Genuine Digital Squelch with PIC Microcontroller Zoltan Meszaros HA5OB http://www.ha5ob.ham.hu/ddcd_e.htm Q=2.4576MHz, JP close=1200Bd, open=2400Bd -------------------------------------------------------------------------------- What the heck? Evaluating the signal coming from the modem, this circuit decides, if it contains a real data stream, or only noise. Does it a lot more efficient way, than any previously known circuit. -------------------------------------------------------------------------------- What the use of it? This circuit can be used with any modem, that lacks a proper DCD output. Good majority of terminal programs has already got software DCD, but it needs significant processor time, giving redundant load to the CPU with interrupts. This digital DCD supplies precise information about the state of the channel to the TNC, PC or RMNC hardware, so the CPU has more time for other tasks. Even better if data coming out of the modem is gated by this DCD circuit, so it only enters to PC (SCC or SIO), once DCD set active. It's extremely useful with BPQ nodes, or using PCFlexnet or JNOS/TNOS. With a standalone TNC or RMNC hardware, there is no software DCD, so the receiver must have the squelch always on. Devoted typically for voice use, the FM squelch introduces a 150-200 milliseconds delay switching forth or back. There are several small add-on boards available (utilizing e.g.. the XR2211 PLL demodulator IC), which are advised for existing TCM3105 or AM7910/11 modems. Unfortunately their performance is far from being ideal. DCD output of them goes active also when the XR2211 PLL locks on any carrier with a pitch between 1000 and 2500 Hz, thus giving a fake DCD in case of a whistle, on certain sounds of the human voice, etc. This digital DCD using PIC microcontroller is absolutely free of these malfunctions described above, as it supplies active DCD only in presence of valid digital data on the channel. If a packet station failed to switch back to receive and accidentally left the transmitter keyed up (emitting a continuos carrier), the conventional FM squelch or the XR2211 PLL DCD circuit prevents using the channel, even if the interfering signal is just above the noise. This PIC Microcontroller Digital Squelch circuit (as it analyzes incoming signal and puts DCD active only if a real digital signal is present) will find the channel free in this case, so you can keep on working on top of the low power interference, while others just stand by! -------------------------------------------------------------------------------- What existing hardwares you can use it with? TNC2 TNC3 BAYCOM serial or parallel modem (U)SCC board RMNC channel board HAPN 4k8 modem ... or any kind of modem with a speed between 300 and 4800 Bd -------------------------------------------------------------------------------- Main features Low power consumption (typically 1mA only) Receiver can be used with open squelch (i.e.. no additional delay) Fast attack time (only 15-20 milliseconds typically on 1200 Bd) Easy hook-up to existing hardware No tuning or adjusting needed at all