Audio Standards

The standard for Audio Transmission is called G725. The standard includes the following:

  • G.711 It is the oldest compression algorithm for Audio Transmission. It is mandated by all H.3xx recommendations (except H.324).
  • G.722 This compression algorithm is optionally recommended. It codes enhanced quality (7 KHz analog bandwidth) audio into 48, 56, or 64 Kbps. G.722.1 codes enhanced quality (7 KHz analog bandwidth) audio into 24 or 32 Kbps.
  • G.723 codes at 6.3 and 5.3 Kbps data rate, which is required for H.324 and optional for H.323.
  • G.728 This compression is optionally recommended, which converts toll-quality (3KHz analog bandwidth) audio into 16 Kbps.
  • G.729 It converts toll-quality (3KHz analog bandwidth) audio into 8 Kbps.
  • G.703 Is a standard which uses Pulse Code Modulation (PCM) to digitize the audio signal, which a requires bandwidth of 64 kbps.

The Video Standards


  • This is the governing standard for video transmission. It determines whether communication exists at the Common Intermediate Format (CIF) level or the Quarter Common Intermediate Format (QCIF) level and thus provides a uniform process for receiving Codec to interpret compressed video signal. H.261 is based on Discrete Cosine Transform, DPCM and motion compensation techniques.

H.261 includes the following standards:

  • H.221 is responsible for protocol, video/audio synchronization & frame structure.
  • H.230 is same as H.221 but is used for MCU communications.
  • H.242 is responsible for Inter device communication. Synchronization of the communicating devices is handled by H.221 once the connection is established throughout the conversation.

H.261 Annex D is the protocol for transferring high quality still images.


  • It is a newer compression algorithm optimized for lower data rates. It’s performance is better than H.261, mainly for data rates below 128 Kbps. H.263 has five resolution modes, mainly SQCIF (128 x 96), QCIF (176 x 144), CIF (352 x 288), 4CIF (704 x 576) & 16CIF (1408 x 1152). H.263 is a mandatory standard for H.324 and optional for all other H.3xx recommendations. H.263-enabled systems are required to decode both SQCIF and QCIF resolution modes and encode either SQCIF or QCIF modes. All other resolution modes are optional.


  • H.320 is the standard for video conferencing which uses "switched services" like ISDN. It governs basic video communications and audio over digital channels & commonly used time division multiplexed (TDM) circuits.


  • H.323 is the standard for video conferencing using an IP network as the communication link. It uses the Real-Time Protocol (RTP/RTCP) & enables use of video conferencing over IP networks, LANs and Internet. The systems connects to each others by dialing an IP Address rather than a telephone number (as in H.320 communications).
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