Old POTS videophones, slow scan TV...

Thread Starter

Externet

Joined Nov 29, 2005
1,568
Hi.
With the speed and definition limitations, video was transmitted on plain old telephone lines restricted to only 3KHz bandwidth.
What would be the expected result transmitting video on 136 KHz carrier; about the same, substantially better ?
Any performance examples, experiments, equipment that you may know of ? -No audio needed- -No color needed-

What resolution and frames per minute could be sent using a given bandwidth ? How is that calculated ?
 

-live wire-

Joined Dec 22, 2017
912
A standard video can range from 2 Megabytes absolute minimum to more than 500 Mbytes. M/mega = a million, K/killo = a thousand FYI.

3KHZ!?! Think about it. That means 3Kbits max per second. A video can easily be many, many megabits (far more standard is megabytes, 8 bits). So 10Mbytes = 80 Mbits. That divided by 3kbits ber second = 80 000 000 / 3 000 = 26 666 seconds! A standard carrier is 1-20MHz! So get a real one that's a few MHz unless you want to wait days to recieve/send messages. There is no getting around it. Do not waste your money on the 3kHz one. 136khz/3khz = 45x faster. Sill pretty slow though (9 mins for a 10MB file). So get a real one, like version!

Just google "1280p" or whatever picture per data to find the bytes. Multiply that by fps and seconds.
 

nsaspook

Joined Aug 27, 2009
7,980
First you would need to know the encoding and modulation methods. With 64QAM you get 6 bits per symbol. For the 136 KHz carrier let say you have a given symbol rate of 100 KHz. That gives us 600KHz to play with before Forward Error Correction and packet bit requirements needed for reliable transmission on a network. With 3/4 FEC, for every 3 bits of data, you are sending out 4 bits, for packets maybe 7/8 of the bits per packet is actual data. That gives us about ~400KHz for actual video bandwidth. A good video codec can use that bit-rate to transmit a pretty good standard def video image.

 
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Bernard

Joined Aug 7, 2008
5,740
I know that it can't be done,but my telephone & high speed internet comes over unconditioned twisted pairs from a 50 pair cable ?
 

Thread Starter

Externet

Joined Nov 29, 2005
1,568
Thanks, gentlemen.
See you are referring to 'standard video' ; QAM ; digital ; bit rates ; high speed internet ...

Am talking about analog video, low resolution, monochrome, slow old stuff...
The kind used by early videophones and SSTV; transmitted on voice bandwidth. Yes, 3KHz. at 1000 lines per minute, 128 x 128 resolution.
Where each frame typically used 8 seconds. As in the sixties to seventies. Where 1500 Hz was black, under 1500 Hz was blacker than black; 2300 Hz was white.

Am not after modern performances, but what to expect doing it with 136 KHz instead of audio bandwidth.

upload_2018-4-6_9-15-18.png


Perhaps am having trouble expressing myself. I suppose that if 136 KHz is modulated with voice band, the resulting resolution and speed would be the same as video on voice band after demodulation. But what if a much wider-than-voice modulation is used ?

upload_2018-4-6_9-46-3.png
 
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Alec_t

Joined Sep 17, 2013
11,858
Ignoring error correction overheads, assuming you transmit a frame of 128 x 128 pixels, each pixel is encoded as a tone averaging 60kHz and the duration of each tone burst is only one cycle (unrealistic), the transmitted frame would take 128*128/60 mS = 273mS, i.e. a frame rate of 3.6/sec. In practice the tone burst would probably need to be longer to ensure reliable decoding. Add on the error detection bits and the practical frame rate would be more like 1/S for a guess.
 

nsaspook

Joined Aug 27, 2009
7,980
Perhaps am having trouble expressing myself. I suppose that if 136 KHz is modulated with voice band, the resulting resolution and speed would be the same as video on voice band after demodulation. But what if a much wider-than-voice modulation is used ?
No, your question was clear: What resolution and frames per minute could be sent using a given bandwidth ? How is that calculated ?

As you can see the difference is still in the encoding and transmission method. Now you see just inefficient primitive analog video was when you can get a "56k modem" that uses the exact same bandwidth.
 
