
GSM Compression
ADPCM Compression
LPC10 Compression options
LPC10 compression is highly sensitive to noise. To get better result, adjust mic input level to avoid overly-loud signals, and eliminate background noise which can interfere with the compression process.
H.261 video compression
H.263 video compression
H.263 negotiable options
Unrestricted Motion Vector mode. In this mode motion vectors are allowed to point outside the picture. The edge pels are used as prediction for the "not existing" pels. With this mode a significant gain is achieved if there is movement along the edge of the pictures, especially for the smaller picture formats. Additionally, this mode includes an extension of the motion vector range so that larger motion vectors can be used. This is especially useful in case of camera movement.
Advanced Prediction mode. This option means that overlapped block motion compensation is used for the P-frames. Four 8x8 vectors instead of one 16x16 vector are used for some of the macro blocks in the picture, and motion vectors are allowed to point outside the picture as in the UMV mode above. The encoder has to decide which type of vectors to use. Four vectors use more bits, but give better prediction. The use of this mode generally gives a considerable improvement, especially because OBMC results in less blocking artifacts.
Syntax-based Arithmetic Coding mode. In this mode arithmetic coding is used instead of VLC coding. The SNR and reconstructed frames will be the same, but generally fewer bits will be produced. The average gain for inter frames is 3-4%. This gain depends on the sequence, the bit rate and other options used. For intra blocks and frames, the gain is higher, on average about 10%.
PB-frames mode. A PB-frame consists of two pictures being coded as one unit. The name PB comes from the name of picture types in MPEG where there are P-pictures and B-pictures. Thus a PB-frame consists of one P-picture which is predicted from the last decoded P-picture and one B-picture which is predicted from both the last decoded P-picture and the P-picture currently being decoded. This last picture is called a B-picture, because parts of it may be bi-directionally predicted from the past and future P-pictures. For relatively simple sequences, the frame rate can be doubled with this mode without increasing the bit rate much. For sequences with a lot of motion, PB-frames do not work as well as B-pictures in MPEG. This is because there are no separate bi-directional vectors in H.263, the forward vectors for the P-picture is scaled and added to a small delta-vector. The advantage over MPEG is much less overhead for the B-picture part, which is really useful for the low bit rates and relatively simple sequences most often generated by video phones.
These options are negotiable. This means the decoder signals the encoder which of the options it has the capability to decode. If the encoder has any of these options, it can then turn them on, and for each of the options used the quality of the decoded video-sequence will increase.
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