Why does my cell phone break up when I have hold music?

February 11, 2008 2:07 PM

Why does my cell phone break up when I have hold music?

I have the LG enV cellphone with Verizon service in the US. I can hear fine when I am talking to someone, but as soon as I get hold music, the line cuts in and out, sometimes cutting the hold music so low that I can't tell it is there. This even happens if I put my mic on mute. It does not seem to matter if I am using a headset or not.

Can you suggest what might fix this and/or what is causing it?

posted by slavlin to technology (14 comments total)

The codecs and processing routines on your mobile phones are extremely optimized towards human speech. They will throw out any sound that it doesn't think is speech in order to both conserve network bandwidth and to cancel out noise. Music, especially at a low volume, gets thrown out by the codec. Sometimes if the music has a singer it will come across better.

There's no way to fix this. I've noticed this on both CDMA and GSM phones.
posted by zsazsa at 2:15 PM on February 11, 2008 [4 favorites has favorites]

Cellular codecs are not generic sound compressors. They are specifically designed for human voices speaking human languages, and they work really well for that.

Unfortunately, what that means is that they don't work at all well for any noise that doesn't sound like a human voice, like music. I would bet what you're hearing is codec failure.

That's very much a function of the specific codec. Verizon used to use the 13K codec, which was more resilient. However, it also used 13,000 bits per second at full rate. They'd rather not do that. So now they use the Extended Variable Rate Codec (EVRC) which sounds nearly as good but only uses 8,000 bits per second at full rate. I bet EVRC doesn't handle non-voice as well. That's the price you pay for greater compression of voice.

MP3 is a generic compressor, and it produces mediocre results using 100 kilobytes per second. EVRC typically averages at about 500 bytes per second. The only reason it can do that is because it's extremely specialized. Unfortunately, music isn't part of its specialty.
posted by Steven C. Den Beste at 2:16 PM on February 11, 2008

Sorry. MP3 is mediocre at 100 kilobits per second. EVRC averages less than 4 kilobits per second. (8 kilobits is full rate, but the majority of packets are not full rate.)
posted by Steven C. Den Beste at 2:19 PM on February 11, 2008

I actually was just put on hold a few moments ago and noticed the exact same thing. I've got a Moto rRzr z3 on T-mobile.
posted by Camel of Space at 2:22 PM on February 11, 2008

Additional data points: clever algorithms can squeeze voice down to 2400bps. The GSM standard was (still is?) 9600bps. A mediocre rate for mp3, equivalent perhaps to AM radio, would be 64kbps.
posted by Leon at 2:32 PM on February 11, 2008

It doesn't matter what type of phone you have. What matters is the codec that your mobile provider uses to encode the call content before it puts it over the air interface to your phone.

In other words, it's not that your phone can't hear the music on hold, it's that Vzw (or whomever) isn't sending the music with very good fidelity at all. The "sending" codec is discarding the parts that don't sound like speech, and you're hearing what's left.

Codecs - they're a blessing and a curse.
posted by johnvaljohn at 2:39 PM on February 11, 2008

I worked in cellular customer care when digital first came out, and we were told in strictest confidence by my then-employer that the first generation vocoders in our digital offering didn't work for people from east Asia. So even an unusual phoneme from a human voice can screw up a crappy vocoder.
posted by popechunk at 2:41 PM on February 11, 2008

Wow, I've always wondered why that happened (Motorola Razr v3m, Sprint).
posted by spikeleemajortomdickandharryconnickjrmints at 3:14 PM on February 11, 2008

Huh - I've also been wondering this for a long time, but never thought to ask it. Also, every time it's happened recently I've always wondered if it was because my phone (a Nokia 5300) was a dud - glad to hear this is a common, almost expected, thing.
posted by cgg at 3:14 PM on February 11, 2008

I know you said that you press mute, but I'm not convinced that this does anything to prevent your phone from processing sound (even if it's just a tiny bit) coming into the mouth piece receiver hole thingy. I think you should try covering the receiver's hole on the phone to lessen competition for your speaker (like everyone is saying, it tries to favor human speech)...
posted by albatross5000 at 3:15 PM on February 11, 2008

The GSM standard was (still is?) 9600bps.

GSM uses 19.2 kilobaud. A GSM carrier is 200 kilohertz, divided into 8 slots. With timing guard bands and RF guard bands and packet overhead, that leaves 19200 bits per second per slot.
posted by Steven C. Den Beste at 3:26 PM on February 11, 2008

clever algorithms can squeeze voice down to 2400bps.

Yes, and it would be intelligible, but it would sound like crap. It wouldn't be commercially acceptable. EVRC at 8,000 bits per second is about as good as you can do without driving away customers.

...or so I thought. Apparently I've been out of the loop too long, and there's been two further generations of codec development at Qualcomm since I left the industry (called "SMV" and "4GV").
posted by Steven C. Den Beste at 3:32 PM on February 11, 2008

The one thing nobody's mention yet is that the audio on mobiles cuts in an out all the time, but you don't notice it as much in a conversation because; You're talking a lot of the time and there are lots of little times where there simply is no sound.

You notice it much more when you know you should be hearing continuous audio.
posted by krisjohn at 3:50 PM on February 11, 2008

I'm glad you asked this question and so many knowledgeable people answered it. My Helio Drift does the same exact thing and it drove me nuts. Now I know, and knowing's half the battle!
posted by phredgreen at 2:55 AM on February 12, 2008