The references to "low band" and "high band" in the literature tell you that it was an analog recording technique. To be specific on both the HS-100 and on the quad video recorders of that day, they used a system of FM modulation of the one Volt, base band, NTSC (or PAL or SECAM) video signal. There were different modulation standards for low band monochrome, low band color, and high band which encompassed both.
I can not recall the exact frequencies involved but the low band color modulation had an FM deviation of only 1Mhz from sync tip to peak white. The high band had several times that deviation plus a base frequency around 10 Mhz which was a good deal higher then that of low band. This, of course meant that the high band recordings had a far superior signal to noise ratio.
This compression of the full bandwidth of a 4.5 Mhz video signal into a smaller bandwidth did require pre and post emphasis. And there were side bands although the upper ones were limited by the overall bandwidth of the system.
These numbers were primarily determined by the ability to produce the video heads with gaps that were small enough and consistent enough.
A side story that I just recalled: while working at a UHF station in New Orleans the sales department wanted to record a two hour sport event, probably football games. Other machines were busy so only one was available and there was no break in the games. The quad recorders of the day were limited to the reel size which held only 1 hour and 30 minutes of tape. I contacted Ampex's tape salesman and inquired about larger reels of tape. He said they could provide them,but no VTR could handle them. We had RCA TR-22s with a sloped front panel. I bought one reel of 120 minute long tape and sure enough it scraped on one of the indicator lights at the top of that front panel. I removed the plastic lens from that light and the one on the other side and there was just barely enough clearance for the reels to turn.
Also the reel tensions had to be very carefully adjusted for the greater mass of the larger reels. Even then,instead of the normal 7 or 9 seconds, we had to allow at least 30 seconds for the machine to come up to speed and stabilize before the recording was good. We recorded and played back the entire season of games with no problems - except people holding their breath for that 30 seconds. And an occasional finger lightly dragging on the reel to dampen the oscillations.
I bet we were the only TV station in the country buying 120 minute reels of 2" video tape. Perhaps the world.
An even more impressive breakthrough, it was the Ampex HS-100 instant replay video recorder. It recorded NTSC color analog video on a hard disk, which enabled repeating the last 30 seconds, freeze frame, and forward and backward slow-motion.
At $110,000 in 1967 dollars, ($1,062,134 in today’s dollars) its use would be restricted to TV networks broadcasting a major sports event. And definitely NOT portable!
http://www.vtoldboys.com/editingmuseum/hs100.htm

4.2mhz bandwidth meaning around 8mhz sample rate at 30 seconds = 240mb in 1967 is it was digital.
Still impressive if it was an analog recording with frame freeze and jumping or 30 seconds worth of video.
Maybe it was a huge drum drive with a sliding head?
I cant find any specs for the Ampex HS-100/200 anywhere.