cell phone networks
May 18, 2013 12:32 PM   Subscribe

Does any cell phone network engineer perhaps know why you can't hear your own voice when speaking on a cell?

I have noticed that when I speak on a landline, I can hear my own voice attenuated in the receiver. It helps in monitoring one's speaking volume. That doesn't seem to happen on a cell. I can't hear my own voice in the receiver and therefore frequently end up yelling or speaking more loudly than I would normally.
Does anyone know why this is the case? And is there a fix to it? Something the carriers might do?
posted by jtexman1 to Technology (12 answers total)
The behavior is called "sidetone"— it's engineered into landlines on purpose for that exact reason.

My understanding is that cell phones do have sidetone (locally generated by the phone; the carrier can't do it because cell codecs introduce too much delay) but that the amount you get varies from phone to phone.
posted by hattifattener at 12:40 PM on May 18, 2013

posted by _Mona_ at 12:41 PM on May 18, 2013 [2 favorites]

Then what's happening when I hear my own voice echoed back on delay on a cell phone?
posted by maryr at 1:08 PM on May 18, 2013

Then what's happening when I hear my own voice echoed back on delay on a cell phone?

Typically it's an error, and/or the person you're talking to has a very sensitive mic or a very loud speaker (or both). The audio goes from your mic to their speaker, to their mic and then back to your speaker. The delay is the round trip delay - from you to them and back. It's annoying as shit.
posted by RustyBrooks at 1:20 PM on May 18, 2013

That's probably the phone on the far side having imperfect echo cancellation. Your voice goes to the far end, comes out the speaker, is picked up by the same phone's microphone, and returns to you. Unlike traditional land lines, cell phones have a pretty long inherent delay (10ms here, 20ms there, it adds up).
posted by hattifattener at 1:20 PM on May 18, 2013

The last time I studied state of the art digital phonography, btw, and this was in the early 2000s, there were echo-cancellation algorithms used in digital phonography a LOT - even pre cellphone when a lot of landline stuff was digital. The echo is always there to some extent, but is cancelled by these algorithms. The "error" I'm referring to above is the cancellation not working.
posted by RustyBrooks at 1:21 PM on May 18, 2013

The echo you hear can also be caused by calling into older phone systems that have old fashioned echo cancellers, and they are broken.

From what I understand, on landlines or cell phones, the talkback you hear is generated in your phone, not by the phone system. There would be echos if it was generated anywhere else.
posted by gjc at 1:25 PM on May 18, 2013

Old-fashioned landlines are fast enough that the sidetone could be generated at the CO or somewhere, but yes, it was actually generated by the phone.
posted by hattifattener at 1:36 PM on May 18, 2013

A landline call is carried on one wire. (It's known as "the local loop".) Outgoing sound and incoming sound are on the same circuit, so when you speak, the earpiece gets that sound just like it gets the other person's voice.

A cell phone uses virtual digital connections, obviously not wire. There's a digital stream going out, and a different one coming back. They aren't naturally mixed.

No, there's nothing technical to be done about this. From the point of view of the phone company, it's working as designed.
posted by Chocolate Pickle at 3:03 PM on May 18, 2013

That's not really true, Chocolate Pickle. As I said earlier, sidetone is explicitly engineered in, because a well balanced hybrid produces very little sidetone. And many (but not all) cell phones do produce sidetone.
posted by hattifattener at 3:11 PM on May 18, 2013 [1 favorite]

While I'm happy to have switched to Android for a number of reasons, I really miss the sidetone on my old Blackberry bold. I can't use a headset with the Android because of the lack of sidetone. There is no good reason why modern cell phones don't provide sidetone other than it has to be designed in whereas in old analog phones it came for free (and in fact had to be controlled so as not to be annoying). I frequently toy with the idea of designing a small widget to go between headset jack and phone socket that would provide the tiny amount of feedback to give good sidetone with an in-ear headset. But alas I'm not a very good analog electronics designer.

If any Google engineers that work on the Android audio codecs are reading this - please pretty please put a sidetone option in!
posted by Long Way To Go at 5:03 PM on May 18, 2013

Sidetone and echo are both the same forms of feedback except for the time delay. Sidetone has a delay of 5 ms to no more than 20 ms and provides useful feedback to the talker. When the feedback delay is longer than about 20 ms, it can be perceived as a disturbing echo to the talker.

In a landline, the hybrid at the central office is designed to provide a little bit of reflected feedback, about 8%. Since the central office is no more than a few miles away, the feedback is almost instantaneous and considered useful sidetone.

In cellular technology, the transmit and receive signals are separated and can have long delays of 20 to 100 ms, both because of distances and because of digital processing delays. So if the equivalent of the central office (or cell tower) were to try to feedback part of the audio, it would be perceived as an echo because of the processing delay. So for cellular traffic, feedback is intentionally suppressed because the delay would be an annoying echo, whereas for a landline, feedback is intentionally created because it is an almost instantaneous useful sidetone. This instantaneous feedback is one of the advantages of the analog landline.

Since the cellular system cannot provide the sidetone feedback due to processing delay, it must be produced locally in the handset if it is to occur at all. In analog landlines, sidetone came for free from the central office.
posted by JackFlash at 6:28 PM on May 18, 2013 [2 favorites]

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