G3 faxing is a digital process. Therefore, before transmission, the page information is digitized by the fax device’s scanner. First, the page is divided into horizontal lines known as scan lines. The scanner then moves across each line and based on the brightness level creates black and white picture elements (pixels). The typical scan line has 1728 pixels. Scan lines are created traveling from left to right and top to bottom.
The terms pixels per inch (ppi) and dots per inch (dpi) are often used interchangeably when discussing fax pages. Technically, however, dpi refers to the output of an image by a printer, whereas pixels correspond to scanned images. Fax devices can usually send and receive at multiple resolutions. The two most common are normal or standard resolution, which is 200 x 100 dots per inch (dpi), and ﬁne resolution, which is 200 x 200 dpi. At the ﬁne resolution setting, a scanned 8.5 x 11 inch page has 1728 x 2200 pixels. This means that the number of pixels that will constitute a full page at fine resolution is 3,801,600. With each pixel represented by 1 bit, this equates to more than 475 KB. Even when sending all of this information at the fastest G3 fax speed of 14.4 Kbps, it will take well over four minutes to transmit this size page. If even more detailed resolutions are used, the amount of transmission time increases greatly.
Fortunately, every exact bit that makes up a page is not transmitted. There are a few options available for compressing the binary page information. The ﬁrst and most commonly supported option is Modiﬁed Huffman (MH). All G3-compatible fax devices must support MH encoding. Another option is Modiﬁed READ (MR). MR offers a more advanced compression than MH and is more efﬁcient even though it employs MH principles. The third compression option is Modiﬁed Modiﬁed READ (MMR). MMR requires error-free communication, but it typically offers the best compression. Here is a quick comparison between the three different fax page encoding types. For more detailed information on each of these encoding algorithms, please see Fax, Modem, and Text for IP Telephony
Comparison of MH, MR, and MMR Fax Page Encodings
Compression Efﬁciency (for an 8.5 x 11 page of text)
Speciﬁcation (ITU-T Recommendation)
Dimensional coding type
Combination of Huffman and Run Length Encoding
Superset of MH that exploits the similarities between successive lines
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