How Narrow is "Narrow-Bandwidth"?

Forum for discussion of narrow-bandwidth mechanical television

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What do you regard as a bandwidth limitation for NBTV?

Less than 10 kHz.
1
8%
Less than 15 kHz.
0
No votes
Less than 20 kHz.
9
69%
Less than 50 kHz.
1
8%
50 kHz or more.
1
8%
I do not care because I have switched to non-electric TV.
1
8%
 
Total votes : 13

How Narrow is "Narrow-Bandwidth"?

Postby Stephen » Tue May 08, 2007 11:48 pm

With all the different schemes and proposals for NBTV, I am wondering what everyone thinks "narrow-bandwidth" TV really means in terms of bandwidth. When I think of alternate schemes that might provide higher resolution, frame rate or colour, I personally regard it as anything that fits in a high fidelity audio channel, say 20 kHz, but I can imagine that there may be a wide range of opinion on this.
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Postby DrZarkov » Wed May 09, 2007 12:35 am

I think anything that fits into an audio-channel. You can go a little bit lower, but then you reach a point where nothing is recognizable. If you go higher, you have technical problems, especially with recording. On the other hand a 60 lines TV with 25 frames is not really "medium resolution" like the German system of 1935 with 180 lines or the late Baird systems with 240 lines.

Of course I would like to have a mechanical TV with 625 lines (in fact a common video-recorder is a kind of mechanical TV, a modern DSP video-projector is mechanical HDTV), but that has nothing to do with NBTV.

Where das SATV and ATV starts? I think SATV is about 100 kHz? So a "natural" limit of NBTV would be somewhere between 20 kHz and 99 kHz? Maybe we could make two classes of NBTV, low definition up to 20 kHz or 48 lines and medium definition up to 240 lines?
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Postby Roland » Wed May 09, 2007 11:01 am

DrZarkov wrote:I think anything that fits into an audio-channel. You can go a little bit lower, but then you reach a point where nothing is recognizable.


I'd agree with this as the ability to record the signal as an audio channel is a major (practical) attraction to me. I also like your suggestion for medium definition TV as it is worth remembering that mechanically scanned TV was not limited to 30 lines.

I didn't vote for non electric TV as although I'm seriously looking into this now - I don't intend to abandon the electronics side in the long run. However having said that - the attaction of this hobby has always been for me the mechanical scanning and not the fact its narrow bandwidth.

:-)

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Difficult choice.

Postby Steve Anderson » Wed May 09, 2007 1:04 pm

I haven't voted in this poll as I find it very difficult to do so.

The 10kHz limit that I have seen mentioned from time-to-time was simply a limitation imposed by the domestic receivers of the time. Baird and others were constrained by this and had to come up with a system that would 'fit in' to the available bandwidth.

As the push for better resolutions went on so the required bandwidth expanded. Tube/valve and receiver manufactures worked together to produce (eventually) receivers that were affordable to the masses.

At the other extreme we have SSTV (Slow Scan TeleVision) as devised by Copthorne McDonald (no relation of Ronald). Now whether to consider this 'television' or not I'm not sure. With the original frame rate of one picture in around 7-8 seconds all movement is lost, it's really a series of stills. Yet it does fit the translation of 'television', i.e. 'Seeing at a distance'.

As for bandwidth, it's baseband only goes up to around 1kHz, a carrier is then FM modulated between 1.2kHz and 2.3kHz. This fits down a phone line and all radio transmission paths. It really is 'Narrow Bandwidth'.

A 120-line system (still at 12.5 fames/sec) would require a bandwidth of 150-200kHz...is this narrow? Compared to the 5.5MHz of full-broadcast 625 it certainly is!

So going back to the poll, it's a hard one to answer...

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Postby AncientBrit » Wed May 09, 2007 10:58 pm

I'm happy with anything less than 20kHz.

The real challenge is getting colour to fit within this bandwidth.

There are several possibilities for transmitting colour.

A subcarrier system has already been proposed, as has RGB multiplexing on the double byte encoding in a CD format.

One I have not yet seen proposed would be a version of MAC, multiplexed analogue component, where 3 signals are time compressed to fit within a single NBTV line.

It would make sense for the channels to carry Y and two colour difference signals, with the Y signal occupying 0.5 of the NBTV line time, and the difference signals occupying 0.25 line time each.

Regards,

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How Narrow is "Narrow-Bandwidth"?

Postby Stephen » Thu May 10, 2007 12:18 am

I have always thought that "slow scan television" was no more than fast facsimile. Television is more than seeing at a distance. It is seeing images with tone graduation in full motion at a distance. Even John Logie Baird recognised this. He did not give a demonstration of his system to the Royal Society until he had achieved a scanning rate sufficient to impart smooth motion and tone graduation in January 1926.

I think that is is fair to make some demarcation between "narrow-bandwidth" and "wide-bandwidth" TV in a similar fashion to the commonly accepted distinction between wideband and narrowband FM. What the commonly accepted demarcation for narrow-bandwidth TV is I do not know and that was the primary reason for this poll.
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Postby Steve Anderson » Thu May 10, 2007 1:03 pm

Stephen,

Yes, all understood. I'm not proposing this, but if our 'interest' were to be renamed I would suggest ACTV (Audio Circuit TeleVision). I think that fits in comfortably with most peoples' thinking.

