NBTV wireless on FM?

[Viewmaster]

Those that know the venue (I have never been), could you give me an idea of the dimensions of the room/hall, just so I can get a feel for the range required. Also what lighting is used (probably flourescent) and a very rough estimate of the level, really bright/average/somewhat dull. Just so I can estimate the ambient light which does affect the range of an IR link. Are there any windows to the outside world that let in bright sunlight?.....Hang on a sec, this is in the UK, forget that question.

Steve, I have only been there once, last year, so the actual choice of rooms may change from year to year.
Anyway, last year there were 2 rooms leading into each other. I would estimate that each room was about 30 ft x 30-40ft.
The lights were on in one room and I recall they were flourescents of office brightness.
Off these two rooms, down a short corridoor I seem to remember,was a rest room with a drinks (soft!) machine. This was larger than other rooms.
Others who have been many times may recollect more details than I have..
Albert.

[Steve Anderson]

Steve, I have only been there once, last year, so the actual choice of rooms may change from year to year.
Anyway, last year there were 2 rooms leading into each other. I would estimate that each room was about 30 ft x 30-40ft.
The lights were on in one room and I recall they were flourescents of office brightness.
Off these two rooms, down a short corridoor I seem to remember,was a rest room with a drinks (soft!) machine. This was larger than other rooms.
Others who have been many times may recollect more details than I have..
Albert.

Thanks for that Albert, it's a start if nothing else. If I aim for a link of say 20m (60 or so feet) that should be enough. I'll assume the lighting is at conventional office levels etc.

The venue being split between two rooms raises a minor headache, but not insurmountable, it depends if the link is needed in both rooms or just one.

I had considered the actual rooms allocated may change from time to time, or even one day a complete change of venue. We'll just have to see what develops.

The hardest part of any IR link is the rejection of ambient light, both of the DC variety (sunlight) and AC lighting run off 50Hz mains giving 100Hz of ripple. There's also harmonics of this 100Hz.

In more recent times there is also quite high frequency modulation of light output from the newer 'energy-efficient' flourescents which have an electronic ballast running at around 80kHz.

Even though most IR photodiodes are housed in a opaque plastic which is IR transmissive, the power ratio between a LED and several lights/sunlight is still a challenge.

Steve A.

[Viewmaster]

If I aim for a link of say 20m (60 or so feet) that should be enough. I'll assume the lighting is at conventional office levels etc.

The venue being split between two rooms raises a minor headache, but not insurmountable, it depends if the link is needed in both rooms or just one.

Steve A.

Thinking about it again, most of the working equipment was in one room only and the lights were down to enable everyone to see the displays.
In the other there were only about 3 items running as I recall but that room had full lighting on.
Peter was the only person in the rest room with static equipment only, when I attended in the morning.
Obviously this all depends on attendance which was down last year on previous years I believe.

Albert.

[Steve Anderson]

I've a bit of soul-seaching on this IR link thing, and although I'm very interested, I think the uptake is going to be small and I think quite frankly for a once-a-year event like the convention, is it really worth it?

After all the distances are quite small, so why not just run some string around the room on the day? Take a sniff and display it. The complexity between a 10m range system and one that can traverse 10km is not that great, just some of the numbers are bigger (and some very much smaller too).

If one were considering a video hop between two hills 10km apart or more, then it's a challenge well worth thinking about. But just 20m indoors doesn't justify the effort required when a few metres of copper will do.

I'm sorry but that's the way I see it, a nice idea and concept, but as ever a solution looking for a problem.

Now, when one talks about 10km between the wind-swept hills of the UK, count me out! I used to spend a lot of time on some of the higher perches in Hampshire and Wiltshire, nah, I'm too old for that sort of business now.

...and no, I'm not going to tell you what I was doing there!

Steve A.

Psssst...If you must really know, it was very QRP operation on 1296MHz, around 10mW....but keep it to yourself, the local farmers thought I was irradiating their livestock! Thankfully I never got blamed for the crop circles.

[Viewmaster]

I agree. Many going there do not read this forum so would not know about it anyway.
Albert.

[Steve Anderson]

I agree. Many going there do not read this forum so would not know about it anyway.
Albert.

Yes, apart from a small number, there are very few members of the NBTVA who are also a member here. I've often wondered why. There's a link to this forum from the NBTVA website, but not getting the printed newsletter any more I don't know if there has been any mention of this forum in it.

This forums first anniversary is near the end of next month, and one would have thought that the word would have got out by now.

Steve A.

[DrZarkov]

Steve, your mention of QRP brings it back to the start of this thread. Many of us (I mean NBTVA members) have a HAM-licence. Why not transmitting on shortwave, not in SSB, but in "good old" AM. The transmitter must be weak enough not to disturb anyone, that should not be a problem. For the non-amateurs a common radio would be enough to receive the pictures. That's they way it was done in 1930, I don't see why it shouldn't work now.

