Discussion in 'Satellite TV, Sky TV & FreeSat' started by jimmyboy, Jan 22, 2008.
Have a look at these
The basic explanation is
"4:2:2 means the chrominance is horizontally subsampled by a factor of two ... signal at the coder prior to transmission and re-inserting it at the decoder. ..."
Suffice it to say that 4:2:2 transmissions can only be decoded by specialist receivers.
Standard transmissions are 4:2:0
Live feeds that haven't been edited and aren't for the viewing public. Hence the reason why they are encrypted and can only be decoded by specialist receivers (£600 plus).
4:2:2 IS NOT ENCRYPTION, 4:2:2 does NOT mean its unedited.
And no where costs £600 +
Lol. I stand corrected. Are most 4.2.2 transmissions not encrypted ?
The £600 came from the price for a Qualitv receiver, which I thought was the cheapest receiver you could but to receive 4.2.2 transmissions !!
No-Way of knowing what chroma is in use for encrypted feeds but I assume for broadcast it would be 4:2:2 on 7e atleast.
Quali TV boxes go for £60 second hand of ebay now, and a PCI satellite card costs £30 for a computer.
Thanks for the info. I didn't realise Quali tv boxes were so cheap now.
Off to ebay then.
i would recommend getting the skystar 2 pci card to decode these feeds as the tuner in the quali box are useless
To answer the OP's original question.
4:2:2 is the standard system used to sample broadcast video for distribution around a broadcast centre. The 4 refers to the sampling rate that is used for the luminance (Y) i.e. the brightness signal, the 2:2 refers to the sampling frequency used for the two colour difference signals (R-Y/Cr and B-Y/Cb) i.e. the colour/chrominance.
This means that the colour information is sampled at half the resolution horizontally as the brightness - which is fine for most things (though it can cause issues with chroma-keying where you need high resolution colour information) as the eye/brain is less sensitive to colour than brightness. However vertically, in 4:2:2, the resolution is the same for both the luminance and chrominance. This has advantages in broadcast applications - particularly in interlaced systems where the vertical resolution is effectively halved on moving information where there is movement between the two fields within a frame... (*) If 8 bit sampling is used then 4:2:2 subsampling from 4:4:4 reduces the number of bits required to code each "pixel" from 32 to 16.
However for DVD, HD-DVD, BluRay and both SD and HD broadcasts to the home, 4:2:0 is used. This reduces the chrominance resolution vertically as well, by sending just one colour signal - either R-Y or B-Y for each line, and alternating them. (Incidentally the French SECAM colour system did this as well). This means that the vertical chroma resolution is now halved as well giving a balanced horizontal and vertical chroma resolution, and also requiring less space to carry - as now only 12 bits per "pixel" are required - as for every 4 luminance samples (in a 2x2 array) there is only one R-Y and one B-Y sample...
What has this got to do with satellite feeds?
Well 4:2:2 satellite links, using a different MPEG2 Profile to 4:2:0 direct to home links, are used by broadcasters for their high quality links from outside broadcast locations back to their network control areas. These links are likely to be at a higher data rate - say 24-34Mbs rather than the 3-6Mbs used to broadcast to us - and the 4:2:2 video is higher quality, and will survive around a broadcast area in quality for longer.
4:2:2 MPEG2 is not usually decodable by domestic SD satellite receivers - hence it appears to be encrypted - but in fact it isn't always. Some HD MPEG2 receivers can decode SD 4:2:2 as can most PCs with DVB-S cards and the right MPEG2 software codec (with 4:2:2 support - like the Elecard ones?)
Bottom line - 4:2:2 at a high data rate is usually used by broadcasters as it is higher quality - and thus when the signal is recompressed to broadcast to us the quality is higher than if 4:2:0 were used.
(News broadcasts may often use 4:2:0 at lower rates though)
(*) In interlaced systems the resolution of a 50Hz SD the luminance signal can drop from 720x576 to 720x288 on fast moving information, and in 4:2:2 you get a resulting 360x288 chrominance signal. Worst case scenario with 4:2:0 is that you get 360x144 resolution chroma on interlaced fast moving content, which is why 4:2:2 is used within broadcast centres. This is because a 720x576/50i signal behaves like a 720x288/50p signal on fast moving information - the trade-off with any interlaced system is that the vertical resolution and motion resolution are not independent.
That's a great synopsis, Steve.
I just want to add, for anyone buying a secondhand QualiTV box, please check the electrolytics around the power supply area as these fail over time; if you don't have the techncial knowledge, have someone who does do this. I would be inclined to swap all the power supply electrolytics for higher temperature rated ones before putting the box into use.
Thread in USA Satforums section (of which I am a longstanding member) under "Other Receivers" has more about this :
Chris Muriel, Manchester
Very good post explaining the differences between 4:2:2, 4:2:0, etc.
Without going too far off topic, does this apply to HDMI transmissions between devices? Just wondered as I have those options as well as RGB(0-255) and RGB(16-255) on my DVD recorder and when left to negotiate over HDMI with my TV, it defaults to 4:2:2.
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