In article ,
Tim Brown wrote:

In article ,
Ron Hardin wrote:

Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660,
does anybody else?

The R8B hears it except with DSB synch detection, so there's some
relation between I and Q channels that cancels the hum.

In article
Ron Hardin wrote:

It's still there at 4:30am EST. It's not audible at all if you're
hearing
both sidebands but obvious in quiet phone call pauses (no music, not fan
noise). It's some difference between the upper sideband and lower
sideband
that gets modulated by hum; so if you don't hear them equally the hum
starts to creep in, and at some point becomes noticeable. (So propagation
at night can make the hum come and go if you're hearing both sidebands,
by
depressing one and raising the other.)

WFAN's transmitter is diplexed into a common antenna with WCBS's (880
kHz) transmitter. Part of the diplexing arrangement includes impedance
matching networks that are supposed to equalize the antenna load
impedance reflected towards the transmitter output over the sideband
bandwidth. I suppose they could be slightly out of adjustment, causing
asymmetrical sideband response. One side effect of asymmetrical response
would be the generation of AM-to-PM.

At the joint transmitter site are older Continental transmitters that
function as backup for the regular Harris DX50 transmitters. Maybe they
have one of the Conti's on occasionally?

Most of these high power transmitters run off of 480V 3-phase power.
Ripple frequency with a straight 3-phase rectifier would be 180 Hz, not
120 Hz. A popular supply arrangement is the 3-phase 6-pulse rectifier
which outputs a 360 Hz ripple frequency. 120 Hz ripple frequency would
most likely originate in either an audio or low level RF stage powered
from a conventional 2-phase rectifier.

I'll take another listen tonight.

Okay. I set the receiver to USB and tuned in WFAN this evening and can
now hear the "hum". The frequency sounds more like 125 Hz, not 120 Hz,
which is significant. Assuming that they still have their CQUAM exciter
patched into the transmitter, my guess is that we are hearing the 5th
and other odd harmonics of a 25 Hz stereo pilot square wave. This would
be phase modulating the carrier, which is why it is not audible when
receiving the station with a conventional envelope detector.

One way to deal with this would be to make a really clean recording of
some "hum" infested program material, burn it onto a CD-R, and mail the
CD and a -nicely- written letter to the corporate director of
engineering at Infinity. I'll wager you, it will get fixed really fast!

Tim Brown wrote:
I'll take another listen tonight.

Okay. I set the receiver to USB and tuned in WFAN this evening and can
now hear the "hum". The frequency sounds more like 125 Hz, not 120 Hz,
which is significant. Assuming that they still have their CQUAM exciter
patched into the transmitter, my guess is that we are hearing the 5th
and other odd harmonics of a 25 Hz stereo pilot square wave. This would
be phase modulating the carrier, which is why it is not audible when
receiving the station with a conventional envelope detector.

One way to deal with this would be to make a really clean recording of
some "hum" infested program material, burn it onto a CD-R, and mail the
CD and a -nicely- written letter to the corporate director of
engineering at Infinity. I'll wager you, it will get fixed really fast!

Here's some short .wav recordings from around 5am (apparently ``The Continent'' Carlin
does overnights on weekends, nice career arc; luckily he has a speaking style that
produces a lot of dead air)

http://rhhardin.home.mindspring.com/HUMUSB.WAV (31k) synch det USB (4:52am EST)
http://rhhardin.home.mindspring.com/HUMDSB.WAV (17k) synch det DSB (4:54am EST)
http://rhhardin.home.mindspring.com/HUMLSB.WAV (27k) synch det LSB (4:56am EST)
http://rhhardin.home.mindspring.com/HUMAM.WAV (28k) AM (4:59am EST)
http://rhhardin.home.mindspring.com/HUMAMU.WAV (27k) AM detuned to 662.400 (5:08am EST)

I see lines at 120 240 360 480 600 720 840 1080, the lines above 480 being faint,
with ``birding'' software that has a granularity of about 20Hz, but in particular
it doesn't seem to have any lines that would reflect a low frequency around 20 Hz.

These recordings all went through a Timewave 599zx DSP to get the AGC to jack up dead
air loudness. The lines are all visible in any program material but the ear doesn't
notice them when something else is there too, so much.
--
Ron Hardin


On the internet, nobody knows you're a jerk.