From Error Control to Error Concealment Dr Farokh Marvasti Multimedia Lab King’s College London

Outline n Philosophical Issues n Previous Work on Forward Error Control/Concealment n Error Concealment in the Source Encoded Coefficient Domain n Error Concealment in the Time (Speech) or Space (Image & Video) Domain n Suggestions for UMTS and Future Work

Philosophical Issues The Better the Source Encoder, the Worse the Error Concealment (EC) Trade off Between FEC and EC: The More Powerful the FEC, the Less Frequent the EC Measure of Performance: 1-FEC-Prob of Error or SNR. 2-EC- Subjective Evaluation FEC is a science and EC is an art!

C Shannon “However, any redundancy in the source will usually help if it is utilized at the receiving point. In particular, if the source has already redundancy and no attempt is made to eliminate it in matching to the channel, this redundancy will help combat noise”

J Hagenauer “We claim that whenever some form of concealment of the decoded source signal is applicable, which means that some redundancy is left in the source signal, “The Source-Controlled Channel Decoding” can also be applied. It is better to avoid errors rather than to conceal them.” “One can use more sophisticated error concealment by using the channel decoder soft output…i.e., hard decision and its reliability.”

Previous Work: Vary, et al MAP Estimation: C = Max P(C i |C recived ) over all i Min-MSE: Ch hard decision soft output: Reliability A PosterioriEstimator C estimate P(C i |C rec )

My Previous Work in FEC/EC n FEC Using GF of Real Numbers n Oversampling and Nonuniform Sampling Th for Error Correction n Works for Erasure Channels and Impulsive Channels n Extension to Error Concealment n Brief Explanation and Examples of Multimedia Signals

Summary of the EC Work at Lucent n Image & Video EC: Presentation by Two of my Ph.D... Students on July 23rd at 1:30pm n 1- EC for compressed Video (H.263 & MPEG 4) by Hasan n 2- Robust Video Codecs for UMTS by Nick

EC for FR GSM n Direct FFT Approach does not work too well for prediction. However, it does show correlation of coefficients within frames n The Best Linear Prediction is derived from Yule Walker equations… (Alexis) n Ad-Hoc Methods: averaging the previous and the future frame coefficients works better than simple substitution

EC for FR GSM n Combining the previous N frame and the future M Frame Coefficients with a weight factor works best n Mathcad Examples:

Comparison of Yule-Walker with Substitution

Enhanced GSM, G.729, etc. n It is not always a good idea to do interpolation/prediction in the source encoded domain (e.g., the 76 GSM coefficients). n If previous frames are stored in time domain or its frequency transform (FFT, DCT, Wavelet, etc.), extrapolation seems to be easier to perform. See next slide. n *This approach does not depend on different standards.

Prediction of 18 future samples from the past 54 samples of a real female speech signal

Problems with FFT n High accuracy of computation is needed. Otherwise, the prediction becomes unstable, see next slide n Research on other exponential or other kernels are under study.

Algorithm for Extrapolation

A different Kernel: exp(j) The above kernel was implemented on a DSP. It is good for burst error correction but not for EC.

Frequency Domain Approach n Take Karhunen Loeve Transform of Previous Speech Frames and Try to do Prediction in the Transform Domain. DCT, FFT, etc are Suboptimal Solutions. n The Advantage is that Both Types of Correlation (Intra and Inter Frame) are Taken into Account

Future Work Complete the Yule Walker Prediction Develop the Time Domain Extrapolation Study the Frequency Domain Approach Joint Optimization of Channel Coding, Source Coding and EC.

Conclusion: Lessons for UMTS n VAD Should Send A Code for Silence Frames Immediately. This is Essential for EC. n Smaller Frame Sizes are Better for EC at the Expense of Less Compression. n More Frame Delay Should be Accepted in the Standards for Better EC. n Silence Frames Can be Used for the Retransmission of Previous Frames if Some Delay is Allowed.