AM to VSB changeover - UK Vintage Radio Repair and Restoration Discussion Forum

Karen O · 21st Apr 2019, 8:42 pm

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)?

julie_m · 21st Apr 2019, 8:59 pm

As far as I understand, VSB was designed to be received on a receiver meant for full AM.

FERNSEH · 21st Apr 2019, 9:07 pm

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.

Graham G3ZVT · 22nd Apr 2019, 8:47 am

A legacy of that was the larger frequency jump between channel 1 and 2, compared with the other band I channels.

Panrock · 22nd Apr 2019, 9:35 am

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

FERNSEH · 22nd Apr 2019, 10:34 am

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.

21st Apr 2019, 8:42 pm	#1
Karen O Rest in Peace Join Date: Jul 2011 Location: Bridgnorth, Shropshire, UK. Posts: 787	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
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: AM to VSB changeover As far as I understand, VSB was designed to be received on a receiver meant for full AM. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

21st Apr 2019, 9:07 pm	#3
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	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
Graham G3ZVT Dekatron Join Date: May 2010 Location: Greater Manchester, UK. Posts: 18,710	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. __________________ -- Graham. G3ZVT

22nd Apr 2019, 9:35 am	#5
Panrock Nonode Join Date: Feb 2005 Location: Worcestershire, UK. Posts: 2,532	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
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	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. Attached Thumbnails