Number . on r- F..ll'Mc~ ~ Touch-Tone Squelch Remote speaker control with a DTMF decoder. by Patrick Wong VE3RGW D T MF (dual-lone multi-freq uency ) is a very s ucce ssful and popula r remote control system. We can hardly imagine a hand-held radio without a touch-tone pad. Fi rst ge neration DTMF decoders typically used passi ve LC fil ters , active filters or p hase -locked loop techn iq ue s to decode DTMF tones. They were either expensive or they suffered serious drift with changes in temperature or humidity. DTMF decoding ICs are m uch cheaper and reliable now. This article describes how to build a general purpose DTMF decoder with a lo w-priced chip from MITEL. The circuit supports DTMF squelch based on a three-d igit station ID (full 999 combination). It also supports decoding o f four additional commands-slation ID, plus one more d igit which can be used to perform remote control on external devices (on/off or single-shoot trigger operation). One expansion port is included to allow future development. The core of the whole equipment is the MT8870 DTMF decoder. This is a Slate-ofthe-art single-chip DTMF receiver incorporating switched capac itor fil ter technology and an ad vanced digital counting/averaging algori thm for period measurement. Conventional methods of freq uency decoding are usually based on freq ue ncy-counti ng or phase -locking tech niques. These methods are time consuming and inaccurate when handling unsteady, complex signals. Switched capacitor filter techno logy is based on a completely di fferent approach. It works on the theory that any AC signals are rapid changes of potential energy under specific timing. AC components, provided they bear the same frequency characteristics, will h ave the s ame timin g o n instantaneous potent ial variation. For example, a signal o f I Hz frequency will experience a peak positive value once every second. The duration o f one second will be constam among any AC signals bearing a I Hz frequency, even though they have d ifferent waveform s. Suppose a ci rcuit is designed to sample the potential value of a signal at preset intervals (say, at exactly 90 degree phase-shift timing fo r a speci fic frequency). If signals are really present bUI the total sum of all samples al ways experience zero . we ca n conclude that an AC component of that specific frequency has been identified . Each fixed timing will be good for only one Irequcncy. Since DTMF is a combination o f four row tones and four column tones. it will need at least eight sets of such time-switching circuits. In the real world. there are more than eight sets of such circuits in order to handle acceptable frequency tolerance in the incom ing DT MF si g nal wh ile d ecoding results arc devel o ped on an averaging of outputs from all these filters. Error Detection Control There are two kinds of DTMF detection errors: TALKOFF and TWIST. TWIST is the failure of decoding a valid DT MF signal due to non-linear frequency response of a transmission media. Among the two enemies. lWlST is easier to deal with s ince it can be controlled by audio equalization on the whole transmission path. These eq ualizations can be done internally inside the DT MF decoding chip, or externally as additional filters. According 10 rectory specification. MT8870 can stand TWIST tolcrancc of +/-6 d B. That is quite sufficient to compensate for the pre-emphasize and deemphasize distortion caused by FM media. T h is sho uld nOI be a p roblem unless the transmission is highly distorted. TALKOFF is the incorrec t recognition of the DTMF component in human voice as true DTMF signal. This is an unavo idable rector since human voices do contain valid DTMF combinations. Fortunately, the presence of these valid components is unsteady. Unlike real DTMF from a touch-tone keyboard or generator, these "human" DTMF signals cannot maintain a constant combination. so they can be isolated by DELAY d iscrimination. If a decoded DTMF signal can stay on constantly for a certain d uralio n which exceeds tbose normal periods experienced in human voice, then it can be ide mified as a real DTMF command. Look at the schematic circuit. There are some e xternal compone nts ho o ked 10 MT8870 (C 2 .R I I ,R1 2,D9 ) . Pi n 16 of MTR870 is the EST (early steering) o utput. It will n ag logic h igh whenever there is a DTMF combination present in the input signal (incl uding TALKOFF error detection). o'.f ~ ~t " • •• •• " w •• C .. " •• • ~ . • , ,,- ,..".. ." - ., ~~.... " - "1· 11 70 ,j • ~ , . t v -I / " , • • o .~ J:.... ... , '. , ~ '049 l~~OUT SpC,R E 8U"E" • ' - "'1' 1 01 I . ~ .. - · 9 ........ • ." .. :;",49 '" " • I, l» O. ". • ,. " j,' 'z ...., ,, •~ .. 4049 ,.. ,. , " ' , • ~ . , , .. • • • ·~ ~ • ~ v .. •• ,• • . • c ' Sl OOGol,.(.~T 01" ........ , • , .. • ,. , • ~ · , t" " ' " " " ,. .. .. • " .. .. ~ • • "' ,1.. "z * ~ z 1" '.. I'·' ~.. , , "\.. .""" • j ' .. .. .. • • , • " .. s:.. ~ . v • " I ." ~.. ",oJ .. < G 20 73 Amateur Radio Today " June. 1992 Figure J. Schematic diagram for the DTMF squek: h controller.
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- Page 3 and 4: ONG LE~ Greatly I Is" AMlSSB Signal
- Page 5 and 6: THETEAM PUBLISHERIEDITOR Wayne Gree
- Page 7: their PC to discover the "ell' visu
- Page 10 and 11: LETTERS Dave KD8VI, Lakewood OH I j
- Page 12 and 13: Number 4 onyourFeedbeclI CIII'd 8,0
- Page 14 and 15: inary address is input to the devic
- Page 16 and 17: make it easier to remove, you can m
- Page 18 and 19: !'Unbet' 5 on yovr FeedbKk card VE3
- Page 20 and 21: Figure 2. The wire beam for /5 and
- Page 24 and 25: Pin 17 is the GT (guard time) contr
- Page 26 and 27: output and charged up C I). If the
- Page 28 and 29: Number 7 on your Feedback e.rd Buil
- Page 30 and 31: •• ,. ,.,..,. " " .. :.~'" ,.-
- Page 32 and 33: help reduce the " thump" problem. F
- Page 34 and 35: You may elect to tinker with the in
- Page 36 and 37: VE3CYC's Wire Beam Colltitlued/rom
- Page 38 and 39: G S.',"m,,, ,.. ,""", , " Ol 'us,
- Page 40 and 41: . ICOM'" R7000 SWEEl'lNG : 1300 CHA
- Page 42 and 43: 73 Review by Bill Brown WB8ELK Numb
- Page 44 and 45: 73 Review by Mike Baker WSCM The He
- Page 46 and 47: planned to use. This top shelf is n
- Page 48 and 49: HAMSATS Number 10 on your Feedback
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- Page 52 and 53: _ «r • .,. .. "" lIea. .....- I"
- Page 54 and 55: SANELLI TECHNOLOGY Sanelli Technolo
- Page 56 and 57: Arv Number 13 on your Feedback card
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- Page 60 and 61: SPECIAL EVENT;,m..",,"",,,..db,,k,,
- Page 62 and 63: VE eeeme. Vendors need to reserve s
- Page 64 and 65: QRP Number 16 on your Feedback card
- Page 66 and 67: ABOVE & BE"YOND~F-- k ~" C. L Hough
- Page 68 and 69: 73 INTERNATIONAL Numbet 19 011 y(U'
- Page 70 and 71: TALK WITH THE KNOWLEDGEABLE PEOPLE
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ASKKABOOM MichaelJ. Geier KBfUM do
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NEVER SAY DIE Continu8(/ from page
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