A method of compensating for a round-trip transmission delay in an audio system comprising: a portable communications device; an audio accessory; and a cable, suitable for connecting the portable communications device and the audio accessory, having at least one wire and being detachable from at least one of the portable communications device and the audio accessory. At first times, a first synchronization data pattern is transmitted on the at least one wire from said device to said accessory, wherein said first synchronization data pattern comprises first signal level transitions on the at least one wire, synchronized to a master transmission clock. At second times, a second synchronization data pattern is transmitted on the at least one wire from said accessory to said device, wherein said second synchronization data pattern comprises second signal level transitions at timings that are set based on a delay value stored in said accessory. In the device, timings of said second signal level transitions as received at said device are determined relative to the master transmission clock, and timing delay control data based on said determination is transmitted from the device to the accessory. In the accessory, the timing delay control data is received, and the stored delay value is updated based on the timing delay control data.