Wednesday, June 1, 2011

Letter to KPAC

Dear KPAC Broadcast Engineer,

I have been bothered by a high pitched buzz when playing KPAC in stereo, but not in mono.  After doing a number of experiments, I have determined that there is a spurious 10.5 kHz tone in the L-R stereo multiplex subcarrier as broadcast by KPAC.  I have digitally recorded a silent interval, and found the problem can be eliminated with a 10.5kHz digital notch filter having a Q of 10.  A slight peak there is also visible in a 12th octave spectrum analysis of L-R.  Though it is hard to see in a spectrum plot amidst the random noise, it is annoyingly audible.

The problem does not seem to originate in any of my tuners, my stereo, or my receiving location.  I have tried several different tuners, in two different locations including one fairly close to the transmitter, and have also tried using only headphones.  I do not get this problem on any other station, including stations with HD or RDS subcarriers.

The tuners I have used include some of the best ever designed, including a Kenwood KT-6040 that was designed in the 1990's to handle the RDS subcarrier at 57kHz; it is described as "birdie free" by well known FM experts.  I have also used a Pioneer F-26, highly praised by FM experts as one of the 3 best tuners ever made, and which was fully refurbished and aligned in April 2011 by the nation's leading tuner restorer.  Interestingly, it seems the better the tuner is the more clearly and annoyingly I hear the 10.5kHz buzz.  On lesser tuners, the buzz sounds like a strange kind of hiss which is more easily ignored.  (Or perhaps the strange undulating hiss on those tuners is yet another problem I haven't figured out yet.)

The buzz can be eliminated by listening in mono or using "high blend" to reduce high frequency stereo noise at the expense of stereo separation.  But my favorite tuner, the Pioneer F-26, does not have a high blend control, and I do not like reducing stereo separation.

Many people might not notice this problem because many tuners nowadays auto-blend until all noises are gone; that is what nearly all car radios do now.

By and large, the sound quality and musical content of KPAC is excellent and that is why I have spent so much on analog FM tuners and in investigating this problem.  I hope the problem can be fixed by servicing or refurbing existing equipment rather than buying new digital equipment which may be of inherently inferior quality.  I am especially opposed to the use of HD digital radio, which in my experience is always inferior to old fashioned analog FM radio.

You can read about some of my experiments on my audio blog.  I have also posted questions to the Yahoo group FMtuners, and the verdict was that I should report this problem ASAP to the station engineer.  I am sorry it has taken this long.

I appreciate your attention to this issue, and it would be great if you could keep me informed as to the status  and/or possibility of quick resolution.  I may be able to get help from a network of FM experts if needed (such as obtaining service manuals and insider tips for specific older station equipment).  If you find my analysis to be faulty, please let me know, and in that case I would like to know what subcarriers you use on KPAC, which may be helpful in correcting the problem on my end.  I expect I will shortly be able to post an audio/video demonstration of the problem I have found so far to my blog.  I could email you the 1.7 second L-R wave file now if that would be helpful.

Charles Peterson
TPR Member #

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