How come you can't hear yourself speaking on a cell phone?
August 19, 2012 10:08 PM   Subscribe

Why can't you hear yourself on a cell phone? Land lines, even old analog ones, feed the sound of your own voice back to you, so you can more easily gauge how loudly you are talking. Why don't cell phones do that?

I am interested in this because of course we have all had to listen to people shout into their portable phones, and it seems like the lack of feedback is a major issue. What gives?
posted by wnissen to Technology (15 answers total) 3 users marked this as a favorite
I don't know the answer, but the feedback you speak of is known as "sidetone".
posted by frontmn23 at 10:14 PM on August 19, 2012 [1 favorite]

For a while in the early 2000's I had a phone that, due to some quirk of the speakers, broadcast my own voice back to me in addition to the other speaker's voice.

It was terrifying and incomprehensible.
posted by Sara C. at 10:16 PM on August 19, 2012 [2 favorites]

Err, sorry, this was a cellular phone. Sorry if that wasn't clear.
posted by Sara C. at 10:16 PM on August 19, 2012

On a landline phone, sidetone is a side effect of combining a 4-wire circuit (2 wires for microphone, 2 wires for earpiece) into a 2-wire circuit (the phone line). A balanced transformer (or its electronic equivalent) circuit called a hybrid is the bit that does this - but, unless the match between the 2-wire side of the hybrid & the 2-wire line is perfect, you'll always get some signal leaking between the earpiece & microphone sides.

It was also found in the early days of telephony that a little bit of signal leakage between sides of the hybrid is less disconcerting / more natural. Hence, there's a a little bit of mismatch deliberately designed into telephone hybrids to encourage that.

A cell phone doesn't have the shared path that a landline has - it has a transmit path from you to the base station, and a receive path from the base station to you - so the mixing never occurs by default. In general, cell phones do mix the 2 signals internally, to 'fake' sidetone & produce the more natural 'phone' sound - but, for various reasons, it's rarely at the same level as occurs on a landline.
posted by Pinback at 10:35 PM on August 19, 2012 [5 favorites]

Your premise is somewhat flawed - maybe not all the time, but I have definitely had this happen: my own voice coming back to me, with a slight time delay, probably due to the time delay of processing and transmitting the signal.

My guess for why it doesn't happen so often has to do with the physical arrangement of speaker and microphone on a cell phone - they are much smaller, and the microphone is possibly more directional than on a conventional analog phone, so the microphone on the other end doesn't pick up the sound of the speaker as much.

Other than that, cell phones employ considerable amounts of noise cancelling to improve sound quality, it would not be unreasonable to have the phone on the other end do some signal processing magic to avoid feeding your own voice back to you.
posted by Dr Dracator at 10:42 PM on August 19, 2012

CP: only to minimise the effect of far-end coupling (acoustic or mechanical) which results in echo & eventually feedback. The two are still mixed internally to provide some minimal sidetone at each end.

As Dr. Dracator observes, it's not always successful...
posted by Pinback at 10:48 PM on August 19, 2012 [1 favorite]

Apologies for answering your question indirectly, but delayed auditory feedback (under some conditions) is extremely disruptive to speech. This may be what affected Sara C.
posted by Nomyte at 10:55 PM on August 19, 2012 [2 favorites]

Yes, it is.
posted by Sara C. at 11:20 PM on August 19, 2012

On a landline phone, sidetone is the signal leakage you get from mouthpiece to earpiece. Echo is when some of the signal is reflected back by the far end of an unbalanced two wire circuit. If the echo is from the other end of a 14,000 foot wire, it's not a problem. Your brain integrates it into the sidetone and you hear it all as one thing. When the reflection is coming from somewhere on the other side of a long link, say an analog phone in England, you hear it as echo because of the excessive delay.

Usually, you don't actually hear it because the phone company employs echo cancelling circuits to filter it out. Sometimes it'll take a few seconds to train, so you may hear echo at the beginning of the call, but not later.

Digital cell phones use compression to reduce the amount of data sent over the air. This compression takes time. So much time that if you happen to hit an unbalanced analog circuit somewhere down the line, you'll hear it as echo even if your call is completely local. Thus, the cell network also employs echo cancellers. When they're not working, it's rather difficult to speak, as Sara C. mentioned.

Since the sidetone would be too delayed if it came from the network side (not to mention being a waste of precious bandwidth!), it's generated locally in your phone. Sometimes it's adjustable in engineering test menus and the like, but usually not. If there's newer firmware for your phone, you might update it. I've seen plenty of firmware updates that include sidetone fixes.

I suspect people often shout because the sound quality sucks on most phones/networks, making people subconsciously raise their speaking volume to be heard. This despite volume not being the actual problem. Same thing as shouting over static.
posted by wierdo at 11:29 PM on August 19, 2012 [3 favorites]

When I shout into my cell phone, it's because I can't hear the person I'm talking to. I forget that they're not on the bus with me (or whatever) and aren't subject to the same external noise. My landline, on the other hand, was in the house, so it was never a problem; I could hear myself and so didn't shout.
posted by windykites at 5:18 AM on August 20, 2012

It probably saves the battery.
posted by dgran at 9:05 AM on August 20, 2012

I'm not sure exactly why cellphones don't have sidetone, but I'll just note that on some crummy VOIP phones and nearly all softphones, there's no sidetone either. So it doesn't seem to be entirely a cellphone thing, but a digital telephony thing.

Honestly it may just in part be laziness misplaced optimization on the part of the engineers; introducing sidetone on a digital phone (where the signal path from the microphone and the one to the speaker never mix in normal operation) would require an extra step, and in all but the latest generation of phones probably separate circuitry. It's something you'd have to actively introduce, as opposed to on analog phones where it can't be altogether eliminated and thus managing the amount of it is a necessary step in designing a phone.
posted by Kadin2048 at 12:56 PM on August 20, 2012

Most people consider sidetone a bug, not a feature. I always hated it and am so glad not to have to deal with it!
posted by Sidhedevil at 6:02 PM on August 20, 2012

I disagree, Sidhedevil. I've always found it to be very desirable- in phones, as well as in radio communications with a headset-microphone, and it's universal on aircraft radio/intercom systems simply because its presence serves as an indication that things are working- as well as providing a means of letting me know that I'm speaking loudly enough to be heard! See the wiki entry- Sidetone
posted by drhydro at 9:56 PM on August 20, 2012

I worked at an office IP phone company for a while, and getting sidetone right was a specific engineering goal. It's definitely not a bug, although there are a lot more ways for it to go bad than good when it's artificial.
posted by mendel at 6:11 PM on August 22, 2012

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