Connect with us

Understanding the analog to digital transition of TV

Discussion in 'General Electronics Discussion' started by dietermoreno, Jun 16, 2013.

Scroll to continue with content
  1. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    I ate dinner at a Mexican restaurant near my house that had a big Yagi on top of its roof, and I saw when the cashier scrolled through the channels it had more channels than I had ever seen on over the air TV before. I know that it was over the air TV because it only had local channels.

    Questions:

    (1) So is digital 100% static free, in that either the signal comes in or it doesn't come in?


    (2) Why did TV never adopt FM for transmitting the video component even though it adopted FM for transmitting the audio component?

    I think Mr. Armstrong would have been very pleased to see the VHF channels be AM and the UHF channels be FM. The reason I say that the UHF channels would be FM rather than the VHF channels is that lower frequencies allow a radio wave to travel farther and the higher fidelity of FM makes up for the higher frequencies not traveling as far. The VHF channels could provide legacy programming for individuals who had not upgraded to an FM TV set yet. The UHF channels could have the VHF channels simulcast in FM.


    (3) What's with the decimal numbers in the digital over the air broadcast? Why are the decimal numbers needed? Does the .1 mean SD digital, the .2 means HD digital and the .0 means analog (unused)?


    (4) Why do digital signals travel farther than analog signals? (at least that is the case here)


    (5) So do digital over the air broadcasts all use UHF and not VHF? Why do digital over the air broadcasts not use VHF?
     
    Last edited: Jun 16, 2013
  2. davenn

    davenn Moderator

    13,837
    1,952
    Sep 5, 2009
    Its static free but not drop out free, when you see the picture become pixelated ... thats dropout

    It's all down to bandwidth (BW). On terrestrial TV AM video and FM (stereo) sound is the most economical and fits into an 8MHz channel. Video signal has a much greater BW than audio signal and it's FM would require much greater transmission bandwidth so for economy purpose AM suffices. audio is FM, than AM because adequate bandwidth is already within the AM video signal bandwidth.



    no idea what you are referring to


    because a digital signal is less affected by noise and being FM makes it even less affected by noise. its easier to retrieve a low level digital data stream at a distance than it is to do the same with an analog signal when noise completelt degrades the signal


    cuz there isnt enough bandwidth on the low VHF channels for the wide bandwidth digital signals

    Dave
     
    Last edited: Jun 16, 2013
  3. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    So my "idea" to use FM for transmitting the video component of the TV broadcast has been realized! (in digital)

    So digital TV is FM TV, just in digital, and it wasn't possible to have FM analog TV because of the band width requirements to drift from the center frequency to have a suitable frequency modulation index that would yield a high fidelity (so FM radio stations use a 300khz spacing in the U.S. even though the signal desired to modulate is only about 100khz, so that would be a frequency modulation index of 3; now if analog AM TV stations in the U.S. had a 6MHZ bandwidth of the signal desired to modulate and a modulation index of 3 was used, that would require 18MHZ bandwidth, and the low VHF TV band is only 28MHZ bandwidth so only one analog FM TV channel could fit so its not possible) but digital can use compression algorithms to compress to a suitable bandwidth, and UHF has been chosen for DTV instead of VHF because it has less impulse noise and also less interference from the FM radio band (what the Wikipedia article says), although the high VHF band is sometimes used for DTV, and since a digital tuner is used the digital tuner will "understand" the number of the analog channel that is actually broadcast at a much higher frequency [than the old analog TV channel] actually on an analog UHF TV channel and display the channel number on the screen.
     
  4. davenn

    davenn Moderator

    13,837
    1,952
    Sep 5, 2009
    well I dont know about "YOU" idea to use it

    all of us amateur radio operators have been using FM for analog TV transmission and reception for many many years. Its nothing new

    Dave
     
  5. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    So you mean you built a home made video phone system for amateurs radio operators? So did you use the SW band which is usually used for amateur radio audio but instead you transmitted television signal at the same frequency that the amateur has been alotted?

    I didn't know amateurs were allowed to transmit in FM. Wouldn't amateurs transmitting in FM use up more spectrum than the amateur has been alotted?

    So also kind of like YouTube before YouTube?



