Sinclair Spectrum Computer SSTV

Forum for discussion of SSTV topics. Slow Scan television (SSTV) is a picture transmission method used mainly by amateur radio operators, to transmit and receive static pictures via radio in monochrome or colour.

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Re: Sinclair Spectrum Computer SSTV

Postby Steve Anderson » Mon Sep 09, 2024 11:41 am

acl wrote:Hi Steve, Any progress on SSTV lately? Regards Chris Lewis

Well. yes and no. I've produced a couple of up-converter designs which are simpler than the first version a few years back. One is basically the same, SSTV to 625, the other SSTV to 800x600 VGA. It might be possible to combine the two and have a selectable output standard. But that's not a priority at the moment. These days the VGA version is probably more useful....VGA is the same in the US as the EU.

As before these are for the original Cop McDonald standard, a 1:1 square aspect ratio, monochrome, 120/128 lines at either a 15Hz or 16.7Hz line-rate, often called the 8-second mode.

Most of the simplification is in the area of the RAM, two 8-pin serial RAM chips instead of a 28-pin (or more) parallel version. This also reduces the pin-count on the two processors to 28-pin 0.3" devices. One might become a 20-pin device.

They are just paper/software designs at the moment, I've yet to plug the soldering iron in.

There is also a possible upgrade to the SSTV demodulator. Although technically an improvement, I somehow doubt this will be visible on-screen. It is also somewhat more complex using an FIR subcarrier filter. Though just the FIR filter alone IS a worthwhile improvement. The other changes are subtle and may not be noticible.

Steve A.

After that a JPG-to-SSTV device would be useful, but this is probably better/easier(?) done on a PC. But not by me, I've not written code for a PC before, and I don't intend to start now! The original JPG-to-SSTV version (not published) involved a number of manual steps to get the original JPG configured acceptable for conversion to SSTV and subcarrier modulation.
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Re: Sinclair Spectrum Computer SSTV

Postby M3DVQ » Tue Sep 10, 2024 9:45 pm

acl wrote:I think the Sinclair spectrum has some powerful surface devices helping the Z80 out controlling the micro SD card


The CPLD there will be converting parallel bytes from the data bus into the serial format used to talk to the SD card, as this is much more efficient than "bit banging" the protocol as sometimes done with microcontrollers. It also provides all the logic for paging in and out extra ROM and RAM.

Interfaces like these don't have any powerful offloading of file handling etc however. All the code for reading and writing blocks to the SD card, and opening and reading the files within a FAT filesystem and so on is written in Z80 code and done by the speccy's 3.5MHz CPU :)
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Re: Sinclair Spectrum Computer SSTV

Postby M3DVQ » Tue Sep 10, 2024 10:09 pm

M3DVQ wrote:I can create you a .sna you should be able to load ok next week.


Try this:
g1ftu-sstv-g6acl.zip
(10.66 KiB) Downloaded 52 times
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Sinclair Spectrum Computer SSTV

Postby acl » Wed Sep 11, 2024 12:48 am

Thank you so much, it worked a treat. Now it should work on all my Spectrums. I noticed one would keep coming up 'tape loading error' occasionally when loading from the fixed output of the digital tape emulator. Perhaps needs recapping. Could you advise which spectrum emulator you used to save as a .snp file

Regards Chris Lewis
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Re: Sinclair Spectrum Computer SSTV

Postby M3DVQ » Wed Sep 11, 2024 7:43 am

I used Fuse but they should all be capable of saving 48K .sna snapshots
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Re: Sinclair Spectrum Computer SSTV

Postby M3DVQ » Wed Sep 11, 2024 7:48 am

Occasional R Tape Loading Errors are a fact of life on the spectrum. There's no error correction or anything in the basic tape loading system so any bit of interference, crackly contacts, etc just breaks the load. I avoid using tape input wherever possible these days :lol:
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Re: Sinclair Spectrum Computer SSTV

Postby Steve Anderson » Wed Sep 11, 2024 11:29 am

acl wrote:Hi Steve, Any progress on SSTV lately? Regards Chris Lewis

Regarding SSTV progress/updates, I'll start a new thread on the impending SSTV up-converter(s) within a couple of days so as not clutter this thread up. It'll be entitled, "SSTV Up-Converter Mk VI". Mk II to Mk V never made it to fruition. They were more of a paperwork, software and simulator design exercise...

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Re: Sinclair Spectrum Computer SSTV

Postby Steve Anderson » Thu Oct 03, 2024 3:11 pm

One of the most infuriating things I find regarding PC monitors is the lack of modes listed that they do and do not support. The majority will support the more common VGA modes, but before setting out designing your next display masterpiece it would be helpful to know if the display does work in the standard proposed.

Most displays these days have a screen area aspect ratio of around 16:9 or thereabouts. What if you feed it a 'standard' 600x800 60Hz VGA signal? Does it stretch it to 16:9 or display it as 4:3 with black bands either side? Or can you select via a menu?

I have two 24" monitors attached to my PC in Dual-screen mode, one a Philips, the other a Samsung. Neither mentions the modes they support or not in their manuals, as well as if the aspect ratio can be altered or not.

Why do I want to know? They both work fine at 1024x768 (or whatever), but for the next up-convertor I need to know. I can drop the PC into various display modes to test those display modes the PC can output, but not all. I would feel more confident if I generatd the signal, then there's a modicum of confidence. So a simple "VGA Test Box" may be on the horizon, it removes all the variables/unknowns possible with the PC-VGA interface.

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