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General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
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11th Sep 2020, 1:29 pm | #1 |
Diode
Join Date: Sep 2020
Location: Swindon, Wiltshire, UK.
Posts: 5
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AM distortion
Hi, I've just joined today after repairing 3 wirelesses - I'm reluctant to use the word "restoring". I haven't reached that level of skill yet! So hello to everyone.
I'm sure that I read somewhere that stations on AM can sound a bit distorted because of the broadcasters' signal processing. In my area I can only receive one music station "Smooth" and there seems to be a little but annoying distortion. Before I wind up the scope - am I chasing an inherent broadcasting problem? |
11th Sep 2020, 2:56 pm | #2 |
Dekatron
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,676
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Re: AM distortion
Yes and no.
Yes the broadcasters use devices such as the Orban Optimod to squeeze all the dynamic range out of their output. But no, it's not just AM radio that is affected, FM and DAB are also hit by this. By its nature AM radio is not going to be Hi-Fi and a bit of processing can actually make it sound subjectively better. If you consider that Hi-Fi needs to reproduce approaching 20kHz at the top end of the audio spectrum, you may be surprised to hear that the theoretical highest frequency AM radio, as broadcast in Europe can manage, is only 4.5kHz
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-- Graham. G3ZVT Last edited by Graham G3ZVT; 11th Sep 2020 at 3:08 pm. |
11th Sep 2020, 3:45 pm | #3 |
Nonode
Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
Posts: 2,196
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Re: AM distortion
The conventional diode envelope AM demodulator is inherently non-linear and produces a noticeable degree of audio distortion. Back in the day when the transmitted AM signal bandwidth was around 10kHz, there was generally sufficient high-frequency content in the programme material to mask the more unpleasant distortion content. Nowadays, with the transmitted bandwidth severely limited to 4.5kHz and modern receiver sharp IF filters restricting it further, the detector distortion components are clearly exposed with no beneficial masking from the programme content.
Martin
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11th Sep 2020, 3:53 pm | #4 |
Nonode
Join Date: Mar 2010
Location: Dukinfield, Cheshire, UK.
Posts: 2,034
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Re: AM distortion
I believe all simple diode detectors introduce audio distortion, especially at mod depths approaching 100% but I cannot recall the actual mechanism, although I recall something called 'slew-rate distortion' at higher audio frequencies caused by the time constant of the RF filtering which follows the diode. I'm sure it's all on the 'net somewhere!
A synchronous demodulator would be much better I'm sure, but no mass-market radio manufacturer of 4-valve superhets would go to the expense.
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Andy G1HBE. |
11th Sep 2020, 3:59 pm | #5 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
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Re: AM distortion
It also can depend on the receiver you are listening on. If it is very old (1930's), it might use an anode bend detector and they are notoriously bad in dealing with modern transmissions. Local stations seem worse than National stations like the BBC. As an example, Gold can sound dreadful on my 1930's radio but BBC Radio 4 sounds perfect. It's all down to the amount of compression used by individual stations.
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27th Sep 2020, 3:34 pm | #6 |
Diode
Join Date: Sep 2020
Location: Swindon, Wiltshire, UK.
Posts: 5
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Re: AM distortion
Thank you for the advice. I've a Bush VHF90 which sounds very good, a Bush DAC34, and a Bush TR82 which probably suffers most distortion which is not in the AF section. The cores are jammed in the i.f. cans on the latter - so I'm going to super glue a rod to them and try some silicon spray to help release them.
I changed the case of the VHF90 to an undamaged white one. In order to protect the case from heat, I've "attached" a brushless PC fan to the back and made a metal stand. (the mod is reversible) I hope this will prevent cracking of the case. I may mess with wattless voltage reduction but I'm very keen to start on my VHF54. Which I would like to smuggle into the house as a "nice piece of furniture" as we used to say. I used to repair TVs years ago and I remember the Philips G6 could have been used as a room heater. Although I spent much of my life in electronics - I feel I have forgotten more than I knew! |