Quote:
Originally Posted by NewVista
Probably not surprising, but what is wrong with SSB for HF chroma components? as HF Luminance is SSB.
What would be ASTC chroma res (6mhz chnl) vs COFDM chroma res (in 8mhz chnl)?
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All receivers have DSB demodulators for chroma. I haven't done the sums nor have a reference to hand to say what happens when you feed them with a SSB or VSB signal.
ASTC chroma vs COFDM chroma is almost an irrelevant question. Assuming we're talking about standard definition the input to the coder is in each case a standard "601" 4:2:2 signal as defined in SMPTE125M or its Eurpopean eqivalent. The output of the decoder is in the same format. The maximum possible chroma BW is 3.75MHz with a brick wall filter. This is followed by data compession using MPEG. This usually involves subsampling the signal to 4:2:0.
Finally we get to the significant difference between ASTC and DVB, the channel coding. 8VSB for ATSC and COFDM for DVB. Without going into the differences between them or the arguments this has caused it's just a method of carrying a certain bit rate reliably from TX to RX. It has no influence whatsoever on Y or C BW.
Apart from the likely decimation of chroma on the vertical axxis to make 4:2:0, on still pictures what comes out will be very close to what goes in. Any artefacts will depend on how heavily you compress the data. Such artefacts will not normally include any loss of BW. For moving pictures there are additional artefacts which may become visible if too much comrpession is used. Again loss of BW just doesn't happen.
Failure of the channel coding produces different effects. In COFDM this is typically the picture freezing and/or breaking up into blocks. I don't know what happens when 8VSB runs out of eror correction.
Channel width of 6MHz vs 8MHz is simply a consequence of band planning in the repsective countries. It just sets a limit to the bit rate that can be carried using a given channell coding system. I'm not familiar with ATSC but in DVB several programmes will be carreid in each 8MHz channel. These sets of programmes are called multiplexes. The total number depends on how heavily each is compressed and exactly which COFDM modulation schene is chosen. In COFDM parameters such as guard band can be chosen to give higher bit rate or better ruggedness.
Proponents of 8VSB and COFDM modulation have argued the respective merits of their systems but provided you can send the bits from TX to RX without pushing the error correcton over the edge they will have no effect on the pictures. I don't know all the arguments but COFDM is inherently rugged in the presence of multipath while 8VSB needs sophisticated equalisers at the RX which weren't available when it was launched. I think COFDM makes greater demands on TX linearity. The COFDM decoder is more complex as it involves large FFTs. Moore's Law soon dealt with that problem. When you factor in the equalisers needed by 8VSB that probably evens up the complexity. 8VSB is more resistant to doppler effcts if the TX or RX is moving. Not usually a problem for domestic TVs
What is almost certain, but possibly not too important for terrestrial TV, is that COFDM is the most efficient modulation scheme for getting the highest bit rate over a given imperfect channel. It's also very flexible since parameters such as guard band, amount of error correction, bits per symbol and number of carriers can be easily varied without changing the TX or RX. This makes it ideal for ADSL.