17-06-2024, 07:59 AM
The chip aux in, or built in modulator isn't using intercarrier in a conventional way. The final carrier frequency in the IC has two modulators, one for video and one for the sound subcarrier. The sound subcarrier can either be aux-in or the small number of preset subcarriers.
It's a separately modulated subcarrier, not intercarrier like the 6MHz added to the video modulator on a VHS 625 modulator.
The output block diagram may also be simplified as it doesn't show how the image frequencies of the two final RF modulators are dealt with. Also though on VHF the modulator RF in is more like a square wave, even the UHF modular frequency seems to be the VCO / 2. Thus the single main VCO seems to run at about 900 MHz to 1,700 MHz.
The diagram shows low pass filters, yet these don't seem to affect the RF out. So perhaps it's a pair of Weaver SSB modulators (one each for video baseband and sound sub carrier) with then carrier injected. The fact that the carrier level is adjustable separate to the modulation is suggestive. I suspect a variation of the Weaver method allows a vestgial video sideband. It has low pass filters internally. See diagram
I should order the one with Aux-in and look at it on the spectrum analyser.
It's a separately modulated subcarrier, not intercarrier like the 6MHz added to the video modulator on a VHS 625 modulator.
The output block diagram may also be simplified as it doesn't show how the image frequencies of the two final RF modulators are dealt with. Also though on VHF the modulator RF in is more like a square wave, even the UHF modular frequency seems to be the VCO / 2. Thus the single main VCO seems to run at about 900 MHz to 1,700 MHz.
The diagram shows low pass filters, yet these don't seem to affect the RF out. So perhaps it's a pair of Weaver SSB modulators (one each for video baseband and sound sub carrier) with then carrier injected. The fact that the carrier level is adjustable separate to the modulation is suggestive. I suspect a variation of the Weaver method allows a vestgial video sideband. It has low pass filters internally. See diagram
I should order the one with Aux-in and look at it on the spectrum analyser.
