SAN FRANCISCO: Apple has said that it was "stunned" that its iPhone 4 used erroneous formulas to calculate signal strength, and promised to issue a free software patch to resolve the issue that has already triggered lawsuits.

In a letter directed to users of the latest iPhone model, the company said the signal drop was likely exaggerated because its formula was "completely wrong," displaying four or five bars in areas with a very weak signal.

It also reminded user they could return their smartphones within 30 days of purchase for a full refund.

Apple shot down users and outside engineers who said the signal problems were due to faults in its new antenna system, which is incorporated in the casing.

Some complained that cupping the smartphones, which launched a week ago with blockbuster sales, in a way that covers the lower left corner strangles telecom service signal strength.

But Apple said users' "big drop in bars is because their high bars were never real in the first place."

The company said the incorrect formula was present in the original iPhone -- released in 2007 -- and promised to fix it by conforming to AT&T guidelines for signal strength display through a free software patch that would be issued within a few weeks.

The software update will also be available for the iPhone 3GS and iPhone 3G.

As part of its antenna system to improve signal strength, the iPhone 4 has silver edging.

"Gripping almost any mobile phone in certain ways will reduce its reception by one or more bars," Apple said.

It noted that some iPhone 4 users had reported a drop of four or five bars when tightly held in a way that covers a black strip in a corner of the silver edging designed as part of the antenna system to improve signal strength.

"This is a far bigger drop than normal, and as a result some have accused the iPhone 4 of having a faulty antenna design," Apple added.

"Upon investigation, we were stunned to find that the formula we use to calculate how many bars of signal strength to display is totally wrong."

In many instances, Apple's formula mistakenly displays two more bars than it should for any given signal strength, the company said.

The update also make the first three bars slightly taller so they would be easier to see.

Apple maintained the iPhone 4's wireless performance remains "the best we have ever shipped." 

Following Apple’s statement in short :—-

Dear iPhone 4 users:
Apple iPhone 4 has become the most successful in history of a product release. Global evaluation were based on the measurement, iPhone 4 has become the world’s best smartphone. Our users have told us that they love this phone. So when we noticed the problem on the iPhone 4 signal coverage, we are very shocked, and immediately investigate this. Here are our findings.

First, almost all phones uses one or more of the columnar format to display the signal strength whether it is an iPhone 4, iPhone 3GS, or Droid, Nokia and RIM phones, every handset has adopted this approach. But some users complained that when they grip the lower left corner of the black metal frame section, iPhone 4 decrease the signal columnar cells 4 or 5. This signal intensity decreased more than the normal level, the result is some users have condemned the iPhone 4 antenna of a flawed design.

At the same time, we continue to focus on media, and received hundreds of letters and user’s e-mail for iPhone 4. They reflect, iPhone 4 of the answer quality is better than iPhone 3GS, they are very happy. This is consistent with our expectations and test results. 

We are really shocked because the signal grid survey found that this problem is very simple and surprising. After investigation, we surprised to find, we calculate the signal strength of the formula shows an error. Our formula in many cases shows the error of multi-display more than 2 cell signal. For example, sometimes displayed four cell signaling and while holding it in a certain manner the signal drops the number of grid cells but the user thinks that it is because the signal is very weak in the region. 

To fix this problem, we adopted the AT & T of the recent calculations to show the signal strength . Signal strength will still remain the same, but the signal strength display iPhone 4 will be more precise, to provide users with better signal strength instructions. In addition, we will use 1,2,3-cell signal strength mode, so users can more easily see the signal strength.

We will publish updates in the coming weeks , it will contain the correct formula. As the problem existed in the previous iPhone, so the software update will be available to iPhone 3GS and iPhone 3G users.

We have returned to the laboratory carried out all re-test, the result is the same – the iPhone 4 in the wireless performance is that we have released the best products. There is no interference by this problem for most users, the software update will make your cell phone signal be more precise. For those users affected by this issue, we apologize for our inconvenience.

If you are unhappy with the purchase of iPhone 4 in 30 days time, you can return an undamaged iPhone to any Apple retail store or an Apple store to return, we will full refund.

We hope you love the iPhone 4.

Thank you for your patience and support.