Moderators: Dave Moll, Andrew Davie, Steve Anderson
Klaas Robers wrote:There have been radio amateurs, in the time that SSTV was popular, that used a separate 1200 Hz oscillator, which was keyed into the SSTV FM signal when a sync pulse was given. Then of course, the FM subcarrier was discontinued and you saw after demodulation a positive or a negative glidge at the beginning of a sync and in most cases also at the end of the sync. This was visible at the right edge and at the left edge of the picture. But if for sync detection a separate tuned circuit at 1200 Hz was used, it gave a more stable sync. I never did this because, if you wanted to recover the sync directly from the demodulated video signal, it gave also a jittering sync due to the glidges.
On the other hand, if you insert a baseband sync pulse in the video signal and send that through the VCO / FM-modulator, The phase of the 1200 Hz is different for each sync pulse and may give also some jitter on the sync. Yes, sync detection on an SSTV-signal is not that easy. I think that an electronic flywheel could have solved this problem, but I have never seen that implemented.
Klaas Robers wrote:What you do is inserting a new sync into the base band video signal. That is better than inserting a 1200 Hz burst into the FM subcarrier SSTV signal. In the way you do it, the SSTV FM-signal is made new in your 4046, so glidges in the FM signal are not occuring.
I still do not understand why you are doing this.
When you insert new syncs that are not synchronous with the 1200 Hz syncs in the original SSTV signal, your video content (picture) will be unstable in the horizontal direction. In case the new sync pulses are synchronous with the original 1200 Hz SSTV-sync, I see no reason why you should reinsert them.....
Steve Anderson wrote:I must admit that I don't understand quite why a non-synchronous (in terms of sub-carrier) sync pulse needed to be inserted. Surely the modulators/VCOs of the period could cover a 2:1 ratio? Simplification possibly? No idea!
Now the discontinuities introduced by this method will introduce glitches in the demodulated signal, probably equally on each edge of the sync pulse on average, though maybe not on every single line.
The SSTV stuff I've done previously hasn't really displayed much in the way of sync jitter, I guess it is there but there are other artifacts and distortions I'd rather eliminate or reduce first.
Getting that sub-carrier to blend each end of a line into and out of of the sync 'burst' of 1200Hz is no easy challenge!
Steve A.
Steve Anderson wrote:OK Harry, good thinking and good luck! It's an area I have no experience in...Albert is probably our resident expert...
Steve A.
Klaas Robers wrote:Harry,
it is more complex than you might think at first sight. Some recording heads give a side way displacement proportional to the voltage that you apply, others give a displacement SPEED proportional to that voltage. And to make things even more complicated: some play-back cartriges give a voltage proportional to the displacement of the "needle" (crystal and ceramic), while others give a voltage proportional to the displacement speed (magnetic dynamic). So just inserting a resistor voltage divider between the recorder amplifier output and the play-back amplifier input, will not guarantee that you get a flat characteristic.
Besides that sound, music, has different amplitudes for low, mid, high tones. So a flat recording characteristic might not be the optimal solution.
In the 78 rpm era almost each disc producer (label) had his own optimal recording characteristic, and the better play-back amplifiers had about 10 different compensation networks to get an optimal reproduction. They all had the name of the record label that you were playing and you should switch it by hand. But I have read that in quite some cases a play-back compensation network of a different Label sounded better than the very label that you were playing. Besides that: video is not music, so the optimal compensation, or equalisation, will surely be different. Good Luck!
This all came to an end with the introduction of vinyl Long Playing 33 1/3 rpm and 45 rpm records. Then the RIAA committee standardized an optimal equalization. And since about 1950 this became the standard. In ceramic and crystal play-back cartridges this is more or less mechanically built in, so a "straight" amplifier will do. But for the better MD (Magnetic Dynamic) elements quite some equalization is needed. Amplifiers from 1950 to 2000 would have a "phono" input with this RIAA characteristic.
If you would like to record NBTV-signals, you should dive deeply into this matters. Then the characteristic should be flat from 2 Hz to 10 kHz.
In time for sure but i know i have some way to go ...that advice above will be kept in mind ..once i get around to doing sweep tests for that ...what i have at the moment is a touch short for 10hz i can get up to 8 khz i don't think this head is good enough for NBTV but fine for SSTV.But for the SSTV audio-FM signals it is less important, because if there are amplitude changes during play-back the limiting input amplifier will compensate automatically for that. Your "disc channel" should be only more or less flat for frequencies from 300 to 3000 Hz (telephone bandwidth). It worked even over an amateur radio channel with quite some noise, distortion and interference in the "aether". (You know, there is no "aether", but we still talk about it as if it is existing.) But speed variations, never occuring in the aether, are forbidden. That will be immediately visible.
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