Biamp first introduced an analog telephone interface with the AudiaFLEX TI-2 card, followed soon after with the Nexia TC. Recently we’ve released the STC-2 card for Tesira, which shows our ongoing support of analog telephony. In this blog post we’ll discuss some basic terms of telephony and then dive in to programming and troubleshooting the interfaces.

Analog telephony terms

  • Plain Old Telephone Service (POTS) refers to the standard low speed and limited bandwidth (180 to 3200Hz) analog service used to deliver telephone services to many homes and businesses. Current modulation on a twisted pair of wires is used to communicate the audio information in full duplex. TI-2 and Nexia TC interface with POTS lines.
  • Public Switched Telephone Network (PSTN) refers to the worldwide public telephone network. Initially composed of analog trunks, the PSTN is now almost entirely digital and includes cell phones. It is the main backbone interfacing with POTS.
  • Private Branch Automatic eXchange (PBX/PABX), also known as a phone switch, is mainly used to route and interface multiple internal phone extensions to the PSTN. PBX are becoming less common but are still present in many corporate offices.

Biamp telephone interface cards (TI-2 [Audia], STC-2 [Tesira])
The TI-2 and STC-2 cards are full-featured dual channel analog telephone interface cards including line echo cancellation, caller ID, ring detection/Validation, auto-answer, autodisconnect, DTMF and call progress decoding, just to name a few features. The connectivity is on a removable terminal block connector with “spring cage” contacts. A SET connector allows for a telephone handset to be connected in parallel with the telephone line interface.

Nexia TC telephone interfaceConnectivity Diagram
The Nexia TC has a single channel interface with similar features as the TI-2 but with RJ-11 connectivity. Only the 2 center pins of the connector are being used. A SET connection is also available.

Note: In North America, TIP conductor is typically red and the RING conductor usually green. If colors are different than the norm, use a voltmeter and expect a negative DC voltage between the TIP and RING (usually between -54V to -40V).

Loop drop vs. call progress
Biamp’s analog telephone interfaces can automatically detect call disconnection at the far end using the following two methods:

  • Loop Drop: Monitoring the current on the line, the interface automatically hangs up whenever the current drops below a certain level. Loop drop is the typical method of auto-disconnection used on most POTS lines and some PABX.
  • Call Progress Tones: Typically used on PABX infrastructures, call progress tones are generated to indicate call termination with the far end. Each country has its specific set of call progress tones (also call tone plan) so be sure to correctly specify your country code from the property sheet of the TI-2/STC-2/TC receive block.

Telephone Interface Blocks
Both the TI-2 card and the Nexia TC interface use the following software blocks:

Receive BlockReceive Block
The Receive block controls the far end input signal, ring level, automatic answering of the incoming calls and includes the following logic nodes:

  • Hook Switch (HS) logic input to manually control the Hook State: Logic High (1) for OFF hook, Logic Low (0) for ON hook
  • Ring Indicator (RI) logic output goes High (1) during an incoming call: a useful feature to warn 3rd party devices. (e.g. Control processor)

Transmit BlockTransmit Block
Signal sent to the far end is controlled from the Telco Transmit block. Controls include Mute and output level adjustment. Note: The output level can only attenuate the signal. If you need to increase the gain of the transmit signal, include a level control block on the DSP chain.

Dialer BlockDialer Block
The Dialer block interface includes:

  • Dial number and Press Send to initiate a call
  • Speed dial for up to 16 numbers (logic trigger also available)
  • Hook Flash (See below example)
  • Redial the last number (logic trigger also available)

DTMF Decoder BlockDTMF Decoder Block
The Dual Tone Multi Frequency (DTMF) decoder block decodes dual frequency tone signals sent over the line (e.g. 1, 2, 3..etc). The logic outputs go high (1) whenever the corresponding DTMF tone is decoded.

Control Status BlockControl Status Block
Useful to monitor the telephone interface, the Control Status block has multiple features including:

  • Four Auto-disconnect modes (None, Loop drop, Call progress and Loop Drop + Call Progress)
  • Monitoring of the interface is also easily achieved using the multitude of logic outputs.
  • e.g. A command string block connected to the Hook State (HS) logic output could monitor the hook state and indicate its status to the control processor without having to constantly poll the DSP.
  • Voltage and Current monitoring are useful troubleshooting tools to test your telephone line. The typical voltage on POTS/PBX lines varies but one should expect between – 54V to – 40V when ON Hook (no dial tone) and –20 to –5V when OFF Hook.

More information about analog telephone interfaces, including advanced programming tips, can be found in our archived newsletter. The companion .dap file is also available for download. Happy programming!