Reader Feedback: The red phenomenon
Mar 1, 2005 12:00 PM
The red phenomenon
Hi Michael:
I am looking for information with regards to the phenomenon of red tearing/blooming on DTV receivers.
Certain channels/transponders appear to exhibit the problem more than others. What is the cause of this? I know red is one of the more difficult colors to truly represent in the NTSC domain due to bandwidth limitations within the NTSC domain.
Now, with digital video at 4:2:2, for example, there is more chroma sampling/bandwidth available than ever before. Is it a codec issue within DTV systems or the FTA RXs? Or have some DTV broadcasters exceeded the NTSC color gamut, which then has overtasked the encoder/decoders?
Looking forward to hearing from you. Cheers,
Jerry Radford
APTN Broadcast services
winnepeg, canada
Michael Robin responds:
Jerry:
Your problem seems to be related to a distribution system, which has some incompatible elements. Solving single-pass analog system problems is relatively simple because of the known (supposedly) types of video distortions, which create well understood (in most cases) picture impairments. Solving problems affecting a complex system, comprising several different analog and digital technologies, as, apparently, the case you are referring to, requires a clear understanding of the signal path, from signal source (i.e. camera) to the receiver. The approach would be the following:
Draw a block-diagram showing all the system path elements.
Ensure that the elements are compatible. If this is not the case, redesign the system. Compatibility failure may result in signals being “illegal” when transformed into another format.
For every system element, list the performance indicative parameters and their numerical values as per the manufacturer's specifications. Verify that each component meets these specifications. Failure to meet them may result in the signals being “illegal.” Bear in mind that different technologies (i.e. analog composite, analog component, digital video and compressed video) are tested with different types of signals, methods and test equipment.
Eventually, all elements are compatible, meet the manufacturer's specifications and are assembled into a working system.
If you are lucky, the system works, or seems to work. Some people might stop here and go home, but they would have missed an important point: To ensure that the system works well under all predictable conditions, an end-to-end system acceptance test using stressing signals has to be carried out. If everything doesn't work out, the system has to be redesigned to meet the expected performance and then checked again.
If you are not carrying out acceptance tests, then you are taking a chance. The system's performance may be inadequate under some conditions, which you have omitted to generate or simulate, and you will get a call-back.
As to the problem that you are concerned with, be advised that red is no more difficult to encode in NTSC than any other color in the spectrum. I have encountered saturated red conditions when a SONY BETACAM field recording from a war-torn area of the world is brought to a European Production Center to feed an MPEG encoder. Now, if the local MPEG encoder has analog component inputs, they are normalized to the EBU N10 format i.e., the B-Y and R-Y components have a value of 700 mV p-p for a 100-percent saturated color bar signal.
The SONY BETACAM recorder B-Y and R-Y outputs would in all likelihood be normalized to the SONY North American level of 920 mV p-p for a 100-percent saturated color bars signal. The visible result would be oversaturated (purple) faces instead of pink faces. This should never occur in a normally operating Teleproduction Center, especially if it is well designed, maintained and operated.
I hope I have answered some of your questions.
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