It also allows non-mechanical systems to be included even though most are interested more in the mechanical side. However, one or two of my posts have been of a non-mechanical nature and thus far no-one has complained.

But what's an audio circuit? Telephones and HF SSB, 300Hz to 3kHz roughly. AM radio, 40Hz to 5kHz. CDs, 20Hz to 20kHz.

Then there is the SACD and DVD-A, both of which go to almost 100kHz! (Two formats I think will go the way of the DAT, DCC and Mini-Disc, but I'll probably be proved wrong).

So with the above in mind I have voted for the 20kHz option assuming the format is 32 lines, 3:2 aspect ratio and 12.5 frames/sec.

The original standards for SSTV are very similar to NBTV. It has full tonal graduation and uses a waveform that is almost idential to NBTV with the exception of the line duration being 60mS instead of 2.5mS. (66.7mS in 60Hz counties).

At first it had 120 lines in a 1:1 aspect ratio. As the binary age came along this was increased to 128 lines, giving one complete frame in 7.68 seconds, 8.533 seconds in 60Hz counties.

As time has gone on resolution has been increased and colour added, now some single-frames can take almost 5 minutes! So I agree, it's not television as conventionally thought of.

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P.S. Where do web-cams fit in?
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Postby Steve Anderson » Thu May 10, 2007 2:00 pm

One I have not yet seen proposed would be a version of MAC, multiplexed analogue component, where 3 signals are time compressed to fit within a single NBTV line.

It would make sense for the channels to carry Y and two colour difference signals, with the Y signal occupying 0.5 of the NBTV line time, and the difference signals occupying 0.25 line time each.


The same concept as used in the professional analogue recording format, Beta and Beta SP (Not BetaMax, the domestic format). With an available bandwidth of 20kHz you could 'just about' get 10kHz luminance bandwidth and 5kHz for each of the colour-difference signals.

The timing could be quite critical from a mechanical camera, but in principle it could work.

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Postby Viewmaster » Thu May 10, 2007 6:41 pm

Steve Anderson wrote:Stephen,

Yes, all understood. I'm not proposing this, but if our 'interest' were to be renamed I would suggest ACTV (Audio Circuit TeleVision). I think that fits in comfortably with most peoples' thinking.

It also allows non-mechanical systems to be included even though most are interested more in the mechanical side. However, one or two of my posts?


'Audio Circuit TV' sounds rather dry and uninteresting, doesn't it? It certainly would not appeal to the general public, some of whom might become interested if the title were improved.
Nor does it reflect the historical, romance of mechanical TV at all. The sheer wonder of sending moving pictures with a simple spinning disc.

Maybe mech TV should have one title and non mech another although that might be confusing.
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Postby DrZarkov » Thu May 10, 2007 7:09 pm

First I do not want to change the name, but indeed "mechanical TV" sounds more interesting and "romantic". NBTV sounds very technical, but the name is o.k.because the narrow bandwidth is a chance, not a handicap. As I saw in the handbook, the old name of the club was LDTVA. But "low definition" is just an effect, not the purpose of mechanical TV.

If it is about early TV systems, the name "mech TV" would not be o.k., there are very early experiments (e.g. by Max Dieckmann in 1906) with CRTs. Of course most of us prefer mechanical systems, so NBTV is the right choice in name, I think.

So I would definite NBTV as something between SSTV and SATV.
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Postby Steve Anderson » Thu May 10, 2007 7:09 pm

'Audio Circuit TV' sounds rather dry and uninteresting, doesn't it? It certainly would not appeal to the general public, some of whom might become interested if the title were improved.


Hmmm, not sure if NBTV is any better at conveying the mechanical roots of the subject to the poplace either. The mention of 'bandwidth' would probably lead people to think that this has something to do with IPTV (Internet Protocol TeleVision). Which of course it has nothing to do with.

The NBTVA was formed well before the Internet became part of Joe Publics' life, the word 'bandwidth' was almost exclusively used by engineers. Today things are different.

Maybe mech TV should have one title and non mech another although that might be confusing.


I agree, one title should cover all. But as I said earlier, I'm not advocating changing the name at all.

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Bandwidth and scanning.

Postby Stephen » Fri May 11, 2007 12:05 am

This poll is generating some very interesting comments. Whilst the results seem to suggest that the general view of the participants is that "narrow-bandwidth" for the form of television that we are discussing means no more than a baseband high fidelity audio channel, there is a wide range of opinion on this.

Furthermore, there is the issue of terminology, as television engineers would probably consider any signal limited to the hundreds of kHz as narrow-bandwidth, and they might describe a video signal limited to the bandwidth of a bandband audio channel as sliver bandwidth. In fact, I do not think that most television engineers would consider a 32 by 48 or 30 by 70 format even usable. I think they would be surprised if they saw the results.

