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 Post subject: Re: Old anolg TV bandwidth verses new digital TV.
PostPosted: Feb Sat 22, 2020 1:22 pm 
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Hi Penthode.

Can you post a link to the article you referenced? There doesen’t seem to be any readily available literature that explains digital TV. Where are the Howard Sam’s equivalent of books about TV that were so prevalent back in the day.

Dennis


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 Post subject: Re: Old anolg TV bandwidth verses new digital TV.
PostPosted: Feb Sat 22, 2020 2:34 pm 
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Dennis, Don't know if this is what you need, but try this link.
https://www.mathscinotes.com/2012/05/hi ... sion-math/

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 Post subject: Re: Old anolg TV bandwidth verses new digital TV.
PostPosted: Feb Fri 28, 2020 6:10 am 
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I did not quote any particular reference. The information is gathered from various sources and from practical experience launching and maintaining digital broadcast TV stations. I shall look for pertinent resources.

With regards to the Viterbi encoding and convolutional coding, it is included to improve spectral efficiency. That is to prepare the transport data prior to 8 VSB modulation in order to cram as much data as possible over the the air yet still achieve an acceptably receivable signal. This in turn maintains as high a Signal to Noise level as possible to the receiver to help ensure that it may demodulated without error.

There remains confusion between the MPEG2 video compression and the MPEG2 transport.They are two entirely different things which unfortunately share the same name. Many broadcast engineers incorrectly refer to the MPEG2 transport stream as 'ASI'. ASI is a serial digital interface used for interconnecting and conveying the MPEG2 transport.

In the TV station, the video encoder (for the multiple program streams or 'subchannels') are multiplexed on to the MPEG2 transport stream. Think of the transport stream as a train of boxcars conveying a few bytes header of labelled information identifying what is in the boxcar plus the remaining bytes as payload data. You will have the majority of boxcars carrying video, a boxcar conveying separately audio and another boxcar carrying ancillary data such as closed captioning or Active Format Description (e.g. 16x9 letterbox vs 4x3 full frame etc) or the EPG (electronic program guide). These boxcars each contain 188 bytes of data. The boxcars which comprise the MPEG2 transport is fed to the Exciter which does all the fancy convolutional encoding and forward error correction immediately prior to the 8VSB modulation. The symbol rate is approximately 10.7 Megasymbols/second. 8 level encoding means 3 bits of data per symbol. (E.g. 3 binary bits allow you to count from 0 to 7). 10.7 Megasymbols times 3 bits per symbol equals roughly 32.1 Megabits per second total for ATSC 1.0 over the air. The forward error correction which corrects for received errors takes nearly 13Mb/s away which leaves 19.39 for studio program data.

As for receiving and recording on a TiVo or home DVR, the 8vsb demodulation will retrieve the MPEG2 transport exactly bit for bit as it originated from the studio. The 19.39 Mb/s MPEG2 transport may be demultiplexed and decoded for viewing or transport recorded for later decoding and viewing. The important thing to remember is that the MPEG2 video encoding is a lossy process and the DVRs record the MPEG2 transport containing the MPEG2 studio encoding to minimize video and audio quality loss.


Last edited by Penthode on Feb Fri 28, 2020 1:44 pm, edited 1 time in total.

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 Post subject: Re: Old anolg TV bandwidth verses new digital TV.
PostPosted: Feb Fri 28, 2020 7:13 am 
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Penthode wrote:

There remains confusion between the MPEG2 video compression and the MPEG2 transport.They are two entirely different things which unfortunately share the same name. Many broadcast engineers incorrectly refer to the MPEG2 transport stream as 'ASI'. ASI is a serial digital interface used for interconnecting and conveying the MPEG2 transport.


+1 on this. KCBS used to employ the MPEG2 transport stream but the powers-that-be wanted to switch to ASI, as this seems to be the way the industry is going. However, Gary Sgrignoli, a consulting engineer who is one of the developers of the 8VSB system, decried its use. He pointed out that the MPEG2 transport stream has a bi-phase clock, which helps to correct timing errors. ASI does not have this (it is Asynchronous by definition) and it is thus subject to timing errors. Unfortunately, jitter that results is not removed by any of the processing done in the exciter and is passed on to the customer as a degraded signal. The errors are not extreme, or it wouldn't be possible to get away with it.... nevertheless, it is not ideal. ASI, however, is both cheaper and easier to implement.... so guess which one is most commonly used....

