UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Television Standards Converters, Modulators etc

Notices

Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

Closed Thread
 
Thread Tools
Old 24th May 2006, 8:54 pm   #1
jim_beacon
Retired Dormant Member
 
jim_beacon's Avatar
 
Join Date: Mar 2003
Location: Bishop's Waltham, Hants, UK.
Posts: 939
Default 405 line audio specs.

Hi,

does anyone know the specification of the audio channel used with 405 line TV? As we have put so much effort into getting pictures on the sets, it would be nice to get the sound right as well.

I'm looking for the audio bandwidth, and whether any compression was used (if so, what were the characterisitcs), and finally, the transmitter mod depth (and was it measured with a stndard line level, peak audio level, or whatever other method the BBC could dream up!).

Thanks

Jim.
jim_beacon is offline  
Old 24th May 2006, 10:03 pm   #2
ppppenguin
Retired Dormant Member
 
ppppenguin's Avatar
 
Join Date: Dec 2003
Location: North London, UK.
Posts: 6,168
Default Re: 405 line audio specs.

From memory, the sound carrier was 6dB below peak vision carrier. Bandwidth was at least 15kHz, limited more by the production equipment than any inherent characteristics of the channel. I suspect that compression was minimal in the early days, it was called a sound mixer (the person!) with hands on the faders.

Using the BBC standard PPM scale, I would have expected 6 on the PPM to correspond with 100% modulation.
ppppenguin is offline  
Old 24th May 2006, 10:13 pm   #3
Steve_P
Dekatron
 
Steve_P's Avatar
 
Join Date: Sep 2003
Location: Bolton, Greater Manchester, UK.
Posts: 6,644
Default Re: 405 line audio specs.

Sorry Jeff, but 6Mhz is the figure used on UHF.

For VHF, look here: www.penders.cwc.net/otindex.html and click on Standards Info on the left. Should tell you all you need!

Cheers,

Steve P
__________________
If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?
Steve_P is offline  
Old 25th May 2006, 6:58 am   #4
ppppenguin
Retired Dormant Member
 
ppppenguin's Avatar
 
Join Date: Dec 2003
Location: North London, UK.
Posts: 6,168
Default Re: 405 line audio specs.

Quote:
Originally Posted by Steve_P
Sorry Jeff, but 6Mhz is the figure used on UHF.
Steve, where did I say 6MHz? I mentioned 6dB below vision carrier and 6 on a PPM. On system I (as used on UHF in the UK) the sound carrrier is (I think) 20dB below vision.
ppppenguin is offline  
Old 25th May 2006, 9:22 am   #5
Panrock
Nonode
 
Panrock's Avatar
 
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
Default Re: 405 line audio specs.

Hi Jeffrey,

Quote:
Originally Posted by ppppenguin
From memory, the sound carrier was 6dB below peak vision carrier.
From the pre-war 'black book' I seem to remember the ratio was 7dB - but this really is splitting hairs and it could have been changed later anyhow.

Let's say the ratio of peak vision to mean sound carrier was 6dB... Doesn't this then imply that peak sound and peak vision powers were actually identical - or have I missed something?

Steve O
Panrock is offline  
Old 25th May 2006, 9:39 am   #6
ppppenguin
Retired Dormant Member
 
ppppenguin's Avatar
 
Join Date: Dec 2003
Location: North London, UK.
Posts: 6,168
Default Re: 405 line audio specs.

Quote:
Originally Posted by Panrock
Let's say the ratio of peak vision to mean sound carrier was 6dB... Doesn't this then imply that peak sound and peak vision powers were actually identical - or have I missed something?
AFAIK this 6dB means peak vision voltage to mean sound voltage so I think you are right. Unless they mean 6dB power ratio in which case the voltage ratio is 12dB. In any case that was an awful lot of power to put into the sound transmission. This must have given much greater range to the sound than the vision.
ppppenguin is offline  
Old 25th May 2006, 4:46 pm   #7
Steve_P
Dekatron
 
Steve_P's Avatar
 
Join Date: Sep 2003
Location: Bolton, Greater Manchester, UK.
Posts: 6,644
Default Re: 405 line audio specs.

Sorry Jeff.

At the moment I am in a lot of pain due to my arm. I hope it works after all this!

By the way, in the 80s the sound carrier on 625 was reduced from -6db of Vision to -10db of vision. There was a massive shutdown at Eyemouth (I was in the team at Selkirk at the time) because it hadn't been adjusted to cope and stayed asleep....

Cheers,

Steve P
__________________
If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?
Steve_P is offline  
Old 25th May 2006, 6:33 pm   #8
jim_beacon
Retired Dormant Member
 
jim_beacon's Avatar
 
Join Date: Mar 2003
Location: Bishop's Waltham, Hants, UK.
Posts: 939
Default Re: 405 line audio specs.

Quote:
Originally Posted by Steve_P

For VHF, look here: www.penders.cwc.net/otindex.html and click on Standards Info on the left. Should tell you all you need!
Steve,

your site gives the relative sound level and the sound carrier offset, but not deatils of the bandwidth of the audio channel, whether or not it is compressed, or the mod depth.


Jeffrey,

thanks for your tips, I'm off to read up about PPM's - us comms engineers like our levels pinned down!

Thanks to you both

Jim.
jim_beacon is offline  
Old 28th May 2006, 12:25 am   #9
Hybrid tellies
Nonode
 
Hybrid tellies's Avatar
 
Join Date: Jan 2004
Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area.
Posts: 2,965
Default Re: 405 line audio specs.

I seem to remember reading somewhere that the bandwidth of the 405 line AM sound cannel was either 100Khz or possibly 200Khz.
Why so wide? apparently it was to reduce the effects of impulsive noise from car ignition, light switches and thermostats etc which caused real problems on the lower frequency VHF band 1 channels.
__________________
Simon
BVWS member
Hybrid tellies is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 1:10 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.