Keeping radios working following analogue switch off.

[Framer Dave]

Originally Posted by Station X

The question as to whether a low power MW transmitter is legal or not is probably better not asked. In this case ignorance may be bliss.

Point taken. I am reminded of the sign on a circus menagerie trailer: "Only fools disturb sleeping tigers"!

Dave

[Boom]

It used to be the case (probably still is) that an offense was only commited if an aerial was attached to a transmitter. However a 10Watt transmitter feeding into a dummy load has quite a (legal) range.

'Closed loop' devices as used in hospital pager systems also needed no license. There is no reason why such a loop shouldn't legally be used to cover a house.

[AlanBeckett]

The thing I find potentially sad is that half the fun is tuning across the dial to find the stations. Simply rebroadcasting a single station on a fixed frequency won't be very interesting. Is there any scope for a more 'broad band' approach'?
Alan

[kalee20]

I thought that as well. However, there's no reason why several small MW transmitters could not be made, each radiating on a different frequency and being fed with different programme material.

I've not tried reducing the frequency to the LW band, but this should also be possible (clearly any room-sized radiating aerial will be grossly inefficient but that will only help to keep power down). In fact the challenge of maintaining decent audio bandwidth rather appeals to me!

[KeithsTV]

The alignment information for the Bush BAC31 says to use a 3 turn loop 2-3 feet from the receiver fed from a signal generator. If a loop was fed with one or more modulated carriers this could feed several receivers placed close to the loop or several loops placed close to the receivers.

Keith

[Boom]

It is only an idea that I had and never got around to actually trying anything but some time back I was toying with the idea of feeding the counters on the AMT3000 with a switching signal to kick it between 2 (maybe more) channels. So long as the switching was done fairly quickly above af then it *should* work in theory *if* the PLL can lock up quickly enough.

Of course a problem arises that the audio input would also need switching in sync' depending on what channel is in use.

The idea was to feed a pair of DAC90As as a stereo setup from just one AMT.

Probably a total non-starter anyway.

[Phil G4SPZ]

I use a home-brewed (not by me) one-valve AM unit using a 12SA7 which was a Chas Miller design. Works great with a few feet of wire fed through 100pF.

[Chris G0EYO]

I haven't read the Digital Britain report and wondered if their plans said anything about using DRM on MW and SW. It is meant to be digital for AM transmitters and will probably require dual standard radio receivers. I would like to put money (but not too much) that they never move AM off MW. I remember the fuss when they tried to close down 198kHz some years ago.

Chris G0EYO

[Mike-repairman]

Well I built that little circuit ( the revised improved version from the link on post #15) and had a modicum of success.

The oscillator worked with a 455kHz xtal (only one around the required frequency I could lay my hands on in a hurry) but connecting it to the second stage via the 0.01 killed it. Not sure why; will investigate when I have more time. Perhaps it needs a little more gain.

The transistors quoted in the diagram, 2N4401, have an alternative, BC337, which have about the same gain, but not having any of those I substituted them for BC136 which are vaguely similar and should work.

If anyone else builds up the circuit please let me know how you get on.

Could be useful if we can get it to work.

Thanks,

Mike.

[Alf]

I use a little circuit I built some time ago, a 2N3819 FET using a medium wave oscillator coil, modulated by a valve type audio o/p transformer, but used in reverse. Hi Fi, it certainly isn't, but it does the job.

[AndiiT]

Hi, Some time ago and for reasons that I can't recall at the moment I tried to "re-broadcast" the signal from a DAB radio using one of the low power FM transmitters (one of the cheaper types that connect to an mp3 players headphone socket to allow transmission to a car radio) mentioned elsewhere in this thread and found that I lost nearly all of the DAB stations, probably due to harmonics from the FM transmitter interfering with the Band 3 DAB frequencies.

Admittedly the DAB signal isn't too clever in this area due to the terrain but if using one of these small transmitters to re-broadcast from a DAB radio some distance may be required between the Digital source and the FM transmitter.


Regards
Andrew

[chipp1968]

I have a Metzo unit I bought some years ago at great expence £100 ?re transmits any input on mw , though I neverhad much luck with it , must dig it out and try it again .
Anyone know these sets ?

