Specifications

1
CHAPTER 1
Introduction
DESCRIPTION
The Model SPK300L, a Portable Speaker User Station, is a component used in the TW INTERCOM SYSTEM. Each User
Station is a communications unit along a multi-unit conference bus.
Figure 1, “TW System Concept Block Diagram,” on page ii, shows User Station interconnection, and User Station connection
to the system power supply.
User Station interconnection can be: 1) centrally wired, with each cable coming from a central point, or 2) distributed, where
all the user stations are looped together from one to another, or 3) a combination of both. The centrally wired interconnection
not only reduces interchannel crosstalk, but also allows for easier expansion into an assignable channel, multi-channel system.
Figure 1, “SPK300L Block Diagram,” on page 3, shows user station functional components, input/output connections, and
controls.
The SPK300L User Station has the following
functional components:
•a microphone preamplifier with limiter
•an electronic microphone switch
•a "bilateral current source" line driver
•a listen volume control
•a headphone amplifier
•a speaker amplifier
•a speaker switch
•a channel selector switch
The Microphone Preamplifier/limiter converts the small microphone signal to a strong line level signal conditions the signal
strength from loud and soft talkers to be almost the same sends the signal to the line via the microphone switch and a "bilateral
current source".
The Bilateral Current Source adds signal, via the channel select switch, to the line without affecting any signals already on
the line. The bilateral current source also extracts the listen signal from the line and sends it to the headphone amplifier via the
volume control. Some of the user's own voice signal ("sidetone") is also fed to the headphone amplifier.