Treasure Your Quadra 840av

- 2001.08.16

I received a very interesting letter with a lot of information about the Quadra 840av. James wrote:

I read your article Quadra AVs and Some Cool Things You Can Do with Them. You forgot to mention lots of amazing things about the Quadra 840av.

I love them and wrote programs to control them, because only the Quadra 840av has full featured QuickTime VDIG effects.

For example, the 840av video input can mix live video with the screen and simultaneously output a combined picture on the video out to a television at 640 x 480 or larger resolution.

That's not all. The 840av can use a mask to crop the areas to show through, and that mask need not be absolute. The Mask may be translucent or gradual using 256 levels of translucency (from an alpha channel layer). That has never been offered on any other card supporting VDIG components for QuickTime.

You can blend a live TV signal over an area of your screen if you wish to and watch TV and type at the same time on the same screen. A special extension from Apple allows you to select transparency percentage using a control slider in any and all QuickTime digitizer programs ever written. It is "AV Digitizer Options" v.1.0b1. You need this astounding free Apple extension.

But that's not all - the 840av also supports Chroma-key color index pixel substitution. You can specify which colors to allow to overlay or which to exclude. I think the limit is more than one color, perhaps 6, but it's been a while since I've used the feature.

This color key mode only works in 256 color mode, but the video being mixed will display in thousands of colors on the output.

The video chips in the 840av are miraculous. They can do more programmatically than any video card made since then.

In fact, the Power Mac 8500, which was supposed to do some of what the 840av did, was a sick joke.

You can hold down a menu with the mouse held down on a Quadra 840av, and the menu can obscure the video beneath it, as it should. This video will keep displaying live at 60 fields per second while holding the mouse button down. The crappy 8500 cannot display live video to the screen with the mouse held down in a menu. Ha! Try it with any program, including Apple's normal video capture and playback tools (Video Monitor 1.0.1). The inability to sustain live pictures is a severe disappointment.

The NeXT Dimension computer had no video mixing or display problems, and the 840av was supposed to copy most of the NeXT abilities, including offering a special ultra high speed DSP (Digital Signal Processor) modem and programmable DSP.

The 840av DSP is really cool. Other than speeding up Photoshop filters, it can add special sound filters live to your output lines, such as echo and stereoizer. Apple gave out a few free extensions to allow 840av users to use the 840av with dynamically modified sound (through mic or CD or line in, etc.) and the sound from the auto-gain inputs would be processed using the DSP chip and not slow down the Mac at all.

No Power Mac ever shipped from Apple has a secondary processor for DSP sound, just the 840av.

The 840av had fast internal SCSI, special double-speed NuBus slots, and much more. You can put two standard FWB SCSI Jackhammer NuBus cards in a Quadra 840av, and each will pump almost 20 megabytes per second into your computer.

With four Seagate 12450W hard disks, a Quadra 840av in 1995 regularly streamed data at over 34 megabytes per second to the screen memory. 34 megabytes per second! In 1994 a Sun workstation - a fast Sun workstation - couldn't write that much data to a null device that simply throws out data as it receives it in Unix. (It's called the dev/null and is usually faster than a hard disk, because it does nothing with the data.) Can you imagine such a speed in 1994? No IBM sold in 1994 could pump a third of that sustained transfer speed, not even for thirty thousand dollars.

Speaking of the screen, the screen video memory on the 8500 is pathetically slow. Worse yet, two FWB PCI cards in a 7500 are way slower than the 840av for SCSI RAID.

The 840av is astounding in many, many ways.

The 840av could pump data from one area of the computer's RAM to another without using the CPU! This is usually called DMA (Direct Memory Access), but in this case it could do it on a single span of 16 bytes. No DMA controller ever sold was optimal moving only 16 bytes at a time. The magic ability of the 840av to do this comes from the MOVE16 instruction. It is a special instruction that uses the cache controller to move data - not any registers in the Motorola 68040 CPU (neither integer not float registers are used).

The MOVE16 instruction is tricky to use, and few programmers used it, other than people like me, but by using it I was able to more than double my animation routines.

I miss the 840av technology that Apple abandoned. No Apple computer and no video card shipped since could do the tricks or match the features of the 840av.

The 840av can record audio from a turntable with no buzz or hum or interference from the motherboard or noise from its hard disk.

Try recording sound on a Power Mac - ugh! Listen to the quiet parts on a Power Mac - yucch!

The 840av was for sound and video, not just sound. It understood human speech; it had a chip called "singer" that had lots of tricks. It can record 16-bit stereo sound beautifully. You need a very expensive sound card on a Power Mac to come close to the 840av for sound input.

Treasure your 840av. Use it to encode video or sound that you later edit and process on a Power Mac. Use it with system 7.6 or 8.1 - or whatever stable OS you please.

Have fun. Spread the word about how the 840av was Apple's best machine, because it inarguably is, especially for live video effect manipulation.

Further Reading

• Quadra AVs and Some Cool Things You Can Do with Them, Adam Robert Guha, Apple Archive. Thanks to digital signal processors, the Quadra AVs remain useful video machines.
• Six Year Old Mac to the Rescue, Scott Atkinson, Mac Musings. The Quadra 660av has what it takes to capture stills from video for a television Web site.
• Mac AV list, an email list for 660av and 840av users
• 660av/840av Video Input Specifics, James Wang
• Quadra AV FAQ, James Wang
• Macintosh AV Series: Video Features and Subsystems