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  #61  
Old 09-09-2014, 12:12 AM
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Originally Posted by ppppenguin View Post
Poynton,..says that the practical [NTSC] broadcast chroma BW is only about 600kHz.
.
Probably not surprising, but what is wrong with SSB for HF chroma components? as HF Luminance is SSB.

What would be ATSC chroma res (6mhz chnl) vs COFDM chroma res (in 8mhz chnl)?

Last edited by NewVista; 09-09-2014 at 08:41 AM.
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  #62  
Old 09-09-2014, 01:11 AM
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Originally Posted by NewVista View Post
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.
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Last edited by ppppenguin; 09-09-2014 at 01:18 AM.
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  #63  
Old 09-09-2014, 08:30 AM
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Well I'm thinking now in terms of HD quality (assuming no multiplexing, which they love to do) in majority of COFDM countries with 7 or 8 MHz channels. And comparing DVB-T vs 6mhz ATSC in terms of ratio of assigned resolution of Y/C - and if it still reflects the old theory that "Chroma res doesn't matter that much" - whereas we see picture simulations of progressively higher C res and, what do you know, it does matter!

So, it seems to turn out Y/C quality is more a function of sampling formats than given availability of luxurious broadcast bandwidth (DVB-T 8mhz channel)? - was curious about this. And some mastering formats have Y=C!!

The upshot of this being that the philosophy of desired chroma res has evolved from as low as one-fifth luminance to as high as full luminance res!

Last edited by NewVista; 09-09-2014 at 08:49 AM.
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  #64  
Old 09-09-2014, 11:27 AM
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Please re-read the relevant part of my last post.

Can I reiterate the point that RF channel BW is almost irrelevant in a digital world. Obviously the wider it is, the more bits per second you can carry for a given error rate. Hence for a given compression system and modulation method you can carry more programmes in a wider RF channel. If you use a more modern compression system such as MPEG4 you can carry more channels or get higher quality or any tradeoff you like between them. Likewise if you use a more efficient channel coding scheme. I'm not sure how 8VSB stacks up against COFDM but I know it's not readily feasible to have a more efficient scheme than COFDM. Again I'll re-itereate that the only purpose of channel coding is to get all the bits from the TX to RX with acceptably low error rate and in an acceptable amount of RF bandwidth.

Multiplexing is simply a means of conveniently utilising the carrying capacity of the RF channel. A single 8MHz channel (or 6MHz channel for that matter) has ample capacity to carry a number of MPEG2 compressed SD programmes or a smaller number of MPEG4 compressed HD programmes. The exact number depends on how hard you are willing to compress the video. A single programme carried in that much RF BW would be a gross waste of spectrum. I don't know if multiplexing is used with 8VSB. If in the US you are using 6MHz of RF to convey a single programme then that's gross waste of RF spectrum. Maybe you've got more of it over there than we have in Europe I know the real reason why multiplexing is unpopular in the US. It's a socaialist plot to force the sharing of transmitters between competing TV stations.

Chroma BW is a decision made at the start of the compression process. Since the source material will usually be 4:2:2 (for SD) or multiples thereof for HD horizontal chroma res will be half of Y res. Vertical chroma res will be the same as Y res. Which is why it's usual to reduce vertical C res to give a 4:2:0 picture at the input to the compressor. Or multiples thereof for HD.

It is possible to use full BW chroma, the SDI specs and MPEG specs have options for 4:4:4 and multiples. While this may be useful in film production it has no place in transmission.

The choice of half res chroma was made back in the early days of digital experiments. I think it was settled as 4:2:2 around 1990. Can't be bothered to dig out the papers. A lot of work was done with chroma a third of luma BW but this was considered inadequate for downsteam processing and chroma key. It's proven to be a good and practical choice though a few users may have a need for full BW chroma.
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Last edited by ppppenguin; 09-09-2014 at 11:37 AM.
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  #65  
Old 09-09-2014, 03:28 PM
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Quote:
Originally Posted by ppppenguin View Post
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.
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  #66  
Old 09-09-2014, 03:44 PM
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Quote:
Originally Posted by NewVista View Post
...what is wrong with SSB for HF chroma components? as HF Luminance is SSB.
Basic color analog signal theory:

A single sideband part of the signal produces a quadrature signal component (all frequency components shifted by 90 degrees). A synchronous demodulator will ignore this component if a single SSB signal is transmitted.
However, the chroma signal has two signals transmitted at 90 degree phase difference. Each synchronous chroma demodulator then ignores any quadrature component of its desired chroma component (e.g., R-Y), but sees the quadrature component of the other chroma component (e.g., B-Y).

This is why the original NTSC specs extended only one component (I) into a vestigial sideband region. The I demodulator sees the lower I sideband, and no Q quadrature high frequencies are present because they aren't transmitted; the Q demodulator is narrowband and therefore does not see the quadrature components due to the wideband I signal.

Transmitting wideband on both chroma axes [edit: and then cutting off part of the upper sideband] introduces quadrature distortion of higher frequency chroma, but since receivers are commonly narrowband, they do not see this distorted color detail (or any color detail!). In PAL, some quadrature distortion is tolerable because of the cancellation of phase errors, so more detail can hypothetically be squeezed out of the lower sideband of non-symmetrical chroma sidebands.

