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VOCAL's software library includes software for complete Group 3 facsimile control. The basic requirements for Group 3 facsimile refer to V.21 channel 2 (300 bps half-duplex modulation), V.27ter (2400 bps and 4800 bps half duplex modulation), T.4 1-dimensional Huffman coding of images and T.30 as the underlying control procedure. The image is sent with a modulation from the "High Speed" set of modulations, V.27ter as the minimum required support. Other Group 3 facsimile "high speed" modulations include V.29 (7200 bps and 9600 bps) and V.17 (7200 bps, 9600 bps, 12000 bps and 14400 bps). (Although V.29 specifies a 4800 bps modulation, it is not relevant to Group 3 facsimile.) V.33 is not specified as an option under T.30
T.30 specifies an additional 14 modulation rates between 2400 bps and 33600 bps (in increments of 2400 bps) that can be used from the V.34 modulation. This is referred to as V.34fax. As with V.34 data modulation procedures, V.8 startup is required. V.34fax includes procedures for full and half duplex data channel operation; half duplex is mandatory for V.34fax support.
T.30-E uses an encoding methodology for continuous-tone (multilevel) images based on the lossy mode of encoding (based on the Discrete Cosine Transform DCT) of JPEG (T.81) image encoding standard. T.30-E uses Forward Error Correction defined in T.42 and adopts a device-independent color space representation, the CIELAB space, which allows unambiguous exchange of color information. T.30-E also indicates the procedure for negotiation of the capabilities for transmission of continuous-tone color and gray-scale images. It specifies the definitions and the specifications of new entries to the Facsimile Information Field of the DIS/DTC and DCS frames of Recommendation T.30.
Information is specified pertaining to image digitization resolution (in bits/pel), spatial resolution, sampling ratio of color components, JPEG capability, color capability, and image data scaling that is subject to negotiation in the pre-message phase of the T.30 protocol.
The technical features of encoding and decoding the continuous-tone color and gray-scale image data are described in Annex E/T.4. It describes two modes of image encoding (lossy gray-scale and lossy color) which are defined using Recommendation T.81.