Regards,

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 Post subject: Re: Old anolg TV bandwidth verses new digital TV.
PostPosted: Feb Fri 28, 2020 7:27 am 
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Jthorusen wrote:
+1 on this. KCBS used to employ the MPEG2 transport stream but the powers-that-be wanted to switch to ASI, as this seems to be the way the industry is going. However, Gary Sgrignoli, a consulting engineer who is one of the developers of the 8VSB system, decried its use.
Regards,


But KCBS still uses the MPEG2 transport! What Gary Sgrignoli was referring to was the synchronous interface, SMPTE310 versus asynchronous interface ASI. Both convey the MPEG2 transport. The trouble was in earlier days the 8VSB clock jitter was increased by using ASI but careful management of the 8VSB clock in later exciter designs minimized the problem.

Alas this has been mostly circumvented by SMPTE2022-2 which is a newer standard for conveying the MPEG2 transport via IP.


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 Post subject: Re: Old anolg TV bandwidth verses new digital TV.
PostPosted: Feb Fri 28, 2020 7:37 am 
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Greetings to Penthode and the Forum:

Yes... it has been nine years since I was at the transmitter and I had forgotten the name of the transmission standard: SMPTE-310M. My pet peeve with all things digital.... the conventions are purely arbitrary and change at the drop of a hat.

Regards,

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 Post subject: Re: Old anolg TV bandwidth verses new digital TV.
PostPosted: Feb Fri 28, 2020 11:16 am 
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You know .... as I was witnessing, living, and involved in (updating equipment, designing new systems etc) the Analog to Digital transisitionS (yes, first there was just plain old digital, then there was HD, then there was the failed HD/3D, and now ... well who knows what's coming next..

I watched, and worked with, the first digital devices. A 7RU box from Sony that could contain perhaps 8 or 10 D/A or A/D converters, and pulled enough power to brown out Manhattan. Next generation, 2RU cards that drew about a microwatt. (yes, that's poetic license). Now a single chip, perhaps 4 or 8 of these on one card.

Anyway, the bottom line throughout all this is that CONVERSION is necessary. And conversion is always lossy. I've mentioned before.... if you see an HD signal from an HD camera on an HD monitor at the point of origin (a studio or truck) it's like seeing 3D. You'd swear you were inside the picture. By the time it gets to your home monitor after multiple compression/decompression, transport, repeat again, then your monitor does it's own conversion, it is a horrible, ugly version of it's former self. Oh well.

anyway.... we were all told how "simple" it was going to be. 35 different formats later.... it ain't so simple

What was wrong with analog, anyway lol. Enhanced analog was just on the horizon at the time....... Ok, sure, you can fit 14 dozen channels in the space of one before, but really ???

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 Post subject: Re: Old anolg TV bandwidth verses new digital TV.
PostPosted: Mar Sun 01, 2020 4:42 am 
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Jthorusen wrote:
Greetings to dtvmcdonald and the Forum:

Also, I'm not sure what you mean by "carrier". There is no carrier as such; only a pilot.

I also don't understand this:

Quote:
OF course one could analog filter at IF and then heterodyne the carrier down to about 500 kHZ and
analog record that and it would work fine.


500 KHz is too low a frequency to carry the digital data load. Of necessity, even after using tricks like 8-level coding or COFDM, you would need at least the original 6 MHz bandwidth to convey the information.

Regards,


ATSC is vestigial carrier suppressed sideband AM. The pilot IS an actual carrier, just somewhat suppressed.
You can phase lock to it (or without it, as I explained) and simply reinsert it and it works fine (if the
channel is flat as a pancake of course.)

To the second point, its VSB. If the carrier is at 500kHz only the vestigial part goes down to zero.
Any wraparound is so low level it does not matter. You would of course need analog
bandwidth to 6.5 MHz. When played back in any case it would be
digitally filtered by the equalizer. In any case ... analog recorded signals most certainly CAN be decoded.
My decoder decoded analog recorded OTA signals from the pre-deployment tests that were digitized after
playback with analog phase lock with no trouble ... the mechanism to track doppler shifts fixed
any wow or flutter.


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 Post subject: Re: Old anolg TV bandwidth verses new digital TV.
PostPosted: Mar Sun 01, 2020 5:37 am 
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Jthorusen wrote:

ATSC is vestigial carrier suppressed sideband AM. The pilot IS an actual carrier, just somewhat suppressed.
You can phase lock to it (or without it, as I explained) and simply reinsert it.


ATSC 8VSB modulation is vestigial sideband suppressed carrier AM with the pilot carrier. I gather you suggest heterodyning the RF to a baseband level. This is academic and not how a consumer DVR works. You do not normally record the physical layer instead of the transport layer.


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