Using Video Recorders for Backup

Stuart Cheshire, 3rd August 1994

With hard disks getting bigger and cheaper all the time, it is getting harder and harder to back them up, especially for those like home users who are on a tight buget. If money is no object, a tape drive like Exabyte or DAT is ideal, but for most people they are too expensive.

Here's an idea for home users who own an AV Mac:

Use your home video recorder.

To backup, have the Mac display a sequence of patterns on the screen, which you record onto video tape on your home video recorder. The patterns encode the data in your files, much like the way Retrospect uses magnetic patterns on tape to encode the data in your files. To restore, connect the video recorder to the AV input of the Mac and have the Mac read the patterns off the tape via its video digitizer, and recreate the file(s) you want to recover.

How much could we store on a VHS video tape? Let's do a back of the envelope calculation:

Your AV Mac can digitize 320x240 video at 30 frames per second.

Let's ignore colour, since the VHS recorder won't record that very accurately, and lets assume that although the digitizer reads 256 levels of grey, we can only rely on recording 16 grey levels on the VHS tape with accuracy. Let's also allow for some horizontal smearing of the scan lines and only read 60 samples per line instead of the full 240.

With these somewhat conservative estimates, we get:

320 lines x 60 samples per line x 4 bits per sample x 30 frames per second = 2304000 bits per second = 2.3Mb/sec = 288 kbytes per second.

That makes 17 megabytes per minute, storing 2.7 GB on a 160 minute VHS tape.

That's faster that I get with my old 10meg/minute DAT drive, stores more per tape, and the tapes cost less.

Also considering that most owners of AV Macs probably own video recorders anyway, it's essentially a free backup system, while DAT drives are still $800.

Dantz might not want to add this option to Retrospect, but it seems like an obvious shareware opportunity for someone. A high-speed high-capacity backup system virtually for free -- probably the only piece of hardware you'll need to buy is a cable to connect your video recorder to your AV Mac.

It seems I'm not the first to think of this. Jerome Calvo recently told me about a company that did this a decade ago:

Date: Fri, 16 Aug 1996 17:14:47 -0700
From: "Jerome Calvo" 
To: Stuart Cheshire 
Subject: re:Video Recorders for Backup

In the 80, Corvus was selling hard disk (5MB, 10MB and 20MB) and
network server for Apple II and PCs, and VCR backup was an option.
I have installed and used a couple of those, but the system was not very
reliable, no feedback to adjust tracking...and this was 10 years ago.

Here's another interesting mail message:

Date:    Mon, 09 Jun 1997 15:22:42 +0400
To:      cheshire@CS.Stanford.EDU

From:    Vladimir Ivaschenko 
Subject: Using videorecorders for backups

I wanted to tell there here in Russia there is a company which is doing
these streamers for over 3 years now. They're called ARVID. Current best
model (I have it at home) writes at 325 Kb/sec. (count up how much data
I can place on standard 180-min cassette). The error-rate is low, and
really I didn't ever had a file which I couldn't read due to errors (it
may reread it several times (once I used a videocasette made in '88),
but works ok). You just attach video-in/out & infrared cables to your
videorecorder and it does everything for you, including file system etc.
It can control any videorecorder & has many in its database,
and even if it hasn't, you can program it using your remote control
unit. Very popular thins here in Russia.
And the price : ARVID-1051 (325 kb/sec) costs $70, 
          while ARVID-1031 (200 kb/sec) costs $50.

It costs _much less_ compared to other streamers (compare how much
costs a streamer tape and clear videotape).

Vladimir

I've also heard of a company called Danmere that has a VHS tape backup system which achieves a backup rate of 9MB/sec, which is the same order of manitude as my rough estimate above. However, theirs is a hardware product, and it only works on Intel "ISA" (IBM AT slot) PCs, whereas I was suggesting a software-only product that would work using the existing video hardware that is built in to almost every Mac. Still, if you have an ISA slot PC it could be an ideal solution for you.