Several proposals for Video--on--Demand (VoD) systems assume batching of the user requests and subsequent multicasting of the selected videos. We propose that the waiting time, which is essential for the sake of batching, can be overlapped with a more elaborate call admission scheme. The call admission scheme provides sufficient bandwidth at any point in time, in order to achieve guaranteed delivery of the video stream without any buffering or additional delays. The call admission is based on the construction of deterministic time--dependent envelopes. Essentially, statistical multiplexing is replaced by deterministic multiplexing. Bandwidth efficiency is maintained through the particular envelope construction which minimizes the amount of over--allocated bandwidth. The construction of the envelopes can be performed using either a computationally intensive exact algorithm or by a fast heuristic. Both the envelope construction and the call admission procedure are presented and simulated in detail. The results indicate the feasibility, benefits and tradeoffs of the proposed scheme.