FSK

[mstechca]

After doing some research,
I just discovered that it is fairly easy for me to make an FSK modem.
FSK seems to use 2 frequencies for transmission. one for mark, and one for space. (presumably logic 1, and logic 0).

I do have some questions.

1. Does FSK use only 2 frequencies and half-duplex transmission, or does it use 4 and full-duplex transmission?

2. What are (or how do I calculate) each of the frequencies involved with FSK?

Once those are answered, maybe I can make a modem and possibly connect here with it.

 

[_3iMaJ]

1. FSK would use 4 different frequencies in full duplex transmission.

2. The frequencies are arbitrary, you can pick whatever you want.

 

[lord loh.]

2. The frequencies are arbitrary, you can pick whatever you want.

How does the receiving modem then know which to consider mark and which to consider space. A standard frequency value makes sense.

 

[_3iMaJ]

Thats under the assumption that you've designed both.

 

[mstechca]

2. The frequencies are arbitrary, you can pick whatever you want.

I can't because I want to be able to connect to another modem.
Aren't there standard frequency assignments for FSK?

Packet radio has a universal frequency of about 144Mhz, so what is it for FSK?

How does the receiving modem then know which to consider mark and which to consider space. A standard frequency value makes sense.

or does it (modem) make the assumption that whatever is transmitted to it on the "mark" or "space" frequency treats it as if it was incoming data?

 

[_3iMaJ]

The receiver simply uses a PLL to make the appropriate decision on what data was sent.

 

[tkbits]

2. The frequencies are arbitrary, you can pick whatever you want.

I can't because I want to be able to connect to another modem.
Aren't there standard frequency assignments for FSK?

Packet radio has a universal frequency of about 144Mhz, so what is it for FSK?

For what kind of transmission?

In the US, there is the Bell 103 standard for use over telephone lines at 300 baud or lower. But I believe there is another telephone standard for "TTY" modems used by the deaf community.

 

[mstechca]

The receiver simply uses a PLL to make the appropriate decision on what data was sent.

I know that, but what are the frequencies for the "Mark" and "space" tones?

 

[_3iMaJ]

You're making the assumption that this is a common transmission scheme. I'm not certain it is. I know they used to use QAM on modems, not FSK. Theres a whole lot more that you'd have to know about the receiver than just what frequencies its looking for. You need to know what their synchronization sequence it's looking for. You have to know what type of compression its using, and what type of error correction its using.

Your problem is far more complicated than just the center frequency.

Edit: The reason that they probably don't use (as I don't know for sure) FSK is because it isn't a bandwidth efficient scheme, its similiar to FM and if you recall uses inifinite bandwidth.

 

[Nigel Goodwin]

You're making the assumption that this is a common transmission scheme. I'm not certain it is. I know they used to use QAM on modems, not FSK. Theres a whole lot more that you'd have to know about the receiver than just what frequencies its looking for. You need to know what their synchronization sequence it's looking for. You have to know what type of compression its using, and what type of error correction its using.

Your problem is far more complicated than just the center frequency.

Edit: The reason that they probably don't use (as I don't know for sure) FSK is because it isn't a bandwidth efficient scheme, its similiar to FM and if you recall uses inifinite bandwidth.

You're talking far more modern modems, with much higher speeds, 300 baud was just two tones, one for mark, and one for space - you can get 1200 baud reasonably well down a low spec phone line in this way, but higher rates required more complicated schemes.

I'm sure a simple google would easily find the tones used?, which were different in the USA and Europe (if I recall correctly?).

 

[_3iMaJ]

I'm still sure they use some sort of error correction, synchronization, and compression. Possibly not error correction, but the other two are a must.

 

[Nigel Goodwin]

I'm still sure they use some sort of error correction, synchronization, and compression. Possibly not error correction, but the other two are a must.

No, there's no need at the low rates employed - in any case, such things are done at a higher level in modems that include them. Syncronisation is done at the RS232 level, on the start bit - however, it was usual to use the parity bit, as a very minimal error check.

 

[_3iMaJ]

How would you know where one word ended and another began? How do you consider the significance of bits? If some bits are more significant than others, that would give rise to compression.

