Practically speaking, how is the timestamp computed?

Practically speaking, how is the timestamp computed?

For audio, the timestamp is incremented by the packetization interval times the sampling rate. For example, for audio packets containing 20 ms of audio sampled at 8,000 Hz, the timestamp for each block of audio increases by 160, even if the block is not sent due to silence suppression. Also, note that the actual sampling rate will differ slightly from this nominal rate, but the sender typically has no reliable way to measure this divergence.

For video, the timestamps generated depend on whether the application can determine the frame number or not. If it can or it can be sure that it is transmitting every frame with a fixed frame rate, the timestamp is governed by the nominal frame rate. Thus, for a 30 f/s video, timestamps would increase by 3,000 for each frame, for a 25 f/s video by 3,600 for each frame. If a frame is transmitted as several RTP packets, these packets would all bear the same timestamp. If the frame number cannot be determined or if frames are sampled aperiodically, as is typically the case for software codecs, the timestamp has to be computed from the system clock (e.g., gettimeofday()).