# Video over DSL with LDGM Codes for Interactive Applications
## 5 Related Work  
The impact of impulse noise on an ADSL2\(+\) link is documented in [42]. Evidently there is severe \"blockiness\", where runs of macroblocks (MBs) have been lost and error concealment at the codec decoder has failed to replace the MBs in an unobtrusive manner. In [16], a simple 2D FEC code (with similarities to Pro-MPEG COP #3), RS codes, and LDPC were evaluated by embedding them in Linux RTP protocol stacks that included RTP packetization with IP/UDP headers. Packet erasures were uniformly distributed. The coding rate was high at 2/3. The authors reported that above 30% PERs, no codes could repair all packets but LDPC was only slightly worse than RS in error recovery. The simple 2D FEC code was noticeably worse than both LDPC and RS. Comparing latencies, RS codes introduced maximum average delay of 544 ms, followed by LDPC with 1000 block lengths at 402 ms and the simple 2D FEC codes at around 50 ms. \begin{table}
\begin{tabular}{l l l l l} \hline  & **Ln = 5** & **Ln = 5** & **Ln = 10** & **Ln = 10** \\  & **PER = 1\%** & **PER = 5\%** & **PER = 1\%** & **PER = 5\%** \\ \hline Max. & 90\% & 73\% & 67\% & 50\% \\ Min. & 81\% & 67\% & 52\% & 42\% \\ Avg. & 85\% & 70\% & 58\% & 44\% \\ \hline \end{tabular}
\end{table} 
Table 4: Recovery capability of improved LDGM. \begin{table}
\begin{tabular}{l l l l l l l} \hline
**Before FEC** & **1** & **2** & **3** & **4** & **5** & **Mean PSNR (dB)** \\ \hline \(k=200\) & 34.53 & 33.95 & 34.91 & 34.93 & 35.64 & 34.79 \\ \(k=300\) & 36.54 & 36.41 & 33.81 & 34.69 & 32.84 & 34.86 \\ \hline
**After FEC** & **1** & **2** & **3** & **4** & **5** & **Mean PSNR (dB)** \\ \hline \(k=200\) & 38.22 & 36.67 & 39.00 & 39.00 & 38.86 & 38.35 \\ \(k=300\) & 38.85 & 36.55 & 39.00 & 38.88 & 37.35 & 38.13 \\ \hline \end{tabular}
\end{table} 
Table 2: Objective video PSNR for 100 B packets before and after FEC. \begin{table}
\begin{tabular}{l l l l l} \hline  & **Ln = 5** & **Ln = 5** & **Ln = 10** & **Ln = 10** \\  & **PER = 1\%** & **PER = 5\%** & **PER = 1\%** & **PER = 5\%** \\ \hline Max. & 87\% & 71\% & 63\% & 45\% \\ Min. & 77\% & 62\% & 49\% & 35\% \\ Avg. & 83\% & 67\% & 55\% & 41\% \\ \hline \end{tabular}
\end{table} 
Table 3: Recovery capability of LDGM. Lower block lengths for LDPC reduced latency but decreased error recovery. These results confirm that a compromise code, such as LDGM (or low block length LDPC), is a good option for interactive video. Computational overhead for the simple 2D FEC code and LDPC remained below 8% whatever the PER while as the error rate increased RS code computational overhead clamped steeply. As part of the OpenFEC project, [43] examined the same codes as in [16] but with measured error traces from ADSL links rather than the random drops of [16]. These results confirmed those of [16] for random losses but showed that even at loss rates below 10% and with a code rate of 2/3, when error bursts occurred not all packets could be recovered. As in [16] an LDPC code with a block length as low as 170 was found to be competitive with RS codes in error recovery terms. Because not all packets could be recovered even at low loss rates, the ability to retransmit was recommended, which is also the recommendation of [29]. In [32], a number of interesting points of comparison are made between some of the channel codes mentioned herein. The main limitation of the LPDC family of codes in comparison to rateless non-patented online codes [44] is that they are not rateless in the sense that the maximum number of parity packets must be defined in advance. LDPC is further disadvantaged, compared to rateless and LDGM codes in that it has to store all the source parity packets during the encoding process. The family of codes performs most efficiently when the FEC overhead is low to medium. Staircase LDGM codes were preferred by the authors' of [32] but they employed large block sizes (as they had in [13]) and were unaware of possible improvements such as that of Section 3.2.