 

[Gorgon]

How would you know where one word ended and another began? How do you consider the significance of bits? If some bits are more significant than others, that would give rise to compression.

Hi,
FSK is used in asynchronous communication, in idle mode the transmitter is sending '1' all the time. The startbit is always a '0', then you have a number of data bits, (LSB first) parity and last 1 or 2 stopbits '1' an so on...

This is normal async serial communication and you must always know the format in both ends(speed, number of bits and paritiy).

There are lots of standards around the world, the US have the Bell standard, we other got the different CCITT Vxx standards, V21, V22, V23 ++.

V21 and v23 are pure FSK standards. 300/300Bd and 1200/75(150)Bd or 1200Bd half duplex.

The best way to implement a standardized modem is to use a standard modem chip. Silicon Systems used to have a whole range of these in the K-series. AMD also had the so called 'World chip' modem.

TOK

 

[Gorgon]

The receiver simply uses a PLL to make the appropriate decision on what data was sent.

I know that, but what are the frequencies for the "Mark" and "space" tones?

300/300 Bd (All number in Hz)
Bell 103 calling: 1270(1)/1070(0) answer: 2225(1)/ 2025(0)

V21 calling: 980(1)/1180(0) answer: 1650(1)/1850(0)

75/1200Bd
Bell 202 Calling:387(1)/487(0) answer: 1200(1)/2200(0)

V23 Calling: 390(1)/450(0) answer: 1300(1)/2100(0)


This is taken form the K-series design manual from Silicon Systems.

TOK

 

[mstechca]

After more research,

I have discovered that in AFSK mode, the transmitter uses one frequency for transmission. I think it is about 144Mhz. This mode transmits two audio tones at 2 frequencies. One frequency is 1200hz and I think that is the space tone (binary 0). The other one is at 2200hz.

In FSK, Two carrier frequencies are used for transmission, and the website (I can't remember the URL of it) states that FSK can go at higher speeds than AFSK.

But I still need to know the two carrier frequencies used. One carrier frequency must be a mark, and the other, a space.

What do I transmit on the mark frequency for a valid mark signal (in FSK)?

What do I transmit on the space frequency for a valid space signal (in FSK)?

 

[audioguru]

But I still need to know the two carrier frequencies used. One carrier frequency must be a mark, and the other, a space.

No. Only a single carrier frequency is used. It is modulated with different frequency audio tones.

 

[tkbits]

The mark frequency is the mark signal, and the space frequency is the space signal.
=====
So I went a-searching, and this is my summary of wireless communications...

FSK - the mark and space frequencies are selected by the transmitter. Some web sites call these frequencies "carriers", but there is no content - the frequencies are the content.

AFSK - a specific version of a more general technique. The mark and space frequencies (in the AF range) are used to modulate a single carrier (in the RF range). Demodulate to get the original mark and space frequencies.
=====
When I hear "modem", I always think "wired". Is this wired or wireless?

If this is wired, the "telephone" standards might be the only standards. (See Gorgon's post.)

 

[_3iMaJ]

What type of modem are you trying to communicate with? Some specs on it would help. FSK is a relatively simple modulation technique.

 

[Nigel Goodwin]

After more research,

I have discovered that in AFSK mode, the transmitter uses one frequency for transmission. I think it is about 144Mhz. This mode transmits two audio tones at 2 frequencies. One frequency is 1200hz and I think that is the space tone (binary 0). The other one is at 2200hz.

In FSK, Two carrier frequencies are used for transmission, and the website (I can't remember the URL of it) states that FSK can go at higher speeds than AFSK.

But I still need to know the two carrier frequencies used. One carrier frequency must be a mark, and the other, a space.

What do I transmit on the mark frequency for a valid mark signal (in FSK)?

What do I transmit on the space frequency for a valid space signal (in FSK)?

You need to check the terms of your amateur radio licence, this will tell you what modes are available on what frequencies - data transmission is ONLY allowed on certain frequencies. As far as I can recall, AFSK is normally used, FSK may not be allowable?.

If you're NOT a licenced radio amateur you shouldn't be discussing the amateur bands, and you probably shouldn't be planning data transmissions.

FSK is often used though in licence free radio modules.