LW and MW timing

[raditechman]

If you notice a time difference between two Radio 4 FM transmissions. If they are from different transmitters then I suppose it is the distribution that the BBC has ised to relay the programme.
However, I think I have read somewhere that modern radios with DAB and FM may add a delay to the received FM signal as they pass the signal through part of the DAB processing stages in the set. Maybe someone here can explain that much better.

John

[G6Tanuki]

Originally Posted by raditechman

I think I have read somewhere that modern radios with DAB and FM may add a delay to the received FM signal as they pass the signal through part of the DAB processing stages in the set. Maybe someone here can explain that much better.

The "FM Delay" is not deliberate - it's a possibly beneficial side-effect of the design of modern radios.

In many instances the "FM" part of a modern DAB/FM radio is implemented digitally too! Digitize the incoming FM RF and feed the resulting 'audio' waveform to a class-D audio amplifier [which is basically a switched-mode power supply in disguise!]

It's cheaper this way than the traditional local-oscillator/mixer/IF-stages/discriminator/linear audio-amp FM radio approach, since in a DAB radio you've already got the frequency-synthesizer/preset-memories stuff, the processor for the digital stuff and usually a class-D amp to handle the audio too. "FM" is then just a matter of running some different code!

The downside of this 'software defined radio' approach is that running all this CPU-intensive stuff imposes a high power-drain whether on FM or DAB, so batteries don't last long.

[GeoffK]

I finally convinced the BBC Essex engineer there is a problem on its 765KHz MW output, with its delay and drop outs, and he is making a recording of it to send to their transmission partners, whoever they are. But why weren't they checking it in the first place.

[Andrew2]

Originally Posted by G6Tanuki

The "FM Delay" is not deliberate - it's a possibly beneficial side-effect of the design of modern radios.

In many instances the "FM" part of a modern DAB/FM radio is implemented digitally too! Digitize the incoming FM RF and feed the resulting 'audio' waveform to a class-D audio amplifier [which is basically a switched-mode power supply in disguise!]

It's cheaper this way than the traditional local-oscillator/mixer/IF-stages/discriminator/linear audio-amp FM radio approach, since in a DAB radio you've already got the frequency-synthesizer/preset-memories stuff, the processor for the digital stuff and usually a class-D amp to handle the audio too. "FM" is then just a matter of running some different code!

The downside of this 'software defined radio' approach is that running all this CPU-intensive stuff imposes a high power-drain whether on FM or DAB, so batteries don't last long.

Is it possible (at consumer prices) to directly digitise the incoming RF at 100 MHz-ish? I know my various comm receivers only do this up to about 30 MHz, changing to down-conversion with a digital IF to get higher.
If some radios do do it this way, that would explain why my Sony portable DAB/FM radio chews its way through batteries, even on FM. Sick of sticking new batteries in I began using it on FM only, expecting much longer battery life. No difference, so I measured the consumption: 100mA on DAB, 120mA on FM!

[G6Tanuki]

I don't know if DAB/FM sets sample directly at the signal frequency - something like the Realtek RTL2832U chip works at input frequencies up to 40MHz and is popular for "do-it-yourself" software-defined-radio-for-a-tenner projects [see http://www.m9t.co.uk/].

Interestingly, I note that one of the features offered in a typical DAB radio chipset:

http://www.nxp.com/products/media-an...chip:SAF3600EL

is "Integrated DAB-FM/ DAB-DAB time alignment and seamless blending" so I guess the manufacturers *do* consider the time delays !

[Biggles]

I have a cheap SDR radio module which does VHF and well above admirably. I think it was about £100 quite a while ago, so I would imagine by now there are many devices to cover FM broadcast frequencies at a fraction of the cost of mine. It has a very impressive performance, with all the added functions of spectrum analyser etc. So the technology is definitely available now, presumably at consumer prices. The current consumption may well still be a hurdle though.
Alan.

