Moderators: Dave Moll, Andrew Davie, Steve Anderson
Dave Moll wrote:As to using VHS for recording formats other than the 625-line (or whatever) for which it is designed, my understanding is that it only works successfully on earlier machines that don't attempt clever post-processing of the signal. This is, however, only what I have heard from others. I have never tried recording 405-line output (such as from my standards converter) - maybe I should give it a go some time.
Klaas Robers wrote:Be aware that a VCR records and plays back fields, not frames. As you know the standard resolution TV systems all use interlace, with 50 (60) fields per second. VCR records this as fields, so it expects 50 (60) frame syncs per second. So in fact it records 312.5 lines at 50 Hz or 262.5 lines at 60 Hz.
If you use 120 lines at 25 Hz, which is called "progressive scan", that is "not interlaced" the frame sync comes at 25 Hz, not 50 Hz and a VCR will have problems with that in synchronising the head wheel.
Steve Anderson wrote:Yep, for an EU VCR the frame or field frequency must be 50Hz, 60Hz for the US and some other countries. A 25Hz (30Hz) field or frame (without interlace) rate will not work. Basically vertical sync must be at 50Hz (60Hz) whether interlaced or not. What happens between the vertical pulses the VCR doesn't really care about - unless it has a TBC that you cannot bypass.
The other thing to consider, and this may vary from machine to machine, is the low-frequency response. 625 requires a response down to at least 25Hz, most machines go somewhat lower, but it's worth thinking about. Active clamping on the back-porch part of the signal is usually the way to preserve this. The luminance signal on VHS/Betamax is recorded via FM which could have a response down to DC, not that dissimilar to SSTV.
The other little 'gotcha' can be the head switching glitch about six lines before the end of the frame/field. depending on your monitor or TV you may see a small white dot wobbling around at the bottom of the screen. This is where the heads are switched 180 degrees apart on the head-drum. If the switch causes a spike into the sync region this can cause problems with simpler downstream electronics.
Steve A.
Klaas Robers wrote:If you have no more than 30 lines, it is seen as waste if you spoil one line for a frame pulse. The more lines you have the less you spoil. Indeed, it is not simple to detect a missing sync pulse, but for the mechanical scanning this is not needed. And for CRT, 405 lines was the first practical system, and a frame pulse was inserted. Of course with magnetic deflection you need some fly back time, so the missing lines are not a problem.
I also wondered about the brightly lit open case with the PMT. It must have been that for the photos the high voltage of the PMT was switched off.
Harry Dalek wrote:Looking at the other videos i am sure he has it working as the testcard is on the monitor ,i looked into this and am trying to find schematics for the BK TV ANALYST which made these i came across one or 2 models ahead and the power supply to the PMT is very low -270 v that makes sense as the flying spot tv screen is pretty bright for a PMT
Klaas Robers wrote:No Harry, it is a flying spot scanner. What you see is a (large) testcard slide closely in front of the CRT, which displays a white field. The PMT then picks up the flying brightness of the slide. That is how a flying spot scanner works. The trick is that the line rate and the frame rate can be adjusted from the front. In this way it can generate a video signal of whatever standard. In real use the cabinet is closed of course. But then you cannot make a photo.
Return to The Tasmanian Devil VCR139....A for Andrew
Users browsing this forum: No registered users and 2 guests