Maintaining sound and picture synchronisation in a digital video environment

In a complex broadcast environment, there are significant problems in trying to maintain the correct synchronisation of the various programme elements throughout the production chain. Individual delays introduced by equipment soon lead to obvious timing errors between the sound and picture and this problem is exacerbated as the signals pass through different production and distribution areas. The BBC has addressed this problem in the analogue production environment by carrying delay information embedded within the signals which is updated whenever any significant delay is added. Where timing correction is required (for example at a studio output), the delay information is used to determine which signal to delay and by how much to achieve synchronism. Since most new major broadcasting installations are now digital in nature rather than analogue, this problem must now be solved in the digital domain. The same technique of using delay information is equally valid and this paper addresses the question of where in a digital signal should the information be carried.