I have to throw my $0.02, So my research says..... I always had a preference for the SSI-204 and SSI-202 but my later designs use MT8870s for the simple reason that they are still being made by a variety of manufacturers and the 204's are no longer availible unless you find a surplus vendor which will have them for 5 to 13 bucks, They also go by CD22204 and a MC number, MC14513? Can't remember.
The SSI-204 falses fairly easy. But on the plus side it decodes about 5 mS faster than the MT8870. Down side is it is tough to keep it happy. You have to put a stiff capacitor (47uF or greater) in the power +5 line other wise on a power surge in the circuit it does this weird stuff like it will stop decoding, Force the stobe pin high, then sit and start to "bake" in the socket. Wear an oven mitt when experimenting. Using the MT8870 reference design, the fastest reliable tones I could decode are 37mS long with 17mS of silence in between digits. It will decode with 32mS on time but it is not reliable on strings, With strings 32mS and 13mS off, it has problems with low and high tones such as 1,2,4,#,C,D everything else will decode okay. Board space as a factor, the 204 uses a 14 pin dip and needs only a crystal, oscialtor resistor and optional low pass filter of a resistor and capacitor (6.8K and 0.01uf). where the 8870 is 18 pins and needs a few more external components to operate and the optional low pass filter. The 8870 also has pins so you can implement hardware "guard time" which is kind of nice as it helps with the falsing, and you can control the gain of the incoming op-amp which my eleminate an extra chip from your board.. All in all the best DTMF decoder I have ever used is that of the ACC RC-850, Not the fastest but it is the most immune to noise I have worked with. It uses 2 chips, I believe they were MT8860 and MT8865, Which are no longer made. Basicly one chip pre-seperated the tones into 2 groups and fed it to the other to be coded into Hexadecimal. But you can be S-2 to S-5 into the repeater and still have reliable decode. Yahoo! Groups Links To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ To unsubscribe from this group, send an email to: [EMAIL PROTECTED] Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
