Digital Radio Communication (compare & contrast to analog)

Any expert especially in digital communication should answer.Is it possible to transmitt digital signal for radio broadcasting on air.If not then why DRM and DAB say that they have digital transmitter

The signals carry data which is converted back to analog in the receivers but the RF is still 'analog' using QAM or QPSK. Instead of the audo being represented by different carrier levels or frequencies, it is digitized and the digits are sent over the air instead. The primary reason is it is possible to compress digital data (like in a ZIP file) so more can be sent within a given bandwidth than if it stayed analog. DRM for example gives quite high quality stereo audio with a text channel for information and it is sent in a channel only about 50% wider than a low quailty AM broadcast.

Brian.

Dear
I have learned from text books that digital information or signals can not be transmitted over air. Only electromagnetic signals (analog) can be transmitted over air because transmitting antenna and receiving antenna (like in radio transmission) work only on analog not digital. Kindly explain so that concept about this should be cleared



Are you saying audio in DAB or drm transmission is in digital form while RF is in analog form? How it is possible because which modulation will be used for this purpose

The book isn't quite right.

ALL 'over the air' signals are analog but the modulation they carry can be digital. For example, take the simplest digital modulation which is keyed AM, suppose a digital '1' bit is sent by turning the RF on and a '0' by turning it off. The electromagnetic wave is still analog, probably a high frequency sine wave but by turning it on and off it is possible to send digital information. Another example would be FSK (Frequency Shift Keying), where the RF is always present but the '1' and '0' are sent as RF at slightly different frequencies.

DAB and DRM use another system which still sends the audio in digital form but a system called quadrature modulation is used. It is still broadcast as analog waves but they are changed in frequency, phase and amplitude to allow more digital bits to be sent. So a digital audio broadcast works like this:

audio --> digitizer (DAC) ---> RF modulator ---> antenna ~~~~~~ antenna ---> RF demodulator ---> digital to analog converter (DAC) ---> audio.

The benefits of doing it are that the data from the DAC can be compressed and have error correction bits added to it. At the receiver the ADC can expand the data again and 'repair' errors using the correction bits to ensure data integrity. Overall, a much higher audio quailty can be achieved than simple analog modulation and it can withstand much higher interference levels because of the correction bits.

DRM broadcasts on short wave are stereo and as good as MP3 quality and at the same time carry program information as HTML, similar to web pages. In contrast, with only slightly narrower bandwidth, conventional signals are mono with poor high frequency response, suffer from interference and have no program information.

There is lots of information about DRM at www.drm.org.

Brian.

I have still confusion if modulation is digital then after modulation we have to send it over air.Then how it is possible to send digital modulated signal in analog form (electromagnetic) as you say ALL 'over the air' signals are analog.?Now case is
audio --> digitizer (ADC) ---> RF modulator ---> digital modulated signal --->antenna ~~~air~~~ Receiving antenna

May be it would be easier to understand, but there are no pure digital signals in reality, because even most famous TTL defines 0's and 1's as voltage ranges (0 as ANY voltage below 0.8 V, 1 as ANY voltage higher 2.4 V, anything between 0.8 and 2.4 is undefined). Both 2.4 V and 5 V will be read as logical 1, despite this huge difference in voltage levels.
The principle of digital modulation is the same. You make some analog signal with some tolerances, for example in FSK modulation the frequency range 1100-1300 kHz will be considered as 0, 1700-1900 kHz as 1.
This is very-very simplified explanation, but I hope the main idea is clear.

I think the confusion is over the RF signal which is always analog (there is no digital alternative) and what is being conveyed by the RF signal (the modulation). It is perfectly possible to send digital information over an analog medium, in fact that's how most modern broadcast systems work, including TV, audio, wi-fi, bluetooth and cell phones.

DAB and DRM can still be picked up by a conventional AM or FM receiver, they are just signals like everything else on the radio bands, but all you hear is a buzz or hissing sound, it takes the quadrature demodulator to extract the bits from the broadcast and then some software processing to convert it back to its original audo sounds.

Brian.

What do mean by analog medium. Kindly tell me only about wireless communication especially radio communication.For example Nautel (NX 100) transmitters are said to be digital while I have noticed they use only PDM (PWM) technique which is analog.Kindly guide how modulation occur both RF and audio are in PDM form during modulation?

Surely you remember the modems used with telephone lines to send and receive digital data?

Modems are "modulators demodulators"- you can listen to the data if you pick up the receiver. They are basically analog representation of digital information.

Common radio uses a carrier and the sound travels on the carrier as a modulation. In the radio, the first step is to get rid of the carrier (roughly) at the tuner stage itself.

In the same way digital data travels on a carrier. In good old days, 0 used to be represented as one audio frequency and 1 used to be represented as another audio frequency (known to both transmitter and receiver).

It is not accurate to describe modulation as digital or analog. Modulation is simply changing the carrier slightly (carrier must not get lost by the change).

If you look at the RF waveform (modulated by a digital signal) you will appreciate that it does not look like digital or analog- because the modulation itself is more analog than digital.