Negating LF problems of NBTV.

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

Re: Negating LF problems of NBTV.

Postby smeezekitty » Fri Aug 07, 2020 7:54 pm

Steve Anderson wrote:P.S. If anyone wants to suggest a future project that I may be able to put together, ideas welcome...e.g. 'B' above, remove the requirement of the PC, record/playback on a SD card as part of the unit. If you've got some older SD cards of 2GB or less hold on to them for this, I really don't want to mess with the e-FAT format, FAT16/FAT32 is OK, though the cards will not use any of the conventional formats...they'll be used simply as a glorified EEPROM - which is really all they are.

Using the same format as the PC-based version of 'B' above, a 1GB SD card will record over three hours of NBTV audio and video.

...however there is a caveat to this... using the proprietary SD card interface requires an expensive (for us lot) licence, the SPI interface is licence-free but of course slower.

FAT32 doesn't limit you to 2GB. It actually supports up to 2TB. Also, the SPI interface on an SD card is plenty fast enough for NBTV
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Sat Aug 08, 2020 1:35 pm

smeezekitty wrote:FAT32 doesn't limit you to 2GB. It actually supports up to 2TB. Also, the SPI interface on an SD card is plenty fast enough for NBTV

Yes, correct, but there is a reason for mentioning it - just wish I could remember what it was...it's been a while since I dabbled around with SD cards/discs/etc..

It'll come back to me at some point.

Steve A.

Ah! It's something to do with the SD card initialization as I recall, I'll look through my notes from a few years back...something to do with the difference between standard capacity cards and high capacity cards. That breakpoint is either 2 or 4GB...again from memory...

The read/write speed needs to be more than 100kB/s via the SPI interface...I would think that all should be able to meet that.
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Re: Negating LF problems of NBTV.

Postby Andrew Davie » Sat Aug 08, 2020 1:37 pm

@smeezekitty your email address recorded with the forum is bouncing as mailbox unavailable.
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Re: Negating LF problems of NBTV.

Postby smeezekitty » Sun Aug 09, 2020 3:26 am

Steve Anderson wrote:Ah! It's something to do with the SD card initialization as I recall, I'll look through my notes from a few years back...something to do with the difference between standard capacity cards and high capacity cards. That breakpoint is either 2 or 4GB...again from memory...

That's true. And the addressing mode is also different. But the differences are not a big deal and easy to handle
The read/write speed needs to be more than 100kB/s via the SPI interface...I would think that all should be able to meet that.

I believe that the SD cards support SPI clocks of at least 8MHz which would give you about 1MB/s

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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Sun Aug 09, 2020 1:18 pm

Most of the information r.e. SD & MMC cards I got from this site - it's a bit vague in places but it's a start:-

http://elm-chan.org/docs/mmc/mmc_e.html

The 'proper' reference is from the SD Card Association, but if printed it would make an ideal door-stop - good for bed-time reading too.

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Re: Negating LF problems of NBTV.

Postby acl » Mon Aug 10, 2020 10:57 pm

Hi Steve,

Glad you see you back home at long last. I think that signal generator was used on the SSTV camera we discussed some time ago.

https://www.youtube.com/watch?v=mdEdHKR6lCA

I am still interested in the SSTV convertor and am willining to help out with the prototype if you need help.

Regards Chris Lewis
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Tue Aug 11, 2020 1:31 pm

acl wrote:Hi Steve, I think that signal generator was used on the SSTV camera we discussed some time ago.


Yes, I saw it in among the gubbings and thought, "What's that?" Found it on the 'net and ordered four more - very useful little device.

acl wrote:"I am still interested in the SSTV convertor and am willining to help out with the prototype if you need help.


I assume you mean the SSTV-625 up-converter? It's work-in-progress but currently stalled due to a whole pile of other things pending, and not just NBTV/SSTV stuff. It would sure help if you (or others) want to be beta-testers/builders of the SSTV-625 up-converter. There's also a 625-SSTV down-converter on the horizon with the intention of also making it 625-NBTV. Same concept as Peter Smith's version of 2000 (CQTV189). PM me if you'd like a copy, or you can download issues of CQTV over one year old from the BATC website:-

https://batc.org.uk/

The biggest problem with the 625-NBTV/SSTV down-converter is finding a suitable A-D device that is fast enough, at least six bits, with hobbyist-friendly packaging (DIP) and at a sane price. Peter used the 6-bit CA3306, now made of Unobtainium. The latest datasheet I have for it is dated Dec 1993.

Two CA3306 chips can be cascaded to form a 7-bit A-D if desired.

Steve A.

Unobtainium (Uo) - a highly desirable material that is hypothetical, scientifically impossible, extremely rare, costly, or fictional, or has some of these properties in combination.

A local supplier has a stock of around 70 of the CA3306 chips at about GBP 2.50 ea. I doubt any more after they've all gone.
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Thu May 06, 2021 5:35 pm

The best part of a year has passed with no progress here...now a paid project has come along with very similar requirements, though it's not NBTV it's is a perfect adaption to our uses. The client only requires DC-1kHz (or so), extending that to around 15kHz is no rocket science.

The storage media would be a PC with a USB port, file handling as you would any other...much as all the above postings...

File extension could be anything, but .nbt or .nbtv seem appropriate here...perhaps also a text/ASCII header (human and machine readable) containing information about sample rate, 1 or 2 channels (video only or video plus audio), maybe other stuff too.

I'll have to review where I got to and make the required modifications to my original plans...especially in the filter area...they may mutate into software filters which would vastly cut down the hardware required...but first hardware filters, then software versions...it maybe time to look at dsPIC micros which I haven't used before..they're designed for digital signal processing, especially filtering...

