Demo: downscaled and compressed movie (avi) of single description vs four multiple descriptions, same aggregate bitrate (H.264) and packet size, 30% packet loss rate.
Info: contact authors
Unlike layered coding (base+enhancement layers), multiple description coding creates independent descriptions. The more descriptions decoded, the larger the output quality.
In this simple examples, four balanced descriptions are created by separating odd and even rows, and taking every other pixel (a process known as polyphase downsampling). This is done in a pre-processing stage before encoding. The merge of decoded descriptions can also be done in a post-processing stage. Hence it is compatible with any standard, as MPEG-2.
In this example the number of packets and the aggregate bitrate is the same. When there are no losses the decoded quality favours single description. The opposite happens in case of losses. The resilience comes from the fact that it is unlikely to loose the same portion of the same image in all descriptions.
It is not fair to compare single description, which is designed gor efficiency, against multiple descriptions, which is designed for resilience. A fair comparison is done with error-resilient single description so that the decoded quality in case there are no losses is the same.
In this example the single description is protected by reed-solomon (100, 90) forward error correction. Two descriptions are generated by separating odd and even rows.
The decoded quality when there are no losses still favours single description. However when the packet loss rate is above 5% multiple description takes over.
It must be noted also that, even when the average quality of multiple description is lower, it has also lower variance, which is very important for the user.
An enhancement of the aforementioned scheme has been developed at University of Udine. Filters are used before downsampling.
As an example three descriptions can be created by separating odd and even rows, and taking also the average. Any two out of three description will suffice to decode the whole frame.
Another very clever scheme has been developd at Politechnic of Turin. It exploit one resilience tool of the new video codec H.264: redundant slices. Redundant slices are secondary slices based on the same reference as the primary slices. Typically the quantization step is increased, so that the secondary bitrate can be finely controlled.
Two descriptions are created by simply interleaving primary slices of odd/even macroblock rows and secondary slices of even/odd macroblock rows.
PoliTO DiVA project Two multiple descriptions by interleaving primary and secondary redundant slices in H.264.
STM PDMD demo Linux/Cygwin Two/four descriptions by down sampling before video encoding.
Pamela Zontone (UniUD)
Emanuele Quacchio (PoliTO)
Created: 3rd April 2007. Updated: 29th June 2007.