NTSC Waveforms and Spectrum Analyzer Signals

Here are a couple of photos of NTSC video waveforms that are used to check channel flatness, and the resulting spectrum display from a modulator that has an RF flatness issue at the output.  This first waveform is a MULTIBURST shown on a Tektronix 1480R waveform monitor.

Here is the resulting video channel from a modulator that has the video input from the photo above.  Take note of the downward slope of the RF sidebands from the multipulse video input.  They should all be the same amplitude -- FLAT.  The spectrum analyzer is displaying 10 dB per vertical division and 1 MHz per horizontal division (1 cm squares).

This next waveform is called SINE X over X.  It is written as SIN X / X on video signal generators that can produce it.

Here is what makes that waveform so magical.  The linear set of sidebands visible in between the video carrier on the left and the sound carrier on the right is the result of the waveform in the photo above.  When that linear set of sidebands are FLAT, then the modulator is properly aligned.  This waveform acts as a video sweep generator.  In this spectrum analyzer image, the SIN X/X waveform is on every video line, not just one line in the vertical interval of the television signal.

I also wanted to add this spectrum analyzer display photo showing ONE analog NTSC television channel adjacent to QAM 256 digital television signals.  Yes, the big cable companies talk about getting "Digital Cable" for more channels. You need to understand what they are talking about.  It is the processing of the various MPEG video digital bit streams at the headend and again inside that "cable box" that provides all those channels to your television set.  The processing of those signals at the headend results in an RF carrier with up to 256 levels of phase-angle and RF amplitude modulation on a carrier wave that fits in ONE 6 MHz television channel.  It looks like a haystack on an RF spectrum analyzer.  It is an RF analog-modulation format that represents a serial digital bit stream containing several video signals viewable after the "cable box" decodes those bit streams for your TV set.   The three signals on the left side of the spectrum analyzer display below make up ONE analog NTSC color television channel.  Inside a cable television system, the sound carrier is transmitted 15 dB lower in power than the video signal to allow a television set to work successfully with adjacent television channels.  The "fuzzy haystack" QAM 256 signal is on the right side of the display screen can have TWO high-definition channels on it, or up to 13 standard definition  video channels on a Comcast Cable system like the one in Chattanooga where I took this photo.  QAM signals can also carry high-speed data signals for internet connections.  There is a "notch" between two adjacent QAM signals at the right side of the display below.   The notch is the guard band that separates on QAM signal from the one in the RF channel next door.  At the far left side of the display is another QAM signal on the new channel lower than the NTSC analog signal.  The width of the notch display is a function of which spectrum analyzer I.F. bandwidth filter is used for this snapshot.

Using a digital signal level meter to monitor a QAM 256 channel, the constellation of ANALOG amplitude and phase-modulated data points can be displayed.   A normal distribution of the signal vectors looks like the graphic image seen below.  If you tried to put a QAM signal on channel 2 from 54 MHz to 60 MHz, the group delay of two-way diplex filters in the RF amplifiers would distort the patterns of dots to the point where they may fall outside the boxes below and the channel would develop "tiling" or a "blocky" image that jerks or fails completely.   Each box below represents a number from Zero to 255 or 00 to FF in hexadecimal computer code.  All those 256 possibilities are the result of converting television signals to a digital bit stream of numbers representing MPEG encoded and compressed data that can be restored as high-definition or standard definition TV pictures.  Cable modem internet service can also be transmitted as a QAM-256 RF signal.

In case you missed this photo, here is a comparison of an off-air analog RF NTSC television channel and an 8VSB over-the-air digital television channel.   This close-up of the Tektronix 7L14 spectrum analyzer shows local analog and digital television channels in the Atlanta area.  That analog channel near the center of the screen is VHF channel 11, with an 8VSB digital TV transmission just to the left on channel 10 (192-198 MHz).  It is the broad, FLAT looking signal with little peak at the left.  The flat portion of the signal trace from the far left of the screen going less than two squares toward the signals is the noise floor where no television signal is present.  The center frequency tuning readout is out of calibration.  There is a pot to adjust it, but I did not think to do that before snapping these photos.  That tallest analog RF carrier is at 199.25 MHz.  There is a weaker digital television signal on channel 12 at the right side of the display screen.  That channel is coming from Chattanooga, Tennessee about 100 miles north.

For all the SCTE and other broadcast folks who come to this page, I write my technical articles to "translate from high-tech to plain language" for those who are just starting to "look behind the curtain" on how cable TV works.  I always enjoy watching the reaction of non-engineers when they begin to understand how we do the magic.  All they want is a clear and sharp picture.  My wife still watches standard definition channels because she remembers the channel numbers.  She stops listening to me when I use "techie talk".  She is happy when I fix her computer problems.