Updated "ROM-Scanners" and the like.

Forum for discussion of electronic television. Generally, stuff to do with CRTs and not using mechanical displays.

Re: Updated "ROM-Scanners" and the like.

Postby Steve Anderson » Wed Dec 08, 2021 9:14 pm

So, here's the basis of the device in terms of hardware. There is a simple filter to add downstream, a TL082 plus a few passives, but really this is it...there's also a few (6, RC0-5) pins available on the micro for other functions...what happened to JP101? I don't know - the hazards of cut and paste...

Steve A.

I've just noticed, JP101 is present, near the top left. I must have written the above before the caffeine had kicked in...

Later...although it is shown in the datasheet, there's something wrong here around D101...datasheets are sometimes wrong!! I have amended the drawing below to show what I think they really mean, but it only applies if you're going to program the devices yourself. So far I'm happy to make my NBTV/SSTV software freely available to those who want it. But those who want a pre-programmed and tested devices, just ask, it's not a big issue. However, this one hasn't even been started yet!! Updates as and when...

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Re: Updated "ROM-Scanners" and the like.

Postby Steve Anderson » Mon Dec 13, 2021 2:31 pm

Chris Lewis (acl) is looking into using a small pre-made PCB from companies like Picaxe for a very similar project (See latest Newsletter Vol, 44 No, 4), though there are many copies and clones on the usual ebay-type sites.

Have a look on the Picaxe site https://picaxe.com/ or Google SPE033 & SPE035. (pdfs attached) These modules accept a micro SD card with the audio stored as stereo MP3 or WAV files at most common sample rates. Though you wouldn't use MP3 for this due to crosstalk and compression artifacts. You can add a simple push-button interface or a UART-style I/O for PC or microcontroller control.

One (slight) downside is undoubtedly the outputs are AC-coupled, of course that can be dealt with the usual DC-restorer arrangement for the video channel. Or, I suspect, a simple modification to the module to provide a DC-coupled output. (Short out one capacitor).

A note of caution: Chris has found that some of the modules (not from Picaxe) are substandard with awful sound (and hence video) quality. One would assume that purchasing these from Picaxe, although a bit more expensive, they should be operationally full-spec. Time will tell.

My concept with just the bare-bones module follows...

Chris notes that the video output is also inverted - an easy fix....

The module includes a small audio amplifier (SPK1, SPK2) to drive a loudspeaker, however it's a permanent mono mix of both channels, so not much use here. Though a simple mod may be able to fix that too.

Steve A.

The next thing is - How do we get DC-coupled WAV files onto the micro SD card in the first place? Good question - working on it...

I'll also look into using USB flash-drives, more user friendly (not so small and fiddly, or to drop/lose) than those micro SD cards.

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Updated "ROM-Scanners" and the like.

Postby acl » Mon Dec 13, 2021 7:15 pm

Hi there Steve,

PICAXE now have gone down the route of using the Chinese clones as they work for 99% of cases. We need to be able to search for an OEM supplier that offers the higher quality modules consistently.

Regards Chris Lewis G6ACL
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Re: Updated "ROM-Scanners" and the like.

Postby Steve Anderson » Mon Dec 13, 2021 8:44 pm

Oh! And they're willing to risk what is (was?) a good reputation for that? Though there may be a better chance with getting a replacement/refund with Picaxe, I would say little to no chance when ordering from from the usual suspects. Plus Picaxe are based in Bath, UK, so you have a better chance with all the consumer protection laws. They are after all acting as a distributor.

If it is a 99% chance of getting good 'uns, that's not so bad. But that doesn't seem to have been your experience...

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Updated "ROM-Scanners" and the like.

Postby acl » Mon Dec 13, 2021 10:17 pm

Hi Steve,

PICAVE are using now using clones the 99% was that the majority of applications worked fine with these modules as MP3 quality was acceptable. Klaas is using one I built and it works fine for SSTV but not for NBTV. If we can source the genuine module we can use it for NBTV. I managed to blow one of the modules by just swapping the output cables round. Must have been a common connected somewhere.

Regards Chris
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Re: Updated "ROM-Scanners" and the like.

