Efficient low-delay video encoders are of fundamental importance to provide timely feedback in remotely controlled platforms such as drones. In order to fully understand the theoretical limits of low-delay video encoders, we consider an ideal differential predictive coded modulation (DPCM) encoder and provide the explicit derivation of the sum-rate-distortion region for a generic number of successive correlated Gauss-Markov sources along the line of the work by Ma and Ishwar. Furthermore, we provide an upper bound on the minimum distortion achievable in case an arbitrary number of sources are not available at the decoder.