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

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Reply
 
Thread Tools
Old 28th Nov 2017, 6:40 pm   #1
merlinmaxwell
Dekatron
 
merlinmaxwell's Avatar
 
Join Date: Jul 2008
Location: Maidenhead, Berkshire, UK.
Posts: 7,419
Default Ampex Quad scanning

A quick question...
How many lines where stored per scan of a 2" Quad tape?
I ask this after watching "Secret War" (R.V.Jones) and noticed quite a few comet tails on the title video, all fairly stable.
__________________
Cats have staff, it's dogs that have owners.
merlinmaxwell is offline   Reply With Quote
Old 28th Nov 2017, 8:54 pm   #2
PaulM
Pentode
 
Join Date: Nov 2006
Location: Lincoln, UK.
Posts: 249
Default Re: Ampex Quad scanning

By 'comet tail', do you mean white dots in a regular pattern which is what a quad can produce, or on video highlights?

The term usually applies to highlight smearing on tubed cameras and arises from a bright light/reflection not being fully discharged.

In a quad, there's a potential big problem with head switching because it's in the visible frame time (unlike, say, a VHS two head machine where the switching can only be seen at the bottom of the picture). If the head switch timing is wrong, there can be no video during the head changeover and the FM limiter produces noise, seen as white. There's twenty segments per field on 625/50 quad, so each field has 312.5/20 lines = 15.6 lines (double that per frame). The visible artefact is usually a zig-zag pattern of dots, usually 'trembling' a little as the servos can never be 100% stable.

Best regards,

Paul M
PaulM is offline   Reply With Quote
Old 28th Nov 2017, 9:58 pm   #3
red16v
Pentode
 
Join Date: Jul 2006
Location: Winchester, Hampshire, UK.
Posts: 155
Default Re: Ampex Quad scanning

As PaulM says you record 15.625 lines per sweep - but you cannot replay that because you would have a head switch (disaster) in the middle of the active line. So, on replay the actual number of lines read off the tape varies per head as you have to either advance or retard the switch point to occur in the horizontal blanking period. The sequence of integer lines read back per head revolution (starting with head 1) is 15,16,16,15,16,15,16,16 so over 8 head passes you have your average of 15.625 lines.
red16v is offline   Reply With Quote
Old 28th Nov 2017, 10:49 pm   #4
PaulM
Pentode
 
Join Date: Nov 2006
Location: Lincoln, UK.
Posts: 249
Default Re: Ampex Quad scanning

Yes, that's quite correct, I was keeping it to the basic theory. In practice the switching point was a bit imprecise (especially with earlier machines) and visible switching can often be seen on archive recordings. Precisely switching things on a head wheel running at 15,000 rpm with the technology of the day was pretty demanding.

Odd that this has just come up! I'm currently doing a 2" Quad transfer as a favour for somebody at the moment (RCA TR70B) and I'm seeing head switch artefacts which the head switch delay timing pot won't remove. It's a Fuji tape and in very good condition (no sticky-shed at all - wonderful). Otherwise it's all good. Yes, my machine appears to be working fine on other known-good tapes, including colour bars. Bad recording, I think.

Cheers,

Paul M
PaulM is offline   Reply With Quote
Old 29th Nov 2017, 10:14 am   #5
red16v
Pentode
 
Join Date: Jul 2006
Location: Winchester, Hampshire, UK.
Posts: 155
Default Re: Ampex Quad scanning

Obviously I don't know anything about your recording on the Fuji tape, but we should remember that when recording there is no head switching taking place. It was quite usual to the record signal to be fed continuously to all 4 heads at once (or indeed opposite pairs) *- playback being a different kettle of fish entirely. Maybe your Fuji recording is a dub of a poor original playback as you suspect? It would explain why adjusting the head switch position is not effective. Even if the playback head switching point is 'duff' I would be very surprised to see it on the output of the machine - it should all still be contained within the back porch and this should all be being blanked off in the output proc amp circuitry. Too may variables I guess! Anyway, well done on keeping the machine in good working order.

* As you may know Ampex allegedly had the patent on record head switching which was an absolute boon when editing. It meant you could perform a video only insert edit without affecting the audio tracks. On the RCA machines the record heads recorded over the entire width of the tape including the audio tracks obviously. So, if you were performing a video only insert edit the video heads recorded right over the existing audio tracks too! It meant you got a very distinct audio 'buzz' over the output audio for the duration of the video only edit. The only solution was to lay the sound off to a recording device (aka quarter inch tape) and lay it back after you had performed the video only edit.
red16v is offline   Reply With Quote
Old 29th Nov 2017, 2:03 pm   #6
PaulM
Pentode
 
Join Date: Nov 2006
Location: Lincoln, UK.
Posts: 249
Default Re: Ampex Quad scanning

The artefacts do move when adjusting switching point delay offset, although given the age of the material (late 60s) it's likely a dub as I don't think Fuji was making 2" tape then? I don't know that for sure, but the tape looks far too clean and good! The problem is bearable, but annoying. Another possible to making switching visible is mechanical alignment but I've ruled that out as I have plenty of head panels and they all do the same - but what if it was on the recording machine? Too many things to go wrong, I've always thought.

I've never had to do any Quad editing myself, so the 'buzz problem' (which I'd heard of) hasn't arisen. If there were enough space in the workshop, enough hours in the day and days in the year I would like to run one of my Ampex Quads up (2x AVR3, 1xAVR2, 1xVR1200), but the RCA just sits there and works (most of the time), so no contest really.

Cheers,

Paul M
PaulM is offline   Reply With Quote
Old 29th Nov 2017, 6:35 pm   #7
merlinmaxwell
Dekatron
 
merlinmaxwell's Avatar
 
Join Date: Jul 2008
Location: Maidenhead, Berkshire, UK.
Posts: 7,419
Default Re: Ampex Quad scanning

Quote:
By 'comet tail', do you mean white dots in a regular pattern which is what a quad can produce
Yes, but these are black. 15/16 lines seems to fit what I am seeing.
__________________
Cats have staff, it's dogs that have owners.
merlinmaxwell is offline   Reply With Quote
Old 29th Nov 2017, 7:37 pm   #8
red16v
Pentode
 
Join Date: Jul 2006
Location: Winchester, Hampshire, UK.
Posts: 155
Default Re: Ampex Quad scanning

Quote:
Originally Posted by PaulM View Post
I've never had to do any Quad editing myself, so the 'buzz problem' (which I'd heard of) hasn't arisen. If there were enough space in the workshop, enough hours in the day and days in the year I would like to run one of my Ampex Quads up (2x AVR3, 1xAVR2, 1xVR1200), but the RCA just sits there and works (most of the time), so no contest really.
No quad editing!!! Perhaps Santa may give you something currently in his garage to cure that if you’ve been a good boy this year.
red16v is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 6:31 am.


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 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.