A couple of years ago I went about making our 1292.1 dstar repeater dual mode, that is dstar or fm voice depending on whether the receiver saw a CTCSS tone or not.
The receive part was easy. Just tap the discriminator and bring the demodulated signal to a BNC jack on the front of the repeater. I could then hook an audio monitor to this jack (isolated with a 47K resistor) and listen to dstar data, or an fm tone, depending on what was applied to the receiver. The IF bandwidth is around 12.5 khz for narrow fm (deviation +/- 2.5 kHz) or dstar that is 26 thereabouts dB down @ +/- 3 kHz. from center. The 26 dB channel width is derived from amateur service rules in the US. Commercial and real world folks use around 60 dB to define the skirts for occupied bandwidth, but, that is a whole nutter line of discussion. Anyhow, with the demodulated signal in hand, it is easy to connect many different repeater controllers that use discriminator audio to derive squelch or COR and add a CTCSS decoder. It was also easy to make the repeater disconnect from the dstar controller when ctcss was detected and stay disconnected for a bit. The problem was component size in the transmitter. The ID1 contains emphasis, and deviation limiting circuitry that apply contoured audio to a varactor diode modulator in the transmitter. The 1.2 repeater does not have any of these components besides the varactor diode installed. There are pads, etc and even the AMBE chip is missing (hence no means to take dstar to AF in receive or AF to dstar in TX in a repeater). This makes the repeater only a pass through repeater with no decoded or encoded voice available as John Hayes mentioned in an earlier post. The ID1 has 2 different paths for signal to be applied to the varactor modulator, DSTAR and Voice that are switched depending on what mode is selected. I went ahead and added one very tiny capacitor (after removing sheilding around the varactor modulator and decided to QUIT ! I don't have the facilities to install such tiny components in such a dense circuit board and just plain got cold feet about ruining a working repeater. The point of this post and the link to GMSK is that the radio transmitter uses a varactor diode to directly modulate the frequency. The signal applied to this modulator, voice, modem tones, ttl data, or gmsk data will caused the frequency to shift up and down accordingly. The modulation scheme determines occupied bandwidth and GMSK is intended to help transport a digital signal in less bandwidth than other means. Hope this clears up any misunderstandings about GMSK and FM. 73 and Happy 4th of July in the US. de nu5d -- "It is a poverty to decide that a child must die so that you may live as you wish." Mother Teresa
