Hi, I'm just wondering does anyone know any good source where I can get some information regarding multiplexing various FSK modulation format? Thank you.
I have two different modem using different FSK modulation format. One is using the old 1200 baud, FSK communication system and another one using the 9600 baud FSK communication system. What I'm trying to do is to multiplex these two diferent signal source so they can both transmitted on the same pair of wires. Do you know any technical details on this matter? As I can't find enough information about what will be involved. I'm quite new in this field so I don't have much experience and expertise. Thank you for your help. Cheers
There are some problems here if you are intending to use modems for telephone lines, or the same sort of frequency spectrum as aphone line.
1200 baud is definitely a "true" FSK standard, i.e. it has two discrete frequencies representing logical "0" and "1" respectively.
9600 baud modems don't use FSK directly because the bandwidth is insufficient (you would need two much higher frequencies to detect 1's and 0's properly at that data rate). Instead, 9600 baud modems use phase encoding to stay within the limited bandwidth.
When I asked about multiplexing you did not state if you required simultaneous use of each modem, or just alternate use. If I remember correctly the frequencies used by each type of modem overlap, so I think concurrent use will be very difficult -if not impossible - without lots of band-pass filtering.
Thank you for your reply BrianG, I really appreciate it.
The wires is own by my company, we use it to control our control system from the master control room. The cables type is the 16mm2 TSP cables.
The carrier frequency used by the 1200baud FSK modem is at 1200 Hz with a bandwidth of 2200, the carrier frequency used by the 9600 baud FSK modem is at 9600 Hz with a bandwidth of 16800. So I don't think that the frequencies will overlap. Another thing I forgot to mention is that the system is a signal on power system, therefore we also need to consider the effects on the signals through the combiner and the splitter.
Anyone can enlighten me further on this topic. As I just got minimal knowledge and information on this topic. A good book or a article might be best.
OK, so clearly your 9600 baud modem is not designed for use on phone lines!
As the modem frequencies are widely different, each modem should have sufficient internal filtering to allow them to share a common connection over private wires. As you are using a signal-on-power system then providing its bandwidth or modulation method will support the highest frequency without significant attenuation (allow perhaps 25KHz-32KHz for the 16800Hz tone) then you should be able to use the existing modems with a simple resistive combiner / splitter at each end.
Cbarn's suggestion is quite feasible but more complex. To combine two independent data streams you would need to provide special TDM (time-division multiplex) equipment at each end to encode and decode the data for each control circuit path.
Hi, it depends on whet you are trying to signal, mainly what data rate you are hoping for. If its quite low a simple micro at each end would do, otherwise it gets more complex. You could do a search for "time division multiplexing" you might find something.
Are you sure about the details on your modems? The frequencies you state are clearly highly unusual and as BrianG states are not within the PSTN/telephone line spectrum.
Do you have any information of standards to which the modems are designed?
To tomatge:
The 1200baud modem is designed according the bell202s standard. The 9600baud modem is a RS-485 modem. The signals are not transmitted on PSTN/telephone line, they are super imposed on power. So they are transmitted on power on a 4mm² electric cables
To BrianG:
The communication/signal filter in the power and signal combiner is actually a band-pass with the range of 1kHz-2.5kHz. Is it possible to run another band-pass filter (9kHz-20kHz) in parallel so that to accomodate the signal from the second modem?
Thank you for all your replies, I've learned alot from this discussions
The question of adding another bandpass filter for the 9600 baud modem may be academic since your signal-on-power system uses a.c. mains power cables.
All of these type of systems that I have come across use a "carrier" system, i.e. they use low power radio frequencies superimposed on the same wires as the power, so that the modem tones are not sent directly down the mains wires but in each direction via a transmitter and a receiver respectively. (They usually operate with a frequency modulated carrier so that amplitude effects from mains spikes are minimised).
It's the overall carrier system bandwidth that will be the limiting factor, so you need to look at the manufacturer's specification to see if you can transmit the frequencies needed for the 9600 modem from one end to the other.
