# A Novel Consistent Quality Driven for JEM Based Distributed Video Coding
## 3 Proposed JEM Based DVC (JEM-DVC)
### Proposed Consistent Quality Driven (CQD) for JEM-DVC  
The scheme that can provide consistent video quality plays an important role for most advanced viewing systems, since viewers are usually sensitive to the fluctuation in the quality of decoded video frames. However, it is the fact that the quality of the decoded key frames is generally much better than that of the decoded WZ frames. This leads to the fluctuant or inconsistent video quality playing between the key and WZ frames, and thus reduces the user's QoE. Figure 5 shows an example for the problem of quality fluctuation. In Figure 5, the subjective quality of the \(i^{th}\) (\(i=14,\ 15\)) decoded key and WZ frames of _Foreman_ sequence are shown. These frames are encoded using the DISCOVER DVC coder. The error or distortion images in Figure 5e,g show the differences between the \(i^{th}\) (\(i=14,\ 15\)) original key and decoded key frames, respectively, and Figure 5f,h show the differences between the \(i^{th}\) (\(i=14,\ 15\)) original WZ and the decoded WZ frames, respectively. The decoded WZ frames generally contain much higher distortion areas as compared to the decoded key frames do, as illustrated in Figure 5. In other words, the image quality of the decoded key frame is generally much better than that of the decoded WZ frames. The problem becomes more serious when the key frame is encoded while using modern video coding standards, such as H.265/HEVC and FVC/JEM, since the image quality of the decoded key frames in these cases is even much better than that of the decoded key frames that were obtained by the DISCOVER DVC codec. Figure 4: The rate of optimal mode selections in cases of large coding unit (CU) size (\(CU_{size}=64\times 64\) and \(32\times 32\)). Figure 3: Directional prediction modes of Intra coding scheme proposed for JEM-DVC. It is necessary to propose a new scheme that can control more effectively the quality of both key and WZ frames than the prior traditional schemes in order to solve the problem, especially for the new architecture employed in JEM-DVC. Generally, the coding rate and the quality of the decoded key and WZ frames are controlled while using QPs and QMs, respectively, where the key frame is encoded while using a set of QPs ranging from 0 to 51, while the WZ frame is encoded using the fixed set of eight QMs, \(\{QM_{i},\ i=1,\ 2,..,\ 8\}\)[12]. Figure 6 shows the fixed set of QMs, \(\{QM_{i},\ i=1,\ 2,..,\ 8\}\). In Figure 6, matrix \(QM_{1}\) represents the lowest bitrate (and the highest distortion) situation, while matrix \(QM_{8}\) corresponds to the highest bitrate (and thus lowest distortion). Within a \(4\times 4\) quantization matrix, the value at position \(k\) in Figure 6 indicates the number of quantization levels that are associated to the DCT coefficients band \(b_{k}\). These quantization matrices are utilized to determine the rate-distortion performance of a DVC coder and they are assumed to be known by both the encoder and decoder. More details on the description of QMs can be referred to in [11]. In this work, we propose a novel scheme, named consistent quality driven (CQD), which includes two quantization methods to minimize the fluctuations in the quality of decoded frames: key frame quantization (KF-Q) and WZ frame quantization (WZF-Q). The KF-Q and WZF-Q methods can effectively determine the appropriate quantization values that were assigned for QP and QM, respectively. The proposed CQD scheme is described, as follows:  
* Key frame quantization (KF-Q): In the JEM-DVC architecture described in Section 3.1, key frames are encoded while using JEM Intra coding and the quality of the decoded key frame is typically controlled using a constant QP. Let \(QP_{i}\) be the QP value that is defined at the frame level to encode the \(i^{th}\) key frame, \(K_{i}\). Subsequently, in order to effectively reduce the quality fluctuation between the decoded key and WZ frames, \(QP_{i}\) is chosen, so that the average PSNR of the decoded key frames is similar to that of the decoded WZ frames.