    I saw an article out of a magazine from 1928 that teaches you how to build your own mechanical TV receiver, which is compatible with a standard MW broadcast receiver and SW amateur receiver by simply adding in some gain stages after the dector tube and connecting the output of the power gain stage you added to a light bulb and then the light bulb flashes on and off instead of a speaker cone moving in and out from the rectified amplitude modulations and the light bulb flashes on and off at different intensities and the light bulb flashing on and off is synchronized with each flash is a scan line and a complete picture is 48 scan lines at 7.5 frames per second and these scan lines are created by a fan motor spinning a plastic disc that has one hole cut in it for each scan line and the disc spins at a speed that creates 8 frames per second. It uses 5khz bandwidth on a standard SW receiver for the video component and the audio component is received on a standard MW receiver.


    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    http://en.wikipedia.org/wiki/WRNY_(defunct)


    So the video component of mechanical TV used simple amplitude modulation that is compatible with existing radio receivers, while the electronic TV uses quadrature amplitude modulation that is not compatible with existing radio receivers and requires a separate receiver for the TV.

    So we had a mechanical to electronic transition in 1935, and we had an analog to digital transition in 2009 (in U.S.).

    So radio amateurs using electronic television in the SW band at the frequency allotted for the amateur station would have to custom build a SW QAM transceiver which could be connected with a coax to the antenna connection of an existing TV and have the TV tuned to channel 3 and use the custom SQ QAM transceiver to change the channel instead of using the channel tuning controls on the TV, correct?






    Funny thing is, looking at the Wikipedia article for picturephone and the links it provides, in 1928 when mechanical television broadcasts began in the U.S. as an experimental technology, magazines said that television was not suitable for the masses and said that television was only suitable for the amateur radio operator, at the same time that AT&T was furiously trying to develop a video phone and AT&T said that the video phone would replace the telephone.

    Funny how bad we are at predicting what will become ubiquitous and what will be forgotten.

    I'm guessing that the reason that the video phone never took off while TV did was because it was so expensive to buy the set and to buy the service for the video phone and video phone service is not a good replacement for face to face communications so it was not deemed necessary, while the TV was mass produced for a cheap price and the over the air broadcasts were free and the TV offered television shows, movies, sports, and news that could not be shown without a video content so it was deemed necessary.
     
    Last edited: Jun 17, 2013
  6. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    An update in my knowledge:

    Here's some digital signals on VHF I hear on my TV radio:



    I think FM is not in use for DTV in the U.S. and instead 8 bit vestigial side band modulation (8VSB) is used, with 3 different voltage levels for 3 mark states and each mark state can carry a one or a zero so 2 bits ^ 3 mark states = 8 total bits.

    8VSB images:
    [​IMG]

    http://www.theonlineengineer.org/TheOLEBLOG/8vsb-a-tutorial/

    [​IMG]
    Center frequency offset slightly from the lower side band edge with almost the whole 6 MHZ as the upper side band.

    However, I read a paper by an electronics engineer named Benjamin Mueller that says that an analog frequency modulation carrier can be used to encase OFDM data carriers to protect from doppler shift and amplitude modulated noises.

    So perhaps OFDM DTV in Europe does use an analog FM carrier.

    OFDM DTV images:
    [​IMG]

    [​IMG]

    At least in the images, it does appear that the OFDM carriers when all combined together resemble having a center frequency with energy drifting to the right and to the left of the center frequency, either double side band AM modulation or FM modulation, and double side band AM modulation is not used for DTV.

    DSB modulation would have double the bandwidth of FM modulation (compared to VSB modulation) with non of the benefits of FM modulation.





    I have a TV radio that was designed for listening to TV audio before the digital transition (it will play audio even if there is no video signal, unlike my TV that won't play audio without a video signal, so if I connect a CD player to my TV I also must connect a video signal like the test signal from a DVD player), and I hear beeping and buzzing digital tones on apparently 7 channels on the High VHF band above and below the weather band on the dial, compared to I was only able to receive 5 High VHF channels in the analog days (CH 7, CH 9, and CH 11 from Chicago acceptable in color and CH 12 and CH 13 from Rockford, IL and Milwaukee, WI only in black & white).

    I'm not sure how I'm hearing AM modulated TV data tones when the TV radio was only designed to receive FM audio.

    Perhaps there is a phenomenon for receiving AM modulation on an FM receiver that I'm not familiar with.

