English NTSC TV's.

[ppppenguin]

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)?

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.

[old_tv_nut]

Originally Posted by ppppenguin

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.

I agree with all of this except the efficiency comparison. DVB-T and ATSC are essentially equal in efficiency for the same robustness level of channel coding (but DVB-T has multiple trade-offs of efficiency vs. robustness available). DVB-T2 uses more efficient error protection coding, which will very likely be adopted for ATSC 3.0 as well.

[ppppenguin]

Originally Posted by old_tv_nut

I agree with all of this except the efficiency comparison. DVB-T and ATSC are essentially equal in efficiency for the same robustness level of channel coding (but DVB-T has multiple trade-offs of efficiency vs. robustness available). DVB-T2 uses more efficient error protection coding, which will very likely be adopted for ATSC 3.0 as well.

Thanks for the info on relative efficiency of the systems. COFDM makes it inherently easy to trade robustness for bit rate. Even to the point where single frequency networks can be used, because co-channel interference can be rejected in the same way as multipath. It's just a matter of choosing a guardband that adequate and accepting the resultant loss of bit rate. SFNs have not been used in the UK. I don't know about elsewhere.

It's worth noting that COFDM isn't needed on satellite broadcasting systems. The channel is inherently not subject to multipath or selective fading. These are the things that are dealt with by COFDM's multiple carriers, each with low bit rate, plus guard bands. A satellite channel has very high losses due to distance, plus rain fade. These cannot be helped by COFDM. There are also rare occasions when the sun aligns with the satellite. Reception is then impossible for a short period.

[wa2ise]

Originally Posted by ppppenguin

8VSB is more resistant to doppler effcts if the TX or RX is moving. Not usually a problem for domestic TVs

Tall towers, that the UHF DTV transmitting antennas are mounted at the top of, tend to sway in the wind. This sway can be as much as a good fraction of a wavelength of the carrier frequency up on UHF. That, combined with some significant ghosts, can make the conditions at a receiver be constantly changing. Which adds extra fun in receiver decoder design...

[ppppenguin]

Originally Posted by wa2ise

Tall towers, that the UHF DTV transmitting antennas are mounted at the top of, tend to sway in the wind. This sway can be as much as a good fraction of a wavelength of the carrier frequency up on UHF. That, combined with some significant ghosts, can make the conditions at a receiver be constantly changing. Which adds extra fun in receiver decoder design...

I think that's only a problem for 8VSB where the equaliser has a lot of work to do. The speed at which the top of the tower moves isn't high enough to give doppler shift that would trouble any plausible COFDM signal.

[old_tv_nut]

Originally Posted by wa2ise

Tall towers, that the UHF DTV transmitting antennas are mounted at the top of, tend to sway in the wind. This sway can be as much as a good fraction of a wavelength of the carrier frequency up on UHF. That, combined with some significant ghosts, can make the conditions at a receiver be constantly changing. Which adds extra fun in receiver decoder design...

Doppler performance in COFDM (DVB) is limited by inter-carrier interference between the many closely-spaced carriers. Single carrier (ATSC) Doppler performance is limited by the tracking ability of the receiver equalizer, which is greatly affected by the frequency of transmission of a reference signal. In ATSC for fixed terrestrial reception, the reference sync segment is transmitted approximately 40 times per second, too slow for mobile reception. The mobile adaptation of ATSC transmits a reference approximately 800 times per second during the mobile portion of the data. COFDM achieves mobile performance by using modes with fewer, more widely spaced carriers, and through the use of reference "pilot" carriers that are inserted with the necessary density in time and frequency.