voice codec & video codec

Multimedia Communication Assignment #1

1. Search for types of voice codec and summarize main features of each codec.

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Codec = Coder + Decoder

Codecs are used to convert an analog voice signal to digitally encoded version. Codecs vary in the sound quality, the bandwidth required, the computational requirements, etc. Each service, program, phone, gateway, etc typically supports several different codecs, and when talking to each other, negotiate which codec they will use.

Type	Feature
G.711	G.711 is a high bit rate (64 Kbps) ITU standard codec. It is the native language of the modern digital telephone network. Although formally standardized in 1988, the G.711 PCM codec is the granddaddy of digital telephony. Invented by Bell Systems and introduced in the early 70's, the T1 digital trunk employed an 8-bit uncompressed Pulse Code Modulation encoding scheme with a sample rate of 8000 samples per second. This allowed for a (theoretical) maximum voice bandwidth of 4000 Hz. A T1 trunk carries 24 digital PCM channels multiplexed together. The improved European E1 standard carries 30 channels. There are two versions: A-law and U-law. U-law is indigenous to the T1 standard used in North America and Japan. The A-law is indigenous to the E1 standard used in the rest of the world. The difference is in the method the analog signal being sampled. In both schemes, the signal is not sampled linearly, but in a logarithmic fashion. A-law provides more dynamic range as opposed to U-law. The result is a less 'fuzzy' sound as sampling artifacts are better suppressed. Using G.711 for VoIP will give the best voice quality; since it uses no compression and it is the same codec used by the PSTN network and ISDN lines, it sounds just like using a regular or ISDN phone. It also has the lowest latency (lag) because there is no need for compression, which costs processing power. The downside is that it takes more bandwidth then other codecs, up to 84 Kbps including all TCP/IP overhead. However, with increasing broadband bandwidth, this should not be a problem. G.711 is supported by most VoIP providers.
G.726	G.726 is an ITU standard codec. This codec uses the Adaptive Differential Pulse Code Modulation (ADPCM) scheme. Like G.711, G.726 has its roots in the PSTN network. It is primarily used for international trunks to save bandwidth. Where G.711 uses 64 Kbps, G.726 uses 32 Kbps, providing nearly the same quality. It is also the standard codec used in DECT wireless phones. The bit rate can be 16, 24, 32 or 40 Kbps, but 32 Kbps is the de facto standard. Asterisk currently supports the 32kbps standard only. In addition to the one that comes with asterisk, one asterisk compatible exists in spandsp version 0.0.2pre26 and later. By some rumors this should be closer to the standards specification than the one from digium.
G.729	G.729 is an ITU standard codec. Has two main flavors: A and B, see the ITU G series page for more info. It offers toll quality speech at a reasonably low bit rate of 8Kbps. However, it is a rather "costly" codec in terms of CPU processing time, therefore some VoIP phones and adapters (notably the Linksys/Sapura/Cisco brands) can only handle one G.729 call (channel) at a time. This can cause calls to fail if the user attempts to use three-way calling, or place simultaneous calls on both lines of a two-line device, and G.729 is the only allowed codec.

2. Search for types of video codec and summarize main features of each codec.

A video codec is a device or software that enables video compression and/or decompression for digital video. The compression usually employs lossy data compression. Historically, video was stored as an analog signal on magnetic tape. Around the time when the compact disc entered the market as a digital-format replacement for analog audio, it became feasible to also begin storing and using video in digital form and a variety of such technologies began to emerge.

Type	Feature
H.261	Used primarily in older videoconferencing and video telephony products. H.261, developed by the ITU-T, was the first practical digital video compression standard. Essentially all subsequent standard video codec designs are based on it. H.261 came out in 1990, intended for video conferencing over Public Switched Telephone Network (PSTN) synchronous circuits. H.261 is designed to run at multiples of 64Kbps data rates from 1x to 30x. This corresponds to the capacity of a DS0 channel, which is used to carry a single phone conversation of 8 Bit ULaw 8Khz audio. These Correspond to ISDN (2 DS0's) and E1 (31 DS0's) & T1 (24 DS0's) circuits. I was very common to use a technique called ISDN bonding allowed 2 or more IDSN lines to be used together to provide higher data rates. Companies like Picture Tel dominated this space and helped write most of these standards. H.261 is not designed for a packet based network and doesn't work well over Frame Relay or TCP/IP Internet. Although some early streaming solutions commandeered its video compression algorithm, discarding much of communication layer and using TCP/IP instead.
MPEG-1	Used for Video CDs, and also sometimes for online video. If the source video quality is good and the bit rate is high enough, VCD can look slightly better than VHS. To exceed VHS quality, a higher resolution would be necessary. However, to get a fully compliant VCD file, bitrates higher than 1150 kbit/s and resolutions higher than 352 x 288 should not be used. When it comes to compatibility, VCD has the highest compatibility of any digital video/audio system. Very few DVD players do not support VCD, but they all inherently support the MPEG-1 codec. Almost every computer in the world can also play videos using this codec. In terms of technical design, the most significant enhancements in MPEG-1 relative to H.261 were half-pel and bi-predictive motion compensation support. MPEG-1 supports only progressive scan video.
MPEG-2	Used on DVD, SVCD, and in most digital video broadcasting and cable distribution systems. When used on a standard DVD, it offers good picture quality and supports widescreen. When used on SVCD, it is not as good as DVD but is certainly better than VCD due to higher resolution and allowed bit rate. Though uncommon, MPEG-1 can also be used on SVCDs, and anywhere else MPEG-2 is allowed, as MPEG-2 decoders are inherently backwards compatible. In terms of technical design, the most significant enhancement in MPEG-2 relative to MPEG-1 was the addition of support for interlaced video. MPEG-2 is now considered an aged codec, but has tremendous market acceptance and a very large installed base.
H.263	Used primarily for videoconferencing, video telephony, and internet video. H.263 represented a significant step forward in standardized compression capability for progressive scan video. Especially at low bit rates, it could provide a substantial improvement in the bit rate needed to reach a given level of fidelity. It was designed for low bit rate communication, early drafts specified data rates less than 64 Kbits/s, however this limitation has now been removed. It is expected that the standard will be used for a wide range of bitrates, not just low bit rate applications. It is expected that H.263 will replace H.261 in many applications.
MPEG-4	An MPEG standard that can be used for internet, broadcast, and on storage media. It offers improved quality relative to MPEG-2 and the first version of H.263. Its major technical features beyond prior codec standards consisted of object-oriented coding features and a variety of other such features not necessarily intended for improvement of ordinary video coding compression capability. It also included some enhancements of compression capability, both by embracing capabilities developed in H.263 and by adding new ones such as quarter-pel motion compensation. Like MPEG-2, it supports both progressive scan and interlaced video.