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Old 24th Oct 2019, 3:07 am   #21
Synchrodyne
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Default Re: AM Broadcast Signal

Quote:
Originally Posted by Hybrid tellies View Post
This graph shows the response of an AF band limit filter used by most AM transmitters to limit interference to adjacent channels. As you can see it does not cut off at 4.5Khz but tails off at 6Khz.
To add to that, here is what I have on file in respect of the UK situation.

The original BBC FL4/55 filter, of 1973, steep cut from 5 kHz combined with a 3 dB lift at 5 kHz:

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The IBA ILR curve, from 1977:

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And according to this, by 1993, the BBC had eased up slightly on the “brickwall”, with the steep cut -3dB point moved out to 5.8 kHz:

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Old 24th Oct 2019, 9:42 am   #22
Hybrid tellies
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Default Re: AM Broadcast Signal

Thanks Synchrodyne, very interesting.
So it looks like the sound quality of some of the AM transmitters has improved slightly since the 1970's especially with the improved compression techniques.
On some of my best radios including my Hacker Hunter RP38A, its now just a joy to listen to BBC R4 on 198Khz.
I notice there is still quite a difference between the commercial stations with Absolute on 1215Khz sounding quite reasonable but some of the Smooth stations sound harsh and flat with the top end of the Audio cut off with to much compression. I wonder if this would be shown on an SDR.
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Old 24th Oct 2019, 10:19 am   #23
Ian - G4JQT
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Default Re: AM Broadcast Signal

I was lucky enough to work in La Paz in the Andes about 20 years ago doing a media survey. I took with me an HF225 receiver amongst other things.

One local station I heard on MW sounded almost like an FM station. I opened the rx bw to maximum (can't remember now but probably 15kHz) and it sounded excellent! A good strong, not too limited or compressed AM station on a wide-band IF setting sounds excellent. One reason why our so-called 'pantry transmitters' can sound so good.

In Europe the bands are (or were) very crowded and bw restriction is absolutely essential to limit adjacent channel interference, particularly at night. But in less densely populated areas this is much less important, and dare I say it, regulation is probably more casually enforced...
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Old 24th Oct 2019, 1:06 pm   #24
russell_w_b
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Default Re: AM Broadcast Signal

I've just dug out my pre-1997 (last time I had anything to do with domestic MF) notes and performance spec for BBC TX AM broadcasting: 60Hz - 5kHz plus/minus 1db (WRT 1kHz at 40% mod).
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Old 26th Oct 2019, 2:51 am   #25
Pfraser
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Default Re: AM Broadcast Signal

Ian G4JQT:

Quote:
One local station I heard on MW sounded almost like an FM station. I opened the rx bw to maximum (can't remember now but probably 15kHz) and it sounded excellent! A good strong, not too limited or compressed AM station on a wide-band IF setting sounds excellent. One reason why our so-called 'pantry transmitters' can sound so good.
Indeed. The Radio Monique signal on 963kHz sounded 'like FM' on a Hitachi boombox I had.

I gather that Monique used an Optimod processor, thus demonstrating that they didn't have to be set up for maximum impact.

I believe the Orban Optimod 9100 had a day/night switch, for less bandwidth at night.
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