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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Thu May 13, 2021 3:52 pm

...also considering adding A-law companding to the audio channel which will reduce the data rate in/out of the PC by a third...and/or reducing the audio sampling rate too...do we need 15kHz of audio bandwidth for NBTV? Video, I feel yes we do need that 15kHz.

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Re: Negating LF problems of NBTV.

Postby Harry Dalek » Fri May 14, 2021 1:10 pm

I recall the project a while ago , trying to follow what it can do ? i am thinking you input images via the USB to it and it outputs NBTV ?
If so what are the Limits bandwidth it can do ? i see 15KHZ
Looks like a nice little gadget ...if i am correct could it out do a PC sound card ?
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Fri May 14, 2021 2:16 pm

Harry Dalek wrote:...if i am correct could it out do a PC sound card ?

The problem with PC 'sound cards' or those built-in (laptops primarily) is that they are AC-coupled which is fine for speech and music, but ideally NBTV requires a frequency response down to at least 2Hz, preferably DC...at least in the video channel. DC restoration is one answer, but they can get fooled by certain waveforms or picture scenes. Even without that there's still visible artifacts left over from the AC-coupling.

This device is in effect a specialised external soundcard specifically for NBTV. The audio channel is much the same as a soundcard, but the video channel's frequency response goes down to DC, No need for DC-restorers and no left-over artifacts like line/frame tilt.

The actual 'quality' isn't as good as a soundcard. most of which are (or should be) at least 16-bit. 12-bits are used here for the audio channel and just seven for the video. This might not seem very good but six or seven bits is generally accepted as adequate for NBTV video. The audio is arguably somewhat better than the original AM broadcasts provided, even with companding (if used).

That's just about it in a summary. I'm not proposing any 'standards', I hope to be able to demonstrate that the problem can be solved, admittedly at some cost in complexity.

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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Fri May 14, 2021 3:07 pm

...as a follow-on...NBTV has a frame rate of 12.5Hz requiring a frequency response down at least 2Hz. What about SSTV?

The original version used a frame rate of about eight seconds or 0.125Hz! So based on similar thinking will require a frequency response down to...er...who knows? 0.01Hz? Whatever, standard audio equipment isn't going to handle it.

Cop McDonald circumvented this by at first using the SSTV signal to AM modulate a fixed-frequency audio subcarrier (I think at 2kHz), exactly the same as AM radio but at much lower frequencies. Later this was changed to a FM modulated subcarrier between 1200 and 2300Hz. On-air tests showed the FM system was better on the busy HF frequencies. Those 'key' frequencies are still the standard for SSTV some sixty years later, including those from the ISS in hi-res colour.

The same could be done for NBTV, the video now goes to around 10kHz instead of only 1kHz for SSTV. So, just multiply the SSTV key frequencies by 10, i.e. 12kHz to 23kHz. Therein lies the problem. Until recently reliable recording and playback of these high audio frequencies has been difficult and/or expensive.

Now with 'soundcards' being able to sample at 48 or 96kHz it is possible. FM-NBTV on one channel, conventional audio on the other...it's possible...anyone want to have a go?...or a possible future project for myself?

Footnote: However, those frequencies cannot be legally transmitted by radio amateurs...as far as I know...

Steve A.

The more I think on the above, the more sense it makes...maybe a large change of course ahead!
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Re: Negating LF problems of NBTV.

Postby Harry Dalek » Fri May 14, 2021 5:42 pm

OH so you plug this into the USB port and it outputs the Audio video at low dc levels down to 2HZ and up to a standard sound card can do ,sounds from what you say the low down to DC Freq's are important .... now that would be interesting what ever you can work out .
It can input video to ? ...might be a good thing for those interested in copying if it go's well ...with laptops the mic input is just being replaced with a built in mic these days .
What you are doing i sort of looked into as in a USB sound card a while back if i ever needed one but i have computers laptops around that have a mic input .
what you are thinking of doing is rather interesting and more useful....
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Fri May 14, 2021 6:29 pm

I'm not sure all are like this, my laptop has a built-in mic, but I can plug in a headset (with mic) and the PC internal mic is disconnected. It has separate 3.5mm sockets for the mic and earphones/headphones. One thing that is of concern is the mic input, often they're mono only. If it's called a 'line input' as well there's a good chance it's dual channel, stereo, which we need here.

But I love to hate those 3.5mm connectors. So some years ago I bought a behringer U-PHONO UFO202, it's a USB-audio I/O device, in effect an external soundcard. Definitely two channel, stereo, and line-level inputs and outputs. Google it if interested, there are bound to be many others similar too. I did check, they're still available. If you have a desktop PC you'll probably not be needing this if you have a soundcard already filled as they usually have both line and mic inputs.

My only concern is the maximum (only?) sample rate of this unit is 48kHz...we'll see...not sure as I haven't used it in a while...check if the others do have selectable sample rates before you buy...preferably including 96kHz for both channels when in stereo.

Steve A.
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Fri May 14, 2021 8:44 pm

To generate the FM-NBTV signal I'm going to use an AD9833 'module' or 'break-out board' as per photo. These are available from many places on the net for 5-10 US$ each. Provides sine (10-bit), square or triangle outputs in approximately 1Hz intervals to 12.5MHz...though only one type at a time...all that's needed here is sine...

Make sure you get the AD9833 version, not the AD9837.

Added later...there's nothing wrong with the AD9837 version, in fact it's better suited for this use...but few places seem to have them...as a complete sub-assembly...the 'naked' device has a pin-spacing of 0.5mm...good luck with that!! I have some AD9833 modules already, so I'll stick with them...much easier to source...

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