Scanning in the Fast Lane
One of the most useful office machines today is the multi-function machine, which is either a printer/scanner/copier or a printer/scanner/copier/fax. The scanning and copying can use a flat bed or an automatic document feeder (ADF).
Almost all machines include a flat bed, which is great for scanning original pictures, books, newspapers, magazines and any other material that isn’t a single piece plain flat rectangular paper.
An automatic document feeder is only slightly less common than a flat bed, and is the only way to go if you need to scan large volumes of plain paper – invoices, manuscripts, medical records, etc. In addition to scanning multiple pages at a time, anywhere from 10 to 50 pages or more can be stacked in a typical ADF, some automatic document feeders allow you to duplex scan both sides of the page.
Automatic duplex scanning is a big improvement over manually scanning both sides of a page with a flat bed scanner. But there are BIG differences in performance between different machines, and these differences can be a deciding factor in purchasing your next multi-function machine if you frequently need to duplex scan.
Automatic Document Feeder (ADF) Types
RADF: Reversing Automatic Document Feeder – Scans one side of a page, then flips it and scans the other side. This requires a little bit of extra hardware but uses only one scanning image sensor, so it is typically a little cheaper to produce. The time required to rewind the page and flip it slows the scanning process and significantly increases the possibility of paper jams.
DADF: Duplexing Automatic Document Feeder – Scans both sides of each page in one pass. This requires a second scanning image sensor, so it costs a little more to produce. However, scanning both sides takes the same amount of time as scanning just one side.
There are other factors besides the feeder which can impact scanning speed, including:
- Black vs. gray-scale vs. color – color scanners (and almost multi-function machines have color scanners) actually scan black, gray-scale and color images using the same image sensor, but color images are larger and require more processing time.
- Resolution – a 1200dpi scan requires more processing time than a 300dpi scan.
- Image Type – there are additional steps required for image processing, compression, conversion to PDF and other things that vary by image type.
Let’s look at a few typical examples:
HP OfficeJet Pro 8700 series.
The OfficeJet Pro 8700 series includes 4 machines – 8710, 8720, 8730, 8740. All have a full range of features including duplex scanning. The first clue is right on the “Series at a glance” part of the specification page
“ADF two-sided scan speed in images per minute (ipm), black/color”
There appear to be 3 different scan engines involved for the 3 different speed levels (8730 & 8740 are the same). But there is more information hidden in those few little numbers:
- Since we know that the same scanner image sensor and mechanical components are used for black & color, these numbers tell us that the 8710 is limited by feeding/imaging speed and not by processing speed since the speeds for black & color are the same, while the 8720 appears to be limited by feeding/imaging speed on black and by processing speed for color, and the 8730 & 8740 are likely limited by feeding/imaging speed since they scan black almost 6 times as fast as the 8710 and color 3 times as fast as the 8710. That means the 8730 and 8740 have both a more powerful processor and faster feeding/imaging mechanisms than the 8710 and 8720.
- While the printing speed and many other specifications are very similar for all 4 machines – e.g., top printing speeds for the 8710 22 black/18 color pages/minute, 8720, 8730 and 8740 24 black/20 color pages/minute – if you plan to do a lot of scanning then the extra cost of the 8740 is money very well spent.
But wait, there’s more!
Next to the “ADF two-sided scan speed in images per minute (ipm), black/color” line is a reference to footnote 14, which reads:
Two-sided scan speed is up to 4 ipm for the 8710 model, up to 6 ipm for the 8720 model, and up to 23 ipm for 8730 and 8740 models. Scan speeds measured from ADF. Actual processing speeds may vary depending on scan resolution, network conditions, computer performance, and application software.
In other words, the physical limit of two-sided scan speed – the time to start feeding, scan the first side with an image sensor, flip the page, scan the second side, scan the second side with an image sensor and get ready for the next page – is the same as the listed black scan speed – 4, 6, 23, 23 images per minute for the 4 models. But color scanning and many other factors can slow down the real-world scan speed, and that is already factored into the color images per minute for each machine.
Now let’s get to the fine print.
The last page of the specification page is loaded with all kinds of details that you might think really don’t matter. But you’d be wrong. Let’s look at Scan speed. For each machine, speed is listed for Normal (single-sided) and Duplex, both black & white and color at 200 ppi (pixels per inch).
The Duplex values are exactly the same as listed above. No surprises, no funny games. But the Normal values tell the real story:
Compare that to the duplex values listed above:
8710: Duplex 4/4 vs. Normal 8/8 – 1/2 speed for duplex, so the pages get flipped and scanned on the other side
8720: Duplex 6/4 vs. Normal 17/8 – 1/2 speed for duplex, so the pages get flipped and scanned on the other side
8730 & 8740: Duplex 23/12 vs. Normal 17/8 – Wait a minute, that means that duplex is faster than normal!
How can duplex be faster than normal? It is actually quite easy. If the machine uses a Duplexing Automatic Document Feeder and scans both sides at once then the pages will scan at basically the same speed in Normal & Duplex modes but the images will scan twice as fast in Duplex mode because the scanner doesn’t have to flip the page and scan the second side.
A little more detective work and then we’re finally done.
The fastest color speed is a clue to the actual processing power of the machine. The 8720 has a Normal mode top color speed of 8 images/minute, which is the same as the 8730 & 8740, even though the top black speed goes up from 17 images/minute to 19 images/minute. Similarly, the 8720 has a Duplex mode top black speed of 6 images/minute which goes up almost 4x to 23 images/minute on the 8730 & 8740, but the color speed goes up from 4 images/minute to only 12 images/minute, which is “only” a 3x increase. So there is a close match between the processing power of the machine and the scanner speed, but this is an indication that there may be much more in common between the 8720 and the 8730/8740 than just print speed and the bigger control panel.
Research Before you Buy
Using just this one group of machines, and ignoring all other differences, the “right” machine depends on your usage:
- Low volume of normal scanning and very low volume of duplex scanning: Save your money and get an 8710 for around $120.
- Moderate volume of normal scanning and low volume of duplex scanning: Spend a bit more – currently around $180 to get an 8720.
- High volume of normal scanning or moderate to high volume of duplex scanning: An 8730 for around $300 or 8740 for around $400 is well worth it in time saved (faster scanning) and less frustration (no flipping = fewer paper jams).
This is one of many factors that will be included in the complete printer selector, coming soon to this web site.