CBC HD Switches To 720p From 1080i - Page 5 - Canadian TV, Computing and Home Theatre Forums

TheBear · 2011-09-04, 01:36 PM

Some insightful comments in this thread, particularly from posters who are familiar with the back end of the broadcast production facilities. Thank you.
I guess I am one of those not-so-average viewers who can spot a properly produced and transmitted 1920 X 1080 image over a 1280 X 720 image without too much effort, even on a smallish 1080 screen. I wonder how many people are running older (or less expensive) receivers that have a native display resolution of 1280 X 720 and thus fail to see the difference in resolution? Conversely, most average and all high-end TV sets these days have a native pixel resolution of 1920 X 1080, so anything with a lower resolution requires pixel interpolatation by the receiver, resulting in degradation of display quality, no matter how good the 1280 X 720 source is to begin with.

I have contacted CBC via their web interface and have asked for an explanation why CBUT (the Vancouver transmitter) was changed from 1080i to 720p. I have scoured the web unsuccessfully for some sort of official rationale from CBC.

MAXAM · 2011-09-04, 06:50 PM

I seem to recall reading that Shaw Cable converts all 720p broadcasts from ABC and FOX to 1080i. On the topic of CBC switching to 720p, does this now mean my 1080p enabled set will be displaying a 720p picture or does it fill in the missing pixels to bring it up to 1080p?

JamesK · 2011-09-04, 10:19 PM

^^^^
Either the STB or TV can convert between 720 & 1080 lines. I have my STBs set to do the conversion, as when the TV does it, it takes a couple of seconds for the TV to switch modes. Eitherway, you'll see the full 1080 lines being used, though the effective resolution is still 720.

MAXAM · 2011-09-04, 10:35 PM

Thanks JamesK! i think I preferred it when CBC HD was 1080i and my set would automatically convert it to 1080p. The P.Q. was excellent!

snorlax · 2011-09-05, 12:54 AM

This is speculation but I suspect the downgrade to 720p and the coincidental appearance of a 720p in the clear satellite feed of 5 CBC stations are related. See the FTA satellite thread. CBC is probably trying to save money by muxing these 5 channels on 1 transponder thus saving the cost of fibre from Toronto to each of the stations. They could not mux 5 1080 feeds on the same transponder and maintain good HD quality because the required Bit Rate would exceed the bandwidth capability of the transponder. The viewer impact will be minor but the savings significant. I'm sure CBC Toronto is still 1080i in house and will continue to feed Toronto and nearby cable systems, Bell TV (also in Toronto), Shaw Direct (in Mississauga) and their OTA transmitter with that 1080i signal since there would be no savings involved in making that switch but I expect all CBC English stations any distance from Toronto will start using that 720p satellite feed and will switch to a 720p format.

987654321 · 2011-09-05, 06:37 AM

Quote:

CBC is probably trying to save money by muxing these 5 channels on 1 transponder thus saving the cost of fibre from Toronto to each of the stations.

I suspect that you are 100% correct. In addition, the data rate makes this HD Lite - formerly just a Bell specialty in Canada. Similar to a OTA HD feed with 3 SD subchannels.

JamesK · 2011-09-05, 07:45 AM

Quote:

They could not mux 5 1080 feeds on the same transponder and maintain good HD quality because the required Bit Rate would exceed the bandwidth capability of the transponder.

How much bandwidth does 720p save vs 1080i? A 1080i frame has 2.25x the number of pixels, but 720p has to send it's frames twice as often. This means 1080i has only 12.5% more pixels sent per unit time. This is also before other signals, such as audio, are considered, so the bandwidth difference would be even less.

FranKllr · 2011-09-05, 10:28 AM

JamesK, 2.25x is wrong when comparing to interlaced. 1080i60 has 60 1920x540 frame, they usually call them field when talking about interlaced media, over a second.

Pixel wise over a second, I calculated 11% less in post #36.
1280x720x60 = 55 296 000
vs
1920x1080x30 or if you prefer 1920x540x60 = (62 208 000)

Progressive frame suppose to achieve better compression. Might save more then 11% on data requirements.

JamesK · 2011-09-05, 10:48 AM

That why I said "but 720p has to send it's frames twice as often". 1080i sends 30 full frames (not fields - 540 lines, it does 60 of those) per second, while 720p sends 60. With interlace, you have twice as many fields as frames, but with progressive, fields and frames are identical. So, to do the math, with 1080i, you send 1080 x 1920 x 30 = 62208000 pixles/second and with 720p, 720 X 1280 x 60 = 55296000, which gives a pixel ratio of 1.125:1 for 1080i vs 720p. In other words, 1080i video alone reguires a 12.5% bandwidth increase over 780p. However, the audio content and other data would be the same for either mode, which means the actual, over the air, signals would have a difference of somewhat less than 1.125:1. I can't speak to the compression differences, but it still isn't a significant difference between 1080i & 720p. Whether 12.5% or 11%, does it make so great a difference that you can squeeze in an extra channel? For 12.5%, you'd need to have 8 channels (almost 9 at 11%) for the difference to be sufficient to squeeze in one more. Again, this is before audio etc., which would further reduce the advantage of 720p.

skink · 2011-09-05, 11:19 AM

What JamesK is saying is what i read years ago saying that a broadcaster would be better serve bandwidth wise by using 720p.Any hd format is better than 480i anyday!

