Why is digital modulation not enough for radio transmission?

Answer 1

From http://en.wikipedia.org/wiki/Modulation..."The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel, for example over the public switched telephone network (where a filter limits the frequency range to between 300 and 3400 Hz) or a limited radio frequency band."

In analog modulation, it is to modify a carrier, i.e. the amplitude, phase and frequency. If the carrier (a sinusoidal signal) is already at the RF frequency that you want to transmit at, and you modulate that carrier, then that process is known as Direct Modulation. You can modulation a lower carrier frequency and then up-convert it by mixing the modulated signal to the desired RF frequency. This mixing or up-conversion is done by heterodyning. Dual conversion, i.e. mixing to an intermediate frequency (IF) and then again to the RF is also call super-heterodyne. This was invented by Edwin Armstrong (also the inventor of FM).

Digital modulation in many cases also involves coding. A simple kind of digital coding and modulation is called Pulse Code Modulation or PCM. Once you have the digital modulated signal, which is the digital bits of the original signal, then you can use these bits to modulate an analog signal for transmission in the RF frequency. A very very simple example is to us OOK or On-Off-Keying. If the digital bits, which is in binary form, is a "1", you turn on the RF carrier and a "0", you turn it off. Other ways to transmit the RF signal is to modulate the digital coded modulation in other forms, i.e. use Amplitude (PAM or Pulse Amplitude Modulation) or PSK (Phase Shift Keying) etc. These modulated signals are actually in analog form and will need to be up-converted to RF frequency so that it can be transmitted.

Thus RF transmission of a digital encoded signal involves understanding of both digital techniques as well as analog methods.

Answer 2

some literatures deals with two types of digital modulation, digital transmission such as pulse code modulation which is not a real modulation (see: Prokais, Digital Comm.) but coding analogue signal into high rate digital signal. ex, in CCITT conventional telephony, 30 audio 3.4 kHz signal are digitized and multiplexed by TDMA into a 2 Mbps digital signal) however also there are many communication system that works with this frequency or less (MW and SW AM broadcasting ) this signal is not suitable for transmission for the following reasons,
1- this digital signal extends from 0 to may be 4-6 MHz so if it is transmitted by radio it will reserve the radio space for its spectrum, now what about other signals, even modulating other signals with higher rates will not solve the problem since all these signals will have a common components (interference) in there lower part of the spectrum. in other words digital carrier do not provide spectrum translation needed for frequency allocation necessary for non interfereng comm systems, sinsoidal carriers do.
2- electromagnetic radiation is based on time varying waves, if the exciting signal is not changing it can not induce a traveeling wave in the space, sinsoidal signals are varying contineously while square waves exhibits flat steadiness for some time of its duration.
3-digital signals have a sharp transit from one value to another which is not suitable in high power txn equipments.
4- most radio high power generation is easy for sinsoidals while it is more complex for square signals.
5- modulating sinsoids with square waves may reduce the bandwidth required
6- engineers are more comfortable to deal with sinsoidal because it is the rythm of the universe,
7- there are many studies about non sinsoidal carriers even there is a book about it, (I havnt read it but I think it was published by MCgrawhill)

Answer 3

???
I'm totally puzzled by the question. If you mean digital modulation as carrier wave on or off, then it's called Morse Code. If you're talking about modulating analog signals (voice, music, etc.) onto a carrier, then it makes even less sense since all cellphone modulation is digitized analog signals, as well as the verious 'satellite' services (Sirius, etc.)

Doug