Last edited by old_tv_nut; 09-09-2014 at 03:49 PM.
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  #67  
Old 09-09-2014, 03:53 PM
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A note about analog inputs: if a composite signal with wideband chroma is fed to the demodulator directly without going through the RF and IF stages that cut off the upper chroma sidebands, wideband chroma can be demodulated without quadrature distortion. Since the chroma filters in analog sets are not strictly brick-wall, you can typically see some improvement in color detail rendition when using a composite or S-video input instead of RF.
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  #68  
Old 09-09-2014, 04:02 PM
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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...
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Old 09-09-2014, 04:09 PM
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Originally Posted by wa2ise View Post
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.
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Old 09-09-2014, 04:18 PM
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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.
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Old 09-09-2014, 04:20 PM
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Originally Posted by wa2ise View Post
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.
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  #72  
Old 04-04-2015, 02:27 PM
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Various notes

Just had the leisure to check out this thread, and perhaps some collected notes could be of interest also months later:



Quote:
Originally Posted by ppppenguin View Post
PAL was sometimes called "Peace At Last" in the UK.
Another one, I don't know where it originated: "Pay for Additional Luxury".

Quote:
SECAM is utterly different to NTSC except for the use of colour difference signals. It's also a horror story for anything byond simple cuts in the studio. Even a fade requires horrible processes that degrade the picture.
In 1977 a vision mixer had been introduced that avoided the separate chroma path with a process called Amplitude Modulated Chrominance, using an internal 5.75 MHz carrier. This was an invention of ľ Bosch. Seems that they at Thomson-CSF were not too happy about the best SECAM vision mixer (and other good SECAM gear as well) being Made in Germany.

But in practice more and more PAL gear came into use also at SECAM stations. Inavoidable result was at least a final PAL-SECAM conversion, and also cascades of SECAM-PAL-SECAM or even more steps were not uncommon. It is my impression that this did much more harm to the picture quality than the specific weaknesses of the SECAM system (which appear to be overemphasized thanks to clever PAL marketing, just as it is the case with NTSC).

Quote:
There was no need for Sony to reverse engineer PAL. The PAL system was described in the Bruch/Telefunken patents. What Sony did was treat PAL as a sort of NTSC to navigate round the patents.
I was not aware of this aspect of PAL patents so far. Makes me wonder how Comecon manufacturers like Sta▀furt and Tesla (these two definitely made TV sets with PAL decoders) handled it.



Quote:
Originally Posted by Colly0410 View Post
& probably 819 lines then convert to 625 lines, both with positive modulation & AM sound..
And thus even after going to 625 lines being incompatible to the rest of the world.

"SECAM-capable" TV sets were common in West Germany, but I understand that this was just good for East German TV (and French forces TV in Berlin) while only real, expensive multinorm sets (usually also being capable of NTSC-M, put on air in Germany by AFN) could receive the crazy French "L" system.

This led to a rather widespread misbelief that "French SECAM" is different from "East Bloc SECAM". Of course it was the same SECAM III B, and I know a TV engineer who liked to provide evidence of this to surprised layman by tuning into an analogue satellite signal from France and hooking an old Sta▀furt set to the modulator output of the receiver.



Quote:
Originally Posted by wa2ise View Post
If the burst changed frequency line to line, it was SECAM. If the burst frequency was steady, and line to line phase was 180 degrees flip. it had to be NTSC, if it was (something like +45 and -45 IIRC), it was PAL.
Reminds me of a behaviour of German (West as well as East) TV sets with decoders for both PAL and SECAM: At times the PAL decoder opened also on a SECAM signal, resulting in a rainbow picture. A screen shot of this phenomenon produced a rumour of a PAL test being done with the Inselsberg transmitter. Really just a poor rumour, because there was nothing to test here at all: The transmitter would have simply swallowed the PAL burst.



Quote:
Originally Posted by dtvmcdonald View Post
The 50Hz I have seen has all been in 50Hz South American countries, Malaysia,
and England.

I was in Malaysia at the height of the CRT to flat screen "always on"
transition and the flicker difference between
CRT and LCD was amazing.
I know that I'm not the only one who now, when occasionally seing an old CRT TV (not 100 Hz technology), is wondering how we could have beared this flicker at all.

Quote:
Of course, given the terrible 50HZ flicker problem, perhaps
European producers avoided white screens even more
than ours did
I would say no, they did not.



Quote:
Originally Posted by ceebee23 View Post
BUT how would 525/60 PAL look
I.e. what is being transmit in Brazil. That's something I'm wondering about for a long time. And the same goes for the approach of Paraguay and Uruguay to modulate 625/50 video as if it were 525/60, i.e. with 4.2 MHz bandwith.
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  #73  
Old 04-06-2015, 12:31 AM
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Quote:
Originally Posted by Adlershof View Post
I know that I'm not the only one who now, when occasionally seing an old CRT TV (not 100 Hz technology), is wondering how we could have beared this flicker at all.
When I visited the UK and Ireland in 2000, that was what I noticed right away-the flicker on 50 Hz CRT sets. It was very strong to me, as I was only used to 60 Hz CRT displays here in the USA.
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  #74  
Old 04-06-2015, 01:42 AM
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I was not aware of this aspect of PAL patents so far. Makes me wonder how Comecon manufacturers like Sta▀furt and Tesla (these two definitely made TV sets with PAL decoders) handled it.
I would assume that they just ignored the patents, as was common in the Eastern Bloc. There wasn't much the west could do apart from ban imports of such goods.
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Old 04-06-2015, 04:46 AM
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Romania was the only country in the Soviet Bloc that dind't use S.E.C.A.M. The reason where politicall too! The tv sets for Romania had decoders for both P.A.L. and S.E.A.C.A.M., 'cause all countries sorrounding Romania (except former Yugoslavia) used S.E.C.A.M. Oh, and Romania and former Yugoslavia where the only countries in Eastern-Europe that subtitled the movies (nowdays, some movies broadcasted on the Bulgarian televisions are subtitled and in Hungary from time to time movies broadcasted on tv are subtitled).
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