[QuadList] Fwd: FM standards in 525 and 625

[Don Norwood]

Chris:

I wonder if you may be correct that there could have been a change in later years.  Manuals for the VR-1200, 1200B, 1200AFC, VR-2000B and TR-70 all agree on 5.00/5.54/6.8 mhz for 625 low band.

But the AVR-2 and TR-70C agree on 4.95/5.54/6.8 mhz

So the sync tip frequency is slightly different between early models and late models.  Perhaps there was a change in the spec for the sync amplitude that resulted in the change in deviation?  I was able to find this illustration that confirms the 70/30 ratio http://www.pembers.freeserve.co.uk/World-TV-Standards/HBI.pdf but I did not locate any documentation on a change in the early to mid 70's.

Don Norwood
Digitrak Communications, Inc.
www.digitrakcom.com
  ----- Original Message ----- 

    From: Chill315 at aol.com
    Date: November 21, 2013 at 9:36:32 PM EST
    To: quadlist at quadvideotapegroup.com
    Subject: FM standards in 525 and 625


    I just hung up the phone with Ted Langfell, our illustrious secretary.  We are having a discussion about the standards used for recording signals on a quad.  This came about because a manual for an AVR-1 had a frequency for blanking different than a VR-1200 manual.  He believes it was 5.3 vs 5.6 MHz.  He is driving so can not check the email.  

    So here is the research that I did while on the phone.  I continued after we hung up.  Both RCA and Ampex tell us that they conform to the SMPTE and EBU standards. The referenced standards are SMPTE RP-6 for Modulation Levels.  The RP-10 standard describes the Signal Specifications.  The 625 system conforms to CCIR Document DOC. X/27E-3 as set forth from the EBU.

    525 Low band mono is from 4.28 for sync tip,  5.0 for Blanking, and 6.8 for peak white.  Pre / post emphasis is 4 db at 4 MHz.

    525 Low Band Color is from 5.5 for Sync, 5.79 for Blanking, and 6.5 for peak white.  Pre / post emphasis is 9.54 db at 4 MHz.

    625 low band is sync tip at 5.0, 5.54 for blanking, and peak white at 6.8 MHz.  Unknown emphasis curve.  

    I did a math solution and came up with 5.54 MHz for the blanking.  This was based upon a linear relationship based upon the .7 to .3 ratio of video and sync as published by Ampex.  This is the specification that is in question.  

    But the top frequency is for PEAK WHITE.  does that mean over 100 units of video?  If not, how much?  If 120 video plus sync, then blanking is lower in frequency.  An interesting math problem.  And a good discussion.  Also the modulators clamp at Blanking so in reality the deviation is kind of controlled by the video level alone.

    525 High band is 7.06 for sync. 7.9 for blanking, and 10.0 for peak white.

    625 High Band is 7.16 for sync. 7.8 for blanking, and 9.3 for peak white.

    Super High Band is 9.2 for sync and 12.2 for peak white.

    All information is from VR-1200 and TR-4 manuals.  I also referred to Ennes's book on VTR theory.

    So what is the reason for the discrepancy?  A misprint?  The sales literature does not list the frequencies.  Or was there a change by the EBU or SMPTE in the standard after the design of the AVR-1?

    As a side note, we know that there were different standards when both AMPEX and RCA first introduced the machines.  That is why there was a special setting on the TRT series demod's that allowed you to screwdriver adjust the curve to get proper playback.  This also appears on the TR-22 and other low band quads from RCA.  The standards were settled early on by the SMPTE as the governing body.  Both RCA and AMPEX licensed each other so that the machines would be compatible.  So if one developed a new standard, the other had to adopt it for conformity.  We did not need the Beta vs VHS was in broadcasting quads.

    Chris Hill
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