[kalee20]

If we micropower-rebroadcast to our treasured AM radios, we will of course chose an appropriate frequency which is away from any close station. It then gives us the possibility to use a wider bandwidth than the 'proper' AM broadcasters can - we don't need to implement the savage treble cut that they need to fit into their allotted channel space.

As long as the radio has a suitably wide selectivity curve (and old-fashioned straight sets will be better than superhets here), we'll be able to get much better sound quality from them than 'official' stations. This must be the cloud's silver lining!

[dave walsh]

Today's Telegraph has two letters on this. One asks for retention of the analogue system re mobile/car reception. The other describes home re-broadcasting to historic radios via cheap electronic calculator level devices. Apparently a firm in Bath already produces a chip for this purpose? Dave W

[Jaded]

Hi. It's been my experience that stereo FM broadcasts are of better quality, (wider audio b/w?) than DAB and I have heard similar opinions from other listeners. If I'm right, does this mean that using a DAB radio as a source for a re-broadcasting device will produce limited quality sound all round the house?

[dave walsh]

That could be the case J but it's not the issue. This is really a question of utility over quality. If you are sending to a vhf portable or wood cased valve set [not a hi-fi system] that's probably limiting in itself. It's not what you'd expect from the portable/domestic set anyway! I often record DAB to MD then transfer to CD. Probaly not as good as broadcast analogue stereo used to be but [realistically] I'd suggest that it's acceptable to most people and I would be unhappy if it dipped below my [own] standard of clarity. Keeping the old radios going would be the priority in this scenario.
Re sending on MW, I think that a lot of valve sets only go up to about 9k Kc/s [with bass compression] anyway! That's why they have such a good "tone" or we think they do. Dave W

[percival007]

I think it was on Chris Evans' show on Radio 2 that a similar discussion was had.

I unfortunately was driving at the time (Rush Hour) so was not really concentrating on the wireless but, i think it was mentioned that F.M. certainly, was going to be "re-populated" with exremely local stations once DAB took a hold (not going to happen if they continue to degrade the sound quality, but that's another topic i think!).

James.

[Tom_I]

Originally Posted by Jaded

Hi. It's been my experience that stereo FM broadcasts are of better quality, (wider audio b/w?) than DAB and I have heard similar opinions from other listeners.

No offence intended at all, but I have to smile when I see comments like this.

Surely I'm not the only one here who is old enough to remember how people used to complain vociferously, in publications like Gramophone magazine and the less esoteric hi-fi periodicals, about how poor FM stereo radio was, specifically in respect of restricted audio bandwidth.

The fact that the upper frequency range was limited to about 15 kHz, with a sharp roll-off above this to avoid audio reproduction of the 19 kHz stereo pilot tone, was considered to be scandalous (even though many of those doing the complaining were of such an age that they wouldn't have been able to hear anything above 15 kHz anyway).

Now, though, FM seems to be the holy grail, all such shortcomings being forgotten. (I know there are many potential explanations, but I've probably drifted too far o/t already, so let's not go there.)

Let's just say confused? I certainly am, ever more so.

Tom

[kalee20]

Well, for those of us with decent FM receivers, there may be a worsening of sound quality if we use DAB and re-transmit on FM.

But we do have the opportunity to rebroadcast on MW (and LW and SW) and get the best quality from our AM radios than we 've heard for years.

Originally Posted by dave walsh

Re sending on MW, I think that a lot of valve sets only go up to about 9k Kc/s [with bass compression] anyway! That's why they have such a good "tone" or we think they do.

As European MW stations are not supposed to transmit much above 4.5kHz (so that they won't overlap the similar sidebands of the station in the adjacent 9kHz channel), then extending our micropower retransmitter to an audio bandwidth up to 9kHz is a big improvement. The folks with older radios, with things like variable selectivity, and the simple sets, will show the most benefit.

I know we can all do things like retransmit now, but losing stations off analogue air may provide the motivation for us to get on and do it.

[dave walsh]

That is a very good point I hadn't thought of Kalee and I bet you're right about a possible re-broadcasting bonus. Ironically though, I wasn't thinking of the bandwidth at the broadcast end but the quality of the output stage in the older receivers [maybe only 9kc/s of audio] which [I presumed] was not going to reproduce the full digital frequency response, even at 9 kc/s spacing.
It does seem to be perfectly true though that many early S Het or even TRF sets could deliver astounding performance in less crowded conditions.
Cheers Dave