Apple's iPhone handsets have been over-reporting signal strength since they were first launched, the company said in response to reports of the latest version of the smartphone losing signal when held in a certain way.
In a open letter on Friday to iPhone 4 users, Apple said it had been "stunned" to find that the formula it uses to calculate how many bars of signal strength to display "is totally wrong". The company said it had discovered this longstanding flaw — the first iPhone came out three years ago — when investigating reports of the iPhone 4 losing four or five bars of reception "when tightly held in a way which covers the black strip in the lower left corner of the metal band".
"Our formula, in many instances, mistakenly displays two more bars than it should for a given signal strength," the letter read. "For example, we sometimes display four bars when we should be displaying as few as two bars. Users observing a drop of several bars when they grip their iPhone in a certain way are most likely in an area with very weak signal strength, but they don't know it because we are erroneously displaying four or five bars.
"Their big drop in bars is because their high bars were never real in the first place."
Previously, Apple had explained the iPhone 4 reception problem by saying the issue was a "fact of life for every wireless phone". In Friday's letter, the company again said that gripping almost any mobile phone in certain ways will reduce its reception by one or more bars, but conceded that the iPhone 4 showed a "far bigger drop than normal".
"To fix this, we are adopting AT&T's recently recommended formula for calculating how many bars to display for a given signal strength," the company said. "The real signal strength remains the same, but the iPhone's bars will report it far more accurately, providing users a much better indication of the reception they will get in a given area. We are also making bars 1, 2 and 3 a bit taller so they will be easier to see."
A free software update that fixes this misreporting of signal strength will be pushed out to users of the iPhone 3G, 3GS and 4 within a few weeks, Apple said.
Apple added that "the iPhone 4's wireless performance is the best we have ever shipped" and signed off the letter with: "We hope you love the iPhone 4 as much as we do. Thank you for your patience and support."
ZDNet UK has asked Apple to explain the connection between the misreporting of signal strength and the iPhone 4's greater-than-normal signal drop-off when held in a certain way, but had received no answer at the time of writing.
The company is currently facing a class-action lawsuit in the US over the iPhone 4 signal issue.