant to over-the-air broadcasters.

On the other hand, 1080p/60 represents a qualitative improvement over both 720p and 1080i as generally implemented, and is the frame rate most people infer when you talk about 1080p.

To say that we can't encode 1080p into an ATSC system isn't so much a myth as a shorthand that addresses the only variant someone would likely want to encode, and ignores the others.

-- Jeff

1080PsF

HDD-5, HDCAM, & HDCAM SR all support 1080PsF/23.98, 1080PsF/24, 1080PsF/29.97, & 1080PsF/30. The HDD-5 also support all of those frame rates in total P. So I don't understand your comment about no studio decks support those frame rates.

1080PsF

Are you asking about the total signal or the broadcast signal?
True HD is...

2200 pixels per line in 29.97 Frames per second (that includes active video and horizontal blanking)

1125 lines per frame (that includes active picture and vertical blanking)

29.97 Frames per second

10 bits for luminance

10 bits for chroma

so the math would look like this

1125 * 2200 * 29.97 * 20 = 1,483,515,000 bits or 1.484Gbps

But broadcast is different they stuff that into an MPEG frame.

They use only 8 bit color and only 1088 lines and I think only 1920 pixels.

1088 * 1920 * 29.97 * 16 = 1,001,698,099 bits or 1.002Gbps.

It wouldn't matter if the frame is "i" or "p" it would still be the same.

1080PsF

SD is

858 * 525 * 29.97 * 20 = 269,999,730 bits or 270Mbps

720 HD is

1650 * 750 * 59.94 * 20 = 1,483,515,000 or 1.4835Gbps

1080 30 frame HD is

2200 * 1125 * 29.97 * 20 = 1,483,515,000 or 1.4835Gbps

1080 60 frame HD is

2200 * 1125 * 59.94 * 20 = 2,967,030,000 or 2.967Gbps

DTV_Engineer

I think perhaps you misinterpreted what I wrote. I did not claim that there were no studio VTRs that supported those formats; obviously there are. What I did state is that our facility has quite a bit of studio equipment that does not -- in particular, several graphics generators and some processing gear.

-- Jeff