Postby Steve Anderson » Tue Dec 14, 2021 12:33 pm

Hmm, I'm starting to wonder if these modules are worth the hassle. It seems we have become their production testing, they're assembled then shipped and we do the product testing. It saves them the time and expense of doing it.

With all this I'm considering going back to my original concept/design. Slightly more complex, expensive and doesn't store as much*, but if it cuts out this hassle factor I think it's worth it.

There is still the issue of generating dc-coupled files, that applies with both versions, though we've done without them so far...

Steve A.

*But it's still quite a lot! It depends on the flash-memory chips fitted. A single 32Mb (4MB) chip will store 4,096 frames of 8,192 bytes as per Ralph's ROMscanner (32 lines, 256 samples per line including syncs). Pro-rata other memory sizes. 'Movies' plus sound has yet to be determined...
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Re: Updated "ROM-Scanners" and the like.

Postby Steve Anderson » Fri Dec 17, 2021 5:02 pm

This project has mutated quite a bit over the past few days. It still is conceptually much the same, but with the intention of adding moving video plus audio to it (Standard NBTVA-style). The storage container is a standard stereo WAV file which will (should) be playable on anything suitable. However, the video is DC-coupled eliminating the requirement for DC-restoration and the like.

Due to the DC-coupled video element the files would need to be created on this device. Playback on (say) a PC though would more than likely result in an AC-coupled arrangement with no advantage over using the PC's sound card in the first instance. But it is compatible with the usual AC-coupled arrangement. You could consider this as an application-specific external sound-card.

Initially sampling will be at 24kHz for each channel, also 32kHz and 48kHz, yielding theoretical bandwidths of 12kHz, 16kHz and 24kHz respectively in each channel. Sample depth will be 12-bits at first (more than enough for the video), though depending on results the audio may be bumped-up to 16-bits later. Problem: Getting hold of sensible 16-bit A-D's and D-A's here and at a sane cost.

One headache on the horizon is storage. This device will result in large files which will need to be stored and played back. That bit is what I need to work on next.

Steve A.

I may break this out into a new thread and come back to the 'FotoStor' at a later date. Perhaps I'll continue this in the "Recording & Playback of NBTV, DC-coupled." thread I started quite some time ago. It is directly related and perhaps an update and/or continuation of same.
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Re: Updated "ROM-Scanners" and the like.

Postby Klaas Robers » Fri Dec 17, 2021 8:10 pm

About the DC-coupled output:

you should also define at which level the DC in the .wav file is recorded or pre-restored. The video of the NBTVA CD's has also a fixed DC-level, with black at 00 00, video positive and sync negative, however some one else might have chosen for sync bottom = 00 00. This implies that you need any way a brightness control in a monitor, and/or a DC-restorer. And after you have included that, there is no need any more for the DC-coupling.

When playing back the CD's of the NBTVA it is the output circuit of the CD-player that defines the low frequency response. I have seen CD-players that go as low as 2 Hz at -3 dB.
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Re: Updated "ROM-Scanners" and the like.

Postby Steve Anderson » Sat Dec 18, 2021 11:49 am

Klaas Robers wrote:The video of the NBTVA CD's has also a fixed DC-level, with black at 00 00, video positive and sync negative, however some one else might have chosen for sync bottom = 00 00.


I've decided on black=0x00 00, sync negative, video positive for no particular reason, but if you want 'zero' to equal sync-tips all you need is a DC-shift on the output and take your pick, easy. However it may be wise to also include a 'Video Invert' playback switch for the odd occasion an inverted signal turns up, also easy. Also a 'Track Swap' switch when that is required (It is bound to happen).

I'll continue this in a new thread, "Recording & Playback of NBTV, DC-coupled v2.0.".

20/12/21 Note: Now re-named "Move-E-Stor MkI"

Here's a quick look at the two waveforms from a WAV file during creation. Track 1 is the usual NBTV signal at 400Hz, Track 2 is the audio at 1kHz at nominal maximum, -6dbFS. The sampling rate is 32kHz, 80 samples per line including syncs. It is planned to add 24kHz and 48kHz. The 'Front Porch' will vanish in the final version, though I may leave it as an option, along with a similar 'Back Porch'. Maybe 0.5 Gamma encoding too for linear signals as below, decoding also where needed.

Initial Waveform 01.gif


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