Digital signal transmission

Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,353
Good day! I understand that this might be a stupid question, I looked on the internet but I couldnt find it. Can someone please clerify for me, how exactly is a digital signal transmitted. I hear that the digital signal is better, it has more frequencies and everything is passing on digital technology. I understand that an analog signal is just voltage applied on an antenna, the shape of the antenna and the parameters of the signal determine the length of the magnetic field. After that we have another antenna which will catch the signal and amplify it.

How does that work with a digital signal, is the digital signal just square impulses? Whats the difference between an analog antenna and a digital antenna, how come there are more frequencies, is it just frequency modulation of an analog signal? I dont get it, what exactly is a digital signal transmission?
 

crutschow

Joined Mar 14, 2008
27,915
If you are referring to the RF transmission of digital television, digital refers to the type of signal modulation of the RF carrier.

Thus in analog TV the analog video signal amplitude modulates (AM) the carrier.

In digital TV transmission the digital video signal consisting of a stream of 1's and 0's is used to modulate the RF carrier.
The modulation scheme is rather complex and is different in different areas of the world.
North America uses 8VSB and other areas use COFDM. Here's a short discussion of the two.

There is no such thing as a "digital antenna".
The antenna just receives the RF signal, whether it is digitally modulated or analog modulated.
To the antenna there's no difference as far as the signal is concerned.
The only difference is the the most of the old VHF TV channels were moved to UHF so generally you only need a UHF antenna to receive HD digital television signals in the US (which some enterprising ad writers now call a "digital" antenna).

Digital modulation can generally carry more data because it is designed to remove redundant picture information (frame-to-frame stationary parts of the picture) using MPEG-2 compression in the US, and 8VSG or CODFM modulation removes redundant sidebands in the RF spectrum to make better use of the bandwidth in an RF channel.
Thus a HiDef TV signal which might have a 20MHz or more uncompressed analog bandwidth can be transmitted in the old 6MHz StdDef analog channel in the US.
 
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Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,353
1. To confirm the difference between an analog and digital TV is only in the modulation?
2. The antenna needed to receive digital TV is an ultra high frequency analog antenna?
3. Digital TV is digitally modulated to skip the parts which are not heard or seen by the human ear and eye, like MPEG and JPEG compression?
Since some things are skipped, we need a lower frequency in order to transmit the signal, since for 1 sec we can transmit enough with a lower frequency and compression?

4. So its just like a frequency modulation, where we transmit '1s' and '0s' and code them with a frequency (this is the principle and there are no digital signals applied to the antenna or received)?
 

crutschow

Joined Mar 14, 2008
27,915
Yes, basically true except for 4.
The RF signal is actually amplitude modulation, not frequency modulation, with single sidebands carrying the encoded digital information.
 

Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,353
Isnt frequency modulation better then amplitude modulation?

I can build my own digital transmitter and receiver by making a coil for an antenna and sending through it different frequencies or amplitudes to signify "1" or "0"? I can build the electronics, but I will need a little help with the antenna as I am not that good with the electromagnetic theory and electro-mechanical parts.
 

crutschow

Joined Mar 14, 2008
27,915
Isnt frequency modulation better then amplitude modulation?
Define better.
An FM modulated signal is more resistance to static interference and pulse noise, but the tradeoff is it requires a higher bandwidth to achieve this improvement.
Thus, for example, the old NTSC analog TV transmission sent the video in suppressed sideband AM modulation to minimize bandwidth but the audio was FM modulated for better audio quality but in a narrower band than standard FM radio.
 

Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,353
I see. Thank you for all your help! I will have to review my books on FM and AM, I thought that since there are less interferences, FM is better.
By sideband you mean a frequency closer to the main frequency used? This is why the FM radio is in MGHz so we can use a sideband as well?
 
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