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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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21st Apr 2019, 8:42 pm | #1 |
Rest in Peace
Join Date: Jul 2011
Location: Bridgnorth, Shropshire, UK.
Posts: 787
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AM to VSB changeover
I was surprised to learn recently that the switch to vestigial sideband, as a more bandwidth economic replacement for straight amplitude modulation, occurred during the 405 era, and not with the introduction of 625, as I had previously assumed.
What was the consequence of this for people with AM sets? Were they stuffed (to use the vernacular)? |
21st Apr 2019, 8:59 pm | #2 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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Re: AM to VSB changeover
As far as I understand, VSB was designed to be received on a receiver meant for full AM.
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21st Apr 2019, 9:07 pm | #3 |
Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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Re: AM to VSB changeover
The Alexandra Palace transmitter used a double vision sideband transmission characteristic. When it was replaced in 1955 by the channel B1 Crystal Palace TX a vestigial vision sideband was used. This presented a problem with certain receivers which were aligned to the upper sideband. The Baird T5 I'm presently restoring was aligned in such a manner. Another TV receiver that comes to mind was the Ultra V470.
AP was the only 405 line transmitter to use DSB. I believe that in the last few years of the UK analogue system I the vestigial sideband was trimmed back from 1.25Mhz to 0.75Mhz. Please confirm if the latter figure is correct. DFWB. |
22nd Apr 2019, 8:47 am | #4 |
Dekatron
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,724
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Re: AM to VSB changeover
A legacy of that was the larger frequency jump between channel 1 and 2, compared with the other band I channels.
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22nd Apr 2019, 9:35 am | #5 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,534
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Re: AM to VSB changeover
A description I found on file of the VSB curve relative-to-carrier reads as follows (Ch1 frequencies in brackets). I am not sure of its original source:
Fc = 0dB (45.0 MHz) Fc + 0.75MHz (45.75 MHz) = -3dB or lower Fc + 2MHz (47.0 MHz) = -30dB or lower Steve Last edited by Panrock; 22nd Apr 2019 at 9:43 am. Reason: Clarity |
22nd Apr 2019, 10:34 am | #6 |
Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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Re: AM to VSB changeover
Response curves taken from the manual of the Mullard AC1 experimental TV receiver.
This was a TRF receiver aligned to the upper sideband. Note the lift in the HF end of the response curve in order to enhance the video upper frequencies. DFWB. |