The other terminology issue for me is "mechanical" and "electronic" television. I have never thought this terminology made sense, although we seem to be stuck with it. What if we address a CCD array for a camera or a LCD panel with a motor-driven commutator? Is that mechanical television? What if we take Mr. Baird's array of lights that he uses as a display in his first television patent dating to 1923 and substitute electronic switching for the commutator? Does that change it from mechanical to electronic?

It seems to me that what we really mean is that we can use devices of the "optical scanning" or "electronic scanning" type. To me optical scanning means optically moving light to dissect or reproduce a television image. To me electrical or electronic scanning means scanning an electronic image or electrically addressing an array of photoelectric devices - even by means of a motor-driven electrical commutator - to dissect or reproduce a television image.
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Re: How Narrow is "Narrow-Bandwidth"?

Postby Andrew Davie » Fri May 11, 2007 1:37 am

Stephen wrote:With all the different schemes and proposals for NBTV, I am wondering what everyone thinks "narrow-bandwidth" TV really means in terms of bandwidth. When I think of alternate schemes that might provide higher resolution, frame rate or colour, I personally regard it as anything that fits in a high fidelity audio channel, say 20 kHz, but I can imagine that there may be a wide range of opinion on this.


In my past life I've programmed the Atari 2600 video game computer (a 1977 machine) that is fun in the same way that NBTV is fun. The fun is imposing some limitations upon what you can do, and then seeing if you can make things happen. On the Atari, for example, there are some immutable limitations, which everyone must adhere to -- but there are also some variables that can be changed (eg: the amount of RAM, the ROM size, etc).

Now the Atari field has its purists -- those that feel that unexpanded base cartridges should be used (that is, 4K ROM, no extra RAM). This is the fundamental original configuration that the early games used. I was a purist, and pushed the envelope as much as I could. But after a while, I realised the envelope was pushed as far as it would go.

But there were interesting things one could do if one relaxed the self-imposed boundaries just a little bit. What can be done with a little more RAM, for example. Or a much larger ROM, but the same RAM limitations. These sorts of forays into side-fields which might upset the 'purists' are what made it all fun, once one had mastered the basic configuration.

I see an analogy with programming the Atari 2600, and developing mechanical Television systems. It's probably the same in many hobbies. The purists would select 10kHz. They'd also select 30 lines, and having the outermost lines more widely spread.

But it's not about being totally pure. That is but one (valid) part of this hobby. There will be those who want to explore in various directions (for example, the addition of colour piggybacking on the 'standard' club signal, or stereo... or you name it). So I see a 20kHz bandwidth being just as valid as 10kHz. And I see (say) 500kHz just as valid. There won't be many people interested in using 500kHz -- because (for one thing) it would be 'too easy'. Half the fun, as I have found with the Atari, is imposing limitations upon what resources can be used, and then developing something within those limitations.

For me, NBTV is about technique. I love the way that a time-based interaction between a spinning disc with holes in it and a pulsating single light source forms an image! And I love it even more that this was being developed 80 years ago! Yes, I happen to be using LEDs, getting my information from the Internet, using a debugging tool that is light years ahead of what was available then... and my Nipkow disc was cut by a laser. But the fundamental WAY it works is the same as it was back then. Spin a disc with holes at exactly the right speed in front of a light source.

THAT is what NBTV is to me. I don't particularly care if I have a higher bandwidth, more scanlines, or if I'm using a spinning mirror drum instead of a disc with holes. For me it's about the techniques that were historically used at the dawn of television. I think that all of these early methods (including, if we must, CRTs) are perfectly valid claims to NBTV and bandwidth is, in my opinion, just an un-necessary and arbitrary division that's just one of those self-imposed limitations of which I wrote earlier.

Rather than narrow-band television (which is really just a consequence of the technology available back then), I tend to think of what I'm interested in as 'Historical Television' which encompasses techniques, not bandwidth

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Purism.

Postby Steve Anderson » Fri May 11, 2007 12:31 pm

The purists would select 10kHz. They'd also select 30 lines, and having the outermost lines more widely spread.


They should also dump their sync pulses. Now things become interesting!

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Historic (or is it hysteric) TV.

Postby Steve Anderson » Fri May 11, 2007 12:52 pm

Rather than narrow-band television (which is really just a consequence of the technology available back then), I tend to think of what I'm interested in as 'Historical Television' which encompasses techniques, not bandwidth


I couldn't agree more. In the thread 'For those old f**ts amongst us' I posted a series of articles from the UK magazine 'Wireless World' regarding 'electronic television'. Dated 1937 I think does make it historic.

Historic TV is not a bad name, but the public again would probably confuse HTV with HDTV, which are quite opposite! But once again I'm not suggesting the name be changed without good cause. (There could also be confusion with the 'History Channel', part of the Discovery Network).

Having said that, would 'Historic TV' encompass the uniquely British 405-line system, or the bizzare 819-line French system? Both have associations still using these systems, just like the NBTVA.

For those interested here's a link for info on the UK 405-line system..

http://en.wikipedia.org/wiki/405-line

...also...

http://www.bvws.org.uk/405alive/

...and a brief item on the French 819-line system...

http://en.wikipedia.org/wiki/Analog_hig ... on_systems

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