GSM Compression

GSM compression is a telephony standard defined by the European Telecommunications Standards Institute (ETSI). The GSM 06.10 compressor models the human-speech system with two digital filters and an initial excitation. The linear-predictive short-term filter, which is the first stage of compression and the last during decompression, assumes the role of the vocal and nasal tract. It is excited by the output of a long-term predictive (LTP) filter that turns its input--the residual pulse excitation (RPE)--into a mixture of glottal wave and voiceless noise. GSM encoder compress 160 16-bit voice samples into 264-bit GSM frame. GSM 06.10 is faster than code-book lookup algorithms such as CELP. It reduces the data rate by a factor of almost five, from 64kbps to 13kbps, with little degradation of voice-grade audio. Decoding requires only about half the computation as encoding.

ADPCM Compression

ADPCM compression uses Adaptive Differential Pulse Code Modulation to halve the data rate from 64kbps to 32kbps. The compression algorithm uses the correlation between adjacent audio samples to reduce bit rate. It transmits only the differences between samples and their predicted values which have less dynamic range than the samples themselves. Predictor coefficients and reconstruction levels are calculated dynamically using coded signal. It allows to reduce the bandwidth but make this adaptation technique more susceptible to transmission errors. ADPCM is not very computationally intensive, encoding and decoding requires enormously less computation than GSM.

LPC10 Compression options

LPC10 compression uses Federal Standard 1015, it provides intelligible speech transmission at only 2400 bit per second, with a compression ratio more than 26.

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

The ITU-T recommendation H.261 describes the video coding and decoding methods for the moving pictures. The H.261 includes descriptions of a coding mechanism and a scheme to organize video data in a hierarchical fashion. The compression techniques used by the coding mechanism include transform coding, quantization, Huffman encoding and optional vector motion compensation.

H.263 video compression

H.263 is an improved version of H.261 compression algorithm. The idea of image coding is the same but some additional elements were added which significantly increased the compression efficiency. With H.263 it is possible to achieve the same quality as H.261 with 30-50% of the bit usage. Most of this is due to the half pel prediction and negotiable options in H.263. There is also less overhead and improved VLC tables in H.263.

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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