[Andrew2]

Biggles - it was the 'direct digitisation' at VHF that I was wondering about. Those VHF SDR radios (like the RTL sticks and the Funcube dongles) use a tuner module to down-convert before digitising AFAIK. My SDRPlay module does this - it has a conventional front end and then a digital section which takes up to 10 MHz chunks for digitisation. My 'posh' AR5001 receiver uses direct digitisation up to 25 MHz, but beyond this is uses a conventional front end to whack the signals down so the ADC can handle them.

[Biggles]

Ok Andy, I hadn't really thought that through properly. The module has an HF input as well, so that would go with what you're saying. Using analogue front end and conventional down conversion would mean a cheaper ADC.
Alan
M1CEU.

[julie_m]

But doesn't a PLL, which you would think they would be using for the tuning, have a handy built-in FM demodulator anyway?

Mind, there's still the stereo demultiplexing and the RDS decoding to be done .....

[sexton_mallard]

I have recently been hearing a momentary 'jump' in music like a little change in timing on Radio 1 and 2 in the car on the daily commute that I never noticed before between Wrexham and Chester. It then occurred to me after reading this thread it must be the slight time differences to the broadcast between two neighbouring TXes as RDS retunes from TX to TX. There are sometimes noticeable differences in sound quality too.

[lesmw0sec]

Originally Posted by lesmw0sec

I listen to R4 of an evening on MW. I can sometimes hear another TX putting out the same programme quietly in the background - there is a delay of a second or two between them. Quite amusing when the pips come on!

Noticed that again last night. The really odd thing was that the pips were at a slightly different audio frequency as well as having a time difference. The tx delay I can understand, but why should the pip tone differ on two versions of the same programme?

[broadgage]

Originally Posted by Jon_G4MDC

------------

Back to the topic. It wouldn't be so surprising if to avoid landline costs "legacy" (tongue in cheek use of that word) MW TXs became driven by off air RXs from DAB.
Why use satellite?

I don't think that regularly relying on the DAB signal as a source of programme content for MW broadcast would be a good idea. National radio broadcasts are meant to be a reliable system especially in time of war or other emergency.
Any failure of the DAB signal would also deprive a significant area of MW reception as well.

IMHO satellite or duplicated land lines should be used whenever possible, with the re-broadcast of the DAB signal only being considered as a standby facility if other arrangements fail.

but why should the pip tone differ on two versions

There is a pitch shift associated with loudness, see... http://hyperphysics.phy-astr.gsu.edu...und/pitch.html

[G6Tanuki]

Originally Posted by broadgage

I don't think that regularly relying on the DAB signal as a source of programme content for MW broadcast would be a good idea. National radio broadcasts are meant to be a reliable system especially in time of war or other emergency.

That naiiively presupposes your "wartime emergency" listenership are [1] listening on MW/LW and [2] listening to the BBC.

My conjecture is that rather less than 10% of the UK population would be doing so.

Most under-50s think that "radio" is FM/DAB, or it's a playlist they subscribe to or - when at home - on their TV via Sky or Freeview. if not, it's a stream on their smartphone that delivers them some hardcore bangin' choons 24*7 out of LA Detroit or New York.

[Jon_G4MDC]

That is a very, very good (if slightly worrying) point.
I would like to say it is a vote for keeping Droitwich and others on 198kHz.
Er, except, where will you get the sets to listen?
The rest feels like a house of cards.

I meant to add about the pitch shift...

If the 2 signals were out of time ( obviously they were) but on the same frequency - so no or very slow beating between the carriers can
explain a frequency shift. You heard the sidebands of the distant station against the stronger carrier of the local one.
It demodulated against the wrong carrier - distortion results. The distortion accounted for the apparently different pitch?
My 20p.

[Murphy Maniac]

Same in Liverpool. Only over the last couple of weeks have I noticed a distinct echo effect when listening to two radios tuned to what I thought was the same FM signal. 93.3 and 93.7 are my local frequencies and so it is conceivable that the radios are tuned one to each. No idea where these signals originate, but it is most annoying.