In one example embodiment, a card payment system includes a card payment apparatus, having an audio jack, configured to: read account information stored on a payment card, transmit the account information via a microphone contact of an audio jack, and receive an acoustic signal via at least one earphone contact of the audio jack; and a client device, having an audio jack socket to receive the audio jack, configured to: receive, from the card payment apparatus, the account information via a receptor for the microphone contact of the audio jack, and transmit, to the card payment apparatus, the acoustic signal via the at least one earphone contact.