Electronic Circuits and Projects Forum

It would be a tedious exercise to make a modem IC with discrete parts.

Nigel says:

FSK is just a carrier, and you switch the carrier between two different frequencies. AFSK uses a single carrier frequency and you modulate it with one of two audio tones, one for mark and one for space - commonly used frequencies have been posted previously in this thread.

Audiogurusays:

FSK is two audio tones that modulate a single carrier, you can hear it. 1200Hz and 2200Hz are low frequencies that were used at 300 (up to 2400?) baud over low-bandwidth and noisy telephone lines. All kinds of improvements in modulation allows 56k over telephone lines and transmitters. They use data-compression, error-correction and phase modulation. They use a lot more than only two audio tones.

I'm very confused, because Nigel says with FSK, you switch the carrier between 2 frequencies. Audioguru says that FSK is equivalent to what Nigel said about AFSK (two tones modulating a single carrier).

Who is right?

Originally Posted by mstechca

Nigel says:

FSK is just a carrier, and you switch the carrier between two different frequencies. AFSK uses a single carrier frequency and you modulate it with one of two audio tones, one for mark and one for space - commonly used frequencies have been posted previously in this thread.

Audiogurusays:

FSK is two audio tones that modulate a single carrier, you can hear it. 1200Hz and 2200Hz are low frequencies that were used at 300 (up to 2400?) baud over low-bandwidth and noisy telephone lines. All kinds of improvements in modulation allows 56k over telephone lines and transmitters. They use data-compression, error-correction and phase modulation. They use a lot more than only two audio tones.

I'm very confused, because Nigel says with FSK, you switch the carrier between 2 frequencies. Audioguru says that FSK is equivalent to what Nigel said about AFSK (two tones modulating a single carrier).

Who is right?

A simple search soon finds relevent data:

Frequency-shift keying
From Wikipedia, the free encyclopedia.
Jump to: navigation, search
Frequency-shift keying (FSK) is a form of frequency modulation in which the modulating signal shifts the output frequency between predetermined values.

Usually, the instantaneous frequency is shifted between two discrete values termed the mark frequency and the space frequency. This is a noncoherent form of FSK.

Audio frequency-shift keying
From Wikipedia, the free encyclopedia.
Jump to: navigation, search
Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented as changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone. Normally, the transmitted audio alternates between two tones: one, the "mark", represents a binary one; the other, the "space", represents a binary zero.

AFSK differs from regular frequency-shift keying in that the modulation is performed at baseband frequencies. In radio applications, the AFSK-modulated signal is normally used to modulate an RF carrier (using a conventional technique, such as AM FM or ACSSB(R)(LM Mode(R)) for transmission.

AFSK is not generally used for high-speed data communications, as it is less efficient than other modulation modes. In addition to its simplicity, however, AFSK has the advantage that encoded signals will pass through AC-coupled links, including most equipment originally designed to carry music or speech.

Applications
Most early telephone-line modems used audio frequency-shift keying to send and receive data, up to rates of about 300 bits per second. The common Bell 103 modem used this technique, for example. Some early microcomputers used a specific form of AFSK modulation, the Kansas City standard, to store data on audio cassettes. AFSK is still widely used in amateur radio, as it allows data transmission through unmodified voiceband equipment.

I don't know if Audioguru is a licenced radio amateur?, but I was G8MMV, I've not been active for a few years now, but when I last was I used Packet Radio on 2m.

I was never a "ham".

audioguru, I suppose you go through 100's and 100's of books related to radio. I think I'll have to go with Nigel on this one, because I have looked at wikipedia before I checked here again, and if a licensed guy is showing me wikipedia, then I guess wikipedia wins.

Originally Posted by mstechca

audioguru, I suppose you go through 100's and 100's of books related to radio.

I haven't read a single book about radio. I think the prof in university talked a little about radios. Many magazines I read in the library taught me stuff about radios.

Are you going to modulate a single RF carrier with AFSK audio tones, or are you going to somehow switch between two RF frequencis with FSK?

Originally Posted by akg

so this becomes equivalent to an FM signal , where the carrier freq is changed WRT to i/p signal.

Good point!
FM is a very simple way to make FSK. But how would you detect it, it is DC modulation, isn't it? I guess you could scale down its frequencies with a counter then detect them with a PLL.

The problem is that MStechca's transmitter was an AM transmitter with a little FM on the side.
Now he can't afford a real varactor diode to allow it to produce true FM.
Also he is using an AM receiver.

Originally Posted by audioguru

FM is a very simple way to make FSK. But how would you detect it, it is DC modulation, isn't it? I guess you could scale down its frequencies with a counter then detect them with a PLL.

I mentioned earlier in the thread that FSK is generally used with crystal control in both transmitter and receiver, this makes it simple and reliable. A simple FM detector will output different voltages for the two carrier frequencies, and you can easily convert it to logic levels with a comparator.

Many licence free modules use FSK, but they use AC coupling, which prevents a continuous DC output from the receiver.