FranKllr · 2011-09-05, 11:29 AM

Ah yes your are right about the 12.5%.

When searching wikipedia on Interlaced video, I came across something suprising.

Benefits of interlacing

Quote:

However the bandwidth benefits only apply to analog or uncompressed digital video signal. With digital video compression, as used in all current digital TV standards, interlacing introduces some additional inefficiencies, and so the transmission bandwidth savings of interlaced video over fully progressive video are minimal even with twice the frame rate, i.e. 1080p50 signal produces roughly the same bit rate as 1080i50 signal.[3][4]

DrSat · 2011-09-05, 11:40 AM

Quote:

Originally Posted by 987654321

I suspect that you are 100% correct. In addition, the data rate makes this HD Lite - formerly just a Bell specialty in Canada. Similar to a OTA HD feed with 3 SD subchannels.

Regardless of the fact that it is in 720p, the C-band HD feed actually looks slightly better in quality than the 1080i OTA signal from Toronto after comparing both using the same TV. It is certainly not as compressed as the HD channels off of Bell TV or even as an "OTA channel with 3 SD subchannels" so it can't be considered "HD lite".

Comparing the data rate alone is like comparing apples and oranges as the C-Band HD feed uses MPEG 4 compression while OTA only uses MPEG 2 compression. Even at the exact same bitrate, MPEG 4 has much higher quality than MPEG 2

At least they are not rebroadcasting the HD feed off of Bell TV, that would truly make it HD lite!

robman50 · 2011-09-05, 11:51 AM

CBC from Toronto is 1080i.

majortom · 2011-09-05, 12:02 PM

rob, the CBC feeds on CBAND satellite are 720P, including CBLT from Toronto.
That's what DrSat is comparing, 1080i OTA vs 720P Satellite. I'd have to agree with him,
the 720P on satellite is quite good.

some other notes on the CBAND 720P feeds:
The CBC satellite feeds are all running 11.3 Mbps per channel, Constant Bit Rate using MPEG 4 compression (H.264). There is ~ 66 Mbps of bandwidth available on a satellite transponder at the 30000 symbol rate & Modulation scheme they're using. Contrast to NBC who uses a similar capacity CBAND transponder to send EAST, Central, Mountain, West in HD, and Universal Sports in SD to their affiliates. NBC uses H.264 compression, 1080i for the HD, but they use a Variable Bit Rate encoder, plus there isn't programming on each channel at the same time 24/7 due to the time shifting / programming schedules. They're both good strategies resulting in quite good quality.
So I'd say CBC is using it quite efficiently.
If the leasing cost of a single CBAND Transponder is on the order of $250 / hr (rough guess from googlin around), that shakes out to roughly ~ $2.16M / year. Toss in the French transponder so double that.

If those numbers are anywhere near reality, that's quite a commitment if ya ask me. Not to mention they're covering the entire country with it.

robman50 · 2011-09-05, 12:24 PM

Oh okay I understand now.

So its for C-Band then. That's FTA right?

2011-09-04, 10:19 PM	#63
JamesK Veteran Join Date: May 2009 Location: Mississauga Posts: 5,089	^^^^ Either the STB or TV can convert between 720 & 1080 lines. I have my STBs set to do the conversion, as when the TV does it, it takes a couple of seconds for the TV to switch modes. Eitherway, you'll see the full 1080 lines being used, though the effective resolution is still 720. __________________ The following program contains immature subject matter. Viewer discretion is advised.

2011-09-05, 12:54 AM	#65
snorlax Join Date: Jun 2009 Location: Thornhill, Ontario Posts: 396	This is speculation but I suspect the downgrade to 720p and the coincidental appearance of a 720p in the clear satellite feed of 5 CBC stations are related. See the FTA satellite thread. CBC is probably trying to save money by muxing these 5 channels on 1 transponder thus saving the cost of fibre from Toronto to each of the stations. They could not mux 5 1080 feeds on the same transponder and maintain good HD quality because the required Bit Rate would exceed the bandwidth capability of the transponder. The viewer impact will be minor but the savings significant. I'm sure CBC Toronto is still 1080i in house and will continue to feed Toronto and nearby cable systems, Bell TV (also in Toronto), Shaw Direct (in Mississauga) and their OTA transmitter with that 1080i signal since there would be no savings involved in making that switch but I expect all CBC English stations any distance from Toronto will start using that 720p satellite feed and will switch to a 720p format. __________________ Rogers Employee. My posts should not be considered as coming from an authorized corporate source.

