SMPTE (Society of Motion Picture and Television Engineers) is requesting feedback regarding changes to the SMPTE 299M-1997 embedded audio standard, "24-Bit Digital Audio Format for 292M Bit-Serial Interface"; SMPTE 299M is currently undergoing revision within the N26 Technology Committe and a change has been proposed that may affect compatibility of existing devices.
The 299M standard was written to support a 1125 line-video standard, but since its original publication, according to SMPTE, the scope of the 299M serial interface has been extended to support other video formats. The current 299M standard does not support at least one format currently defined for the 292M interface, and the proposed change is a solution to solve the incompatibility with this video format and other formats that may be defined for the 292M interface in the future.
According to SMPTE, in the current 299M standard, the audio clock phase field is limited to 12 bits, or 4,096 video-sample clock periods, meaning that the current standard is incompatible with video formats with a total line length greater than 4,096 sample clock periods. It has been proposed to extend the audio clock phase field to 13 bits to support video formats that exceed this number of clock periods per line. Currently, there is only one video format defined for the 292M interface that exceeds this limit—the 1280x720 24Hz progressive format defined in SMPTE 296M—which has 4,125 video-sample clock periods per line.
To send your feedback on the proposed change, e-mail Carl Girod, director of engineering, at CGirod@smpte.org. Responses will be accepted until the week of May 19, 2003.