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MisterBill2

Joined Jan 23, 2018
7,963
The old video phones that I saw were about one frame per minute and a two inch screen. So the data rate was a lot slower. And those wonderful video weather pictures from the satellites took a few minutes to arrive, but the resolution was quite good. With current technology the typical picture is a few megabytes, but the refresh rate is still limited by bandwidth. And bandwidth is still limited by carrier frequency. So just like the slow pictures that arrive on an old modem with a phone line hookup for the internet, the high resolution pictures will take quite a while. Consider sending a 2 megabyte file with a 1200 baud modem, (1200bytes/second) . With no overhead it would be close to 2000 seconds.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,568
Thanks again, gentlemen. With unexpected luck, I found half of the wheel re-invented :

----> http://wiki.argentdata.com/index.php?title=SSTVCAM

Outputs the audio ! Now am after more luck to find the second re-invented wheel part; audio to video.
Your findings will be appreciated.

To be able to perform---->

For application on a very unusual way:
Camera audio------amplifier------speaker . . . . . . . . . microphone------preamplifier------decoder-----videomonitor
 

nsaspook

Joined Aug 27, 2009
7,980
The old video phones that I saw were about one frame per minute and a two inch screen. So the data rate was a lot slower. And those wonderful video weather pictures from the satellites took a few minutes to arrive, but the resolution was quite good. With current technology the typical picture is a few megabytes, but the refresh rate is still limited by bandwidth. And bandwidth is still limited by carrier frequency. So just like the slow pictures that arrive on an old modem with a phone line hookup for the internet, the high resolution pictures will take quite a while. Consider sending a 2 megabyte file with a 1200 baud modem, (1200bytes/second) . With no overhead it would be close to 2000 seconds.
I worked on the GOES-1(2,3) receiver on the ship as a FLTSATCOM tech in the 70's. We converted the downlink audio signal to WEFAX format to print on an output device.
 

nsaspook

Joined Aug 27, 2009
7,980
The old video phones that I saw were about one frame per minute and a two inch screen. So the data rate was a lot slower. And those wonderful video weather pictures from the satellites took a few minutes to arrive, but the resolution was quite good. With current technology the typical picture is a few megabytes, but the refresh rate is still limited by bandwidth. And bandwidth is still limited by carrier frequency. So just like the slow pictures that arrive on an old modem with a phone line hookup for the internet, the high resolution pictures will take quite a while. Consider sending a 2 megabyte file with a 1200 baud modem, (1200bytes/second) . With no overhead it would be close to 2000 seconds.
I worked on the GOES-1(2,3) receiver on the ship as a FLTSATCOM tech in the 70's. We converted the downlink audio signal to WEFAX format to print on an output device.
 

nsaspook

Joined Aug 27, 2009
7,980
The old video phones that I saw were about one frame per minute and a two inch screen. So the data rate was a lot slower. And those wonderful video weather pictures from the satellites took a few minutes to arrive, but the resolution was quite good. With current technology the typical picture is a few megabytes, but the refresh rate is still limited by bandwidth. And bandwidth is still limited by carrier frequency. So just like the slow pictures that arrive on an old modem with a phone line hookup for the internet, the high resolution pictures will take quite a while. Consider sending a 2 megabyte file with a 1200 baud modem, (1200bytes/second) . With no overhead it would be close to 2000 seconds.
I worked on the GOES-1(2,3) receiver on the ship as a FLTSATCOM tech in the 70's. We converted the down-link audio signal to WEFAX format to print on an output device.
 

MisterBill2

Joined Jan 23, 2018
7,963
I followed that posted link and saw the camera. Then a bell rang and I realized that was used also in an article in QST magazine quite a few months back. So there is an article out there about using this thing. You notice that there are different formats, and none of them are standard TV formats. Probably feeding the audio signal into a PC sound card with the right software running will result in pictures being displayed.So that should round out your wheel.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,568
Trying to avoid computers, so far I found an application to feed the audio to a smart phone to display the SSTV video.
If anyone can advise for a dedicated conversion chip to use a plain LCD composite monitor instead, would be great.
 
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