How long does it take to send a fax?

September 13, 2007 3:59 PM

Does it take longer to fax a full page of text compared to just a single line of text? And if so, is it a significant amount of time? Or are all faxes equal?
posted by Mrs. Buck Turgidson to Technology (6 answers total)

Yes. It does. Fax machines compress the data they send. I think they do some kind of run length encoding, so, instead of saying white pixel white pixel white pixel a thousand times, they can say ok, 1000 white pixels.
posted by rbs at 4:02 PM on September 13, 2007

I believe faxes very efficiently compress whitespace before transmission, so a single line should take less time than a whole page.
posted by Good Brain at 4:03 PM on September 13, 2007

A fax machine scans a document and converts every pixel into either black or white, which it represents as a series of bits. That is then transmitted (using RLE, as mentioned above).

Because of that, areas which are constant white or constant black transmit much faster than areas which are complicated.

Run Length Encoding is a primitive form of compression, OK by not really great, and given that the Fax transmission standard was created back when microprocessors were small, slow, and stupid, it's not a bad approach.

Huffman encoding is drastically better but it takes a lot more computing hardware and much more memory because it can't be done on the fly. To use Huffman encoding it would be necessary to scan the entire document, and then to process it for several seconds before transmission began. And Huffman encoding hadn't been invented when the first faxes were designed and the interface standard was established.

It turns out that Huffman encoding comes within a few percent of the theoretical limit. RLE is nothing like as good, but it's still a lot better than an uncompressed datastream would be.
posted by Steven C. Den Beste at 4:29 PM on September 13, 2007 [2 favorites]

[Huffman encoding for arbitrary bit streams hadn't been invented yet. The original paper by Huffman was written about the time I was born.]
posted by Steven C. Den Beste at 4:31 PM on September 13, 2007

And very dense small-font text takes even longer. The more detail there is on the page, the longer it takes, and we're talking a huge range of times. A single sentence transmission might take a couple seconds, whereas a dense page (like the fine print in a contract) will take 30 seconds or more. That's just for the actual transmission, not the initial training (funny sounds) and headers. At least that's been my experience.

As an aside, it's amazing how robust fax has been as a standard in general. It certainly made me pull my hair out back in the days when I built satellite telephony systems, but any technology that lasts 20 years deserves respect.
posted by intermod at 7:47 PM on September 13, 2007

I remember the rather excellent Secret Life of Machines covering the fax machine and why it takes longer for a picture to be faxed than text. Here's the cartoon version. And why of course why one line would be quicker than one page of text (though I'm not sure how significantly quicker it would be)
posted by fearfulsymmetry at 8:31 AM on September 15, 2007