Jitter: A Failed Transaction

When you swipe a card and nothing happens or the reader responds with a "card read error" message, one of the most common problems for the failure is jitter. Jitter negatively affects the ability of the card reader to correctly decode the information on the magnetic stripe into readable information. Sources of jitter include the encoding process, the card reader, and normal customer use.

The magnetic stripe on a card stores data like a computer stores data, in binary format. Binary just means that the information is encoded into sequences of 1s and 0s. When the clerk at a store swipes a credit card, for example, the reader decodes the sequence of 1s and 0s into human readable information, such as credit card number, name, and address.

Why Does Jitter Occur

In order for the reader to accurately read the information stored on the card, each bit needs to be encoded to a consistent length. When the card is encoded, the iron or barium ferrite that coats the tape is magnetized to create flux changes, otherwise known as zones.

When the card passes through a card reader, it measures the distance between flux changes to determine if the encoded bit is a 1 or 0. If the flux changes happen sooner or later than expected, the card reader has trouble decoding the information on the magnetic stripe. As a result, the card misreads and the transaction fails.

Common Causes

Sources of jitter during the encoding process include:
Defective Tape Out of spec card properties The card moves during the encoding process Electrical or mechanical noise At the point of the transaction, the card reader may introduce jitter through one of the following problems:
Vibration Sticking Slipping Worn or dirty magnetic read head End users also introduce jitter by using the card in day-to-day activities. As the card bends and the magnetic stripe incurs abrasions, jitter increases.

Why Test

On the date you issue a card, jitter is at the lowest level it will ever be for that card, which underscores the need to start with a card that has low jitter levels.

We calculate jitter as the percent change in bit length from one bit to the next. A card may be unreadable with a jitter range that exceeds 15%. Ideally, we want new cards to have a 0% jitter, which helps ensure quality transactions for our customers.

In my next Q-Card Hotline article, I’ll discuss how we test for jitter.