Digitally modulated waveform...

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Externet
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Digitally modulated waveform...

Post by Externet »

We know how a carrier, AM modulated with an analog signal looks like, the carrier varying its amplitude at the tune of the analog signal strength, and as often as the modulation frequency.
In FM, it is a constant amplitude carrier and varies its frequency increasing and decreasing at the tune of the modulation frequency, and deviates from the central carrier frequency as much as the modulating strength.

How does a digitally modulated carrier -say television RF signal- looks on the oscilloscope ?

Miguel
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Dean Huster
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Re: Digitally modulated waveform...

Post by Dean Huster »

I'm used to the "old" digital modulation (pulse modulation) that simply turned the carrier on and off, mirroring the digital signal. Don't know what perverted thing they do with complex digital stuff like HDTV.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

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Bigglez
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Re: Digitally modulated waveform...

Post by Bigglez »

Externet wrote:How does a digitally modulated carrier -say television RF signal- looks on the oscilloscope ?
There is no such thing as 'digital' modulation. The DTV
signal is a carrier modulated by a QAM signal that carries
a digital code. Once received that code is used to
reconstruct the original data, which in turn is the
visual and audiable portion of the program.

The code is compressed (redundant portions removed)
by the MPEG2 codec so it consumes less bandwidth compared
to prior analog techniques. The compression rate can be
modified at will by the broadcaster, as long as the data
stream fits within the channel bandwidth.

For commercial television service this the same as the old
analog standard of 6MHz channel spacing, but the data
stream can carry 19.39 megabits of data - enough
for four SDTV sub-channels.

The ATSC standardfor terrestrial broadcasting in the USA
(and Canada) replaced NTSC, which has remained mostly
unchanged since the early 1950s. Recall that stereo sound
and SAP were added in the mid 1980s. NTSC is now obsolete.

For digital cable (satellite) delivery QAM (Quadrature Amplitude
Modulation) is used, but DTV uses a scheme called 8VSB,
or eight level Vestigal Sideband Modulation. Meaning that
it has eight analog levels, requiring very good analog linearity.

At the end of the day we call these digital television, but underneath
the coding schemes it remains analog.

The data stream as viewed on a 'scope is meaningless. However,
the quality of the link path is measured in BER (Bit Error Ratio),
and can also be accessed on an analog scope by observing
the eye pattern.
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