2011-09-05, 10:48 AM	#69
JamesK Veteran Join Date: May 2009 Location: Mississauga Posts: 5,089	That why I said "but 720p has to send it's frames twice as often". 1080i sends 30 full frames (not fields - 540 lines, it does 60 of those) per second, while 720p sends 60. With interlace, you have twice as many fields as frames, but with progressive, fields and frames are identical. So, to do the math, with 1080i, you send 1080 x 1920 x 30 = 62208000 pixles/second and with 720p, 720 X 1280 x 60 = 55296000, which gives a pixel ratio of 1.125:1 for 1080i vs 720p. In other words, 1080i video alone reguires a 12.5% bandwidth increase over 780p. However, the audio content and other data would be the same for either mode, which means the actual, over the air, signals would have a difference of somewhat less than 1.125:1. I can't speak to the compression differences, but it still isn't a significant difference between 1080i & 720p. Whether 12.5% or 11%, does it make so great a difference that you can squeeze in an extra channel? For 12.5%, you'd need to have 8 channels (almost 9 at 11%) for the difference to be sufficient to squeeze in one more. Again, this is before audio etc., which would further reduce the advantage of 720p. __________________ The following program contains immature subject matter. Viewer discretion is advised.

2011-09-05, 12:02 PM	#74
majortom Veteran Join Date: Jan 2009 Posts: 2,615	rob, the CBC feeds on CBAND satellite are 720P, including CBLT from Toronto. That's what DrSat is comparing, 1080i OTA vs 720P Satellite. I'd have to agree with him, the 720P on satellite is quite good. some other notes on the CBAND 720P feeds: The CBC satellite feeds are all running 11.3 Mbps per channel, Constant Bit Rate using MPEG 4 compression (H.264). There is ~ 66 Mbps of bandwidth available on a satellite transponder at the 30000 symbol rate & Modulation scheme they're using. Contrast to NBC who uses a similar capacity CBAND transponder to send EAST, Central, Mountain, West in HD, and Universal Sports in SD to their affiliates. NBC uses H.264 compression, 1080i for the HD, but they use a Variable Bit Rate encoder, plus there isn't programming on each channel at the same time 24/7 due to the time shifting / programming schedules. They're both good strategies resulting in quite good quality. So I'd say CBC is using it quite efficiently. If the leasing cost of a single CBAND Transponder is on the order of $250 / hr (rough guess from googlin around), that shakes out to roughly ~ $2.16M / year. Toss in the French transponder so double that. If those numbers are anywhere near reality, that's quite a commitment if ya ask me. Not to mention they're covering the entire country with it. __________________ uhf/vhf combo rotor chimney mount homebrew vhf hi hpf ap-8700 preamp 4way split lg lcd dtt901 pctuner mythtv

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2011-09-04, 01:36 PM	#61
TheBear Rookie Join Date: Jan 2008 Location: Cloverdale, Surrey, BC Posts: 27	Some insightful comments in this thread, particularly from posters who are familiar with the back end of the broadcast production facilities. Thank you. I guess I am one of those not-so-average viewers who can spot a properly produced and transmitted 1920 X 1080 image over a 1280 X 720 image without too much effort, even on a smallish 1080 screen. I wonder how many people are running older (or less expensive) receivers that have a native display resolution of 1280 X 720 and thus fail to see the difference in resolution? Conversely, most average and all high-end TV sets these days have a native pixel resolution of 1920 X 1080, so anything with a lower resolution requires pixel interpolatation by the receiver, resulting in degradation of display quality, no matter how good the 1280 X 720 source is to begin with. I have contacted CBC via their web interface and have asked for an explanation why CBUT (the Vancouver transmitter) was changed from 1080i to 720p. I have scoured the web unsuccessfully for some sort of official rationale from CBC.

2011-09-04, 06:50 PM	#62
MAXAM Join Date: May 2005 Posts: 1,083	I seem to recall reading that Shaw Cable converts all 720p broadcasts from ABC and FOX to 1080i. On the topic of CBC switching to 720p, does this now mean my 1080p enabled set will be displaying a 720p picture or does it fill in the missing pixels to bring it up to 1080p?

2011-09-04, 10:35 PM	#64
MAXAM Join Date: May 2005 Posts: 1,083	Thanks JamesK! i think I preferred it when CBC HD was 1080i and my set would automatically convert it to 1080p. The P.Q. was excellent!

2011-09-05, 10:28 AM	#68
FranKllr Join Date: Sep 2009 Location: Shawinigan, QC Posts: 84	JamesK, 2.25x is wrong when comparing to interlaced. 1080i60 has 60 1920x540 frame, they usually call them field when talking about interlaced media, over a second. Pixel wise over a second, I calculated 11% less in post #36. 1280x720x60 = 55 296 000 vs 1920x1080x30 or if you prefer 1920x540x60 = (62 208 000) Progressive frame suppose to achieve better compression. Might save more then 11% on data requirements.

2011-09-05, 11:19 AM	#70
skink Rookie Join Date: Jan 2009 Posts: 19	What JamesK is saying is what i read years ago saying that a broadcaster would be better serve bandwidth wise by using 720p.Any hd format is better than 480i anyday!

2011-09-05, 11:51 AM	#73
robman50 Join Date: Feb 2011 Posts: 166	CBC from Toronto is 1080i.

2011-09-05, 12:24 PM	#75
robman50 Join Date: Feb 2011 Posts: 166	Oh okay I understand now. So its for C-Band then. That's FTA right?