Moderators: Dave Moll, Andrew Davie, Steve Anderson
Klaas Robers wrote:Harry,
before you are going to try to record NBTV, first try to record a square wave signal. If you listen at the replay, you hear almost the same sound as when you listen at the original wave form. But if you look at your oscilloscope to the replay signal, you will not recognize the square wave of the input signal.
This happens because the lower frequencies are recorded deeper in the tape and at some distance from the gap in the recording head. The higher frequencies are recorded on the surface of the tape and right under the gap. What you will see is that the recorded square wave plays back as a more or less triangle wave.
As our ears are not sensitive to wave forms, but sensitive to the volume (strength) of higher and lower sine waves, we do not hear this type of distortion. SSTV is much less sensitive to this, because it is an FM tone. All frequencies are close to each other, 1500 Hz to 2300 Hz, in the audio spectrum. So all tones are recorded at almost the same depth in the tape and at almost the same distance from the gap in the recording head.
This distortion is deadly for NBTV video signals. That is the reason that all video tape recorders work with a FM subcarrier, just like SSTV.
But do not stop your experiments.! Go on with it, because only if you see it yourself, you are convinced and you learnt from it for life time.
Steve Anderson wrote:Both Karen Orton and Chris Lewis have had varying degrees of success using Compact Cassettes. They both had an analogue 'processor' to make corrections to the shortcomings of this medium....time to dig through all those newsletters...
Steve a.
Klaas Robers wrote:Harrt, the sound on a VCR is recorded in exactly the same way as the sound on a cassettte recorder. So the distortion is similar. The VCR-tape is very thin, so there is not so much depth in the magnetic layer. But also for compact cassettes the tape is thin, in order to get enough playing time.
The video in a VCR is recorded as an FM-subcarrier of a frequency of several MHz and by a separate recording head on a fast rotating disc.
Harry Dalek wrote:I would think the VCR video heads would be useless for low audio what would you think would be the lowest video system it could record ? i have heard soem vcrs managed 405 line .
Andrew Davie wrote:Just a thought: What if you were recording onto a hot medium... would it have an easier time embossing the track?
In other words, heat up the CD in the oven for a minute or so before recording. Well who knows... something to try!
Steve Anderson wrote:A 625 VCR should record and playback 405 monochrome no problem, the field and frame rate are the same as 625. There's no colour so that part of the VCR should simply shut down. The field/frame rate is what the capstan and drum servos lock to, the line rate doesn't matter...except - if it has a TBC (Time Base Corrector). Few VCR's had that luxury and it can usually be switched off or bypassed anyway.
IF (a real IF these days) you can get a S-VHS (Super VHS) machine and the S-VHS tapes, these have a greater luminance bandwidth ideal for 405 recording.
Wiki...S-VHS improves luminance (luma) resolution by increasing luminance bandwidth.[2] Increased bandwidth is possible because of increased luminance carrier from 3.4 megahertz (MHz) to 5.4 MHz.[2] Increased luminance bandwidth produces a 60% improvement in (luminance) picture detail, or a horizontal resolution of 420 vertical lines per picture height – versus VHS's 240 lines.
The result is that it's hard to tell the difference between the original 405 and the S-VHS copy.
Really, if you keep the field rate at 50Hz (60Hz US etc..), the VCR doesn't care what the signal information is as long as it's pseudo-video in nature. This applies to standard VHS too, just keep the info within the limits of the format...
Steve A.
The little white wobbly'blob' at the bottom of the screen (not always visible, the head-switching moment) might appear to be wider than in 625 material...it varies so I'm told.
Klaas Robers wrote:...And do the square wave experiments with your lathe as well.
Klaas Robers wrote:Recording a square wave of about 200 Hz is a good test to see how the recording - play-back channel behaves. In the far past I did these type of tests on professional studio-type reel to reel recorders, also on 19 cm/sec and 38 cm/sec, and the result was invariably that the output did no more look like a square wave. The audio recording process with HF bias magnetisation is the source of the distortion. But with no HF bias magnetisation the distortion is even worse!
But doing experiments in this direction is very valuably. I have seen that Karen did this too with cassette recording a few years ago and wrote about it in the Newsletter of the NBTVA. Read that stuff. And do the square wave experiments with your lathe as well.
Return to Construction Diaries
Users browsing this forum: No registered users and 5 guests