JVC Technical document about Dynamic Drum and BEST/Video calibration
 

Some person finally put this on the web. JVC published a number techical guides over the years describing new features on their VHS lineup. This doc was referenced in the later one that covered the 1998 models (HR-Sx500 and so on) and some service manuals, but I had no luck finding it published anywhere.

This doc describes the inner workings of the dynamic drum system, specifically for the pre-1998 models, as they changed the mechanism a fair bit for that lineup (after which there were only minor modifications untill the very late-model decks like the sr-vs30u which used a substantially different mech). The 1998 guide has a whole section on the mech, but doesn't note anything about the dynamic drum system so I presume they didn't alter much of the core principles of it.

As has been subject to a bit of discussion in the past, the doc notes that it is only used for "trick play", and is not active during normal playback. It also notes how it's initialized electronically, which could maybe give some clues to how to disable it without doing to much manual tinkering, tho as mentioned they changed the mech a bit for the later models that we're mainly interested in.

The BEST/Video calibration function is also covered, the playback part seems to be a bit more simple than I thought it would be, altering some EQ parameter (not entirely clear if it's on the rf signal or the demodulated video) if the video signal frequency spectrum deviates from some pre-set default. It's not impossible that they made it more complex later though, but I don't know.

Attaching both here in case they disappear from the original location.

Thanks for the documents, I certainly haven't seen it before, Yeah I figure that DD system is not activated during the normal playback by watching it working. I talked about it in one of the old threads here and at VH. Maybe now I can revise the "DD disable procedure" by at least using the JVC names, I was calling the parts different names.

As to the video calibration, a.k.a B.E.S.T I believe it equalizes the RF signal like any other type of magnetic recording, because what's recorded on tape is modulated RF not video signal.

I posted those documents in the procedure's thread, Thanks a lot.

Oh wow, not only best has to be ON if a head replaced but it has to be ON for worn heads too, So all VCRs by now should have it ON, That's re-assuring though because now I know I can get more use of my VCR's.

I have to admit though despite its problems the Dynamic Drum feature was one of the highest technical achievement put on a consumer VCR, Hats off to JVC for that.

This video sample shows a perfectly working DD on a pre-recorded tape, Some of the speed indicators don't show in the OSD, it's a VCR glitch.

Cannot attach sample here due to size limit.

Yeah it was a pretty interesting feature, as noted in the doc they even included pitch compensation for audio playback. Mitsubishi actually put moveable heads on their U82 SVHS model, which was a step further, but it seems to have been much more prone to failure since it needed a slip ring to control them.