[Stephen]

Why not transmitting on shortwave, not in SSB, but in "good old" AM.

Also AM, unlike SSB, has the unlimited low frequency response needed for NBTV signals. Furthermore, with vestigal sideband filtering on the transmitter, a 10 hHz signal might only occupy a 12-13 kHz bandwidth.

[Steve Anderson]

That's they way it was done in 1930, I don't see why it shouldn't work now.

Volker, absolutely no reason at all. Now a couple of things, firstly let's consider the Radio Amateur situation....at first I'm playing it 'by the book'...

1) Firstly (at least in the UK) it's only the operator of the transmitter that needs a licence assuming that an amateur frequency was to be used. So the people just receiving don't need a licence.

2). The UK band plan doesn't allocate enough bandwidth for NBTV (esp. AM) until into the 432MHz band, the lowest frequency that FSTV can/should be used.

3). Amateur gear doesn't have (usually) the bandwidth we require, up to 10kHz or so, most fall off at around 3kHz, and the low-frequency performance also isn't up to the task.

4). Not many non-Amatuers have suitable receivers.

OK, so let's dump the Amatuer thing.

So to ensure 'Joe Public' compatability we need to choose either good old AM from 550kHz to 1600kHz or FM from 88 to 108MHz. The AM system has the same limitations as point three above, so that leaves just the FM band, again we're back to where we started.

Now your suggestion of extending the range of those small transmitters was entirely valid, and the chance of you being 'caught out' are very small indeed. The power is low, and it's just a one day event.

Now as (I think) Klaas mentioned the stereo separation in these devices is generally very poor and you'll get crosstalk to some extent between the two channels. Even in professional encoders and decoders you're unlikely to get a channel separation of over 40db. With these and domestic receivers it's going to be far worse especially at the extremes of frequency.

The answer? Use two, both in mono (i.e feed the same signal to both channels making mono, in addition the range should be extended if the receivers can be put into mono mode. Say sound on 89MHZ and video on 107MHz...depends on what's on the band already.

This may make any modification to the units not needed, but if you want to receive both vision and sound you'll need two radios!

Steve A.

[DrZarkov]

I think most of us have a transistorradio with shortwave. The shortwave on my radio starts from 3000 kHz and ends at 29,999 kHz. That means I can receive anything from the 80 meter band to the 10 meter band. I don't know how it looks with other transistor radios, but I think it will be the same, more or less. My old "Mende" from 1953 starts at 5 kHz and goes up to 23 kHz. That cuts off the 80 meter and the 10 meter band. If we use AM we don't need special receivers, any radio can do that. About the necessary bandwidth: Of course we must not disturb anybody. But we only need a range of about 20 meter. That means such a tiny transmitter which could hardly disturb anybody. You find a lot of circuits for "test-transmitters". I still prefer this legal way of transmitting.

The other question is: How many of us do actually have a HAM-licence? What I read in the newsletter and here, about 70%? I'm currently making my licence ("E"-licence, the German novice licence allows me quite a lot), and I hope to be ready in the time of the next convention. It is planned that we will pass our test at April. If really so many people have a licence, why not using higher frequencies?

[Steve Anderson]

Hmmm, interesting, I guess things are different in Europe. I haven't seen a radio in decades that has short-wave on it. All car radios and most portable radios here don't have AM of any kind at all.

Keep in mind that in large parts of Asia the FM band is wider than in Europe, typically starting at 78MHz, not 88MHz so more stations can be crammed in.

When I was back in the UK last year I should have had a look at some radios but didn't even think about it.

Steve A.

P.S. I have just spoken to my wife on the phone about this, she works in the broadcast industry. The AM band is still allocated to broadcasting, but there's only a few stations on there, all government 'information' stations that no-one listens to....and it seems they can't even if they wanted to!

[DrZarkov]

Yesterday I've got the newsletter Vol 33 No. 2. There is not only the usual chat in the 80 m band at 3700 kHz, but there are NBTV-transmissions on shortwave! Using AM VSB mode, positiv modulatian, club standard at saturday 8.00 UTC. So I really need a longwire on my roof.

[Klaas Robers]

In fact, it is not forbidden to transmit NBTV in AM on 80 meters. Yes, it occupies 20 kHz, but if the band is not crowded, who cares.

We have got our frequencies to do experiments and to "play" with them. NBTV in AM is real playing, so we are encouraged to do so.

Besides that the Vestigial Side Band trick can be done only once, so in the transmitter OR in the receiver. Not both. In CCIR TV this is done in the receiver. The transmitter is full AM from 1,25 MHz below the carrier to 5 MHz above the carrier. It is quite difficult to filter off the lower side band below 1,25 MHz without unacceptable phase distortions in the part from 0 to -1,25 MHz. The TV-receiver has the vestgial side band curve filter.

The simple TV-transmitters in VCRs have true AM from -5MHz to +5MHz and can be tuned without any problems by the receivers.