Взято с сайта мелкомягких :  Note: AC-3 data is streamed in units called frames. Each frame can be independently decoded into 1536 samples of PCM audio (multiplied by the number of channels in the frame). This represents 32 milliseconds of audio at 48 kHz sample rate. A 5.1 channel AC-3 frame occupies fewer bytes than 1536 stereo PCM samples. There are exceptions to this, but those types of AC-3 frames are very uncommon, cannot be transmitted over S/PDIF, and will not be discussed in this article. Therefore, to prevent delivering the compressed audio bitstream over the S/PDIF interface faster than real time (that is, to prevent delivering 32 ms of audio in less than 32 ms), an AC-3 frame must be padded with zeros until it takes up the same number of bytes as 1536 stereo PCM samples.