FREE Subscription to Dr. Dobb’s Digest: Same Great Content, New Digital Edition
Site Archive (Complete)
Embedded Systems
Email
Print
Reprint

add to:
Del.icio.us
Digg
Google
Furl
Slashdot
Y! MyWeb
Blink
June 08, 2007
Real-Time Enough

A FreeBSD-based software Frequency Shift Keying modem for Caller ID

(Page 1 of 4)
Robert Krten
Robert implements a real-time caller ID implementation that is part of a voice-activated call monitor system he built.
Robert is an independent consultant specializing in real-time and embedded systems. He can be contacted at rk@parse.com or http://www.parse.com/.


Caller ID

Caller ID, which is a feature that tells you who is calling on the telephone, occurs as a Frequency Shift Keying (FSK) signal between the first and second ring. It operates at 1200 baud, using the Bell-202 Standard, which is based on 1200 Hz (a "mark," representing a 1) and 2200 Hz (a "space," representing a 0) tones. It's a serial transmission, with a start bit, 8 bits of data, no parity, and a stop bit. At the data level, it consists of packets of information with checksums.

In this article, I discuss a real-time caller-ID implementation that is part of a voice-activated call monitoring system I built. In particular, I discuss how the caller-ID part of the solution has evolved from a hardware FSK modem in the early 1990s to the software FSK modem in use today. While my particular implementation is based on FreeBSD 5.3, there is nothing operating-system specific here.

My hardware consists of an adapter for a phone line (Figure 1) that converts the telephone signal into something that I can feed into a sound card's input. Because I have two lines, I have two such circuits in my system. I've conveniently used the sound card's left channel for one line, and the right channel for the other.

[Click image to view at full size]

Figure 1: Telephone line adapter.

1 Caller ID | 2 Requirements | 3 The FSK Algorithm | 4 Bit-Cell Management Next Page
TOP 5 ARTICLES
No Top Articles.
DR. DOBB'S CAREER CENTER
Looking for a new job? open | close
Search jobs on Dr. Dobb's TechCareers
Function:

Keyword(s):

State:  
  • Post Your Resume
  • Employers Area
  • News & Features
  • Blogs & Forums
  • Career Resources

    Browse By:
    Location | Employer | City
  • Most Recent Posts:



    MICROSITES
    FEATURED TOPIC

    ADDITIONAL TOPICS

    INFO-LINK



     




    Techweb
    Informationweek Business Technology Network
    InformationweekInformationweek 500Informationweek 500 ConferenceInformationweek AnalyticsInformationweek Events
    Informationweek MagazineGlobal CIOIWK Government ITbMightyByte and SwitchDark Reading
    Digital LibraryIntelligent EnterpriseInternet EvolutionNetwork ComputingPlug Into The CloudDr. DobbsContentinople
    space
    TechWeb Events Network
    InteropVoiceConWeb 2.0 ExpoWeb 2.0 SummitEnterprise 2.0Mobile Business ExpoNoJitter
    Black HatGTECEnergy CampCloud ConnectGov 2.0 ExpoGov 2.0 Summit
    space
    Light Reading Communications Network
    Light ReadingLight Reading AsiaUnstrungCable Digital NewsInternet EvolutionPyramid Research
    Heavy ReadingLight Reading LiveLight Reading InsiderEthrnet ExpoTelco TVTower Technology Summit
    space
    Financial Technology Network
    Advanced TradingBank Systems and TechnologyInsurance and TechnologyWall Street and TechnologyAccelerating WallstreetBST SummitBuyside Trading SummitIT Summit
    space
    Microsoft Technology Network
    MSDNTechNetTotal IT ProTotal Dev ProNET Total Dev Pro CommunitySQL Total Dev Pro Community
    space