Ancillary data is indicated by a 3-word packet consisting of 0, 3FF, 3FF (the opposite of the synchronization packet header), followed by a two-word identification code, a data count word (indicating 0 - 255 words of payload), the actual payload, and a one-word checksum. Other than in their use in the header, the codes prohibited to video payload are also prohibited to ancillary data payload. Specific applications of ancillary data include embedded audio, EDH, VPID and SDTI. In dual link applications; ancillary data is mostly found on the primary link; the secondary link is to be used for ancillary data only if there is no room on the primary link. One exception to this rule is the VPID packet; both links must have a valid VPID packet present. Both the HD and SD serial interfaces provide for 16 channels of embedded audio. The two interfaces use different audio encapsulation methods — SD uses the SMPTE 272M standard, whereas HD uses the SMPTE 299M standard. In either case, an SDI signal may contain up to sixteen audio channels (8 pairs) embedded 48 kHz, 24-bit audio channels along with the video. Typically, 48 kHz, 24-bit (20-bit in SD, but extendable to 24 bit) PCM audio is stored, in a manner directly compatible with the AES3 digital audio interface. These are placed in the (horizontal) blanking periods, when the SDI signal carries nothing useful, since the receiver generates its own blanking signals from the TRS. In dual-link applications, 32 channels of audio are available, as each link may carry 16 channels. SMPTE ST 299-2:2010 extends the 3G SDI interface to be able to transmit 32 audio channels (16 pairs) on a single link. As the standard definition interface carries no checksum, CRC, or other data integrity check, an EDH (Error Detection and Handling) packet may be optionally placed in the vertical interval of the video signal. This packet includes CRC values for both the active picture, and the entire field (excluding those lines at which switching may occur, and which should contain no useful data); equipment can compute their own CRC and compare it with the received CRC in order to detect errors. EDH is typically only used with the standard definition interface; the presence of CRC words in the HD interface make EDH packets unnecessary. VPID (or video payload identifier) packets are increasingly used to describe the video format.