NBTV wireless on FM?

[Phil Hunter]

@Phil: Nice idea, have you tried to send the picture? It seems still to be a good idea for me, because everyone has a small radio, be a IR-receiver needs to be constructed first.

Another question: The cabinet under your Televisor, is it a Columbia Graphonola?

I made a quick attempt at transmitting to the televisor via the radio (by muting the sound output) - however some form of interference was getting through so I was recieving a badly distorted picture.
(Anyone with any comments on this would be welcome)

Unfortunately there is no gramaphone in the case, the only writing states "selectra" It was bought from a antique shop (it had been converted into a wash stand ! ).

Incidently I changed the position of the televisor to stand on top (from it's original hidden location) because it was too low to view unless you sat on the floor, after breaking ribs in a riding accident I chose a more comfortable " armchair" viewing position.

I have just had confirmation of dispatch from Darryl of a SC 32 / 30 converter so hopefully I can soon give feedback of it's performance.

[Dave Moll]

The matrixing circuitry adds a certain amount of crosstalk between the channels that is not noticable with ordinary stereophonic sound. However, with video on one channel and audio on the other, the audio is likely to have a noticable "buzz" and the video a noticable audio pattern. The matrixing circuitry may also have an effect on frequency response of the system.

I did wonder how good the channel separation would be - which is what I would like to check out if I obtain a working microtransmitter.

I hadn't thought about cutting out the matixing, but then this sounds like the sort of technical electronics that I would leave to others more capable in this area than I am.

[DrZarkov]

The problem is, any kind of modification of the transmitter would make it illegal...

[Stephen]

The problem is, any kind of modification of the transmitter would make it illegal...

Would bypassing the audio matrixing really be a modification of the transmitter that would make it illegal? I do not know the answer to that. It would not be a modification of the RF circuitry or even the modulator, because the matrixing precedes that. Also, it might be as simple as introducing the video and audio signals beyond the matrixing circuitry so that perhaps no alteration of the transmitter circuitry is involved. It would be interesting to find out.

[Klaas Robers]

Channel separation of FM stereo is very poor. I guess that you will find at least 10% of the right signal in the left signal. It is difficult to get it better as it depends on the strength of reception. FM radios adjust their channel separation when the signal is getting weaker or stronger. The noise in the stereo part is much more severe than in mono, so if the signal is getting weaker the radio shift gradually to mono. The channel separation is getting worse and going to zero.

Besides that I fear that the low frequencies, i.e. lower than 50 Hz are attenuated in FM receivers. For NBTV this should go much lower, say to 2 Hz or 5 Hz. This IS obtained by the CD line output, but not by FM radios. It is also the problem that NBTV-ers encountered when trying to record NBTV onto cassette recorders. This never gave good results. In fact NBTV recording has been waiting for the CD and CD-R.

[Stephen]

The problem with the stereo channel separation and bandwidth is due to the audio matrixing circuitry for stereophonic sound, since the basebase signal is left plus right and the subcarrier is left minus right. If you can bypass the matrixing in both the transmitter and the receiver you can overcome these obstacles.

In fact, in various areas of the world broadcasters are allowed to have completely independent radio channels on FM subcarriers. These are low deviation monophonic signals that are comparable in quality to regular medium wave broadcasts. I have heard such signals and there is no hint of cross modulation with the regular baseband signal that carries a completely different broadcast.

[Stephen]

Another thing that I forgot to mention that might cause a problem is the audio pre-emphasis in the transmitter and the corresponding audio de-emphasis in the receiver. I think that the pre-emphasis/de-emphasis is on the order of 6 decibels per octave. This is not a problem with audio because audio does not have high frequency content with high amplitude that could cause excessive frequency deviation and result in overmodulation. However, video is totally different in its signal spectrum amplitude distribution and may have high frequency content of significant amplitude. Chances are that the pre-emphasis circuitry is going to cause significant overmodulation distortion. Also the pre-emphasis and de-emphasis circuitry is probably going to cause significant phase shift distortion as well. It would probably be necessary to bypass the pre-emphasis circuitry in the transmitter and the de-emphasis circuitry in the receiver to get decent results.

[DrZarkov]

If you have to modify transmitter and receiver infrared is of course the better option. And without any doubts legal.

[Steve Anderson]

It would probably be necessary to bypass the pre-emphasis circuitry in the transmitter and the de-emphasis circuitry in the receiver to get decent results.

A way around this without having to modify the transmitter or receiver would be to put a de-emphisis circuit at the input of the transmitter and a corresponding pre-emphisis circuit on the output of the receiver. (50 or 75uS depending which country you're in).

This would negate the video over-modulation problem. Any phase-shifting is cancelled out.

Steve A.

[Steve Anderson]

How about (I assume) a perfectly legal central broadcast system based on IR? Using a bunch of high-powered IR LEDs placed high in the centre of the hall facing in all directions should allow reception as long as there was line-of-sight.
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That sounds really good! And it's easy to build, too. Of course not that easy like an old transistor-radio, but easy enough for the now 4 NBTV monitors on our next "Classic-Computing" later this year in Stuttgart.

I'm not sure when your next "Classic-Computing" event is but for the next NBTVA convention in April 2008 it should be possible to get something organised.

I'm quite willing to spend some time on this as I have other uses for it as well (Nothing to do with NBTV).

If this is a serious proposal, I'll devote some time to it. I have more than a few ideas floating around inside my head (there's plenty of room).

I'm trying at a conceptual level to keep things on the receive side as simple as possible as more of those would be required than transmitters.

Steve A.

Space....the thing between ones' ears.

[Andrew Davie]

Space....the thing between ones' ears.

... where no one can hear you think?

[Steve Anderson]

Space....the thing between ones' ears.

... where no one can hear you think?

I know we're going off-topic here....but I've heard one of my colleagues being described as "A vacuous hole in the atmosphere.", another as "A waste of Oxygen and bio-matter."

Pretty damning eh?

Steve A.

[Steve Anderson]

I'm in the process of selecting the optical (OK IR) components for this link. I have ordered some BPW41 photodiodes and some TLN115 LEDs which are spectrally matched at 950nm.

Just for the sake of it I worked out the frequency of 950nm...it comes out at 316THz! (Terra = 10¹²).

I wonder if there is a band-plan for those frequencies? I had a look at the UK Frequency Allocation Table, Issue No. 13 of 2004 and it lists 275-400GHz as 'Not allocated', there's no reference to frequencies above 400GHz.

So I think we're safe.

Steve A.

Interestingly, below 9kHz is also 'Not allocated'. There is a story, and I assume it is a story, that the railroad in South Africa had big problems with distribution of power via the cantenery system they used to power the trains.

There was one length of track that was 1500km long. At 50Hz that is a quarter-wavelength antenna, so all the power was being radiated into space. Nice story, but I think that's all it is.

[Steve Anderson]

Gents,

I thought I would re-awaken this thread as time is ticking by and the convention is really not that far away.

Having said that, with the passing of Doug Pitt will it be at the usual place and around the same date? Was it Doug that had the contacts and organised things?

My point is that if an IR broadcasting system is to be ready in time someone (me as a minimum) needs to get on with it!

Is there any real interest in this? Feedback, as ever, welcome.

I'm also a little unsure if it's to be a video-only system or with sound? This would complicate things somewhat.

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 A.

P.S. I've just had a look at the pictures from April 2007's convention on this forum, the lighting doesn't appear overly bright and what outside windows there are seem to be shaded by trees and shrubs outside. It also looks quite a small venue.

[DrZarkov]

I'm interested! I would like to come to the UK next year for the convention. Only picture would be enough, just keep it simple!