Bandwidth

Discussion in 'General Electronics Chat' started by manulal, Aug 8, 2012.

  1. manulal

    Thread Starter New Member

    Dec 6, 2010
    17
    3
    Hello

    I am finding it a bit difficult to comprehend what the author of a famous text book has written about TV signal band width. Here is the excerpt from his book. Could any of you please explain what exactly he means? I don’t get why the required bandwidth is around 5.5 MHz.



    "Another example of a nonperiodic composite signal is
    the signal received by an old-fashioned analog blackand-
    white TV. A TV screen is made up of pixels. If we
    assume a resolution of 525 × 700, we have 367,500
    pixels per screen. If we scan the screen 30 times per
    second, this is 367,500 × 30 = 11,025,000 pixels per
    second. The worst-case scenario is alternating black and
    white pixels. We can send 2 pixels per cycle. Therefore,
    we need 11,025,000 / 2 = 5,512,500 cycles per second, or
    Hz. The bandwidth needed is 5.5125 MHz."


    When I reached AC circuits and its applications, I stumbled upon the above quoted paragraph and this doubt started bothering me since then.

    I clearly understood the logic behind 11,025,000 pixels per second and since a cycle can have two pixels 5,512,500 cycles per second or 5.5125 MHz.

    So, if I have understood correctly any signal with a frequency exceeding 5.5125 or let us say 6 MHz can theoretically contain all the information required for the TV transmission in the worst case scenario.

    Now, let me mention an example.

    North American channel 2 occupies the spectrum from 54 MHz to 60 MHz. A bandwidth of 6 MHz.

    My question is, provided that any frequency more than 6 MHz can contain the all information in the TV transmission, why can’t Channel 2 use a particular frequency above 6MHz, say 57 MHz, instead of using the spectrum from 54 to 60 MHz?

    I understand from the ebook part of AAC that a modulated AC signal can have a number of harmonics and instaead of a single precise frequency it will be a band of frequencies and all.

    But I don’t think that the explanation in the excerpt is related to harmonics and all but rather the to the information to be carried by the signal at the wost case.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,431
    3,360
    You are ignoring something called modulation.

    A single sine wave signal at fixed frequency such as 54MHz contains no information on its own. It is called the carrier signal.

    Once we begin to modulate the carrier signal (and there are different ways of doing this, amplitude, frequency and phase) there is a frequency spread. That is, the carrier signal becomes more than a single frequency. This spread is the bandwidth requirement. In the case of a TV signal the bandwidth is 5MHz causing the carrier to spread +/- 5MHz, i.e. 10MHz. The -5MHz spread is the same as the +5MHz spread, i.e. it is a mirror image of the +ve spread. Thus to conserve frequency space and power transmitted, the -ve spread is filtered out and not transmitted (called SSB - Single Side Band transmission).
     
  3. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    Close,but no cigar!

    The mode used for analog TV is "Vestigial Sideband" (VSB),in which around 1.25 MHz of the lower sideband is also transmitted along with the USB.

    The rest is quite good,but I might add the following:-

    A baseband video signal for the system described in the text quoted by the OP may well be ideally 5.5125 MHz,but in the real world,the NTSC system limits the response to around 4.5MHz ,in order to save frequency spectrum.

    In drawings,the bandwidth needed for video is often drawn as if it were a solid block of signals,but in fact it is a spectrum.

    Looking at a video signal on a Spectrum Analyser,a series of multiples of the(approx) 15KHz line will be seen,each with sidebands at 30Hz & multiples of 30Hz, with substantially vacant spaces between them.

    The colour signal is modulated on,in the NTSC case an (approx ) 3.58MHz subcarrier,so that the sidebands "frequency interlace" between the luma ones,& is added to the luma signal.

    The process of modulation in the Transmitter now causes upper (USB) & lower(LSB) sidebands extending above and below the carrier.
    Note that nothing happens to the carrier,the sidebands are created by the mixing process in the modulator.

    Each sideband is really a frequency shifted version of the baseband video spectra,except that the lower sideband is inverted,so the the highest video frequencies become the lowest frequencies of the LSB.
    To conserve bandwidth,most of the LSB is filtered out.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,431
    3,360
    Geeze, I didn't want to give the op a complete lesson on NTSC video frequency spectrum:

    [​IMG]


    So where did I go wrong except for the vestigial bit?
     
  5. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    Quote from your posting:
    "Once we begin to modulate the carrier signal (and there are different ways of doing this, amplitude, frequency and phase) there is a frequency spread. That is, the carrier signal becomes more than a single frequency."

    Relevant quote from mine:

    "Note that nothing happens to the carrier,the sidebands are created by the mixing process in the modulator."

    I'm sorry to correct you,but I used to do this stuff for a living!
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,431
    3,360
    Ok, sorry for the poor wording.

    I stand corrected. I should have said:

    The carrier stays at the same frequency. Additional frequencies are created both below and above the carrier frequency by the mixing process in the modulator.
     
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