    So apparently there are 2 more channels squashed in there in digital than in analog with digital using every single channel instead of analog using every other channel.

    but I thought all the VHF stations moved to UHF in the digital transistion. So maybe these stations on VHF I'm receiving are actually old UHF stations that have been moved to High VHF to save room for the big 3 networks (ABC, NBC, CBS)on UHF.

    It is true that I heard no tones on the low VHF band. So it appears that indeed the low VHF band has been vacated, but the high VHF band has not been vacated.

    Wikipedia says that CBS 2 Chicago moved to High VHF CH 12, NBC 5 Chicago moved to UHF CH 29, CH 6 WKQX-LP is allowed to continue in analog for now (it acts as an FM alternative rock station), ABC 7 Chicago moved to UHF CH 44, WGN (Chicago) CH 9 moved to UHF CH 19, WTTW PBS (Chicago) CH 11 moved to UHF CH 47, Fox 32 Chicago moved to UHF CH 31, WCIU (Chicago) CH 26 moved to UHF CH 27, WYCC PBS (Chicago) CH 20 moved to UHF CH 21, WISN 12 ABC (Milwaukee, WI) moved to UHF CH 34, WREX 13 NBC (Rockford, IL) moved to UHF CH 53, Ion 38 (Chicago) moved to UHF CH 43, CW 50 (Detroit, MI) moved to UHF CH 14, WYIN PBS CH 56 (Gary, IN) moved to UHF CH 17, UniMas Univision CH 60 (Aurora, IL) moved to UHF CH 50, WJYS Jesus Your Savior CH 62 (Hammond, IN) moved to UHF CH 32, [CH 68 was not used in Chicago area], CH 74

    Wikipedia says that previously UHF channels used to have to be spaced 6 apart so there was only channels 14, 20, 26, 32, 38, 44, 50, 56, 62, 68, 74, 80, 83 available for broadcast (all channels above 52 were sold to cell carriers in 2009 for the 700 MHZ band - CH 52 - CH 69 -, channels 70-83 were used as translators in North America, CH 83 was removed in the U.S. in 1983 for the 850 MHZ AMPS cellular band and later the 850 MHZ GSM cellular band, also CH 60 was used in Chicago area, and there is scheduled to be an auction of the 600 MHZ band - CH 35 - CH 51 - in mid 2015) because of the limitations of super-heterodyne receivers when TV was invented with all valves, but today with the more accurate tuning of phase locked loop receivers with ICs its no longer necessary. So that's why the "moved" VHF channels can use UHF channels not used for broadcasting before. Obviously after the 2015 auction the UHF channels would need to be spaced even tighter from 14 to 34, unless the independent stations and PBS stations moved to High VHF or even Low VHF.

    Maybe the ultimate goal of the FCC with the 2015 auction is to only carry the big 3 networks and force you to pay for cable or sattelite to watch any other networks.

    http://en.wikipedia.org/wiki/North_American_television_frequencies



    Hhm, so CH 12 (CBS 2 Chicago) is the only High VHF station in the Chicago area, so that makes no sense that I'm hearing 7 different stations beeping and buzzing on the High VHF dial of the TV radio.

    Even more interesting that I'm hearing digital stations to the right and to the left of the weather band, when Wikipedia says the weather band from 164.400 MHZ to 164.550 MHZ is below channel 7 (174 MHZ - 180 MHZ).

    Perhaps a possibility is that below the weather band the "digital stations" I'm hearing are actually digital public service transmissions in the public service and business VHF bands from 138 MHZ - 144 MHZ and 148 MHZ - 156 MHZ. This band uses narrow band FM, so an FM detector could detect it.

    Perhaps another possibility is I'm hearing digital transmissions in either the maritime VHF band from 156 MHZ - 158 MHZ (which is also narrow band FM) or the railroad VHF band from 159.81 MHZ - 161.565 MHZ

    An even stranger possibility is that I'm receiving digital transmissions from my friend's digital wireless guitar who is half a mile line-of-sight-away in the wireless musical instrument VHF bands from 160.6 MHZ - 162 MHZ and 174-216 MHZ (low power at exact frequencies only). The high VHF band is also 174-216 MHZ for CH 7 - CH 13.

    http://en.wikipedia.org/wiki/Very_high_frequency

    I am very confused.
     
    Last edited: Feb 3, 2014
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-