What is a codec in phone calls?

Every internet call runs one. The two ends agree on which before the conversation starts, and the choice is usually invisible until something sounds wrong.

G.711 sends more data and keeps more of the original sound. G.729 squeezes the same voice into roughly an eighth of the bandwidth, and you can hear the difference on music or a noisy line.

Newer options like Opus adapt as they go, dropping quality when the connection struggles rather than breaking up.

Speech recognition cares about this more than people do. Heavy compression strips detail a human ear fills in automatically and a transcription model cannot.

Picture a caller spelling her email address to a front desk: S as in Sam, F as in Frank. People reach for those helper words on the phone without thinking, and the codec is why.

A traditional phone call carries sound between roughly 300 and 3,400 hertz. That range is enough to understand speech, and the standard G.711 call still uses it. The trouble is that the hiss that separates an S from an F sits mostly above 4,000 hertz. On a standard call that part of the sound never gets sent, so the two letters arrive at the far end nearly identical.

Human listeners cover the gap with context. A transcription system hearing a lone letter in an email address has none to use, so it guesses. Spelled names and addresses fail more often than ordinary sentences for exactly this reason.

Wideband codecs, often sold as HD voice, carry sound up to about 7,000 hertz and keep that detail. They only help when every leg of the call supports them.

Few calls use one codec from end to end. A cell network uses its own, the carrier in the middle may use another, and your phone system a third. Every handoff decodes the audio and compresses it again, and each pass loses some detail, a bit like photocopying a photocopy.

You can't control most of that chain. What you can do is avoid adding a lossy step of your own. If you run your own system, check whether it's set to a low-bandwidth option like G.729. That one needs about 31 kbps per call, compared with 87 kbps for G.711. The saving made sense on a slow line 15 years ago and buys you nothing today.

Many desk phones will show you the codec in use. Look for a call statistics screen in the phone's menu during a live call. If it says G.729 and you didn't choose that on purpose, ask your provider why.

Calls reach GreetKeeper through your carrier's network, so the negotiation happens upstream of us on the path your phone provider already uses.

Where transcript accuracy disappoints, audio quality is the first thing worth checking. A heavily compressed call transcribes worse than a clean one, every time.

The transcript shows you where words were missed, which points at the line rather than at the caller.