SSTV / NBTV Player

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Re: SSTV / NBTV Player

Postby FlyMario » Wed May 25, 2022 10:50 pm

Yeah. It's odd... I meant to actually delete that post as when I was in bed afterward I thought about the fact I had used .WAV files on my field recorder to playback on my previous two Mechanical TVs. I did that because I was certain that the sync pulses would get plowed over by the compression algorithm.

I chuckled at myself thinking about the actual quality of the video in the postage stamp output. But yeah... the sync pulses are key.

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SSTV / NBTV Player

Postby acl » Wed May 25, 2022 11:22 pm

Peter,

In my 'video jukebox' I used only WAV files for NBTV. MP3 as Steve described is a heavily compressed format but only worked fine for SSTV.

Regards Chris Lewis G6ACL
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Re: SSTV / NBTV Player

Postby Klaas Robers » Thu May 26, 2022 2:27 am

acl wrote:Peter,

In my 'video jukebox' I used only WAV files for NBTV. MP3 as Steve described is a heavily compressed format but only worked fine for SSTV.

And for audio of course!

That it works for SSTV is mainly because SSTV is an FM-modulated audio tone, where the complete audio spectrum is no wider than the telephone bandwidth, 300 to 3200 Hz. MP3 then has sufficient space to concentrate only on that narrow spectrum.
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Re: SSTV / NBTV Player

Postby FlyMario » Thu May 26, 2022 3:43 am

ACL,

Did the little DFPLAYER Mini's work well for you? I seem to recall using them in Halloween prop's a few years back with no issue. They are really cheap on Amazon which gives me some concern. Like 10 dollars for 3.

After seeing it being used in your machine, it makes me think it will be fine. Also, I can always say... well ACL uses it :)

Thanks!
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SSTV / NBTV Player

Postby acl » Thu May 26, 2022 6:56 am

We had to modify as per instructions but I have bought a supply of them if you want one. However, we have moved on and bought one off eBay for about a fiver which will be reviewed in the next newsletter. Please find attached pictures. These are brilliant. They play both MP3 and .WAV files.
IMG_3769.JPG

IMG_3766.JPG
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Re: SSTV / NBTV Player

Postby FlyMario » Thu May 26, 2022 7:10 am

Beautiful work man! I think I have some dfplayers and from what I can tell they are so many different providers making knock-offs. I will have to play with one and see if they are reliable. Maybe you got a better model. Crazy how the market has gotten. There are probably 30 companies making them LOL

Thanks! I need to become a member so I can read the newsletters.
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Re: SSTV / NBTV Player

Postby FlyMario » Sat May 28, 2022 2:26 am

Turns out that my Dfplayer Mini's are working as expected. Under the scope, the outputs look perfect. I guess I got mine before all these massive knockoffs.
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Re: SSTV / NBTV Player

Postby FlyMario » Sat May 28, 2022 7:13 am

ACL,

Did you have a problem with the output between Ground and the DAC becoming inverted? I looked at the .wav file in audition and it was perfectly formed. But from Dfplayer it is perfectly inverted under my scope. Maybe I can flip it back over with an opamp. Kind of peculiar. Everything else looks quite usable.

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Re: SSTV / NBTV Player

Postby Steve Anderson » Sat May 28, 2022 12:06 pm

With audio gear it's simply a 50:50 chance the signal comes out inverted or correct. There's no need in audio to preserve the absolute polarity, so the circuity is arranged in the simplest and/or the most cost effective manner. If you try a different model player there's still that 50:50 chance, maybe this time correct. The same applies to the record process. There's no rule (that I'm aware of) that says a positive-going input signal must result in a positive-going numerical figure on the card.

This generally applies to all audio formats, records, tapes, you name it. Professional equipment does (as a rule) preserve absolute polarity - but at a cost (not much true). There is a reason for this which in a domestic situation really doesn't apply.

But as you say, a simple unity-gain inverter is all that's needed, an op-amp or one or two transistors...

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Re: SSTV / NBTV Player

Postby acl » Tue May 31, 2022 5:48 am

thumbnail_P1080030%20-%20kopie.jpg
thumbnail_P1080030%20-%20kopie.jpg (135.48 KiB) Viewed 172 times


Here are some comments from Peter Spies. They may apply to your player. If you do modify one and 'cook' the board then I have plenty spare. The sound processing program Audacity has a feature to invert the phase of an audio signal and Gary Millard's video2nbtv also has a video invert option. If your NBTV source is battery operated you can simply swap the signal leads over to get the correct phasing. As with both NBTV and SSTV gear when using plug in wall mounted power supplies for display and source you expect that as the earth pin is normally plastic there shouldn't be any electrical connection path between supplies although you can get some interesting results especially if you are using an earthed 'scope.



1. the coupling capacitor from pin 14 (DAC-L) of the GD3200B chip to pin 5 (DAC-L) of the module was only 0.1uF.
As a consequence the cut-off frequency was much too high for NBTV purposes.
I removed it and replaced it with a short, then used a sufficiently large (47-100 uF) external coupling capacitor.
(I believe the capacitor was 10uF in the older, 'original' versions of the module).
A coupling capacitor is needed, as the DAC-L signal from the chip has a considerable DC offset.

2. I removed the 4k7 resistor coupling the DAC-L signal to the (input capacitor of) the 8002 amplifier chip.
The integrated amplifier now only amplifies only the DAC-R (=NBTV audio) signal. It will drive a speaker.
The need for an additional, external audio amplifier chip was thus eliminated.
(I believe these L- and R- channel coupling resistors were 22k in older versions of the module, and presumably not mounted in the same place)
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