When it comes to digital radios, is the audio the only thing digitized, or is the electromagnetic radio wave physically altered so that it is squared off?
My current thoughts are that the wave is a standard sine wave, but the audio that travels on it is 1s and 0s.
Does anyone really know?
-Josh
Digital Physics Questions
Moderator: Queue Moderator
Hi Josh!
I'm still in the process of making my way through the analog to digital learning curve myself, but I think I can take a shot at an answer for your question.
Digital modulation does not specifically alter the shape of the FM carrier waveform. Rather it modulates it by altering the state of the carrier in some other way.
Specifically, Astro digital modulation uses a four state version of frequency shift modulation, where the carrier is shifted up or down by a fixed amount from the center frequency, in order to represent a given digital value.
In a four state system, in order to increase the transmission rate, the binary bits are encoded as pairs. Each pair of binary bits can have four possible values: 00, 01, 10, or 11.
In Astro these values are transmitted via the following transmitter deviations:
00 Shift +0.6kHz from center
01 Shift +1.8kHz from center
10 Shift -0.6kHz from center
11 Shift -1.8kHz from center
Hope that helps!
I'm still in the process of making my way through the analog to digital learning curve myself, but I think I can take a shot at an answer for your question.
Digital modulation does not specifically alter the shape of the FM carrier waveform. Rather it modulates it by altering the state of the carrier in some other way.
Specifically, Astro digital modulation uses a four state version of frequency shift modulation, where the carrier is shifted up or down by a fixed amount from the center frequency, in order to represent a given digital value.
In a four state system, in order to increase the transmission rate, the binary bits are encoded as pairs. Each pair of binary bits can have four possible values: 00, 01, 10, or 11.
In Astro these values are transmitted via the following transmitter deviations:
00 Shift +0.6kHz from center
01 Shift +1.8kHz from center
10 Shift -0.6kHz from center
11 Shift -1.8kHz from center
Hope that helps!
-
Astro Vocon
- Posts: 3
- Joined: Tue Jan 08, 2002 4:00 pm
Hi Josh,
This is a good question but really needs a bit of basic physics to properly answer.
The more a radio carrier wave looks like a sine wave the less bandwidth it takes up.
If you transmitted a pure sine wave then it would theoretically occupy zero bandwidth. It doesn't in reality because of imperfections in the oscillator producing the sine wave. These imperfections are referred to as phase noise or "noise side bands".
Waveforms that are not sine waves, such as square waves or triangle waves, can be created by a series of sine waves of differing frequency and amplitudes. This idea is called the Fourier theory.
So if we took a carrier wave and, as you put, "physically altered so that it is squared off" we would create a wide band signal consisting of lots of sine waves. The more square the more bandwidth. You can see that it is a bad idea from the bandwidth point of view to find space to park all these sine waves!
The trick with efficient modulation is that we make minimal changes to the carrier wave. Just enough changes so we can recover the original modulation.
Now as HumHead has pointed out Astro modulation moves the carrier wave smoothly between four discrete frequency values in sympathy with the digital bit stream.
Why four states? Well because since we don't want a wideband transmission we need to disturb the carrier as little as possible. The Astro data rate is 9600 bits per second. Each bit can be 1 or 0. If we used two state modulation we would need to shift the carrier 9600 times per second. By using four carrier states (called symbols by the experts) we only have to shift the carrier 4800 times per second because each state represents two bits i.e. 00, 01, 10, or 11.
And why don't we want a wideband signal? Because today most people have to pay for the spectrum they use (one way or another).
Hope this helps.
This is a good question but really needs a bit of basic physics to properly answer.
The more a radio carrier wave looks like a sine wave the less bandwidth it takes up.
If you transmitted a pure sine wave then it would theoretically occupy zero bandwidth. It doesn't in reality because of imperfections in the oscillator producing the sine wave. These imperfections are referred to as phase noise or "noise side bands".
Waveforms that are not sine waves, such as square waves or triangle waves, can be created by a series of sine waves of differing frequency and amplitudes. This idea is called the Fourier theory.
So if we took a carrier wave and, as you put, "physically altered so that it is squared off" we would create a wide band signal consisting of lots of sine waves. The more square the more bandwidth. You can see that it is a bad idea from the bandwidth point of view to find space to park all these sine waves!
The trick with efficient modulation is that we make minimal changes to the carrier wave. Just enough changes so we can recover the original modulation.
Now as HumHead has pointed out Astro modulation moves the carrier wave smoothly between four discrete frequency values in sympathy with the digital bit stream.
Why four states? Well because since we don't want a wideband transmission we need to disturb the carrier as little as possible. The Astro data rate is 9600 bits per second. Each bit can be 1 or 0. If we used two state modulation we would need to shift the carrier 9600 times per second. By using four carrier states (called symbols by the experts) we only have to shift the carrier 4800 times per second because each state represents two bits i.e. 00, 01, 10, or 11.
And why don't we want a wideband signal? Because today most people have to pay for the spectrum they use (one way or another).
Hope this helps.