A method of transmitting a video packet stream at a video stream transmission bitrate from a transmitting device to a receiving device over a network, the method comprising, at the transmitting device: transmitting an audio packet stream to the receiving device at an audio stream transmission bitrate; receiving one or more audio packet metrics from the receiving device associated with receiving the audio packet stream at the receiving device; determining a measure of network bandwidth in dependence on the one or more audio packet metrics; determining whether the measure indicates sufficient available network bandwidth to support an audio stream with a higher transmission bitrate than the audio stream transmission bitrate and, if so, increasing the audio stream transmission bitrate; if the audio stream transmission bitrate has increased such that the audio stream is adapted to the best available quality, then transmitting one or more probe packets at a probing transmission bitrate for testing the available bandwidth of the network, wherein the probe packets comprise data duplicated from the audio packet stream; re-determining said measure during the transmission of the one or more probe packets; and determining whether the re-determined measure indicates sufficient available network bandwidth to support the video packet stream at the probing transmission bitrate and, if the re-determined measure indicates there is sufficient available network bandwidth, transmitting the video packet stream at the probing transmission bitrate; wherein, the transmission of probe packets is carried out at regular time intervals and, if during a probe, significant degradation of the bandwidth is detected, the time to the next probe is increased. transmitting an audio packet stream to the receiving device at an audio stream transmission bitrate; receiving one or more audio packet metrics from the receiving device associated with receiving the audio packet stream at the receiving device; determining a measure of network bandwidth in dependence on the one or more audio packet metrics; determining whether the measure indicates sufficient available network bandwidth to support an audio stream with a higher transmission bitrate than the audio stream transmission bitrate and, if so, increasing the audio stream transmission bitrate; if the audio stream transmission bitrate has increased such that the audio stream is adapted to the best available quality, then transmitting one or more probe packets at a probing transmission bitrate for testing the available bandwidth of the network, wherein the probe packets comprise data duplicated from the audio packet stream; re-determining said measure during the transmission of the one or more probe packets; and determining whether the re-determined measure indicates sufficient available network bandwidth to support the video packet stream at the probing transmission bitrate and, if the re-determined measure indicates there is sufficient available network bandwidth, transmitting the video packet stream at the probing transmission bitrate; wherein, the transmission of probe packets is carried out at regular time intervals and, if during a probe, significant degradation of the bandwidth is detected, the time to the next probe is increased.