dave@mars.njit.edu (Dave Michaels cccc) (07/18/90)
A telephone handset has four wires going to it, two for mike, and two for speaker. How does the phone merge these two into a full duplex pair of wires? Also, I disconnected the transmitter disc trying to make a 'mute' feature on a phone without one, and discovered my friend can still hear me (at a reduced volume) from the earpiece ... hmm, why?
dave%westmark@uunet.uu.net (Dave Levenson) (07/19/90)
In article <9841@accuvax.nwu.edu>, dave@mars.njit.edu (Dave Michaels cccc) writes: > A telephone handset has four wires going to it, two for mike, and two > for speaker. How does the phone merge these two into a full duplex > pair of wires? The telephone contains a hybrid (located in a little box often called a network) which is supposed to direct the audio energy from the transmitter toward the line, and energy from the line toward the receiver. A little bit of audio from the transmitter is deliberately 'leaked' to the receiver, so that you can hear yourself (which is often called side-tone). This leakage path is attenuated so that most of your speech-energy is directed to the far end. This attenuation is called trans-hybrid loss. > Also, I disconnected the transmitter disc trying to > make a 'mute' feature on a phone without one, and discovered my friend > can still hear me (at a reduced volume) from the earpiece ... hmm, > why? Your telephone receiver may act in reverse, as a low-level microphone. Your friend can year the signal it generates due to the above- mentioned leakage-path working in reverse. The volume is reduced partially because the receiver is not very efficient as a microphone, and partially because of the deliberate trans-hybrid loss. Dave Levenson Voice: 201 647 0900 Fax: 201 647 6857 Westmark, Inc. UUCP: {uunet | rutgers | att}!westmark!dave Warren, NJ, USA AT&T Mail: !westmark!dave [The Man in the Mooney]
toddi@yang.cpac.washington.edu (Todd Inch) (07/23/90)
In article <9849@accuvax.nwu.edu> dave%westmark@uunet.uu.net (Dave Levenson) writes: >Your telephone receiver may act in reverse, as a low-level microphone. In fact, you can build a "high-tech" tin-can quality phone by connecting two of these telephone handset "speakers" together with a pair of wires. No amp, no battery. They just provide enough of a miniscule signal to drive one other and will act as either a microphone or a speaker. Amazing. They're also great for crystal radios because they're so sensitive. Using the speaker as a microphone is quite common in inexpensive push-to-talk station intercoms such as the $15 two-wire type from Radio Shack. One end has the amp and battery, the other end is just a speaker (plus a push button which shorts a DC-blocking capacitor to signal the "base") and the push-to- talk button on the base unit essentially just swaps the two speakers. I've also seen PA systems on boats that use a horn-type speaker on the mast to talk to land or another ship which also allow eavesdropping, er, listening via the same horn speaker. Todd Inch, System Manager, Global Technology, Mukilteo WA (206) 742-9111 UUCP: {smart-host}!gtisqr!toddi ARPA: gtisqr!toddi@beaver.cs.washington.edu