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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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22nd Jan 2019, 11:48 am | #1 |
Nonode
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,869
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Modulation Distortion
A question has been asked, on a different workshop forum, about a possible change to the depth of modulation on medium wave transmissions by the BBC and /or others which is causing distortion on some vintage Radios. Can someone comment on this please?
Cheers John |
22nd Jan 2019, 12:28 pm | #2 |
Nonode
Join Date: Mar 2010
Location: Dukinfield, Cheshire, UK.
Posts: 2,033
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Re: Modulation Distortion
I believe that up to the mid-60's, BBC transmitters were modulated up to something like 60% or less to minimise distortion in the simple diode detectors of the time. When the offshore pirates came along with their 'loud' modulation and compression, the BBC fought back and turned up their own mod.
I haven't checked this for veracity, it's just a story that does the rounds. The other thing is the 'power saving' technique now in use. This lowers the carrier level on positive modulation peaks to save the amount of electricity used by the TX. I wonder if some types of detector or AGC may be upset by this? No doubt someone with greater knowledge will be along soon!
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Andy G1HBE. |
24th Jan 2019, 11:15 pm | #3 |
Nonode
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,869
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Re: Modulation Distortion
Thank you Andy. No comments so far.
Cheers John |
24th Jan 2019, 11:37 pm | #4 |
Heptode
Join Date: May 2015
Location: Chatham, Kent, UK.
Posts: 947
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Re: Modulation Distortion
Hi modulation types do upset the agc i remember using controled carrier mod on an AM transmitter and geting severe distortion on a receiver close by because the agc could not keep track of varing carrier levels this resulted in the RX being overloaded. Mod was ok when signal wasn't so strong may be the same reason ?
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