All DVC codecs assume that they have access to the original Wyner–Ziv frame to decide to stop the turbo decoding process, i.e. the original binary sequence x is available at the decoder to compute BERr exactly. In this case, the optimal number of requests r* is obtained by monitoring the value of BERr. That is:
where τ is a threshold that indicates the acceptable residual BER. In the literature, τ is typically set to 10−3 for QCIF video sequences. (Reproduced by permission of ©2007 EURASIP.)
However, this is a hypothetical assumption which is not possible in any practical DVC codecs.
In the turbo coding literature (channel coding scenario), methods for early stopping in turbo decoding have been studied , ; the main target of these methods is a reduction in the number of turbo decoder iterations, decreasing the decoding process computational complexity. In this section, efficient request-stopping criteria will be proposed to control the number of requests for additional parity bits made by the decoder in the context of Wyner–Ziv video coding. Although the proposed criteria are based on the available turbo coding literature, they go a step further since they have to be adapted to a video coding context (source coding scenario), which was not done in the earlier implementation.