[QuadList] QuadQuiz answers--Why RCA chose 6-mil head width for Super High Band experiments

[Ted Langdell]

Harold has the correct answers.

On Jul 1, 2013, at 1:40 AM, trandoc at aol.com wrote:
> Ted,
> 
> Super High Band allowed the use of 7 1/2 IPS to achive the same S/N and needed narrow heads to avoid guard band issues. 
> 
> I guess Type C doomed quad use by this time?


Here's the background:

RCA's Koichi Sadashige outlined RCA's research into Super High Band in a presentation to the Society of Motion Picture and Television Engineers' Technical Conference on April 25, 1974.  The paper, titled "Improving the Performance of Quadruplex Videotape Recorders Operating on the U. S. Domestic Standard" was published in the Sept. 1975 edition of the Journal of the SMPTE beginning on Page 714.

Sadashige said the 1974 presentation was intended to inform users about RCA's research and to ask whether the Super High Band/Pilot system should be presented as a standards proposal.

RCA's research followed surveys of US and world-wide Quad users about current and future needs.

Sadashige reported that, "highest user interest was in the operation at	7 1/2-in/s	(19-cm/s) tape speed with performance essentially equivalent to that achieved at 15 in/s (38 cm/s)."

RCA's experiments involved a TR-70C modified to produce FM Carrier deviation from 9.2 to 12.2MHz. They used a 6 mil (152µm) active track width, and operated a headwheel panel with 45µin (1.4µm) gap length and pole tip protrusion of 2.5 mils (63.5 µm) at the early stage of head life. 

Conventional gamma iron oxide tape was used, and at 7.5 ips, a video signal to noise ratio of 46dB, "comparable to normal 15-in/s operation is obtained," Sadashige reported.

The results also noticeably reduced moiré since the lower sideband components responsible for moiré were in the fifth lower sideband instead of in High Band's fourth lower sideband.

Sadashige explained why they picked  6 mil (152µm) active track width this way:

"For these 7 1/2 in/s operations, we have chosen to use a head track width of  6 mils (152µm) rather than the conventional half-speed track width of 5 mils (127µm). 

Mechanical accuracy of the headwheel construction presently specified in the SMPTE Recommended Practice indicate that the minimum width of the guard band which will safely allow the interchange of recordings and head- wheel to be approximately 1 mil (25µm). Total inaccuracy of tracking due to the electronic circuits is on the order of 0.2 to 0.4 mils (5 to 10µm) in a contemporary tape transport equipped with a dc-drive capstan.

The track pitch of 7.5 mils (190µm) at 7 1/2 in/s thus consists of a 6-mil active track width and a 1.5-mil (38-µm) guard band. The 6-mil track width headwheel panel, while producing 20% more FM output than the 5-mil panel, contributes materially to restoring the half-speed video SNR to the level of the normal- speed operation. An additional advantage is that the 6-mil panel can play back a normal 15-in/s recording with only minor performance degradation.

Our	experiments indicate that	 a SNR of 44 dB or better normally prevails under these conditions. This is a very useful advantage when one considers the possibility of operating all quadruplex systems with only one type of headwheel panel."

RCA investigated using a Pilot-tone locked to incoming color burst in conjunction with Super High Band.  The goal was to pull time base and velocity error information during playback and to correct it. The amplitude of the recovered pilot signal allowed accurately correcting chroma amplitude errors.

The lab work found that there were circumstances where the pilot tone could cause discernible interference in the picture, due to spurious signals being generated by the process.

RCA chose the 9.2 to 12.2 deviation "to keep the entire carrier range above the sum of the color subcarrier frequency and the pilot tone frequency," Sadashige advised.  "Under these conditions, the video pilot  technique functions satisfactorily without being adversely affected by the spurious signals."

A different deviation was used for 625 line modulation.  Responding to a question from Ampex's David Fibush, Sadashige replied:

"The sum of the color subcarrier and pilot frequencies is less than the lowest carrier frequency for 525-line systems, so here the problem of spurious signals does not exist. The problem with 625-line systems is dealt with in essentially the same way: the FM frequency band is chosen so that it does not overlap the sum of the color subcarrier and pilot tone frequencies. Specifically. the FM frequencies for 625-line super-high-band run from10 to 10.85 MHz while the sum of the pilot tone and color subcarrier frequencies is 11.07 MHz."

The presentation also reports on what RCA did that resulted in improved sound quality, aimed at enabling two-channel audio that was able to hit 15KHz top end, and improve SNR.

I've not had time to identify whether the Super High Band/SHB Pilot ever made it to becoming a standard.

Ted

On Jul 1, 2013, at 1:40 AM, trandoc at aol.com wrote:

> 
> Ted,
> 
> Super High Band allowed the use of 7 1/2 IPS to achive the same S/N and needed narrow heads to avoid guard band issues. 
> 
> I guess Type C doomed quad use by this time?
> 
> Harold
> 
> 
> 
> -----Original Message-----
> From: Ted Langdell <ted at quadvideotapegroup.com>
> To: QuadList <QuadList at quadvideotapegroup.com>
> Sent: Sun, Jun 30, 2013 4:59 pm
> Subject: [QuadList] QuadQuiz--Why did RCA choose 6-mil head width for Super High Band experiments
> 
> 
> What was RCA's rationale for choosing to use 6-mil heads for their experiments with Super High Band on a TR-70C?
> 
> Some SMPTE members may know the answer.
> 
> Bonus question:  Why didn't Super High Band (or Ampex's Super High Band Pilot) get any marketplace traction.
> 
> Good luck and hope your part of the country is staying relatively cool.  
> 
> We're at 108, our expected high for today... heading for 109 tomorrow and 111 Tuesday and Wednesday in the central Sacramento Valley of California.
> 
> Is that hot enough to melt Quad Tape on the Sidewalk?  The forecast says it'll be hotter than the Fourth of July.
> 


Ted

Ted Langdell
Secretary
Skype: 	TedLangdell
e-mail:	ted at quadvideotapegroup.com



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