telephone effect for mic

[carrejans]

Hi,
Are there any effects to put after a regular microphone, to create the telephone-effect?
Thanks.

[Mark Hammer]

There's one on GEOFEX for exactly that purpose.  I think you will have to scroll way down the list to where the print gets smaller to find it.  It is essentially a cascaded highpass and lowpass filter.  That's all telephone circuitry did or does.  It took a whole lot of psychological/communications research to find the smallest possible bandwidth that would still leave human speech intelligible.  That's why so many cool audio things came out of Bell Labs at Murray Hill.

[axg20202]

..otherwise known as a band pass filter....

[Mark Hammer]

Well, it's bandpass...of a sort.  The most familiar bandpass filter has an identifiable centre-frequency where the greatest resonance (or least attenuation) is situated.  While it is true that cascading a highpass and lowpass filter gets you a select band, that definition would make any pedal with an input cap that trims content (and DC) below 16hz, and an output stage  that trims top end above 15khz, a "bandpass" filter.  In this particular instance (the telephone tone), we are simply talking about an amplifier whose bandwidth is constrained at both ends, but which has no obvious emphasis that would place it in the same camp as traditional bandpass designs.

I'm not trying to be picky.  I just don't want novices to confuse what goes on in something like a Dr. Q with what goes on in a telephone-emulator circuit, by virtue of a technical term.

[axg20202]

Yep, fair enough. I wasn't trying to jump on your advice to the OP either - sorry if it came off like that. It's just that if I was sitting at my desk looking to patch something in for this effect, I'd be reaching for a bandpass and adjusting the Q and frequency settings etc until it sounded right. I've used this effect a few times when dubbing programme material that includes cutaways of people listening on the phone. I forget the settings right now because I have a preset for it.

Attentuating all those frequencies usually causes a fair drop in level. I would think any pedal-based circuit would need to have a gain stage post-filter to bring the level up to unity.

[e45tg4t3]

hello there,
i´ve got a schematic for an telephone effect out of a book... here´s a scan for you

Best Regards

Benny

[earthtonesaudio]

One more thing that came out of the Bell Labs telephone research:  distortion.  I think the limit for coherent speech was something huge like 50% THD.

Probably a dual opamp would do the trick for you...
Small input cap (cut bass), maybe a TS style gain stage, and a high pass at the end...
Then you take the output of the gain stage (non-inverting) and it goes to one pin of the microphone jack.
Split the output of the gain stage and run it through an inverting buffer (gain of -1) and it goes to the other pin of the microphone jack.

Voila!  Instant telephone tone AND balanced output!

(See Craig Anderton's Electronics Projects for Musicians, the 8-in-one-out-mixer project for more detail)

[mr.adambeck]

It's also REALLY easy (actually the easiest project I've ever done) to convert an actual telephone to have a quarter inch jack, so you can use it as a mic.  Let me know if you're interested in this and I'll write up some instructions.

[R.G.]

One more thing that came out of the Bell Labs telephone research:  distortion.  I think the limit for coherent speech was something huge like 50% THD.

Actually, it was more like 400% THD. It turns out that you can run speech into a comparator so that it turns out only square waves. You can still understand the speech. I read a paper once on the intelligibility of differentiated infinitely clipped speech - the comparator thing.