Why do some channels broadcast in 544x720 or even less?

[CineManno]

I've recently bought a satellite dish and doing some research in video formats. What is striking me is that some channels broadcast in a 544x576 or 480x576 format over the satellite. Why do they do this? I reckon they record in the studio in plain PAL format, i.e. 720x576. So they shrink it to this size before putting it over the satellite. The set top box (sat receiver) obviously has to scale this back again to 720x576 before feeding it to my TV. What is the reason for this shrink and stretch action?

[edDV]

I've recently bought a satellite dish and doing some research in video formats. What is striking me is that some channels broadcast in a 544x576 or 480x576 format over the satellite. Why do they do this? I reckon they record in the studio in plain PAL format, i.e. 720x576. So they shrink it to this size before putting it over the satellite. The set top box (sat receiver) obviously has to scale this back again to 720x576 before feeding it to my TV. What is the reason for this shrink and stretch action?

They do it to conserve bit rate (bandwidth). 544x576 matches typical PAL studio generated standard definition quality. It uses 75% of the pixels used for 720x576. This allows 5 channels @544x576 vs. 4@720x576 at the same data rate.

[CineManno]

Hi,

I would like to understand this in more detail. Suppose I have 2 Mbit/s to spare for a certain channel. Now I have two options:
1) use 720x576 format, encode it, and send it over the satellite and the set top box feeds it directly to the TV
2) downsize it to 544x576, encode it, send it over the satellite, upsize it by the set top box to 720x576 and feed it to the TV.

Why would option 2) provide better visual quality than option 1)?

I understand that spending 2 Mbps for a 544x576 format provides more bits per pixel than spending 2Mbps for 720x576 format. But during the upsizing in the set top box you would lose quality, wouldn't you?

[Alex_ander]

With PAL you resize for display to either 768x576 (4:3) or 1024x576 (WS). 544 is the horisontal number of pixels in a pillarboxed 4:3 image in a WS 720x576 encoding and this ensures the same resolution for 4:3 and 16:9 material on a WS display (stretching factor 1024/720 is close to 768/544). Unlikely 544 is used for encoding WS material.
The reason for a broadcasting company to reduce resolution/bitrate/bandwidth where possible is to place more programs in the payed satellite frequency range.

[jagabo]

The satellite company is trading off sharpness to get less macroblocking.

[RabidDog]

Not to be too picky but

It uses 75% of the pixels used for 720x576. This allows 5 channels @544x576 vs. 4@720x576 at the same data rate.

Even by your own math this should mean that it uses 80% of the bits OR you could get 4 channels for the BW of 3.

But also they could if they wished simply broadcast the 720x576 resolution at a lower bitrate (80%). I prefer jagabo's explanation.

My petard awaits hoisting? 8)

[edDV]

Not to be too picky but

It uses 75% of the pixels used for 720x576. This allows 5 channels @544x576 vs. 4@720x576 at the same data rate.

Even by your own math this should mean that it uses 80% of the bits OR you could get 4 channels for the BW of 3.

But also they could if they wished simply broadcast the 720x576 resolution at a lower bitrate (80%). I prefer jagabo's explanation.

My petard awaits hoisting? 8)

720x576 = 414,720 Bytes for luminance
544x576 = 313,344 Bytes or 75.55% of 414,720 Bytes for luminance

4 frames @ 313,334 = 1,253 Bytes
3 frames @ 414,720 = 1,244 Bytes
so your estimate is closer.

In real satellite and cable systems a process known as "statstical multiplexing" is used to increase the number of channels by distributing bandwidth VBR style so that channels with more motion get more bandwidth while low motion channels get less.
http://en.wikipedia.org/wiki/Statistical_multiplexing
http://www.udcast.com/products/downloads/WP_OSMA.pdf

[RabidDog]

It seems to me, with qam, while that "more motion" thing sounds good, what happens when all the channels are showing hi-motion video ?? give me Hi bitrate HD FTA DVBT or give me death!! a nice political slogan

[edDV]

It seems to me, with qam, while that "more motion" thing sounds good, what happens when all the channels are showing hi-motion video ?? give me Hi bitrate HD FTA DVBT or give me death!! a nice political slogan

That is where it gets statistical. The more channels, the more they average total bandwidth (bitrate) needs to a mean.