Design notes on telephone circuits ( 38 ) :

• Signaling

• Black Magic! Telecom Design Tricks - 40 page book on telephone ringing and related electronics.   
• Circuit translates A law to m law - two common methods exist to compand voice for transmission through a PCM channel   
• DSP Solutions for Data/Facsimile Modems - application note from   
• DTMF Tone Generation and Detection: An Implementation Using the TMS320C54x - This application note describes the implementation of a dual tone multiple frequency (DTMF) tone generator and detector for the TMS320C54x. This application note provides some theoretical background on the algorithms used for tone generation and detection. It documents the actual implementation in detail.   
• MT8870 Datasheet - popular DTMF decoder IC from   
   

• Line interfacing

• 3750 V of Isolation Not Required at the Network Termination - technical note in pdf format from   
• Approximation problem of telephone-line characteristi impedance - This article provides a design procedure to minimize the reflection effect due to telephone-line mismatching and increase the transhybrid attenuation located on the telephone 2w/4w termination. The aim is to prevent reflection and poor trans-hybrid attenuation, rather than repress them.   
• DAAs go for the silicon - The transformer-based telephony data-access arrangement faces strong challenges from optical- and capacitor-based designs that reduce costs and enhance performance.   
• dBm to RMS voltage chart - in pdf format, also available in   
• DC Holding Circuits - features several DC Loop Holding Circuits, one of them meets the UK voltage/current template as specified by NET4. The article may also be downloaded in Microsoft Word format,   
• Gyrator circuit - simulate large coils electronically   
• Handy Line Interface Design Hints!   
• How to measure Total Harmonic Distortion (THD) of a transformer or other passive device - none page Word file from   
• Impedance measurements of telephone lines - This two page technical note shows the impedances measured on the telephone line, for more information read the   
• Low cost, CTR 21 compliand DAA for Europe - application note from   
• Midcom Technical Note Technical Note Index - tech notes about telephone transformers   
• Modem Line Interface Solutions using Vishay SSRs   
• Notes on the Telephone-To-Sound-Card Interface Project - The purpose of this circuit is to allow a sound card (a SoundBlaster AWE 64 in my case) to interface with the phone line so that output from the sound card that normally goes to the speakers goes instead to the phone and can be heard by a remote caller or local phone, and also speech on the phone line will be sent to the sound card via the sound card's microphone interface.   
• Optocouplers Isolate Modem Data Access Arrangement - application note in pdf format   
• Overview of IEC 950 - EN 60950 (BABT), UL 1950 / CSA 950 - A Brief Overview of what it is and what is important when dealing with transformers in pdf format from   
• Reference Telephone Line Interfaces for CMX686 V.22bis and CMX687 V.22 Low Power Modem   
• Return Loss Calculations - four page note explains how return loss can be determined and contains a MathCad analysis, an Excel spreadsheet analysis and a QuickBASIC program listing, zipped file from   
• Transformer Equivalent Circuit - The 'complete' equivalent circuit as used in Midcom's Transformer Analysis Program (TAP)   
• Transformer Modeling Tips - technical note in pdf format from   
• Transformer Seminar Notes - companion to the seminar entitled "The Telecommunications Transformer: low-cost solution to product reliability and continuing regulatory compliance" by Dave LeVasseur, in pdf format, from   
• Wiring inside telephones   
   

• Automatic terminal equipment

• Phone Remote System - a telephone interface that allows the householder to give instructions to appliances from outside, application note from SGS-Thomson   
• Telephone & Answering System   
   

• Telephone ringing

• Ring-tone generator uses high-Q filter - produces high quality sinewave ring signal   
• Telephone ringing circuits - how to detect ringing and how to make phones ring   
• Telephone Ring Generator   
• Telephone Ring Generator - will ring a telephone once every 10 seconds   
• Telephone Ring Generator Using Small Power Transformer - This ring generator will ring a telephone once every 10 seconds. The interval between rings can be lengthened or shortened if needed. The 70 volt/ 30 Hz ring voltage is produced from the 120 volt side of a small 12.6 VAC power transformer.   
• Telephone Ring Generator Using Switching Supply - This telephone ring generator below generates the needed high voltage from a simple switching mode power supply (SMPS) which employs a CMOS Schmitt Trigger square wave oscillator, 10 mH inductor, high voltage switching transistor (TIP47 or other high voltage, 1 amp transistor) and a driver transistor (2N3053).   
• Telephone In-Use Indicator, Telephone Ringer and The Surfers Preserver - This Phone Ringer circuit will work with any ordinary phone including older bell ringer types. The circuit rings the phone in a completely realistic manner until someone answers. When the receiver is lifted the user hears the audio of your choice. DC current is passed through the phone to activate the phone?s electronics.   
   

• Telephone ICs

• Voice Switched Speakerphone ICs (TEA1095) - pdf file   
• TEA7540 Speakerphone Circuit for Handsfree Telephone Set   
   

• Other ideas

• How to make residential phones regulate their own power - With the advent of broadband access, manufacturers of small residential-phone systems are delivering products that no longer depend on the central office to regulate and control the power they use.   

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