405 lines from Raspberry Pi 5

[Mike Watterson]

So using two chips is likely simpler than feeding one chip with an external 3.5 MHZ already AM signal, also not all the SOIC-16 pin versions have the aux-in anyway. I'm tempted to buy the one that has.

[ppppenguin]

If you use a subcarrier for sound modulation on System A won't it run into trouble on sync tips when the vision carrier level is zero? I know this is why you can't use intercarrier demod at the receiver but does it also apply at the sending end? I can't quite get my head around what would be happening here.

[Mike Watterson]

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.

[Mike Watterson]

So the video level isn't an issue for the sound. as it has a separate final RF modulator. a You are right, System A can't use an intercarrier modulatr, one with a single RF modulator shared with video.

[ppppenguin]

It's a long time since I looked on a spectrum analyser but I'm pretty sure the output is DSB.

I thought about doing a Weaver VSB modulator in a FPGA. It looks entirely feasible but it wasn't important enough to pursue.

[PerdioPal]

"but I'm pretty sure the output is DSB." That won't matter here. Even DSB intercarrier will work, other half chopped off by the TVs IF filter.

[Mike Watterson]

It's a long time since I looked on a spectrum analyser  but I'm pretty sure the output is DSB.

I expect so. The TV wouldn't usually care.

[Murphyv310]

Hi
I decided to purchase a used Pi5 8gb to try 405.
I still have two Sony 9-90UBs so want to get this set up
Is there any final info and programming for someone like me who very much struggles?
Copy and paste into the terminal is fine though.

[`FRANK.C]

Hi Trevor
It is a while since I set up mine and to be honest I have forgot what I did.

At that time it wasn't the easiest thing to do. Maybe there have been updates to make it more straight forward? I haven't heard that there were.
But being familiar with Linux is a good help.

In post #61 of this thread I have listed what I had to do at that time to get it to work

In post #54 I have listed the older version of the operating system worked with 405 lines for me.
The current one at that time, would not work. But this was to be corrected in the next release. So it is likely that the current version now will work.

If you are having difficulty in getting it to work I will send you my SD card that 405 lines is working on.

Frank

[Murphyv310]

Hi Frank.
Sorry for the late reply.
I've got it working, just need to do some tidying up. I see also a modulator hat has been developed too. I'll find out about that in due course as I'm busy with other projects and family things at the moment.
Thanks for the offer of an SD card but it's working with an external SSD drive which you can now with all pi's from the later 3b version